cavitation plate depth

I have an aquaintance who is turning his Gemini catamaran into a powercat. He has removed the rudders and is mounting a pair of 40hp power trim/tilt outboards (with 20" shafts) on fixed height brackets on the transoms, he told me he will be mounting them with the cavitation plates in line with the bottom of the transoms. This does not seem deep enough to me to not pitch out of the water at times, the hulls draw about 18" fairbody draft and the tip of the transoms(on my boat anyway) are only about 7" below the waterline as painted. These are fat hulls with a displacement speed of about 7 knots at S/L 1.34 so lets say a cruising speed of 6.3 at S/L 1.2. so a couple of things, 80hp seems way more than needed under any circumstances and even cruising on one seems a little much,would not a pair of 9.9 high thrust 25" shaft Yamaha motors with more immersion be a better choice or 15 hp Hondas, im thinking of several things here, fuel economy, keeping the props in the water and keeping weight out of the back of the bus. Is there a rule of thumb for the depth of the cavitation plate on a displacement boat? im thinking that placing it at the bottom of the transom may be for planing boats, also is there any reason not to place it as deep as you can consistent with not drowning the powerhead when the bow pitches up and also being able to get the leg out of the water when you are leaving the boat.

Steve.

 

First what you call a cavitation plate is an anti-ventilation plate. Its purpose is to prevent air from being sucked into the forward face of the propeller. Cavitation is a totally different phenomena. On a hull that is planing the water exits the stern cleanly. That is, it doesn't wrap around the transom. The reason to have the plate about level with it, is that it creates the least amount of drag. The anti-ventilation plate submerged will create a large amount of resistance. At lower speeds, particularly narrow long boats, the problem is that propellers come out of the water as the vessel pitches. The motor should be have the longest possible shaft and be installed in the lowest position possible. The lowest position is that in which the powerhead won't be submerged while in use.

 

Thanks Gonzo, you just confirmed what i had thought. When i had a Yamaha on my Macgregor 36 cat there were times when it was submerged briefly and it would still come out of the water going out the ship canal in a northeaster even with a 25" shaft. So i cant imagine that having the plate at the bottom of the transom is not going to be venting all the time.

Steve.

 

Outboards are a poor choice to begin with. Weight aft, prop out of the water in a seaway, washed by sea water, cluttered stern, ugly...... I never understood why people consider them on anything larger than a daysailer.

If I had to use outboards Id go with a deep leg...25. Perhaps mounted on a Jack Plate type rig or a pair of genoa tracks and a lift so that for high speed motoring the plate would ride at the water break on the corner of the transom and for rough water work sunk as deep as possible .

 

This guy has a Gemini 3200, there will be no high speed motoring, he has 2x40hp motors but with these hulls i would expect that anything over hull speed will burn too much fuel. Actually outboards have a lot of plusses also and are a reasonable choice on a cat, appropriate sized ones are light, inboard diesels would be a poor choice. I was questioning his choice of the size of the motors (too much HP,too much weight,too much fuel burn) for hulls that cant really take advantage of it and the height that he is setting the cavitation plate being it is a displacement boat,not planing.

Steve.

 

What Gonzo said is important as it relates to catamarans. Cats have a strong tendency to hobby horse because of their (usually) balanced double ended hull form. The normal rule to set the anti ventilation plate at the level of the bottom of the transom should usually be extended lower for cats. Drag of a lower gear case is not a big issue here since high speed is not on the table. Just lowering the motor is not such a good idea since it could cause swamping and will keep the mount and tilt motor in the corroding water and cause earlier failure. A longer leg is the better choice.

General comments and may not be as true for all cats.

 

I cannot really comment until I know what sort of speed your friend is hoping to achieve. But I agree two 9.9hps should get him up around 8-9 knots, which is similar speed to most trawler yachts. If he wants to go mid teens then probably twin 40's isn't enough.

Currently I'm having a 28ft powercat built for my own use, with twin 20hp outboards, expecting to cruise economically at 9 knots, partly because it is way lighter than a Gemini

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

Thanks Tom, this was my take also, as i said my aquintance has motors which i think are way more horsepower than he will be able to use with any kind of economy. I think the advice he was given was more appropriate for planing hulls. The longest shafts seem to be 25" and are only on the smaller hp motors from 8 - 15hp which would also seem to be more appropriate for displacement hulls, then ther are all the 20" long shafts until you get up in hp. The Gemini has a lot of beam to the hulls aft so may not hobby horse to much but on mine the tip of the transoms are only immersed by about 7" so it wont take much.

 

Hi Richard, i dont think he expects speeds above 10 knots, i think he just likes big motors as he had a single 50hp in the middle when it was a sailboat, which again seems a bit much. Im just concerned that he is not going to be able to keep them in the water if on fixed brackets with the cav plates at the bottom of the transoms. BTW, i think your powercat seems about perfect for the balance of reasonable speed and good economy.

Steve.

 

I have found that having a central outboard won't work at speed unless there is a good fairing in front of the engine to stop the aeration. So a 50hp central outboard basically just makes froth and spray. Put it behind a hull and you start getting somewhere.

The biggest outboards we put on one of my designs were twin 90hp's on a 35ft eight berth Banshee. There is a photo on my website of it towing a waterskier at 22 knots. Earlier I had sailed the same boat at 19 knots. That boat had the engines on the transoms. We experimented with a central 70hp, it wouldn't do more than 11 knots. But did 16 knots when it was mounted on a transom.

Fortunately he can run one engine with the other tilted up. That will nearly double mpg and he need only use both engines when manouevering in close quarters.

Thank you for the compliments about my Skoota designs

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

To me a single central mounted 50 hp on a cat like the Gemini is way too much as realistically you are going to be cruising at 7- 8 knots at most, and you can get close to that with a 9.9 to 15hp. The weight difference between, say, a 15 and a 50 is a lot and owners of these boats are not shy about piling stuff on these boats and it all seems to end up in the back of the bus, bigger motors, bigger fuel tanks, generators and a rib with motor in davits, it all ends up at the stern.

 

If the prop letting go is a concern, fitting stainless cupped props will likely help, they are more tolerant of part of the prop being out of the water.

 

When the winds on the nose and you trying to get into that little anchorage before dark youlle be thankfull for that extra HP.

No boat ever made uses full power all the time and fuel consumption is HP produced not HP available

 

Yip typical Yachties use a small motor slogging its wee heart out all the time use loads of fuel and go backwards in a blow and against the tide !!
The motor is for your safety so make it bigger and use it if need be get back to port with its help and one less worry !! bigger motor, bigger prop, more push when its needed .
Me i had a 22 foot trailer sailer with a 25 hp motor , the kids could ski behind it !
I had a long crowded channel on the way home after a weekends cruising so used to pull into a sheltered bay ,drop the mast ,stow all the gear and have dinner and watch other struggly against the tide and wind and we would pass most everything including big powerboats and be back to the ramp ,on the trailer before the others were even half way back .

 

I owned a Macgregor 36 catamaran with a single 9.9 yamaha high thrust 25" shaft 4 stroke and it had all the power i needed, we have ship canal here with a bridge that opens for ships and when its blowing out of the NE the whole of Lake Superior is rolling straight in, that little motor would push us out at not much above idle, those motors have thrust to spare, the problem was keeping it in the water or above the water as the bow reared up or plunged down. In flat water it would nudge 9 knots. There was no time when i would have wanted more power.

Steve.