Scouring damage at Coastal berths

[Low Light Mike]

The topic of berth/terminal damage caused by the prop-wash from the Coastal ferries appears to be a developing news story, and so I created this thread for us people to discuss our ideas of the problems and solutions:

-------------------------------------

Here are some posts that we had earlier today in the massive C'Ren thread:

Apr 15, 2009 6:41:10 GMT -8 Low Light Mike said:

Chris Montgomery's Province-blog has a couple of stories on "Coastal Erosion" at the terminal berths, specifically at Tsawwassen and at Departure Bay.

Here's the web address communities.canada.com/theprovince/blogs/onthewaterfront/default.aspx

I've posted a few comments on that blog; it looks to be a developing story. I wonder how serious the Coastal-Erosion problem could be, and if it would impact the structural-integrity of the new pax-building being constructed at Departure Bay?

Apr 15, 2009 7:36:40 GMT -8 DENelson83 said:

I guess BCF will have to construct some Coastal-proof metal walls underneath the berths.

--------------------------

I'm intrigued by the possibilities of this issue, and the impact on terminal operations. I can think of various doomsday scenarios (a building collapse at Departure Bay berth 3) or a sink-hole in the lane-way to the berth.

I can also imagine closures at Horseshoe Bay during the busy summer of 1 berth; that would cause traffic nightmares at that terminal.

-------------

We've got 5 terminals that berth Coastals. 4 of the 5 have rock-fill near the berths and have terminal structures and building housed above the fill:
- Departure Bay (has the brand-new pax terminal and the under-construction waiting-building right beside Berth 3)
- Horseshoe Bay (has a pax waiting room right beside the berth)
- Swartz Bay (has the SGI pax waiting room right near berths 2 & 3)
- Tsawwassen (has the 2-story pax-waiting room right beside berth-3)

Duke Point has natural rocky shoreline, with the terminal buildings up the hill, away from shore.

---------------------------

What are the chances that a safety-authority of some kind (whoever regulates safety at the terminals) would shut-down part of the terminal, right as busy-season is heating up?

What are the odds of a building collapse for one of the pax buildings near the berth?

Is this going to be a pervasive (affecting at least 4 major terminals) and expensive problem for BCFS?

[Neil]

Interesting story. I'm waiting for the reaction from the 'Society for the Prevention of Unkind Remarks About Coastals'. The usual response of "Blame The Tyee" won't apply here.

If this story is true, how is it that the Coastals have managed to do so much damage at Tsawwassen in only a few months, when the equally powerful Spirits have been using the terminal for many years without similar consequences? And, is this a result of idling while berthed, or the arrivals and departures?

I don't recall reading anything about tests by Flensburger on the erosion capabilities of these vessels. I wonder if this is something BC Ferries could have foreseen when the propulsion systems were designed.

[rt1commuter]

Interesting point vis-a-vis the spirits. It must be source of propulsion that makes the difference. The Spirit's thrust is much more diffuse, since the thrust is coming from two props. The maximum pressure on the shore is then probably lower, even though the sum total thrust is about the same. The coastals, operating with only one powerful propeller may produce a higher peak pressure on the shore.

One things for sure, the prop wash induced by the coastals appears to be much stronger than that of the spirits.

[boardsailor]

I have a impression that of Spirit versus Coastal propulsion arrangement and its effect on berth is misunderstood. Spirit is pushing with to the dock with two props at the stern of the vessel. Coastal is double ender and has props on both ends. While maneuvering to the dock is in Mode II, that mean both propellers are in use on both end of the vessel. Once in the dock, stays in Mode II with forward prop feathering( minimum trust or pull) and aft prop holding ship in the dock. Imagen, forward propeller 17 feet diameter revolving at 130 rpm just about 20 feet from docks wall! The turbulence created is tremendous! Just take a look at the water near dock end of the vessel. The other C class have a ability to reduce forward props rpm once in the dock. Super C are so called constant shaft speed configuration and here is whole problem! Is no provision to reduce shafts rpm and reduce props wash while in the dock.
The set up of machinery is such that due to short time remaining in berth it is not practical to
just shut down forward end( the one in the dock). Why, because while leaving the dock the shore end prop is pushing ship out of the dock. As it is, for Super C, docking and undocking is most time consuming as it is. It takes sometimes longer than offloading and loading combine.
For some reason the benefit of double enders quick approach and departure is not achieved.

