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Old 07-03-2011, 09:08   #1
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Outboard Selection Options

My current boat is a Crowther buccaneer trimaran. it is 10m long and weighs about 3300 lbs. design weight loaded is 5000 lbs. When I bought the boat it had a high thrust 8 hp honda four with alternator electric start remote controls, etc. This is a rather heavy engine (~130lbs.) and was mounted on a stern extension made of heavy timber faired into the shape of the hulls adding 2 feet to the boat with an outboard bracket on the rear. With this setup the boat squatted in the rear and cavitated horribly in any chop. The waterline was way high on the stern and the bow was out of the water with this arrangement. This boat has rather fine ends and the mantra of keep the ends light is in constant refrain from all who know this design. I cut the transom extension off, dropping ~100 lbs from the stern of the boat.

I am now ready to mount power elsewhere, most likely a bracket on the aka (crossbeam). I saw another trimaran online with two engines mounted on either side and thought this could be a better arrangement. The 6 horse engines weigh 55 lbs each and would give the improved maneuverability of twin engines.

I need to get an engine for my dinghy, and think that one of the 6 hp engines would be perfect for my 10 ft porta-bote. I could use one engine on the dink when at anchor and have both for the tri when at sea, etc. This should give me 12 hp instead of the eight I have now, and be about 20 lbs lighter overall. I would give up the high thrust of the 8, but would have two props in the water when needed and be able to just run 1 for economies sake. Should the thrust of the pair be too little, I could change to props with less pitch.

I am about to splash $2700 for a pair of tohatsu's Any one want to chime in on whether this is the perfect solution, or not?
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Old 07-03-2011, 09:27   #2
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Re: Outboard Selection options

Mine will be the minority opinion.

I have the Tohatsu 6, it is (as I am sure you know) it is the exact same motor as the Mercury and Nissan 6... and 5, and 4.

I just purchased a 'new old stock' Yamaha 6hp 2 stroke to replace my brand new 6hp 4 stroke.

The single cyl 4 stroke vibrates too much for me. It also has a tiny zinc that is (IMHO) not suited for use in the water for an extended time.

I sympathize with the weight issue (mine is a mon-hull with an outboard in a well) but the 100+ pounds for the 8hp 4strokes is simply too much.

I will also say that your idea of using one of the motors for the dingy may be problematic... it is a fair amount of work to mount / unmount a motor for this purpose. Might work for you... but think about the shaft length also. You will want a 15" shaft for the dingy, I would suspect a 20" (or even 25") shaft would be better for your primary motor.

Might you instead find a 2 cyl Yamaha 2 stroke (6 or 8hp) to replace your current motor? You can cut the weight in half without losing the yamaha quality or picking up the vibration.

Anyway, something to think about.
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Old 07-03-2011, 09:42   #3
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Re: Outboard Selection options

Thanks for the reply. I don't think switching the engine to the dink is anymore trouble than moving it from the bracket it would be mounted on when sailing if I were just securing it on deck.
I am thinking of using short shaft motors with drop down mounts on brackets so that they will be well clear of the water when not in use but quickly deployed if needed. The brackets for the smaller motors are pretty cheap(~$50 on craigslist).
I kinda like the four stroke economy and torque. That would be really important to me if i had a slow heavy boat like the pearson. How fast can you motor it with current set up?
Wish I could try one one of the singles out and see if the vibration will bother me as much as it does you. perhaps, I will look for a boat rental place nearby and try one.
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Old 07-03-2011, 09:56   #4
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Re: Outboard Selection options

THat is a critical distinction between our boats, as a mono the extra HP is wasted... since it is not going to derive any benefit from more hp... just a bigger bow wave.

Yes, if you remove the motor to sail then the idea of using one for the dingy is more workable. I hope it works out for you.

If I were closer, I would be happy to lend you mine... maybe someone from the forum will be able to do so.

Good luck.
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Old 07-03-2011, 09:58   #5
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Re: Outboard Selection options

AntiqueTri,

Maybe pictures would help explain your problem ?
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Old 07-03-2011, 11:54   #6
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Re: Outboard Selection options

I took this off the boat as it was heavy and messing up the performance.





with that gone, i have the original stern profile and want a swim platform/ladder there.



Now I am ready to mount some power on the boat, but would rather not put it off the stern. Thinking of mounting two 6 hp 4 strokes on the beams and using one on the dink occasionally.



I was looking for some feedback on this setup, so far s/v Faith thinks they vibrate too much for his use.
Any one else have an opinion on this use of these engines?
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Old 07-03-2011, 12:06   #7
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pirate Re: Outboard Selection options

Think we could do with some input from Mark Johnson (NC) here.... he's the tri man that could help re balancing etc... maybe..
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Old 11-03-2011, 05:02   #8
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Hi,

A word of warning about 6hp johnson/ tomhatsu outboards, in my experience

I have mine transome mounted, apparently I have drowned it once. It has never been submerged, only used in rivers not open water. It seems they do not like water, mine certainly does not vibrate less than a 2 stroke.

I would not say its any more efficient, it just manages to get my 24 footer to 5.5 knots (hi thrust prop). The last 1/4 turn of the throttle does not really increase the revs, so would not say it has masses of torque.

I would of got a bigger engine but for weight issues. It seems they are a Hobsons choice.

In my opinion.

Gerrit
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Old 11-03-2011, 06:09   #9
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Re: Outboard Selection options

Mounting your kicker to the aft crossbeam is MUCH better than having it hang on the transom! It is easier to get to, moves the weight fwd, and cavitates less. (NO, it will not motor the boat in circles...)

I would build a STRONG swinging bracket, like the one I built for my previous boat, a Seaclipper 28.

Between kicking up the motor, and raising the bracket out of the splash zone, it worked great. An even better version was done by Jim Brown, as a mod to his Searunner 31, Scrimshaw. He added a "V shaped nacelle" to the bottom of the bracket, so that it would better part the waves, rather than "slap" like mine sometimes did. (Either one works great)!

IF they are still around... BY far the best Aux kicker is the Yanmar 9.9 saildrive. It has a very long leg extension, and has a LARGE prop going slow, rather than a small prop going fast, (like on skiff & dinghy motors).

Lightly loaded, it would push my SC 28 at over 10 knots! Your boat... loaded, it would still go well over 7. It is a remarkably powerful little motor, and even with electric start, is 105 pounds.

Best of luck, Mark
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