One or two outboards on sailing catamaran

I have been studying plans for 40' sailing catamarans like the Easy Catamaran and more. Many of them are driven by two 9.9 hp high thrust outboards like Yamaha long shaft engines. I also see some discussions using only one larger outboard motor. Yamaha has a 25 hp high thrust as well.

So the question I'm stuck on is if one would be better off with two 9.9 hp or one 25 hp outboard on a catamaran like that? Are the two propellers going to give more thrust than one larger etc?

On other points it looks like one would have to add steering to a one motor setup but the two motor setup could be fixed.

 

A single 25HP motor would probably push you a little faster in smooth water, two 9.9's I think would push you faster in any kind of chop or sea, given that all other things like mounting are equal.

A single engine center mounted on a sled would not need steering, and even if you mounted a single on the end of a hull you might not need steering. MacGregor 36's came with single O/B on a hull and while having limitations this worked quite well most of the time.

The mounting configuration of an O/B on a cat is pretty important as any aeration will cause you to loose thrust very quickly. Take a look here for a current discussion involving some options:

http://www.boatdesign.net/forums/outboards/tohatsu-9hp-vs-other-brand-options-48301.html

There have been many discussions about mounting options, do a search.

Steve

 

I recently completed a 1500 mile trip with a 30 ft gemini cat with 2 x 9.9 high thrust motors. One motor was mounted on the original fiberglass hinged bucket on centerline with remotes at the helm and hooked up to the rudders with lines so it steered. The second motor was mounted on the port transom as far outboard as I could mount it, on a standard sailboat bracket, it has no remotes and doesnt steer, itwas only there as a backup as we had no sailing rig. We only ran one motor at a time as when we tried both together it only gave about 1.5 knots difference, but a couple of times we ran both just for more thrust. For the most part we ran on the center motor when there was close quarters manouvering to be done and the wing motor when we could run it for hours just to keep the engine hours close to the same.
My observations are that while the wing motor was mounted off center but behind a transom and the other was on center but mounted on the bucket so theoretically more suceptible to aeration. There was no noticable difference in how they pushed the boat. That said, I am very impressed with how well the boat manouvers on the single centrally mounted motor with steering. I think that a single 25hp which steered would be a good settup on a reasonably light 40ft cat.
Steve.

 

Two small motors sounds appealing. Once anchored a small motor could be unshipped and used to power a tender. Redundancy is also advantageous

Perhaps a 30 ft cat doesnt use a powered tender.

Perhaps a cat can be operated by two unmatched engines, 20hp and a 5 hp

 

Very interesting. How hard were you running the motors when you reached the 1.5 knot speed improvements?

Other things to consider as well is weight. Two 9.9 hp are 230 lbs (power tilt) vs. 200 for the 25 hp. The 25 has an alternator output of 13 amp vs 6 for each of the 9.9 as well.

Another option if the 25 hp is not enough is to go up to the 50 hp High Thrust. It will though add some 60 lbs over the 25 hp.

 

I am currently sailing a 38ft catamaran in Greece, so have missed many threads. But we are in port right now, sheltering from thunderstorms

If you are looking at a 40ft + sized catamaran then twin 9.9hp is not enough power. The minimum sensible is twin 15hp, maybe Big Foot Mercury? Or one central 25hp may be OK depending on the type of hull and how well it manouvers.

I have had single 9.9hp outboards on a number of my own 35ft catamarans. They are marginal in strong winds and big seas. However I would not use anything bigger than a 25hp outboard on a 40ft cat, better to use twin engines. It isn't really practical to use the main engine on a dinghy unless it is very small. It's just too complicated to disconnect the controls and starting cables

A 1.5 knot speed increase using twin 9.9 instead of one engine sounds about right

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

Specify what speed that 1.5 knots increase was at. If that was 10 knots to 11.5 that is impressive. 5 to 6.5, not so much...

 

6knots to about 7.5 which is over hull speed at full throttle. Actually when we started the trip we were running at about 3/4 throttle on one motor and getting about 9+mpg (statute) but as the trip went on we noticed that by moving weight foreward to get the transoms up the speed went up a full half a knot without any increase in throttle setting, then eventually whe we got to doing longer stretches, particularly lake superior it was wide open throttle at 6.5 to 7 knots. I am very happy with the 9.9, but as richard said it is marginal into strong winds, we had several stretches where we were pushing into 25 knot headwind and fairly large seas and we would get knocked back to 2 knots by the waves, we considered firing up the wing motor but didnt think it would stay immersed. I am actually in awe of the thrust of those motors. I wish the manufactures would publish thrust figures for their motors as to me that is more important than horsepower since they dont have high thrust versions of all models. I would guess that a 25 hp would have more thrust than the 9.9 and should be able to run at the same speed at maybe half throttle an be a lot quieter, and maybe just as economical.

Steve.