Cavitation plate depth

[bax]

I have a question for the group.

I was wondering if there was a rule of thumb as to the optimal depth of the cavitation plate relative to the waterline. The location of the outboards on the 9m cat I'm building is almost a third of the LOA forward of the transom to
reduce the effects of pitching. I'd like to have the very tip of the fin on the
motors roughly even with or slightly above the deepest point on the hulls which should minimize grounding damage to the outboads. This of course affects the height of the cavitation plate which would therefor be about 3 1/2" to 4" below the waterline. Is that deep enough?

bax

[daiquiri]

I guess you are talking about the anti-ventilation plate.

If it was a clasical planing boat, I would have told you that it is enough to keep the plate even with (or max. 1-2 inches above) the keel bottom. But in this case things are different, there will be some wave interference in between the hulls and at some point a wave hollow might happen right where your props are. It's depth will depend on the geometry of your boat. The important thing is that the plates are maintained submerged at all speeds, so I believe you should keep them as deep as you can, compatible with the other constraints you have (like your grounding-damage consideration). If that means 4", then let it be 4".
You will discover if the position is correct after few trials.

I hope you have considered the fact that more forward (i.e. closer to the center of gravity of the boat) you place the steering device (the o/b motors in this case), the less effective it will be...

[bax]

Hi Daiquiri,

Yep, you're right, it is an anti-cavitation plate. The motors will remain fixed in the straight line position as steering will be done solely by the rudders under way. Except of course in tight places where I will use forward & reverse to manoeuver. Iv'e chartered a Mainecat 30 which has basically the same setup, and it works great. One thing I forgot to take note of was the depth of the outboards!

[marshmat]

Ventilation and cavitation are different phenomena- the plate's purpose is to prevent ventilation, ie. to stop air from getting sucked into the prop.

I've seen a couple of setups where the engine bracket has a small section in front of it, sort of a very short V-hull shape, to help tame the waves before they reach the outboard. This seems to keep the powerhead pretty happy (no waves coming in the air vents) as well as smoothing out the waves ahead of the prop.

Deeper props are generally better for an application like yours, but it's a rare outboard (pretty much only the 25" shaft models) that can have the plate more than about eight or ten inches below the surface without putting the powerhead too close to the water. Outboards are supposed to kick up on grounding, so the question is, what part (hull or skeg) would you rather hit first

[daiquiri]

Originally Posted by marshmat

Outboards are supposed to kick up on grounding, so the question is, what part (hull or skeg) would you rather hit first

Well said.

[gonzo]

It is an antiventilation plate. Cavitation is a different phenomenon. You need to keep it at surface level or slightly below when the boat is planing. That is probably 1/4-1/2" over the keel

[bax]

Its purpose in life is to prevent cavitation, that's why they call it a cavitation plate. The cavitation occurs due ventilation, that being air sucked down from the surface.

Unlike speedboats, cats do not typically plane, especially sailing cats, when under sail or power. Therefore postioning the plate at 1/4 to 1/2" over the keel is not a good idea. For this situation, you want to get the prop as deep as you can to minimize the potential for cavitation.

As others pointed out, the rest rides on whether you want the hulls or the motor to ground first. I'm choosing both basically. The fin will be about even with the deepest point on the boat, except for the retractable rudders.This boat is a kind of beach cat on steriods, so I'll be running it up to the beach fairly often. In this situation, the bows should hit the bottom while the motor is still free and clear.

[sandy daugherty]

Your thinking is good. Positioning the engines that far forward will greatly reduce the chance of the engines sucking air and suddenly spinning over red line before the governor can bring it back. This causes a LOT more damage to the typical outboard engine, that has a kick up feature built in.
Another consideration is that a prop close to the surface has more slippage than one a few inches deeper.

So deeper is better. Better fuel economy, fewer over-speeds outweigh rubbing the paint off a skeg that is already cool to kick up occasionally.

Hint. Slow down in thin water. As far as the random, semi-submerged shipping container, it has to be hit at just the right depth to only damage the outboard and not slice a few inches of bottom off you vessel. You buy your ticket, and you take your chances!

[gonzo]

Cavitation is produced by too much power applied to a propeller or it turning too fast. It is a vacuum. Ventilation is air sucked from the atmosphere. Cavitation can ocurr in a submarine at 400 feet below the surface, ventilation can't.

[b1ck0]

Cavitation is cold boiling of water. When the fluid is moving too fast, the pressure decreases and the water boils in lower temperature than normal. The cavitation isn't depended from the depth.

[Alan M.]

You want the props as deep as possible, to avoid (or at least reduce) them ventilating in lumpy conditions.

My propellor shafts are 400mm below the waterline. (Honda 20's with the 703mm leg)

Your rudders will touch in deeper water than the outboard will anyway, so don't worry too much about that. Get them in deep.

[sandy daugherty]

Gonzo: manufacturers, dealers and buyers have had it wrong for at least a hundred years. They aren't going to change.

[bax]

Bravo gonzo. Yes, we all know that cavition can occur simply by rpms exceeding the propellor's ability to move the water efficiently past the lower unit. That is not the issue or the question. The question was in regard to optimal depth for real world use on a cat in particular. We're not talking about submarines here.

[gonzo]

Bax: you gave him wrong information. I use a submarine as an easy to undestand example

[bax]

Gonzo,
Not sure what you are referring to about supplying the wrong information. In my original post I asked about optimal depth.