[Nick]

This is something most of the terminal employees have been screaming about for the past 6 months. I'm surprised it has taken this long to come to light.

The point about maximum pressure being less with the Spirits is a good one. Also, at Tsawwassen, the Spirits are pushing into the berth bow-first, so the propellers are at the other end of the berth and away from the vulnerable structures. That said however, at Swartz Bay it is the complete opposite, where they are berthed stern-to.

The Spirits have been using the same berth at Swartz for the past 17 years without any hiccups. The Celebration has had to use berth 2 at Swartz Bay, one of the reasons being the wash it produces against the shore under the ramp structure. When they were doing trials back in October/November, there was so much wash from the bow propeller that the ramp decks were getting soaked.

Now, why the Coastals produce so much wash, I don't know. They are large ships, and need to move a lot of water to slow down, but there seems to be another problem at work here. Without more information, all we can do is speculate.

[Mill Bay]

On the Coastals, the propellers are very close to the surface of the water and at times even break the surface. This not only impacts the efficiency of their ability to function properly in terms of propulsion, it also creates the huge amount of white water. In addition, when docking, the propeller at the bow is suddenly forced to push against the ship's forward momentum which requires a lot of power to produce the necessary thrust. That's where you get the excessive white water from, but all that energy being churned out through the water has to dissipate somewhere, and the first things it hits are the retaining walls behind each berth and the ocean bottom of each berth.

The Alberni comes and goes from TSA all the time, as do other C's occasionally, and there has never been much trouble with her undermining the shoreline. She has to go into mode II as well to enter the dock, so it doesn't seem likely it is the single act of docking that causes so much damage, although maybe the Alberni doesn't create quite as much wash as the Coastals. I would look for the underlying cause to be the cumulative effect of the propeller remaining in operating in roughly the same location in the water for a longer period of time. The cause for this is found in the fixed speed drive motors on the Coastals which prevent soft control of the propeller while the ship is idling. If that is indeed the cause, then we can only say it is BCFerries own fault because they insisted on that arrangement despite judgment calls by FSG about the feasibility of fixed speed motors--the only marketable difference was fixed speed motors were cheaper.

Perhaps, in the course of making a thoughtless gesture, thinking they could impress us all by appearing to be shrewd managers and endeavoring to keep costs down by saving a few thousand dollars on a multi-million dollar project, BCFerries may have thought to put those savings into a hedge fund to pay for erosion repairs which will now likely cost us (the customers they were trying to impress) more than any money they 'saved' in trying to appear as wise investors with other people's money.

[WettCoast]

It is also interesting that Christina Montgomery is quoting The Tyee as if to say the stories written by Andrew MacLeod have substance. She even credits Mr. MacLeod as having the scoop on the Colin Henthorne story. Perhaps the members here who continually put down The Tyee might also want to put down Ms Montgomery?

[Dane]

The fact this is being considered as a design flaw shows a fundamental lack of understanding regarding the issue to me; more later, exam in 11 hours <G>

WCK, to refresh your memory, here is what she actually said wrt the Tyee:

For a refresher, you can find The Tyee's roundup of the ferries' possible flaws here. (emphasis added)

[Neil]

Apr 15, 2009 20:52:11 GMT -8 Dane said:

The fact this is being considered as a design flaw shows a fundamental lack of understanding regarding the issue to me; more later, exam in 11 hours <G>

WCK, to refresh your memory, here is what she actually said wrt the Tyee:

For a refresher, you can find The Tyee's roundup of the ferries' possible flaws here. (emphasis added)

No need to 'refresh anyone's memory', Dane. We can read, too. I think the point that WCK was making was that this story did not come from The Tyee, which is a source criticized by some who seem very certain of their technical insights.

If these vessels are causing serious erosion issues when operating as intended, then the question of a design flaw is a valid one.

[rt1commuter]

I don't know if the constant speed props are what's to blame. The thrust produced is a question of pitch; if the props are feathered then they aren't producing any thrust anyway, and while they may be frothing, no water is directed at the shore. When I hear discussions of 'caves', I'm thinking the erosion is being caused by high pressure on the shore; which I expect is highest during start-up. The net thrust on the shore during slowing down should be substantially less than the thrust during start-up because the boat is moving and pushing against water that moving in the opposite direction to the boat, thus much of the momentum is absorbed changing the direction of the water flow.

