Outboard Size for 40 ft Cat

[smj]

Seems as if the Yamaha 9.9 has the most advantageous lower unit gear ratio with Honda coming in second. Hadn't heard of the Suzuki high thrust and it seems it has a standard lower unit gear ratio?


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[Ken Z]

I would opt for 2 9.9 honda extra long shaft, to keep the prop in the water in a chop, and keep our weight down and weight of fuel. Bigger is better at times, but you can have a lot of thrust with 16-18 h.p.. Yamaha is great , I dont think they make a 25”.

[smj]

Sailjumanji turned us on to this product about 10 years ago. Great ad on to the outboard and well priced. We have added these onto 4 outboard powered cats and love the benefits they give.
http://www.powerthruster.com


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[Mark424]

It's a bit of a head-scratcher because the prop is what's really putting energy into the water regardless of the HP of the engine, if you have more HP than the prop needs, it's wasted. If the prop has too high of a pitch, and thus too high of "slip ratio", it might loose grip and just churn up a froth.

Most small outboards have an approx 9" prop, some makes have 10" prop on their high thrust (I think Honda & Suzuki), the Yamaha 9.9 can handle a 11.75" dia prop. Similarly, the normal outboard gear ratio is 2.08, normal high thrust is a 2.33 ratio, and the yamaha T9.9 is 2.92 (thus more torque going to the prop). Best I could tell from trying to read some academic stuff on prop's is that thrust is proportional to the square of the prop diameter (I'm sure it's a lot more complicated than that), but the idea is that a bigger diameter prop is a lot better. I've never seen a test of various pitch props on something like a cat, would love to see if you can put a higher pitch prop and run at lower rpms.

Add on top of that, that many (including the Yamaha T25) don't come in a 25" shaft which you may or may not need.

+ I'd rather have a motor than under normal cruise I'm running at 1/2-2/3 RPM, a lot less noise and vibration. That argues for a bigger motor with more reserve power and lower normal operating range.

One last option, although very $$$ -> get yourself a 12KW +/- DC genset and run 2 electric outboards (Torqeedo or e-Propulsion). You have one engine running at moderate RPM, less noise/vibration. You have lots of extra power for house needs. You can carry less water weight since you have the power to make what you need. Could probably carry less battery weight. The props on electric outboards are a completely different design, I have no idea if they are substantially better than run of the mill outboard props or not.

[Mark424]

Originally Posted by smj

Seems as if the Yamaha 9.9 has the most advantageous lower unit gear ratio with Honda coming in second. Hadn't heard of the Suzuki high thrust and it seems it has a standard lower unit gear ratio?

The Suzuki 9.9/15/20 are essentially the same engine but only the 9.9 comes in the 25" shaft. The best I can tell the only parts that are different are the computer and the air in take restrictor plate. You could upgrade their 9.9 to a 20, just not sure if the prop could make use of the extra torque.

[BVIyachtsales]

Definitely 25's. two strokes can save weight but we really like the Yamaha 25hp fuel injected outboard. Less might work but going slow gets boring fast and going backwards is scary... it's not always calm when you need to motor.
Hope that helps

[44'cruisingcat]

Originally Posted by Mark424

It's a bit of a head-scratcher because the prop is what's really putting energy into the water regardless of the HP of the engine, if you have more HP than the prop needs, it's wasted. If the prop has too high of a pitch, and thus too high of "slip ratio", it might loose grip and just churn up a froth.

Most small outboards have an approx 9" prop, some makes have 10" prop on their high thrust (I think Honda & Suzuki), the Yamaha 9.9 can handle a 11.75" dia prop. Similarly, the normal outboard gear ratio is 2.08, normal high thrust is a 2.33 ratio, and the yamaha T9.9 is 2.92 (thus more torque going to the prop). Best I could tell from trying to read some academic stuff on prop's is that thrust is proportional to the square of the prop diameter (I'm sure it's a lot more complicated than that), but the idea is that a bigger diameter prop is a lot better. I've never seen a test of various pitch props on something like a cat, would love to see if you can put a higher pitch prop and run at lower rpms.

Add on top of that, that many (including the Yamaha T25) don't come in a 25" shaft which you may or may not need.

+ I'd rather have a motor than under normal cruise I'm running at 1/2-2/3 RPM, a lot less noise and vibration. That argues for a bigger motor with more reserve power and lower normal operating range.

One last option, although very $$$ -> get yourself a 12KW +/- DC genset and run 2 electric outboards (Torqeedo or e-Propulsion). You have one engine running at moderate RPM, less noise/vibration. You have lots of extra power for house needs. You can carry less water weight since you have the power to make what you need. Could probably carry less battery weight. The props on electric outboards are a completely different design, I have no idea if they are substantially better than run of the mill outboard props or not.

Yes, the props are very important. However something that often seems to be overlooked is the importance of the shaft length.

I chose the Honda 20s because they're available (in Australia) with a 28 inch shaft, as well as with high thrust props.

The best, biggest prop in the world won't be of much use if it's out of the water.

[schoonerdog]

Dave,


Curious about what your wide open throttle is for your catamaran (BTW, I love outboard powered catamarans, had one, ridiculously simple to operate, repair, replace compared to the way over recommended diesel cousins). My 44ft catamaran is 8 tons, has twin diesels, and flat out fully loaded is probably around 9 knots, unloaded is a little over 10 knots. Schionning is a wonderful catamaran designer, congratulations!

Originally Posted by Dave_S

My cat loaded for a couple of nights is about 7.25t and 48' long.

I had 2 x 25hp High Thrust Yamahas, flat out around 8 knots cruised at around 6.5

I replaced them with 2 x 40hp Hondas not sure on figures for flat out yet (can't get full throttle from the controls - longer throw on the throttle linkage), cruise at 3,500rpm at a smidge over 8.5kn. Flat out should be 5500 to 6000 rpm, best case I think around 13+ kn.

The 40's are EFI, a big factor in my decision to go up in size. They are not High Trust though but it is the smallest HP rating using the same gearbox as bigger hp motors so they will be strong enough.

The 40's are a bit lighter than the 25's and similar, if not slightly better fuel economy with a speed advantage (6.5kn to 8.5kn), I guess 60% more HP and similar fuel efficiency I am happy. Speed for speed the Hondas will be better.


None of that directly relates to your use but if I were choosing again between two sizes, I'd go with the bigger size, you will have more control in the wind and swell.