Cavitation plates

[dominicebs]

My boat doesn't have a cavitation plate as has been pointed out several times. When I next have out i am tempted to have one fitted. What do they do and is there any good reason why I mod not already have one

[David Schweizer]

What is a cavitation plate? I have heard the term but have never really known what it means.

[Pete & Helen]

My boat doesn't have a cavitation plate as has been pointed out several times. When I next have out i am tempted to have one fitted. What do they do and is there any good reason why I mod not already have one

 

Forum post here that tells you

http://www.canalworld.net/forums/index.php?showtopic=63355&hl=%2Bcavitation+%2Bplate

[DaveGood]

The thread referenced above is mine.

 

We had one fabricated for us by Captain Fizz of this parish ( not seen him around for a while). I can't remember how much he charged us but it was well under a 100 pounds.

 

Our boat is now a lot qieter and vibrates less when under way and definetly uses less fuel and engine runs at lower revs for a given speed.

 

It,s a relatively simple job using commonly available inexpensive materials to fabricate and you don't need the boat out of the water. If you can weld, have some steel plate and a saw that can cut it you could do it.

 

Well worth having done.

 

DaveGood

[MtB]

What is a cavitation plate? I have heard the term but have never really known what it means.

 

 

I've always understood these to be the horizontal plates fixed above the blade on a motorised butty. Also known as 'fish plates'.

 

They stop the water flow from the blade flying up into the air in a big rooster tail as a butty has no uxter plate.

 

 

MtB

[DaveGood]

 

Hi MTB.

 

A cavitation plate is a a second plate welded below the weed hatch cover that sits at the bottom of the weed hatch directly over the prop and flush with the hull. Without it the prop kicks water up onto the weed hatch causing noise, vibration and wasting energy.

 

The cavitation plate is welded directly on to and below the weed hatch cover so it's all one unit, and you lift them out together when removing the weed hatch.

 

You probably have one on your boat and what you described for a motorised butty sounds like a cavitations plate. If so, you just weren't aware that's what they're called and that some boats don't have them.

 

DaveGood

[DaveGood]

Sorry,

 

Re-read your post mtb, you seem to be describing a propeller that is not under the hull with a weed hatch above it, but one where the prop is beyond and behind the hull. In which case what you call a fish plate fulfills one of the functions of a cavitation plate (directing the prop flow energy that would be kicked up vertically into a rearward motion giving more thrust forward). Noise and vibration would matter less, if at all, with a propeller outboard of the hull instead of below it kicking water up to a shaft (the weed hatch) within the hull itself.

 

DaveGood

Edited January 17, 2015 by DaveGood

[Bee]

If you lift the weed hatch out and make a few measurements you could get someone to weld a plate and a bit of a frame on it and then put it back again, Voila! job done. Just don't stick a few tons of winter coal on the boat and sink it whilst the hatch is off.

[MtB]

Dave I'm aware of the bit you mean, in a weed hatch. I'm just saying I've never heard of that panel ever being called a cavitation plate. The term is usually applied to the plates fitted above the blade on a motorised butty. Full and correct name being "anti-cavitation plates" actually.

 

The Nurser butty 'HAZEL' has them, for example.

 

 

MtB

[DaveGood]

Dominicebs,

 

If you don't have one on your boat it's because the builder couldn't be arsed. It's not legally required, just as he's not legally required to (let us say) put in a water tank adquate for your needs. It saved him a few pounds in materials and two or three hours work.

 

You're better off with one then without.

 

 

DaveGood

[DaveGood]

MTB,

 

I have no idea why a cavitation plate is called that. I regard it as a misnomer since cavitation is a phenomena associated with high speed propellers and a "cavitation plate" has nothing whatsoever to with that.

 

On the other hand what you call a "fish plate" has nothing to with fish.

 

Two technical sites I looked at and my surveyor all referred to it as a "Cavitation plate", and I certainly don't consider myself a more eminent authority on these matters then the RYA.

 

if

[cereal tiller]

 

 

I've always understood these to be the horizontal plates fixed above the blade on a motorised butty. Also known as 'fish plates'.

 

They stop the water flow from the blade flying up into the air in a big rooster tail as a butty has no uxter plate.