As for white water, it's somewhat irrelevant to the erosion we're talking about here. I suspect the erosion is being caused by a strong stream of water, and it doesn't matter how deep or shallow it is. Given a deep prop and a shallow prop producing the same net thrust on the boat, the deep prop will produce a stronger force on the shore than the shallow prop will because some energy will be lost to generating froth.

It's important to consider that the Coastals are about the same size as the Spirits; the thrust they produce to get moving has to be about equal to what the Spirits are producing, except from a single source, and, as I suggested earlier a larger maximum pressure on the shore.

[Dane]

Apr 15, 2009 21:09:10 GMT -8 Neil said:

I think the point that WCK was making was that this story did not come from The Tyee, which is a source criticized by some who seem very certain of their technical insights.

If these vessels are causing serious erosion issues when operating as intended, then the question of a design flaw is a valid one.

I am going to have two responses in the one thread here.

To your first quoted comment, which I really hope is not directed at me, I think further elaboration is required as clearly the points made earlier by myself, among others, were not seen as "good enough." To recap my opinion on why the Tyee, in general, is incorrect about many of their claims regarding the Coastal class:

Vibrations - they say it's a problem. I agree, there are indeed some vibrations created from the 20,000+ HP engine strapped to the metal hull, but as a passenger I am saying it's not unusual or uncomfortable (although since my original posting I would say it's unusually strong on the CI pushing into Duke Point). Crew concern has also been noted on the issue, and it would seem that this concern generally comes from those working at the ends of Deck 5/6. This issue was, in my opinion, misrepresented and overblown.

Cavitation - this is where I had the biggest issue. I was the first to admit, and continue to admit that I don't actually understand what cavitation is. That said, I can hack my way through an article with relative ease and the "supporting document" the Tyee used had direct contradictions to the source of the material. That raises alarm bells for accuracy to me.

Noise - The Tyee reported the noise issue as new, and ongoing, after the instillation of mufflers (not mentioned) on the CR and CC. While come complaints have persisted, based on what I have heard, the major issue has been resolved. Failing to make even slight acknowledgment of this speaks to the overall quality of the article. When the Coastals came into service the Deck 7 enclosed lounges were unpleasant to be in at times due to the sound. This is no longer the case.

Fuel Efficiency - FSG met the BCF contractual requirements for delivery of the vessels, so there couldn't have been any big surprises there, or if there were they were agreed to (which I personally wouldn't believe). BC Ferries did advertise, arguably with poor judgment, that there would be a savings. Obviously, compared to the vessels they were replacing there was not. Some members posts seemed to suggest that this was a flaw with the vessel. I maintained, and continue to believe that this is a confounding of issues. Poor marketing doesn't mean poor ship design, which the Tyee implied quite strongly.

There does seem to be issues with some areas of the Coastal class, there have and continues to be issues with effectively every vessel in the fleet. From a passenger perspective, however, these ships do represent a great leap forward from the V's and C's. Another bone of contention I have had is that a lot of commentary on the anti-Coastal front dealing with the passenger experience/amenities was coming from people that had never been on the darn things! From an average Joe viewpoint the Spirits probably continue to be the 'best' mainline vessels, but the Coastals are pretty great too.

Ferries have always been a political thing in BC, obviously. The Tyee is unabashedly anti-Liberal, so anything they bring forward will be demonized in almost any circumstance. Similarly with the Spirits when they came online, just with different parties. Failure of the Fast Ferries also seems to have stemmed hyper-sensitivity in the area from media, politicians, and observers.

Those of us that question the Tyee are not ill-informed as some posts as of recent appear to imply, nor are we arm chair Captains.

--

With respect to the issue at hand....

