Advice on Preventing Prop Ventilation

I have a 42ft Power Catamaran which is prone to prop ventilation. This usually occurs at top revs and especially in a beam-on seaway. I'm looking for ways to fix this problem from people who have had experience with it. Attached are some photos of the underwater area.

 

Interesting hulls. Similar to a seabright skiff box keel. What design is it. It looks like the prop is very close to the surface and that may be the problem .running with a quartering sea it would have trouble keeping the props submerged all the time. Do you have pics of the full length of the hulls. What horsepower and cruise speed.

 

The trailing egde of the keel looks a bit crude.

Dont know much about cats. They are so shallow and wide that in a beam sea the windward prop might break the surface.

Perhaps a different style prop , one that eats air like the racing powerboats use, might work

 

It's a cat. It draws under 1m so the props are close to the surface. 2 x 110hp, 16knot @ 90%.

 

You might contact the designer...he might have some suggestions.

 

I realise that but it doesn't make a cat special. I have had vent issues on mono's that draw under a meter. Usually when the prop is to close to the transom. Is it semi displacement. What is your cruise speed. 12 to 15 knots. Does it trim level or bow down. You may be able to improve it a lot just by trim. Sorry mate I was typing didn't see your last post.

 

A vexing problem no doubt, but do you know which prop is letting go, at which stage of the hull/wave interaction ? It doesn't occur in a head-on situation ?

 

Not a solution, but perhaps fairing the trailing edge of the keel would reduce some of your problem. The turbulence coming off those flat ends can't help.

 

A flat plate over the propeller may help. I fixed a similar problem on a monohull that ventilated the prop and rudder on quartering seas.

 

A shroud would help, but the hulls are long and skinny, so enough vertical movement in these shallow bodies are going to "uncover" the wheels from time to time, especially in certain conditions, like running into or before a beam sea. Fairing the aft end of the box keel will not help the ventilation issue, but will very marginally improve efficiency. The problem is physics and some wave trains are simply just going to expose the props. Tacking across the conditions that cause these issues is the only simple solution, though a well fitted shroud will delay the onset of ventilation.

 

The boat semi displaces bow up a bit. Would trim tabs pushing the bows down help? What does a shroud look like? Would 300mm vertical boards about 2m long along the length of the boat help?

 

It might be helpful to travel alongside in another boat in the conditions where the problem shows itself, just to get a better impression of what is happening, but I'd be inclined to fit a plate like gonzo suggested, there seems to be plenty of room there to do it, just allow the regulation prop tip clearance. Could be enough to prevent air being sucked down, when it is close to leaving the water. It would be a simple enough experiment to do, and should not alter performance adversely. Might even dampen pitching, if that is part of the problem.

 

I suspect trim changes are the issue (pitching) and a plate is an easy thing to try.

 

How does the stern going down and the bow going up affect ventilation? When the boat is going 16kn, if you look over the side where the prop is, there is a hollow in the wash extending under the boat. Is this the bow wave or the boat trying to get on the plane?
Would fairing the rudder help with efficiency?

 

Whenever the hull(s) adjacent to the prop clear the water, you have the possibility of air being dragged down into the prop. A plate can delay that happening.