Refit 31 prout Repower advice

[mikedefieslife]

Most setups are a single outboard mounted where a sillette drive leg would be at the rear of the nacelle. Even then you might suffer a little in wave but not too much with a decent shaft length.

The Nacelle will need to but open (similar to the rear nacelle on the Gemmini cats). Yes you will need steering otherwise you're going to be in for a world of difficulty especially when running astern.

[valhalla360]

Originally Posted by Sparx

Does a single engine in the middle create a larger opportunity for ventilating the propeller when running in a chop? Thinking is that twin engines are mounted closer to the hulls and therefore these engines are riding the waves more or less in synch with the hulls. Where as the single engine in the center has more opportunity to get stranded without enough submergence when both hulls are riding on top of the waves. Comments from someone with experience using a centerline engine would be helpful here.

Depends. You would really have to look at the mounting locations and the wave patterns at different speeds.

On our Gemini, the two wakes met in the middle at cruising speed and thus the prop didn't have problems staying hooked up. With the bucket mount, we could adjust the height of the motor and in big waves, the bucket would rise up with the waves (the two holes at the back wouldn't fill fast enough to keep it down and flotation would lift it but would drain any water that got into the bucket).

The big question is how much fabrication will you need to switch from the current configuration to anything else.

[shane malone]

i think i will go with the twin 15s i knew the 50 was overkill but you have to go up to that to get the 25” shaft. the fabrication is done except im building an engine cover to keep the deck clean. thanks so much for all the input everyone it’s nice to have Different opinions! Can’t wait to get it together I cut the top off and raised it 24 inches for more headroom and cut the engine box out and shortened the nancel 13”.

[valhalla360]

Originally Posted by shane malone

i think i will go with the twin 15s i knew the 50 was overkill but you have to go up to that to get the 25” shaft. the fabrication is done except im building an engine cover to keep the deck clean. thanks so much for all the input everyone it’s nice to have Different opinions! Can’t wait to get it together I cut the top off and raised it 24 inches for more headroom and cut the engine box out and shortened the nancel 13”.

If you want a 25" shaft, they make extension kits, so you aren't stuck with 15hp or 50hp.

Bay Manufacturing is the biggest company doing these.

[weavis]

Originally Posted by shane malone

i think i will go with the twin 15s i knew the 50 was overkill but you have to go up to that to get the 25” shaft. the fabrication is done except im building an engine cover to keep the deck clean. thanks so much for all the input everyone it’s nice to have Different opinions! Can’t wait to get it together I cut the top off and raised it 24 inches for more headroom and cut the engine box out and shortened the nancel 13”.

With separate controls for each engine, it will turn in its own circle..

[Cadence]

Originally Posted by weavis

With separate controls for each engine, it will turn in its own circle..

And no steering cable or steering linkage required. The throttle/shifting cable is much easer to run and various lengths can be had right off the shelf.

[Cadence]

I may have been wrong earlier thinking I had a Honda with a 28" shaft. It was probably a BF 15 power thrust model which has a 25" shaft remote start and charging, power tilt and trim.
I believe a pair would do you nicely.

[Bean Counter]

Twin Yamaha 9.9 Hi Thrust with 25" extra long shaft and low range gearbox (2.92:1) with bigger propellers, Most of the time you will need only one engine to push you along at (guessing 5 - 6 knots) / motor sailing and save a lot of fuel. You then have redundancy, maneuverability when needed, most economical way of petrol motoring.

[Sos]

Originally Posted by Bean Counter

Twin Yamaha 9.9 Hi Thrust with 25" extra long shaft and low range gearbox (2.92:1) with bigger propellers, Most of the time you will need only one engine to push you along at (guessing 5 - 6 knots) / motor sailing and save a lot of fuel. You then have redundancy, maneuverability when needed, most economical way of petrol motoring.

As above

[Pete7]

Originally Posted by Bean Counter

9.9 Hi Thrust with 25" extra long shaft and low range gearbox (2.92:1) with bigger propellers,

This bit is the key. Little wizzy propellers designed for small run arounds won't give the best performance. A nice big slow turning prop will.

Pete

[Cadence]

Originally Posted by Pete7

This bit is the key. Little wizzy propellers designed for small run arounds won't give the best performance. A nice big slow turning prop will.

Pete

I believe the lower units on them are entirely different. Different gear ratio an can swing a bigger wheel. I'm not sure two 9.9 would do it without going to the gas guzzling RPMs to get hull speed but two 15 high thrusts would do fine.

[44'cruisingcat]

Originally Posted by Cadence

I may have been wrong earlier thinking I had a Honda with a 28" shaft. It was probably a BF 15 power thrust model which has a 25" shaft remote start and charging, power tilt and trim.
I believe a pair would do you nicely.

Honda did the 28 inch shaft on the BF 20. However I'm pretty sure that motor was never available in the US.

[VBsail]

My experience is more horsepower is not the answer when trying to move a displacement hull, even a cat that can reach double digits under sail.

My Catamaran came with an old Yamaha T9.9 and brand new Tohatsu 9.8 "high thrust" with the little 8.7" 4 blade prop. I fell into the more horsepower line of thinking and bought a brand new fuel injected suzuki DF20A. It was a great motor for a small boat, but all the horsepower over about 8 was completely wasted. The 20 went no faster and was noticeably worse at slow speed maneuvering and changing momentum. Reverse was terrible. The larger diameter slower spinning prop makes all the difference. I eventually bought another Yamaha T9.9.

I tried 4 props on the 20hp before admitting defeat. The solas 10" 4 blade high thrust prop (pictured) is the same prop as used on the honda high thrust.


I found a high thrust prop on a standard gearbox basically just beats the water to a froth comparatively to a slower larger prop. Al lot of noise and spinning, not as much push.




Yamaha T9.9 uses 11.75" diameter 3 blade prop. Mercury, Honda, Suzuki, & new Tohatsu 9.9 fuel inject "high thrust" all turn a 10" diameter prop faster with standard gear ratios.

[shane malone]

thank you so much for all your experience with this is our first sail boat we figured it would be a cheap way to start and see if this would be more fun for retired life than a trawler

[shane malone]

mite have to look at yamaha too was only considering honda or mercury high thrust models before