We had heard about this earlier, albeit somewhat briefly with respect to wash and terminals. I must admit I am struggling to remember if the discussion was in person, or on the forum. It has been subject to a bit of attention at Horseshoe Bay among the "Sewells crew." At Horseshoe Bay, where Flugel noted on the blog itself would be difficult to work on, some reinforcement was actually done about 10 years ago for the PacifiCat project (and Departure Bay). It would be curious to see if the issues are as pronounced at these two terminals? At Departure Bay Chris mentions Berth 3 - which I believe is not the berth usually used by the CR, but rather the Queen of Cowichan. Each generation of major ferry brings with them new issues like this, and response always seems to be delayed,

In the blog it is mentioned that BC Ferries had assigned one of their top people to deal with the erosion issue as soon as the Coastals came on line. Given the performance requirements BCF had against FSG's product, I find it incredibly hard to believe that the wash was beyond the level expected, rather it seems more likely to me that planning for this was not as complete as it needed to be. In Horseshoe Bay the wake deflector was a policy put forward to address the (at the time) forthcoming Coastal class. There has been some issues at Sewells, and I have heard several people complain, although the wake is about equal to the Surrey Tsunami, which also is the subject of many anti-BC Ferry complaints. I certainly wouldn't want to have the CR going in against that rock wall, that's for sure.

None of these comments are meant to minimize the issue at hand. Erosion under terminals and structures is obviously bad news, and an expensive repair. Hopefully it is dealt with quickly before we get into the swing of full on summer which is around 29 June, I think.

What I do believe is that we are setting out on a bit of a witch hunt, again, which doesn't seem justified within the specific context of the thread.

Can't we all just pick on the NorAd again ;D Only defend her because I feel bad lol.

[Northern Exploration]

Paul a very reasoned, sensible and normal approach to this issue. But hardly a good way to sell papers or generate controversy.

Seems like there is an unseen force at play akin to lunar tides. There is a definite cycle. When things quiet down and BC Ferries hasn't done anything wrong, something has to be magnified out of context and everyone go screaming down the streets, arms flailing in the air, over reacting (that visual makes me chuckle - like those stupid blowup gorillas or whatever cheapo car dealers rent to get attention). Or at least that is what some media wishes will be the reaction. I assume that for the entire lives of the Coastals or until something new takes their place, they will be the target of extra scrutiny - some of which will be based on facts and others on pure nonsense.

In this case, it is entirely plausible that the extra thrust of the Coastals is causing additional errosion, particularly during the period when the ferries are reversing to slow down on approach. It is easy enough to fix and remedies are likely quite known to marine engineers who specialize in these sorts of things. IMO it would be silly to design ferries "afraid" that they would cause errosion at terminals. Errosion is a fact of life around the water. It can and should be dealt with before some bigger problem ensues. Dump some rocks or build some new casements.

Much is said about the wash of the Coastals and the fact that the props may break the surface. This really isn't a negative at all in some situations. Arneson Surface Drives used for speed application on boats (mostly yachts and high speed vessels) are an example of surface piercing props that are very efficient and designed specifically for the extra power they produce. I do not know enough specifically about the props on the Coastals to comment on them but I thought Markus commented on one of the threads that is wasn't an issue based on tank tests and live experience. Maybe he can weigh in what he knows.

[Low Light Mike]

Apr 16, 2009 7:17:34 GMT -8 Northern Exploration said:

In this case, it is entirely plausible that the extra thrust of the Coastals is causing additional errosion, particularly during the period when the ferries are reversing to slow down on approach. It is easy enough to fix and remedies are likely quite known to marine engineers who specialize in these sorts of things. IMO it would be silly to design ferries "afraid" that they would cause errosion at terminals. Errosion is a fact of life around the water. It can and should be dealt with before some bigger problem ensues. Dump some rocks or build some new casements.

Ok, focusing on issues from the above quote:
- If there is a problem and it needs to be dealt with soon, how will this impact ferry operations at the busy terminals during the summer?
- ie can this be fixed before busy time begins?
- is there a public-danger in the meantime?

Obviously I'm just throwing out the questions that come to mind as an armchair observer, and am expecting that we'll have to wait and see what actually happens.

And of course its debatable whether the problem really is easy enough to fix. I have no idea, I'm just speculating and enjoying the various ideas and points of view.

As I've said before, this issue is intriguing to me; although I'm more interested in the "how can they fix it?" than the "why could it happen?" issues.
------------------------------

ps: good on us, as we've found ways to use the words "Tyee", "Pacificat" etc in this day-old discussion.

[Mill Bay]

For inquiring minds...

cavitation
n.

1. The sudden formation and collapse of low-pressure bubbles in liquids by means of mechanical forces, such as those resulting from rotation of a marine propeller.

2. The formation of bubble-like gaps in a liquid. Mechanical forces, such as the moving blades of a ship's propeller or sudden negative changes in pressure, can cause cavitation.

