Optimum engine size

[catalac08]

The generally accepted way to size an engine is to have an engine that will achieve hull speed. However I am wondering if that thinking is more applicable to a monohull with limited frontal windage area and that sails well and that a cat by virtue of its higher windage was a different situation. My boat much as I love her does not sail that well hard on the wind.

My boat is an 8M Catalac catamaran with a large square fronted sprayhood, not pretty but is very effecive and house 2x100w solar panels on the top. I have a 20hp ultra long shaft outboard which mostly is fine and fulfils the hull speed criteria but into strong tides and a headwinds struggles unless I can bear away enough to motor sail. I was recently forced to make a trip at totally the wrong time against a 3/4 knot tide and some headwind right on the nose. It took me 2 hours to cover 2 miles which was very discouraging experience. In that situation another 10hp would have helped me make better progress. The increasing fuel efficiency of fuel injection compared to a carb would reduce the traditional downside of having the larger engine.

Any thought or perpectives on engine sizing for cats?

[GrowleyMonster]

Originally Posted by catalac08

The generally accepted way to size an engine is to have an engine that will achieve hull speed. However I am wondering if that thinking is more applicable to a monohull with limited frontal windage area and that sails well and that a cat by virtue of its higher windage was a different situation. My boat much as I love her does not sail that well hard on the wind.

My boat is an 8M Catalac catamaran with a large square fronted sprayhood, not pretty but is very effecive and house 2x100w solar panels on the top. I have a 20hp ultra long shaft outboard which mostly is fine and fulfils the hull speed criteria but into strong tides and a headwinds struggles unless I can bear away enough to motor sail. I was recently forced to make a trip at totally the wrong time against a 3/4 knot tide and some headwind right on the nose. It took me 2 hours to cover 2 miles which was very discouraging experience. In that situation another 10hp would have helped me make better progress. The increasing fuel efficiency of fuel injection compared to a carb would reduce the traditional downside of having the larger engine.

Any thought or perpectives on engine sizing for cats?

I would get another outboard, for the other hull. I bet a pair of 9.9hp outboards would push you a lot better than a single 20, though you probably wouldn't save anything on fuel.

[denverd0n]

Do you need more horsepower? Possibly.


The other possibility is that your prop is not well suited to your boat. I would do some research on optimum prop specs first, before I went to the expense of buying another/larger motor.

[Davepf]

OP: What is your cruise speed and top speed ? I also have a Catalac 8M and previously had an high thrust 9.9hp that could cruise at 4kts and top speed at 5kts.

I change the outboard for a 20hp (non high thrust but with high thrust 4 blades prop)when I cruised the Bahamas and was able to cruise at 5kts and top speed at 6kts. Fuel consumption is about 5mpg@5kts.

[catalac08]

Thanks for replies.

My engine is in the centre. Fitting another engine would involve a lot of structural glasswork, re-building the steering connector bar which I am not up for just now but thanks for the suggestion but this is much harder on a small cat.
I have a four blade hi thrust prop suitable for a heavy old boat and the Tohatsu gearing is quite good for this application,not quite as good as the aildrive 9.9,s but better than most o/b,s. The 20hp will push top speed about 6.8kn and cruise at 5 happily but the issue is a lack of reserve power when trying to push a tide and headwind. I average 0.7galls per hour cruising so if I got a 30hp engine the better economy being fuel injected may help so may not be more thirsty, except when using the spare power?

[kmacdonald]

To punch into waves and headwinds you need a prop with as much surface area as possible. If you have high thrust props then you have done all you can do without getting another engine that can spin a bigger prop. Extra horsepower alone won't do it.
I'm not sure a 30 HP engine will spin a significantly bigger prop than the 20 HP engine you already have.

[Davepf]

Does your boat squat alot under power ? And how much more weight is the 30hp compare to the 20hp ?

This boat is more a motorsailer than a sailboat. With both engine and sail, fuel economy is fantastic and speed/pointing ability is great 6-7kts easy. I know this is no help when going into the wind !

If the new engine is not too heavy and you can fit a bigger prop, I think you could have best of both world (Fuel economy and power).

[catalac08]

Thanks for comments about prop sizes. Within my thoughts about fitting a 30hp engine is the worry that the props on the 20hp is 10x7 (4 blade) & 9.25x9 (3 blade)and the 30hp props are about the same diameter but a coarser pitch. It is dissappointing that the biger engines do not swing bigger prop.

