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  1. #1
    Join Date
    Apr 2010
    cheshire, UK

    Default What size outboard?

    Well i'm sure this has been done to death, but here goes-
    I have recently bought a 24ft sailboat (Robert tucker Unicorn 24) with a beam of 7'6" and is a tri keel.
    She is without an inboard, so i'm considering an outboard.
    Now the big question is what size?
    i'm no racer, and am happy with using power when the wind is low, and my days off are numbered.
    She will be sat in River Conwy, which can have quite a torrent sometimes, and needs a bit of 'umph' to fight the impeding current.
    I have a nice Honda 5hp, but i feel this would be better as an auxiliary, i also have a 35 hp Johnson 2 stroke for my old fletcher speedboat.
    So would the johnson be complete overkill or not?
    if so, what size outboard would YOU recommend for me?
    Or if the Johnson/Honda would work, what size Prop would be best?
    As i said. i'm No racer, and am happy for a compromise on sail speed if it means i can use her more often with the engine.

  2. #2
    Join Date
    Mar 2011


    I have a 22ft sailboat, with a 4hp outboard. I had a problem with a Spring tide last year approaching Weymouth, which the general feeling on here was due to the Hull being extremely 'dirty'.
    It is a Bilge keel.

  3. #3
    Join Date
    Feb 2010


    The Johnson is ridiculously oversized but the Honda may be Ok. Why not just try it out?
    A high thrust prop would almost certainly be worthwhile...if you can get one for that engine.

    Other than that you might end up looking at a 8hp.

  4. #4
    Join Date
    Feb 2005
    Carribbean currently Grenada


    Tohatsu 8 hp saildrive with 25" ultra long shaft will do nicely.

    Flog the others and treat yourself.
    Monkey patching programmer [retired ]

  5. #5
    Join Date
    Jul 2003
    West Australia

    Default OB power

    Power needed to get the boat up to a decent water speed is not very much. I imagine the Honda will do well. The typical course pitch prop will mean a good speed at medium engine speed so quieter.
    However it is the headwinds that can cause problems. You might be better off sailing. A finer pitch prop for the Honda would do best under really difficult headwinds (and waves). The water speed of the boat will dictate what contrary tides you can beat but regardless of power it is difficult tp beat the hull speed limitations ie around 5 knots. (not impossible but needing a lot of power)

    The obvious answer is to buy an adjustable height o/b bracket and mount on one side of the stern. Try the Honda and see how you like it. Get a bracket with plenty of adjustment. My has about 10 inches from up to down in 3 positions and I use them all. Medium is for normal motoring. Right down for really rough water or someone on the bow. (picking up mooring) Highest is for smooth water motoring with a load of people
    in cockpit. Set the bracket so motor cavitation plate is at water level for normal sailing in mid range.

    You may find the motor is too low down the transom for starting and maintenance. This is where a long shaft engine has the advantage being about 5 inches longer. You may decide you want a LS engine so may need to move the bracket. (and fill the holes). However the Honda is worth trying. good luck olewill

  6. #6
    Join Date
    Oct 2011
    Solent based..


    I have 24' Trident (for sale) but have used her for last 6 Years. I bought a 6hp Mariner with high thrust prop as I have to Outboard off fairly frequently, it was sufficient.

    If caught with a tide to push in flatish weather (with cleanish bottom) she would just approach hull speed. If not just the slightest wind and motorsailing got her up near hell speed.

    Anything bigger engine was to heavy, they are Twin cylinder. If you are going to buy and 8hp you might as well buy anything up to 15hp, as the weight varies very little. 4 Strokes do not like being run at 100% all the time unlike 2 strokes.

    MY little 3.5hp would push the boat at 4 knots but again any wind motor sailing got her up to speed. Then F6 on the nose it took 3/4 of an hour to do the 1/2 mile to the mooring, when the river was to busy to motor sail

    When it comes to outboard height I had 2 heights:
    Up: For sailing storage - engine clear of water nill drag.

    Down: For motoring - As low as comfortable whilst being able to reach controls and not get any waves on the engine.

    I did not bother trying to set it to cavitation plate hight, your pushing big boat through water so you want prop as deep as possible for max thrust. The drag of the outboard leg is not significant in my mind.

    What you do not want is prop popping out of the water when you really do not need it. When some one goes forward to take mooring line? Or having to think if it is low enough in seas or high enough not to get swamped.

    Although again that's where long shaft or super long shaft come in..

  7. #7
    Join Date
    Mar 2004


    8hp max, the weight will affect the sailing performance & be heavy to haul around.

    My 21ft trimaran, 1T, has a 2.5 hp, 4 knots max, gets me on the mooring.

  8. #8
    Join Date
    Aug 2009
    Norfolk Broads


    The MacGregor is only 2' longer, and that 'needs' a 50HP outboard - apparently

  9. #9
    Join Date
    May 2008


    i think you would just about make do with the 5hp but as you know the stream is very strong on a spring flood or ebb and lumpy too near the mouth of the river.

    I now have a Jag 24 (not at Conwy) which has a 10hp honda (but in a well). This combination would be ideal but a 10hp is very heavy to hang "out back"

    I would certainly start the season with what you have first and see how it copes with the motoring and sailing aspects

    Sailing in a windy Scotland

  10. #10
    Join Date
    Sep 2010
    West Sussex / Hants


    I agree the 35hp is a no-go; not only way overpowered and presumably very heavy, will also have very thirsty fuel consumption, thinking of range under power let alone cost !

    The 5hp is OK but not really enough, especially in that area.

    The suggestion of sell both and get an 8hp Tohatsu saildrive - with suitable prop, good ( for an outboard ) electrical charging and remote fuel tank is definitely the way to go.

    Remember you may well need remote throttle & gear controls.


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