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Old 07-02-2014, 19:54   #1
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Outboard on a prout snowgoose?

I'm considering purchasing a prout snowgoose project boat. The boat has the sonic outdrive (in unknown condition) but the engine has been removed. Any thoughts on the idea of repowering the boat with an outboard? I was thinking either a 25 HP mounted where the outdrive is now, or possibly a pair of 9.9's?
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Old 08-02-2014, 04:59   #2
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Re: Outboard on a prout snowgoose?

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Originally Posted by susswein View Post
I'm considering purchasing a prout snowgoose project boat. The boat has the sonic outdrive (in unknown condition) but the engine has been removed. Any thoughts on the idea of repowering the boat with an outboard? I was thinking either a 25 HP mounted where the outdrive is now, or possibly a pair of 9.9's?
having recently installed an engine and outdrive on my prout snowgoose elite,i think this idea has merit.

though i if i were to do this i would use the transom of the engine compartment as the outboard bracket,and cut out an apeture large enough for the out board to tilt inboard when not in use.

you would also need an outboard with an extra long leg extention,to avoid cavitation,and a fairly fine pitch propellor for this reason as well.

the engine compartment would also need to be self bailing,this would not be much of a problem if it has the standard prout engine bed moulding inside,though it would probably be nessacary to modify the existing engine beds to accomodate the outboard bracket,and steering etc.

if you look at the link under my signature there are quite a few photos of the engine instalation i did on my prout elite.
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Old 08-02-2014, 06:31   #3
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Re: Outboard on a prout snowgoose?

Id be interested in parts from the sonic or the whole thing if yo want to sell it but why not just repower it
Its an easy refit with almost any small diesel
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Old 08-02-2014, 07:32   #4
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Re: Outboard on a prout snowgoose?

Several of the Stillette powered Gemini's have made a similar change.
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Old 08-02-2014, 07:47   #5
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Re: Outboard on a prout snowgoose?

susswein, I removed the diesel and Volvo outdrive and replaced it with a new Yam 25 with power tilt. I just cut out the transom where the hole for the outdrive was. I got the long shaft but it really needed to be 6" longer. And 25 hp to too much, even with the high torque lower unit, Never got above 2200 rpm (could not find a lower pitch prop) and would ventilate in larger waves. I think 15 hp would be fine but with a shaft extension and lower pitch on the prop. I also really liked the power tilt. a pair of 9.9s would be great if you could figure out how to mount them well. Oh, and your stern will raise up 4" if you remove the diesel.
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Old 08-02-2014, 10:39   #6
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Re: Outboard on a prout snowgoose?

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susswein, I removed the diesel and Volvo outdrive and replaced it with a new Yam 25 with power tilt. I just cut out the transom where the hole for the outdrive was. I got the long shaft but it really needed to be 6" longer. And 25 hp to too much, even with the high torque lower unit, Never got above 2200 rpm (could not find a lower pitch prop) and would ventilate in larger waves. I think 15 hp would be fine but with a shaft extension and lower pitch on the prop. I also really liked the power tilt. a pair of 9.9s would be great if you could figure out how to mount them well. Oh, and your stern will raise up 4" if you remove the diesel.
Ed
that is the way i would do it thanks for posting the picture.
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Old 10-02-2014, 17:40   #7
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Re: Outboard on a prout snowgoose?

A 25 hp outboard is not going to give you anymore than a 5% in top speed over a single 9.9 hp Yamaha high thrust outboard. The reason is in the numbers. The 9.9 has a much higher gear ratio, and more room for a larger diameter prop. It will just about reach full power at 7.5 knots. Even Yamaha's high thrust 25 fails to gear the prop down to a low enough speed to match a catamaran's performance under power. These motors are designed to push a small trailerable boat to planning speeds and beyond. When limited to the speed a catamaran can achieve, the more powerful engines have to cavitate or ventilate the propeller to reach their best power. Do instead of going faster, you just burn up a lot more fuel, something a cruising boat wants to avoid.

If you prefer anecdotal evidence I can report my experience. I owned a Roger Simpson designed 34' catamaran powered by a Honda 50 on a sled that lowered to the water's surface. It had the lowest pitch propeller available at the time, and would cavitate at any power setting above half throttle. The cat would then slow down as if I'd pulled the throttle back. Adding MORE throttle would result in even SLOWER vessel speed, with a cloud of steam bubbling up from the prop plume. A Gemini 3000 with a single 9.9 High Thrust Yamaha could drive right over us.

So its really a matter of numbers. Shaft length must get the prop deep enough to reduce sucking in a "whirlwater" funnel of air. The prop must be pitched to allow the engine to reach full power, and big enough to do that at a lower, ventilation preventing RPM.