 

 

MtB

And the plate reduces the amount of air drawn down when the boat is reversing,thus causing aeration of the Blade.

CT

[DaveGood]

That makes a lot of sense CT.

 

Have a greenie

 

DaveGood

[David Schweizer]

Hi MTB.

 

A cavitation plate is a a second plate welded below the weed hatch cover that sits at the bottom of the weed hatch directly over the prop and flush with the hull. Without it the prop kicks water up onto the weed hatch causing noise, vibration and wasting energy.

 

The cavitation plate is welded directly on to and below the weed hatch cover so it's all one unit, and you lift them out together when removing the weed hatch.

 

You probably have one on your boat and what you described for a motorised butty sounds like a cavitations plate. If so, you just weren't aware that's what they're called and that some boats don't have them.

 

DaveGood

 

We have already got one of those, suspended breow the weed hatch cover on two welded heavy angle iron pillars, but didn't know what it was called. I thought every boat with a weedhatch had one, why would a builder leave thumping great hole in the uxter plate?

[ditchcrawler]

MTB,

 

I have no idea why a cavitation plate is called that. I regard it as a misnomer since cavitation is a phenomena associated with high speed propellers and a "cavitation plate" has nothing whatsoever to with that.

 

On the other hand what you call a "fish plate" has nothing to with fish.

 

Two technical sites I looked at and my surveyor all referred to it as a "Cavitation plate", and I certainly don't consider myself a more eminent authority on these matters then the RYA.

 

if

I would be surprised if the RYA other than the chaps on the ground doing training know what a motorised butty is.

[Bettie Boo]

Please do not suppose that by posting in this thread that I am in anyway trying to stylize that I know what I'm talking about.

 

However, I do happen to have a pic on my lap top that shows a pic of the "hole" that Captain Fizz welded a cover for onto the bottom of our weed hatch cover, just in case there is any confusion as to the cover Dave is describing.

 

It's the rectangular hole above the propeller - which now has a cover

 

 

 

ETA - to remove an extra "am"

Edited January 17, 2015 by Bettie Boo

[DaveGood]

Hi David Schwiezer,

 

Why would boat builders not fit a cavitation plate? Well, it's not legally required and\or you can shave some money off the final price\take a bigger profit by not bothering.

 

The OP wants to know is it worth having done? I and Betty Boo own a boat that came without one, I researched the possible benefits, including asking here, then commisioned Captain Fizz to fabricate one for us.

 

The answer is yes, it is worth doing, provided the OP is aware that the whoever does the job can do so for less then the cost of one new battery and still make a profit.

 

If he's charged a sum that go's into three figures he's probably being cheated.

 

It's maybe twenty pounds worth of sheet steel and paint plus two to four hours work to cut and weld.

 

Captain Fizz charged us well under 100 pounds, and I thought, he should have charged us more.

 

DaveGood

Edited January 17, 2015 by DaveGood

[Martin Kedian]

If you take a few measurements and pop it over to me I will put a bottom plate on for you while you wait cheers martin

[Sea Dog]

I think the plate you refer to would be termed a (anti) ventilation plate since cavitation is, as someone said above, related to prop design and speed. The issue you are trying to combat (with such plates as might block up that there weed hatch hole in t'pictures) is the prop drawing air down from above, thus ventilating it.

 

Is it actually causing you an issue Dominic (other that people keep pointing it out to you - presumably divers)? Cos if it's not broke, why fix it?

[blackrose]

When I hear the term cavitation plate (or anti-cavitation plate) I automatically think of either a plate that's on the top of the rudder on some boats to prevent water being pushed above the surface of the water and wasting energy, or what I normally call the baffle plate on a weed hatch. I'm sure cavitation is an incorrect use of the term in both cases, but that's the inland waterways for you. We often use such technical terms incorrectly.

 

I'm surprised that any narrowboat builder would fail to put a baffle plate on a weed hatch. Even my Liverpool boat has one. Also, the job doesn't need be done by a marine fabricator. If I can get a new 9ft sq skin tank fabricated and welded into my engine hole for £130 by a gate and trailer fabrication yard at Staines on the river, then you should be able take your hatch to any steel fabricators and get them to weld a plate onto it for 20 or 30 quid as long as you provide drawings and dimensions. A hundred seems a bit excessive to me.

