Choosing Correct Outboard - Need Technical Help

Discussion in 'Propulsion' started by CatBuilder, Apr 30, 2011.

  1. CatBuilder

    CatBuilder Previous Member

    Can anyone suggest the proper outboards to choose for powering my boat?

    I have an "in" at a Yamaha dealer, but can only ask for pricing, not technical support, on the outboards.

    Here are the models:


    Here are the specifications on my boat:

    Length overall: 44'-10" (13.67 m)
    Beam: 25'-0" (7.62 m)
    Draft: 1'-9"/6'-3" (0.53/1/90 m)
    Weight: 11.900 lb (5.398 kg)
    Max Displacement: 18.292 lb (8.315 kg)


    The designer said if I could get 35HP, that would be better than the 25's. He said this back when I was looking at Hondas a year ago and those have changed as well.


    1. What hull speed am I trying to achieve here at WOT? My boat does close to 20 knots under sail in a lot of wind, but I'm not sure what I'm shooting for under power.
    2. Can anyone calculate the speed of the given engines through the water, accounting for RPM, prop pitch and gear ratio?
    3. Why do the gear ratios change so much from one model to the other, if they are all High Thrust models?
    4. Do I need to re-prop any outboard I get, and/or is re-propping a better solution? As in... could I just get a standard outboard that blasts out high RPMs at the shaft and gear it down by changing the pitch of the prop? If so, how would I do that?
    5. Which engine combination from the current Yamaha line would work best for my boat?

    Attached Files:

  2. gonzo
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    gonzo Senior Member

    What is the boat's rated HP?
  3. rasorinc
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    rasorinc Senior Member

  4. CatBuilder

    CatBuilder Previous Member

    The plans show a pair of Yanmar 2GM20's, then a 3GM30's, then a 4JHE's in different pages. :confused:

    I'm going with the outboard option. On a smaller boat of his, he showed a pair of 25hp outboards in the plans. This is a larger boat with more displacement and he was quick to say, "get the 35's."

    So... there is basically no definite answer... as usual... from my designer.
  5. CatBuilder

    CatBuilder Previous Member

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    FAST FRED Senior Member

    Most sail boats will cruise ONLY in the full displacement mode.

    The vessels weight in tons (2240lbs to the ton) multiplied by 3 or so will give the total HP required at cruise.

    Outboards seem to live longer at less than flank , so the HP required should be about 50% to 60% of the engines rated output.

    Seems a small pair of outboards should get you a 6-7K cruise speed.

    Their light weight wont hurt a cat either.

  7. CatBuilder

    CatBuilder Previous Member

    FF: So I'm looking at 50HP total?

    I took the max displacement (assuming I'm loading her to the waterline), then divided by 2240, then multiplied by 3. That gave me 25HP, total.

    Bumping up the HP by a factor of 2, to run at 50% or rated HP gives me a total requirement of 50HP, or a pair of 25HP engines.

    Do you think there is some other factor here for catamarans? Windage? Other factor?
  8. gonzo
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    gonzo Senior Member

    Yes, 50HP would be the equivalent to a pair of inboards. That would put you within the design specs. Too much power is a waste of money, but a safety margin is good. The only drawback with outboards is that they don't charge batteries well. Their alternators are usually pretty small compared to an inboard.
  9. Submarine Tom

    Submarine Tom Previous Member

    You could probably get by with a pair of 9.9's but 25's will give you more versatility.

  10. Frosty

    Frosty Previous Member

    Ide get the 50 because the 50 and the 60 are identical even in weight which means you can make your 50 into a 60--if you wanted.
  11. CatBuilder

    CatBuilder Previous Member

    Frosty, we are talking about a pair of engines, so it would be a pair of 25's to make 50hp total.

    Gonzo, being on sail boats exclusively, I don't use engines to charge batteries anyway, so no problem at all.
  12. hoytedow
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    hoytedow Carbon Based Life Form

    As far as charging goes, even under sail, a wind generator could do that job instead of depending on the outboards.
  13. CatBuilder

    CatBuilder Previous Member

    Thanks, Hoyt.

    This is where I'm a bit of an expert. I know nothing about building the hulls, but once I have hulls up, you'll find all my posts dry up here.

    I have been doing systems on boats for about 20 years now. :D

    I'll be able to move right along and already have them all figured out.

    Being a sail boat, I don't produce power with my main engines. It's prohibitively expensive. Also, wind is a waste of time because the wind gens don't put out enough amps on a continuous basis. They are terrible if you are anchored somewhere for a while. A good anchorage is, by definition, lacking wind.

    Solar and generators are the way to go. I'll be having quite a bit of both. :)
    1 person likes this.
  14. hoytedow
    Joined: Sep 2009
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    hoytedow Carbon Based Life Form

    Once you have finished building others will benefit from this thread and may have questions to which only you would have detailed answers.

  15. CatBuilder

    CatBuilder Previous Member

    I meant all of my questions. I have asked about a million of them here. :)

    Sometimes, I think I'm pestering people or that I get few responses to something because I ask too many questions.

    I have another question and would love input on where to mount these twin 25HP engines on my boat. I think I've reached the saturation point though, and nobody will want to answer.
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