A difficulty sourcing catamaran outboards is that ultra long shafts are hard to find. Tohatsu will do up to 30/40hp with an ultra long kit fitted. Yamaha do the 9.9 but maybe not longer in extra long shafted and that is about it. Interesting also that the 30hp Yam is about 20kg heavier than the tohatsu!
So you have the prop size problem and the leg length limited to one or two makes it is not very easy.
I shall have to do more homework but I am sure others have been this way and sorted out what best to do?

[kmacdonald]

The Yamaha 25 High Thrust has a 12" diameter prop.

[Davepf]

Originally Posted by kmacdonald

The Yamaha 25 High Thrust has a 12" diameter prop.

Yes that would be the best upgrade as the 30hp is not an high thrust version so the prop would still be a small size low pitch aftermaket one.

[Cheechako]

My 42 cat with little extra wind catchers other than a bimini had the 3GM30 diesel engines. I think they are maybe 26 HP? but at significant high RPM. If I had it to do over I would get a bit more HP and preferably at lower RPM. maybe 30-35 hp each. So maybe you do need more HP. A 6hp outboard will move a 30 ft mono at hull speed in flat water, but that doesn't mean that's what it should have.
Two high thrust smaller outboards would be nice but expensive I suppose... I used to high idle the lee engine when I needed to get to weather time wise. The cat then pointed up a good 15-20 degrees over sailing.

[catalac08]

Originally Posted by kmacdonald

The Yamaha 25 High Thrust has a 12" diameter prop.

Thanks I was not aware of that.
I see that Yamaha 9.9 has a 2.9 gearbox and so swings a prop the same diameter as a 20/30hp but lower pitch. So punches well above its weight but having used 8m Catalacs with the Yamaha 9.9 high thrust and a Tohatsu 20hp also with hi thrust prop I can confirm a world of difference in that the Tohatsu 20 is far better.
The difficulty in all this is that you cannot get a bigger engine with
2.9 gear ratio
ultra long (25") shaft
The nearest you can get is a Tohatsu 25/30 with ultra long shaft and 2.17 gear ratio and the prop is still only up to 10" diameter.
The Yamaha high thrust engine 25hp is a 2.42 gear ratio (trying to get dealer to confirm diameter of prop) but only comes in long shaft which I am not sure is long enough.
The Mariner Bigfoot range would be ideal but they do not do them any more.

It seems hard to get what you really, really want!

[VBsail]

I see that Yamaha 9.9 has a 2.9 gearbox and so swings a prop the same diameter as a 20/30hp but lower pitch. So punches well above its weight but having used 8m Catalacs with the Yamaha 9.9 high thrust and a Tohatsu 20hp also with hi thrust prop I can confirm a world of difference in that the Tohatsu 20 is far better.

I am a bit surprised by that. I bought a suzuki DF20A fuel injected when they first came out in 2013. I was excited and really wanted to love it. Weight of my 9.9 with double the hp. It came with 9.25" prop and I put a 10" diameter 4 blade high thrust on it we nicknamed the blender. The 20hp worked great in calm water at a steady speed. Moved along at half throttle, but felt like I had a blender behind the boat when things kicked up or docking. The extra throttle made a ton of noise but that was about it. I tried 5 props on the motor before selling it after 2 seasons and buying another t9.9.

There is nothing really between 100lbs and 200lbs that is low geared large prop, let alone XL shaft.

The yamaha T25 is 200lbs (weighs the same as most 50s) and is only available in 20" shaft. Slightly heavier is the ETEC 40/50/60 with 2.9 gear ratio and 14" diameter prop.

A difficulty sourcing catamaran outboards is that ultra long shafts are hard to find.

Bay manufacturing sells 5" shaft extension kits for almost all models. When I sold the 20 I bought a another new 25" T9.9 and put a 5" leg extension on it to make it a 30" shaft to get a bit more grab.

[catalac08]

Bay manufacturing sells 5" shaft extension kits for almost all models. When I sold the 20 I bought a another new 25" T9.9 and put a 5" leg extension on it to make it a 30" shaft to get a bit more grab.[/QUOTE]


Thanks for your comments some of which I have to agree with. My experience on a 9.9 Yamaha was actually on a 9m Catalac but did not have the opportunity to compare rough water conditions. I have also tried different props and my conclusion, like yours is that I need a different motor. I think my boat is more demanding on the engine than an Iroqois as it is heavier, more bulky in the water and with high windage is more dependent on the motor than the Iroqois which is a much better sailer.





These extension kits are interesting. I am surprised that the drive shafts are stainless as most grades are not suitable, it would be interesting to know the grade of stainless?
In the Uk these extension kits are pretty unknown, do they generally have a good reputation in the US?