Go look up the lowest gearing for outboards. Almost all will be geared at something just higher than 2:1 (engine rpm to prop rpm) but the Yamaha 8 and 9.9 High thrust engines have an almost 3:1 ratio. Since the size of the prop on an outboard is limited by the clearance to the cavitation plate, Honda, Mercury, Suzuki, et.al. would have to redesign virtually the entire bottom end of some of their engines. That's not going to happen to reach such a small market as us Cat sailors. Instead, they threw some small change into process to print some fancy decals, and a lot more on advertising to convince their market that they had a new, improved engine.

Today you can get an Australian SOLAS prop with more blades and lower pitch, but without also changing the gear ratio improvement is limited. So they just quietly agree to never publish the "Bollard Pull" results, even though the have it in the raw data coming from their test centers.
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Old 11-02-2014, 06:28   #8
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Re: Outboard on a prout snowgoose?

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A 25 hp outboard is not going to give you anymore than a 5% in top speed over a single 9.9 hp Yamaha high thrust outboard. The reason is in the numbers. The 9.9 has a much higher gear ratio, and more room for a larger diameter prop. It will just about reach full power at 7.5 knots. Even Yamaha's high thrust 25 fails to gear the prop down to a low enough speed to match a catamaran's performance under power. These motors are designed to push a small trailerable boat to planning speeds and beyond. When limited to the speed a catamaran can achieve, the more powerful engines have to cavitate or ventilate the propeller to reach their best power. Do instead of going faster, you just burn up a lot more fuel, something a cruising boat wants to avoid.

If you prefer anecdotal evidence I can report my experience. I owned a Roger Simpson designed 34' catamaran powered by a Honda 50 on a sled that lowered to the water's surface. It had the lowest pitch propeller available at the time, and would cavitate at any power setting above half throttle. The cat would then slow down as if I'd pulled the throttle back. Adding MORE throttle would result in even SLOWER vessel speed, with a cloud of steam bubbling up from the prop plume. A Gemini 3000 with a single 9.9 High Thrust Yamaha could drive right over us.

So its really a matter of numbers. Shaft length must get the prop deep enough to reduce sucking in a "whirlwater" funnel of air. The prop must be pitched to allow the engine to reach full power, and big enough to do that at a lower, ventilation preventing RPM.

Go look up the lowest gearing for outboards. Almost all will be geared at something just higher than 2:1 (engine rpm to prop rpm) but the Yamaha 8 and 9.9 High thrust engines have an almost 3:1 ratio. Since the size of the prop on an outboard is limited by the clearance to the cavitation plate, Honda, Mercury, Suzuki, et.al. would have to redesign virtually the entire bottom end of some of their engines. That's not going to happen to reach such a small market as us Cat sailors. Instead, they threw some small change into process to print some fancy decals, and a lot more on advertising to convince their market that they had a new, improved engine.

Today you can get an Australian SOLAS prop with more blades and lower pitch, but without also changing the gear ratio improvement is limited. So they just quietly agree to never publish the "Bollard Pull" results, even though the have it in the raw data coming from their test centers.
In calm conditions attempting 80-90% of hull speed, the 9.9 will do fine. You may get a tiny advantage in fuel efficency but in comparing some numbers to other Gemini's who have gone the 9.9 route, it's more about cruising speed than horsepower. Where the 9.9 has problems is when you need the grunt to fight into a strong headwind or or when docking.

We swapped out the orignal 40hp 2 stroke for a 25hp 4 stroke.

With the 25hp, we've wound up a few times fighting a strong headwind and that power comes in very handy. One time on a protected bay (ie: negligible wave action) we had to turn up a mile long narrow channel to get into a marina. The put us dead into a 30 kt wind, flat out the 25hp was giving us around 2kts. I can pretty much guarantee a 9.9 would have been at a standstill or lossing ground.

Also while we try to avoid it, when bashing thru waves a catamaran doesn't have the moment so the extra hp recovers more quickly after each wave.

Around the dock the 25hp will make a huge difference as you can generate more thrust from a standstill. Its the low end power not winding the engine out to max RPM that counts.

There is a point where it becomes overkill. The original 40hp was probably right on the edge of that, after you had clearly reached hull speed, there was still some power left but if you used it, all it did was make a lot of noice, so I can see where a 50hp might push over the edge for a 34-35' catamaran.
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Old 11-02-2014, 07:49   #9
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Re: Outboard on a prout snowgoose?

Maybe a little OT.

I have a Seawind with two 9.9 Yamahas. Upside is economy, simplicity, and the boat will turn in its own length.

