efficiency of surface drives and surface piercing propellers

[David Knight]

I’m designing a 12m planing trimaran and am in grate need of some information regarding the efficiency of surface drives and surface piercing propellers, I could also do with some info on predicting windage at 70 knots. Can anyone help me? Any suggestions would be gratefully received.

[raceday]

David,

Surface drives are very efficient and you will gain an approximate 7 to 10 mph hour by using surface drives. Your drives are part of your hull and do create drag even though they propell your boat. Using a SD will increases the efficiency of your hull because they create less drag. Example: Hustler 388 Slingshot w/ Mercury Bravo Drives has a constant or hull efficiency around 269 but when we added surface drives our constant is 290. As I said before in most our test we always gained around 7- 10 mph with surface drives. If you need any more information feel free to ask.

Good Luck
John Cunningham
www.kanyun.com

[Lin Olen]

Hello raceday. Surface drive efficiency proved by USNavy on Albert Hickman's inverted "V's"' which they used extensively during WWI. Your high speed cruisers should be married to Rotary Sail. Read my posts about Rotary Sails and Tuned Hulls on the forum. Another brainwave of mine was used to stabilize Ken Warby's world record breaker, the pitch stabilising lifting foil. Essential at extreme speeds, but the principle must be understood and correctly applied. Cheers, Lin

[masalai]

Design your boat so you know weights & resistance and the hull form is optimised for your cruising velocity, then get the screws based on velocity & theoretical power required then discover there is no room weight allowance for fuel to give you the range you want, & start again...

12m and 70 knots??? Get an existing professional design and buy the proven boat - after having tested it out wit all your additional needs.... Got lots more money than I have to spare?

[masalai]

David do an internal search on "surface drives" - lots of stuff of interest, many useful ideas and several amusing "bunfights" demonstrating communication difficulties and personal opinion/attitudes to little effect

Stuff goes back several years & many bits you will find relevant/interesting...

[milespm]

my boat is 15m and does 61 knots with 2876 LE401 MAN diesels. It is pushed by Trimax 2200 drives and 1:1 ZF Transmissions. Note: the 10" exhausts exit on top of the props. If not you wouldn't get on plane with extra pitch.

[mr.marine]

i think thats a good point. also, in case you dont know where to buy, try this shop www.mcmarine.net. really good parts and very fast. if you cant find what you need contact them, they are really friendly people. www.mcmarine.net

cheers, bob

[masalai]

Here is one designed to be competitive in the international powerboat racing circuit - this is the latest Maritimo, getting ready for the last race of the season and the first race for this boat - compliant with 2010 racing rules 'prepped' on that day for around 150 mph - first race - not much sponsorship sign-writing done, as still in the setting up phase...

Note the ventilating system needed only to get out of the hole, otherwise out of the way... The exhausts are dry and minimise power loss - bloody loud and quite 'cammy' when idling/warm-up, SCREAM when booted... The leading edge of the screws and the rudder below the half-moon were sharp enough to give a good shave, if your razor was not good enough