... the process where a void or bubble in a liquid rapidly collapses, producing a shock wave.


What it essentially is, is a mechanical force causing air to mix with water creating whatever amount of wash, turbulence or 'noise' in the water. This is why submarines don't want to create cavitation because it will make them visible to anyone else listening for noise in the water.

[Northern Exploration]

I would expect for insurance reasons they have sent a diver down or done some investigating already. Unless of course it is hyperbole. If the situation is real and they haven't thoroughly investigated then a bit of Russian Roulette is being dallied with and that is never a good thing when safety is at stake.

I wonder what body would be responsible for shoreside operations? Ie. Does TC have any say about that?

I have seen temporary shoring done in conditions where a dock just down flow from a set of locks with a fair amount of outflow. Picture a foating dock turned on its side and covered with sheet aluminum and lashed against the side. In this case that was done and put in place quite quickly so to not impact lock operations. The water was flowing behind a rock outcropping and undermining a restaurant. Flow was exasperated by the gates at the overflow dam being worked on and more water diverted.

Another option shoreside that can be done is holes drilled and a concrete slurry is used that displaces the water.

A lot would depend on the underwater conditions I would think and what fill was used behind the coffers.

You are right Flug this will be interesting to see how it works out.

[rt1commuter]

In this case, it is entirely plausible that the extra thrust of the Coastals is causing additional errosion, particularly during the period when the ferries are reversing to slow down on approach. It is easy enough to fix and remedies are likely quite known to marine engineers who specialize in these sorts of things.

Are you certain it's during the slow down period when the maximum forces are encountered? If this is the case, why not use the rear prop to slow the vessel down (or a combination of both, which is what I believe they do now)? I'm trying to decide between the forces being the same, and the force being larger during start up. I have a hard time seeing why they would be larger during the slow down. Not that it really matters .

Apr 16, 2009 9:48:31 GMT -8 Mill Bay said:

For inquiring minds...

cavitation
n.

1. The sudden formation and collapse of low-pressure bubbles in liquids by means of mechanical forces, such as those resulting from rotation of a marine propeller.

2. The formation of bubble-like gaps in a liquid. Mechanical forces, such as the moving blades of a ship's propeller or sudden negative changes in pressure, can cause cavitation.

... the process where a void or bubble in a liquid rapidly collapses, producing a shock wave.


What it essentially is, is a mechanical force causing air to mix with water creating whatever amount of wash, turbulence or 'noise' in the water. This is why submarines don't want to create cavitation because it will make them visible to anyone else listening for noise in the water.

Not air! Not with Cavitation. In Cavitation what forms are bubbles of vapor --- a fast propeller creates zones of extremely low pressure, so low that the water instantaneously evaporates to form a bubble of vapor. This unstable bubble then rapidly collapses as the pressure increases causing noise as well as severe erosion of the prop.

What's supposedly happening with the coastals is different, apparently it's the prop coming so close to the surface that some air gets sucked in; not technically speaking cavitation, but certainly a bad thing.

[DENelson83]

If you want to solve the problem of the Coastals' propellers breaking the surface of the water, why not put them on telescoping shafts with strong metal plates inboard of them?

[D'Elete BC in NJ]

Apr 16, 2009 11:15:03 GMT -8 rt1commuter said:

Apr 16, 2009 9:48:31 GMT -8 Mill Bay said:

For inquiring minds...

cavitation
n.

1. The sudden formation and collapse of low-pressure bubbles in liquids by means of mechanical forces, such as those resulting from rotation of a marine propeller.

2. The formation of bubble-like gaps in a liquid. Mechanical forces, such as the moving blades of a ship's propeller or sudden negative changes in pressure, can cause cavitation.

... the process where a void or bubble in a liquid rapidly collapses, producing a shock wave.


What it essentially is, is a mechanical force causing air to mix with water creating whatever amount of wash, turbulence or 'noise' in the water. This is why submarines don't want to create cavitation because it will make them visible to anyone else listening for noise in the water.

Not air! Not with Cavitation. In Cavitation what forms are bubbles of vapor --- a fast propeller creates zones of extremely low pressure, so low that the water instantaneously evaporates to form a bubble of vapor. This unstable bubble then rapidly collapses as the pressure increases causing noise as well as severe erosion of the prop.