Down side is the boat is a little underpowered at best. With a clean bottom and flat, calm, no current conditions I can do seven knots, while I easily sail 10-12 knots in 15-20 knot winds. On a return trip from the Dry Tortugas I missed my weather window by a few hours. Hitting Big Pine Key I was motoring into the teeth of a 15 knot head wind and fighting current from the Gulf emptying under the Seven Mile Bridge. My speed dropped by three or four knots. I have no doubt in worse conditions I would have not been able to motor to Boot Key and would have been forced to tack or heave to.

A friend took his monohull with a 39 horse Yanmar and had to call Sea Tow as he could not motor into a 50 knot wind. So no matter how big your engine is there will be a point where you can't motor into the teeth of the weather.

Bottom line for me is that two 9.9 outboard allow a cat to turn in its own length and will power it in reasonable conditions at a price is hard to beat. An inboard will cost more and not be as agile as two screws but will power the boat in more but not all conditions.
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Old 11-02-2014, 08:50   #10
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Re: Outboard on a prout snowgoose?

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Maybe a little OT.

I have a Seawind with two 9.9 Yamahas. Upside is economy, simplicity, and the boat will turn in its own length.

Down side is the boat is a little underpowered at best. With a clean bottom and flat, calm, no current conditions I can do seven knots, while I easily sail 10-12 knots in 15-20 knot winds. On a return trip from the Dry Tortugas I missed my weather window by a few hours. Hitting Big Pine Key I was motoring into the teeth of a 15 knot head wind and fighting current from the Gulf emptying under the Seven Mile Bridge. My speed dropped by three or four knots. I have no doubt in worse conditions I would have not been able to motor to Boot Key and would have been forced to tack or heave to.

A friend took his monohull with a 39 horse Yanmar and had to call Sea Tow as he could not motor into a 50 knot wind. So no matter how big your engine is there will be a point where you can't motor into the teeth of the weather.

Bottom line for me is that two 9.9 outboard allow a cat to turn in its own length and will power it in reasonable conditions at a price is hard to beat. An inboard will cost more and not be as agile as two screws but will power the boat in more but not all conditions.
Not counting a short burst of wind with a squall or similar event, I would argue, it's not that unusual to find yourself in consistent 25-30kt winds if they exceed the forecast. In most places it would be highly unusual to find yourself in a consistent 50kt wind that wasn't forecast ahead of time.

Yes, there is always a worse situaiton you can come up with but being able to power reliably into 20-30kt winds seems like a fairly reasonable design criteria.
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Old 11-02-2014, 10:08   #11
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Re: Outboard on a prout snowgoose?

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Not counting a short burst of wind with a squall or similar event, I would argue, it's not that unusual to find yourself in consistent 25-30kt winds if they exceed the forecast. In most places it would be highly unusual to find yourself in a consistent 50kt wind that wasn't forecast ahead of time.

Yes, there is always a worse situaiton you can come up with but being able to power reliably into 20-30kt winds seems like a fairly reasonable design criteria.
We may have to agree to disagree on what is 'fairly reasonable'. I can recall many boats having a Seagull as their only power. Even something like an Atomic 4 on a 40-50 foot boat might have problems in a 20-30 knot wind.

For most of history boats relied on humans with oars/paddles when the wind died. Many folks have circumnavigated with very limited motor power. Of course it takes skill and knowledge to cruise with a boat that can not motor into 20-30 knot winds. Quite frankly I consider the ability to plan a cruise with a built in safety factor the most important skill a sailor can have.

Perhaps more to the point if a sailor finds himself needing to motor into a 20-30 knot wind I would question his seamanship.

YMMV
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Old 11-02-2014, 10:21   #12
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Re: Outboard on a prout snowgoose?

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We may have to agree to disagree on what is 'fairly reasonable'. I can recall many boats having a Seagull as their only power. Even something like an Atomic 4 on a 40-50 foot boat might have problems in a 20-30 knot wind.

For most of history boats relied on humans with oars/paddles when the wind died. Many folks have circumnavigated with very limited motor power. Of course it takes skill and knowledge to cruise with a boat that can not motor into 20-30 knot winds. Quite frankly I consider the ability to plan a cruise with a built in safety factor the most important skill a sailor can have.

Perhaps more to the point if a sailor finds himself needing to motor into a 20-30 knot wind I would question his seamanship.

YMMV
If you are talking an open ocean crossing, I would agree that a sailboat shouldn't have a lot of need for an engine that can be relied upon in a tough situation (still nice to have).

Coming in and out of marinas, anchorages or tight channels, I would question the seamanship of someone who purposely underpowers thier vessel in the modern world.
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