Trailer sailer conversion to displacement cruiser

Discussion in 'Powerboats' started by pullark, Jun 3, 2013.

  1. pullark
    Joined: Apr 2013
    Posts: 2
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: brisbane,Australia

    pullark New Member

    Hi All,
    I've been thinking about a boat-building project but don't have the inclination to start from scratch. My desired vessel would trailerable, be used mainly in sheltered waters with the occasional coastal trip thrown in. As a sailor I'm quite happy traveling at displacement speed. I have a 25hp Kubota diesel and most of the running gear to set it up. I'm thinking a trailer-sailer might be a suitable "donor" for conversion. I'm comfortable with undertaking the work involved subject to getting some guidance about modifications to the keel and rudder design.
    I have a hull in mind that may do the job. It's a Hood 23 which is quite a common boat in Australia (not sure about elsewhere?). Solidly built and cheap to buy. I think the hull is quite attractive in a retro kind of way and I would build a cabin on the standard flush deck design. They were made with fixed keels and also as trailer boats with a swing keel. I've attached a photo of one.
    I have some questions about my proposed project....
    Would 25hp diesel be overkill in a boat this size? I realise I'll be limited to hull speed which is maybe 6 or 7 knots. Motor and trans weighs in around 130kg. I love the little Kubotas. They're bulletproof and run on the smell of an oily rag (approx 2 litres per hour).
    Not sure about centreboard weight but I'm thinking this would be removed or at least reduced in size to offset the motor weight. I would like to incorporate a steadying sail so some weight down low would be good.
    In terms of motor installation, I would build up a small keel around the shaft tube. The hood does have a small keel along its length but I'm thinking the boat would track better under power with a bit more of a keel profile especially at the aft end. Am I on the right track with the modified keel concept?
    As for rudder design I'm not sure. The standard boat has a transom mounted tiller rudder. I would want wheel steering (mounted on bulkhead) so I'm assuming traditional rudder setup would be located at the end of the keel for protection and steerage. How do I determine rudder size relative to prop? I believe a 13 or 14 inch prop would be about right . Gearbox has a 2:1 reduction and converted boat would weigh about 1300kg.
    So...quite a few questions posed. I would appreciate constructive feedback regarding my concept. However, if you think I'm completely misguided please say so ....and why.
     

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  2. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 476, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Welcome to the forum.

    Yes a 25 HP engine would be over kill, by at least 50%, probably more like 65%. This boat will max out at 6 knots and nothing short of an atomic reactor, will get it to go much faster. A 5 HP will get you to hull speed, but there will not be any reserve capacity, so a 10 HP engine would be the ticket, with plenty of reserve to punch through contrary currents, winds and chop. An inboard will eat up a lot of valuable interior volume, so consider an outboard, possibly mounted in a well, so you don't have to see it, smell it or hear much of it. If you arrange the outboard to steer, you can skip all the contrivance with a rudder, as you will not need it.

    You obviously have the shoal draft version. I'm not sure of the centerboard weight, but the boat does carry over 1,850 pounds (840 kg) of ballast. Most of this is likely inside the stub keel, not the board, though don't be surprised if the boat does weigh several hundred pounds.

    The boat will track just fine without an addition to the stub keel. Just run your shaft out behind it or through it, if possible. The rudder should live fairly close to the prop, so it can work in it's "wash". A reasonable rudder for that boat will have about 1.5 square feet (.14 sq. meters) of surface area (each side). It'll be sluggish to respond, but this will be typical of a boat arranged this way (long stub keel, straight shaft and very low aspect rudder). Make the rudder slightly less deep as the bottom of the stub keel, so it doesn't hit things when you run her aground.
     
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