Edited January 18, 2015 by blackrose

[Martin Kedian]

I agree with you I have a coffee jar for such jobs and ask the customer to put into it what they think its worth £20,00 sounds fine

[Sea Dog]

 

I'm surprised that any narrowboat builder would fail to put a baffle plate on a weed hatch. Even my Liverpool boat has one.

 

I was surprised not to find one on my (pre-used) boat, built by Simon Piper - a yard which, together with his Dad, has done quite a number of fairly decent boats in their time. Possibly like the OP, based on my previous experience in boats with such plates fitted, I thought I'd look at the underwater end and take the dimensions when it was out of the water.for blacking with a view to getting one fitted shortly thereafter.

 

When I'd measured it all up, I realised that since I don't have weed hatch access from the upper deck of my Trad (and hence a perfectly dry bilge!), the contraption would have to be hoisted out of the weed hatch tunnel inside the engine space any time I needed access. That would be a real struggle, so I'd have to need it pretty badly to make it worthwhile.

 

This set me a thinking - after a year of reasonably extensive cruising, I've had no ventillation related noise or issues from having an unfaired trunk opening in the uxter plate and, given a properly sealed weed hatch, I don't suppose I would. I think that any air up that tube would have to be dragged down and out of it to vent the prop - and it would then be equalised with water so thelimited supply of air to vent the prop would be a quickly exhausted. There may be some small amount air still up there at the top of the trunk, but it won't be interfering.

 

So, whilst it's not broke I won't be fixing it - but if I find I need to I'll be straight round to see Martin (Kedian)!

[IanD]

Turning the issue upside down, has anyone ever tried putting a similar plate under the prop, like a tapered extension of the hull bottom plate along the skeg? This would make the prop pull water in from the sides of the hull along the swim rather than from underneath the boat; as well as helping prevent prop damage and reducing rubbish pickup from the canal bed, it might also prevent the stern of the boat sitting down so much in shallow water as the prop pulls water from under the hull.

[Dalslandia]

Cavitation can be had on several surfaces, like the rudder, or the hull above the propeller, and on the propeller itself (as said) so a plate at the bottom of the weed hatch tunnel can be called cavitation plate if you get any cavitation on it? Right

the fish tail shaped plate above the propeller on outboards is called cavitation plate, sometimes, by some.

 

a plate under the prop can protect the prop from foreign parts, but also help make it jam the prop.

 

And I am not anti.

[Neil2]

 

I was surprised not to find one on my (pre-used) boat, built by Simon Piper - a yard which, together with his Dad, has done quite a number of fairly decent boats in their time. Possibly like the OP, based on my previous experience in boats with such plates fitted, I thought I'd look at the underwater end and take the dimensions when it was out of the water.for blacking with a view to getting one fitted shortly thereafter.

 

When I'd measured it all up, I realised that since I don't have weed hatch access from the upper deck of my Trad (and hence a perfectly dry bilge!), the contraption would have to be hoisted out of the weed hatch tunnel inside the engine space any time I needed access. That would be a real struggle, so I'd have to need it pretty badly to make it worthwhile.

 

This set me a thinking - after a year of reasonably extensive cruising, I've had no ventillation related noise or issues from having an unfaired trunk opening in the uxter plate and, given a properly sealed weed hatch, I don't suppose I would. I think that any air up that tube would have to be dragged down and out of it to vent the prop - and it would then be equalised with water so thelimited supply of air to vent the prop would be a quickly exhausted. There may be some small amount air still up there at the top of the trunk, but it won't be interfering.

 

So, whilst it's not broke I won't be fixing it - but if I find I need to I'll be straight round to see Martin (Kedian)!

I had a discussion with Dot Piper some time ago about this and it seems Pipers did not fit cavitation plates or whatever you want to call them as they felt they were unnecessary. They certainly make getting the weed hatch lid off more difficult depending on what sort of boat/access you have.

 

There's been discussion on here in the past and AFAAIA no-one has come up with a rock solid argument as to why these plates would make the transmission any more efficient, but they must make it quieter, and that often translates to a feeling of greater efficiency.