What's supposedly happening with the coastals is different, apparently it's the prop coming so close to the surface that some air gets sucked in; not technically speaking cavitation, but certainly a bad thing.

Yes, let's try to explain this once again...and I'll try to keep it simple for a certain person who fades out every time I use a technical term! ;D

Water boils and vaporizes when it goes from liquid to gas. The temperature that this happens at is typically 100C at sea level, and we consider this to be the boiling point of water. A very important value as the Celsius scale uses it as one of its defining values...but I digress. The definition of 100C as the boiling point of water is only true at one standard atmospheric pressure as defined...move up a mountain and the boiling point of water drops. Eventually, if you lower the pressure enough, water will vapourize at room temperature.

Here is what is happening with cavitation. As the propeller turns, currents and eddies are generated. This has no direct connection with the propeller's position relative to the surface of the water, or the ocean bottom, for that matter (Actually, this does matter as it defines boundary conditions...and I see one audience member glazing over... ). In order for thrust to be generated, water molecules need to speed up...to be accelerated. As the water molecules are accelerated, other water molecules rush in to fill the vacancies left by the molecules now speeding away. But these replacement molecules have to accelerate, too, in order to move in to the vacancies.

If these replacement molecules move in at a slightly slow rate than the departing molecules have left, there is a next loss of molecules in a said location...this results in a loss in pressure in the location where we are observing. Now consider this happening over a period of time; eventually the number of molecules and, consequently, the pressure, will drop to a level where the boiling point of water is equal to the ambient temperature of the surrounding water. Consequently, water vapour will start to fill the low pressure area.

However, there is a certain amount of energy required for water to transition from the liquid state to the vapour state...actually, quite a bit of energy...more than it takes to get water from absolute zero to the boiling point...by almost two times. The water vapour draws the required energy from the surrounding water...effectively cooling it. We are now left with a very unstable state...the water temperature is now below that which boiling is supported. Thus the newly formed bubble collapses (often explosively) if there are any disturbances, releasing the energy back into the surrounding water as the water vapour condenses. And the process begins again...

Okay, I've tried to demystify and englishify (de-engineerify) the cavitation process. The process really is as simple as I described above...describing it mathematically is a little more difficult, as is predicting it.

[Northern Exploration]

Apr 16, 2009 21:25:53 GMT -8 @4roads said:

Apr 16, 2009 20:00:24 GMT -8 DENelson83 said:

If you want to solve the problem of the Coastals' propellers breaking the surface of the water, why not put them on telescoping shafts with strong metal plates inboard of them?

If you're thinking "Z" drives, lots of rework. "Z" drives aren't telescopic, but rather similar in scope to an inboard/outboard (shafts and gearing to redirect the power out, down and then out).

telescoping shafts probably wouldn't be as efficient as a straight
through shafts, even if those shafts are sectioned and geared
as is the case with a "Z" drive (or those aboard the Island Sky).

Any event, the biggest problem is expense and for what?

If that was such a great idea, it would have been best set to place when the propulsion system was installed at FSG.

Telescoping equipment requires hydraulic pumps, lines and cylinders, plus bridge located and engineering located monitoring insturumentation which adds to costs which needs to be weighted against efficiency.

When you have shallower draft it is common for the shaft from the engine to be at an angle. The prop on an angled shaft is less efficient. A Z drive allows the shaft to be horizontal rather than sloped and therefore the prop is in its more efficient attitude.

[Northern Exploration]

Apr 17, 2009 2:40:02 GMT -8 D'Elete BC in NJ said:

...and I see one audience member glazing over... ).

It is Friday, almost noon, and it is going to be 20C out. I am heading for a patio at Starbucks or a restaurant and the only glazing I am interested in, is the donut kind.

Propellor spinning = turbulence = make bubbles = make noise = over time can cause prop wear = not necessarily bad for efficiency but can be. Cavitation not the same as surface wash. I think that is what BCinNJ meant to say. Maybe not the most technically accurate but does the job .

I will try to search it later (after patio time) but this was covered I believe when the noise and vibration comments were first made and I believe Markus commented on the nearness of the props to the surface.

