Refit 31 prout Repower advice

[44'cruisingcat]

Originally Posted by VBsail

The solas 10" 4 blade high thrust prop (pictured) is the same prop as used on the honda high thrust.


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Actually it's not, although it looks a bit like it.

The best prop in the world isn't much use if it's out of the water. Getting the prop in deep is number one priority in an outboard installation, IMO.

[VBsail]

Actually it's not, although it looks a bit like it.

Correct. It is a Solas. Its the same as the honda 4 blade solas, not oem.

The best prop in the world isn't much use if it's out of the water. Getting the prop in deep is number one priority in an outboard installation, IMO.

I agree. Mount location is #1. Shaft length helps after that. I added a 5" kit to make a 30" outboard leg for that reason. .



On longer boats with higher hullspeeds extra horsepower is probably useful just to to reach hullspeed. The prout with a 28' waterline should have no issue motoring hullspeed with 10 hp. I actually bought a pair of T9.9s (in the pic) and eventually sold the second (to the local honda/tohatsu dealer for his own nimble kodiak) since one was all I needed.

[Boatguy30]

Is the the Bay 5" kit? Is the shaft bushed at all in the extension?

I use a Yamaha T8 on my 34' cat with a backup 5hp 2 stroke wing engine. Might change that to a 8hp 2 stroke so I can add controls for the occasional close quarters maneuvering

[44'cruisingcat]

Originally Posted by VBsail

Correct. It is a Solas. Its the same as the honda 4 blade solas, not oem.




I agree. Mount location is #1. Shaft length helps after that. I added a 5" kit to make a 30" outboard leg for that reason. .



On longer boats with higher hullspeeds extra horsepower is probably useful just to to reach hullspeed. The prout with a 28' waterline should have no issue motoring hullspeed with 10 hp. I actually bought a pair of T9.9s (in the pic) and eventually sold the second (to the local honda/tohatsu dealer for his own nimble kodiak) since one was all I needed.

Long lean multis don't really have a hullspeed.

[sailorcherry]

I like the Proud Quests, dream cat, under 75k, is a 37 elite and would modify the back porch with some swim steps.

I was wondering, with the mods you have done/are doing why didn't you buy a Catalac? Some twin screw and of course you have much more headroom.

Originally Posted by shane malone

I’ve been taking my time with the project hoping to see water this summer having a very hard time deciding what to do for power was pretty convinced on Twin 15’s but now kind of leaning towards a single 50 mercury outboard they now they make them in 25 inch shafts. Thinking the in flat calm water the single larger prop would be able to run it will work quieter rpm’s over the twin 15 outboards but witch is the best all around. Side-note We plan on running a 15 short shaft for our dinghy so if we did go that route we would have multiple of the same parts

[mikedefieslife]

Originally Posted by 44'cruisingcat

Long lean multis don't really have a hullspeed.

I kept seeing people mention hull speed in this thinking, well that doesn't really apply to catamarans like it does monos, especially those with narrow fine hulls.

[valhalla360]

Originally Posted by 44'cruisingcat

Long lean multis don't really have a hullspeed.

Sure there is. The issue is the "1.3" is actually another variable largely controlled by the length to beam ratio.

The result is for large long lean racing machines, skin friction limits speed before hull speed can be reached.

For smaller non-racing cats like the OP's, there most definitely is a hull speed. If kept light he might get 0.5-1.0 kt extra over a similar length mono but there is definitely a wall where more HP is just silly. It's also highly subject to loading as that will change the individual hull beams.

[valhalla360]

Originally Posted by sailorcherry

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I was wondering, with the mods you have done/are doing why didn't you buy a Catalac? Some twin screw and of course you have much more headroom.

We just sold our Catalac 10m (for life reasons not because we didn't like the boat).

The engine configuration was one of the downsides. Do access the engines, we had to open the hatch, remove all the stuff on the platform, remove the platform, climb down (I'm 6'2" and the hatch was above my head). Stradle the engine with uncomfortable sloping hull bottoms, then climb out and reverse the process. To get to the other engine, you repeat the process. Simply checking the oil was a 20minute job.