[rt1commuter]

Apr 17, 2009 2:40:02 GMT -8 D'Elete BC in NJ said:

Apr 16, 2009 11:15:03 GMT -8 rt1commuter said:

Not air! Not with Cavitation. In Cavitation what forms are bubbles of vapor --- a fast propeller creates zones of extremely low pressure, so low that the water instantaneously evaporates to form a bubble of vapor. This unstable bubble then rapidly collapses as the pressure increases causing noise as well as severe erosion of the prop.

What's supposedly happening with the coastals is different, apparently it's the prop coming so close to the surface that some air gets sucked in; not technically speaking cavitation, but certainly a bad thing.

Yes, let's try to explain this once again...and I'll try to keep it simple for a certain person who fades out every time I use a technical term! ;D

Water boils and vaporizes when it goes from liquid to gas. The temperature that this happens at is typically 100C at sea level, and we consider this to be the boiling point of water. A very important value as the Celsius scale uses it as one of its defining values...but I digress. The definition of 100C as the boiling point of water is only true at one standard atmospheric pressure as defined...move up a mountain and the boiling point of water drops. Eventually, if you lower the pressure enough, water will vapourize at room temperature.

Here is what is happening with cavitation. As the propeller turns, currents and eddies are generated. This has no direct connection with the propeller's position relative to the surface of the water, or the ocean bottom, for that matter (Actually, this does matter as it defines boundary conditions...and I see one audience member glazing over... ). In order for thrust to be generated, water molecules need to speed up...to be accelerated. As the water molecules are accelerated, other water molecules rush in to fill the vacancies left by the molecules now speeding away. But these replacement molecules have to accelerate, too, in order to move in to the vacancies.

If these replacement molecules move in at a slightly slow rate than the departing molecules have left, there is a next loss of molecules in a said location...this results in a loss in pressure in the location where we are observing. Now consider this happening over a period of time; eventually the number of molecules and, consequently, the pressure, will drop to a level where the boiling point of water is equal to the ambient temperature of the surrounding water. Consequently, water vapour will start to fill the low pressure area.

However, there is a certain amount of energy required for water to transition from the liquid state to the vapour state...actually, quite a bit of energy...more than it takes to get water from absolute zero to the boiling point...by almost two times. The water vapour draws the required energy from the surrounding water...effectively cooling it. We are now left with a very unstable state...the water temperature is now below that which boiling is supported. Thus the newly formed bubble collapses (often explosively) if there are any disturbances, releasing the energy back into the surrounding water as the water vapour condenses. And the process begins again...

Okay, I've tried to demystify and englishify (de-engineerify) the cavitation process. The process really is as simple as I described above...describing it mathematically is a little more difficult, as is predicting it.

Well said

[Mill Bay]

I mixed it up when i said cavitation is air being mixed with water... I read a couple explanations that mentioned bubbles of vapor being created in the water, whose collapse creates shocks and 'noise'. I confused myself and understood vapor to mean that air was being mixed in somewhere.

[D'Elete BC in NJ]

Apr 17, 2009 8:02:46 GMT -8 Northern Exploration said:

Apr 17, 2009 2:40:02 GMT -8 D'Elete BC in NJ said:

...and I see one audience member glazing over... ).

It is Friday, almost noon, and it is going to be 20C out. I am heading for a patio at Starbucks or a restaurant and the only glazing I am interested in, is the donut kind.

ha...and I spent a paid day at the beach basking in the sun...

Apr 17, 2009 8:02:46 GMT -8 Northern Exploration said:

Apr 17, 2009 2:40:02 GMT -8 D'Elete BC in NJ said:

...and I see one audience member glazing over... ).

I will try to search it later (after patio time) but this was covered I believe when the noise and vibration comments were first made and I believe Markus commented on the nearness of the props to the surface.

He did...can I find it or any of the other convos about it...NO...what's up with the search on the forum? Try typing in a search for something you know is on here...nada...hmmm...

[kerryssi]

There was a problem with the spirits causing just such damage. It was caused by the captains shoving the throttles full ahead to get out of the berth quickly. Got to keep those schedules you know. They solved it by easing out of the berths, then hitting the throttle. It also takes a lot more throttle to stop a large ship than a small one. Making a hot landing then full reverse thrust did not help the docks, but it did help keep the schedule.

[Ferryman]

Here's some possible evidence at Tsawwassen's Berth 3, Coastal Inspiration/Queen of Alberni berth....Perhaps I'm wrong, but it looks possible to me...