On our prior Gemini I would sit down next to the engine and unclip the outboard cover and set it on the bench. If I had to do something with tools or lose parts, I would string a tarp under the engine to catch them but for simple oil check, just a minute or two.

The Catalac 10m does have more interior space but the galley wench is isolated from the main sallon and the steps are all and steep. The 8/9m versions are much more tight space.

It's all trade offs and personal preference. In this size range for coastal cruising, we would probably go with the following in order of prefence(cost and availability not a concern). Given the right deal in the right place, I might take any of them:
- Gemini
- Endeavor 34/36
- PDQ/Catalac
- Prout

[44'cruisingcat]

Originally Posted by valhalla360

Sure there is. The issue is the "1.3" is actually another variable largely controlled by the length to beam ratio.

The result is for large long lean racing machines, skin friction limits speed before hull speed can be reached.

For smaller non-racing cats like the OP's, there most definitely is a hull speed. If kept light he might get 0.5-1.0 kt extra over a similar length mono but there is definitely a wall where more HP is just silly. It's also highly subject to loading as that will change the individual hull beams.

Guess it depends on your definition. Most people regard hull speed as that point where the power required to climb your own bow wave becomes excessive. Most cats, because of their high LWL: BWL ratio simply don't generate a large bow wave.

Short fat hulls need to plane to significantly exceed " hull speed". Cats can go well beyond that speed without planing. We've seen 20+ knots without planing. Speeds in the teens regularly. Without planing.t

[valhalla360]

Originally Posted by 44'cruisingcat

Guess it depends on your definition. Most people regard hull speed as that point where the power required to climb your own bow wave becomes excessive. Most cats, because of their high LWL: BWL ratio simply don't generate a large bow wave.

Short fat hulls need to plane to significantly exceed " hull speed". Cats can go well beyond that speed without planing. We've seen 20+ knots without planing. Speeds in the teens regularly. Without planing.t

Same definition just that for long lean racy cats, skin friction controls long before you reach hull speed.

I believe you have a 44' cat which is a different ballgame than the 30-35' cruising cats, so your hull speed factor is likely twice what a 44' mono would be and that's why you can see speeds in the teens without planing.

Keep in mind, hull speed is not really a firm number. When people start talking about hull speed down to the 1/10th of a knot, they don't understand. You can keep pouring on the power and go faster and in a good wind, your sail plan likely exceeds what your engines can produce by a good margin.

[VBsail]

I kept seeing people mention hull speed in this thinking, well that doesn't really apply to catamarans like it does monos, especially those with narrow fine hulls

Prout 31 is 28' waterline and 7,000lbs empty.

It will not follow the same characteristics as a racing catamaran. To classify all catamarans together is as accurate as lumping all monos together.

For the OP, Take a look at the Gemini Catamarans group on facebook. They have many documented installs of replacing diesels with outboards. Lots of info there.

80 horsepower on a Gemini, 9kn WOT, 8GPH

https://www.youtube.com/watch?v=e7yiN_8jDts

[Boatguy30]

Prouts are short and fat. They do not go much beyond typical medium displacement monohull boat speeds.

[Boatguy30]

VBSail.

What about that 5" extension?

[VBsail]

Yes, This one was the Bay manufacturing 811A kit. No additional bushing.

[shane malone]

putting an after market shaft extension is out for me the standard 25” will be perfect the prout and gemini are Quite different in hull design. the prout has a narrow exit at stern and the gemini is more like a semi-Displacement hull that being sead you could plane the gemini with more power say 250hp but not the prout the hull has Virtually no lift in Design, that was never the Goal. for my prout the engine Bracket is almost 4 feet from stern Which will have a lot less action and stay more submerged and stable then engines mounted on the transom such as the gemini configuration. i will show more Photos when the snow melts.