28ft trailer sailer, 20 Knots under power

Discussion in 'Boat Design' started by rwatson, Sep 5, 2014.

  1. rwatson
    Joined: Aug 2007
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    Location: Tasmania,Australia

    rwatson Senior Member

    What a surprise - I went back to add some more comments to

    and it told me it was older than 393 days, so I couldn't post to it. Considering how long some boats take to build, I think its being a bit unrealistic :)

    Anyway, that thread still has some important info on the SOR, as does

    Briefly - a self build-able plywood/epoxy trailer-able yacht, with water ballast and good power performance

    The project has suffered from lack of funds, and how much work was involved in the plans.

    For all that, the concept still creeps forward. Just in case there is anyone else interested in the concept, I have attached a few renderings of the idea. One is the view from the inside helming station - a major feature of the design. Along with full height interior, water ballast, 70 hp outboard, able to be transported in a container etc.

    I have a largely completed scale model to test this summer, and I will need to engage more professional help in due course.

    Anyway, its still a current dream. Its a good project to practice my Cad on.

    Attached Files:

  2. latestarter
    Joined: Jul 2010
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    Location: N.W. England

    latestarter Senior Member

    I tried to post on the other thread, a warning came up saying
    This Thread is more than 393 days old.
    [] I am aware this Thread is rather old but I still want to reply.

    It seems you can tick the box to override the warning.
  3. rwatson
    Joined: Aug 2007
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    Location: Tasmania,Australia

    rwatson Senior Member

    good tip. Thanks
  4. Wavewacker
    Joined: Aug 2010
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    Location: Springfield, Mo.

    Wavewacker Senior Member

    Almost posted on a post 3,246 days old asking about a SOF catamaran! The guys first post too, never answered!

    This kinda reminds me of the Mac 26X, but on steroids.
  5. rwatson
    Joined: Aug 2007
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    Location: Tasmania,Australia

    rwatson Senior Member

    Never mind, it could probably do with a new topic heading - as I have quite a bit more progress made these days.

    I guess this project is more like a cross between Roger Macgregor and Phil Bolger. Trying to get the best features of both - including the ability to self build.

    In the design process, we have aimed at just exceeding the new "market target", the Tattoo http://tattooyachts.com/ ) in room, power and sailing performance.

    Not so much a steroid job, more like a regular gym workout :)

    The experience of owning a Mac was a great education exercise, but the idea of avoiding paying all the freight, government duties and dealer charges on a boat from half way around the world, and improving on the design, has become a challenge.

    This summer, the scale model has to get on the water, and prove the concept.

    Earlier this year, Mike Tweedie was kind enough to help me complete his planing analysis of the hull. ( attached ) which gives me great comfort.

    Do you know, maybe it would have been cheaper and easier to build a full size mdf hull, stick an outboard on it and do the test than fritz around with models and stuff. That could be the frustration talking :rolleyes:

    Attached Files:

  6. rwatson
    Joined: Aug 2007
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    Location: Tasmania,Australia

    rwatson Senior Member

    Just for my own ready reference, I thought I would do an analysis of the performance calculations

    "Dingo" Tweedie sent me a great explanation of the previous chart

    "The drag D is the force needed to tow the vessel; it is the total resistance, including air resistance as well. Thrust T is the required force to move the vessel at a certain speed. D is parallel to the water surface, but T is along the axis of the shaft. If your shaft is parallel to the waterline, then then will be the same (as in this case since (greek e) = 0 (angle of thrust line).

    P(effective)[kW] = D[kN] x V[m/s]; it is the power needed to tow the vessel. Of course, this is not true in real life since the brake power needed to propel the vessel is greater due to the inefficiency of the propeller, and the losses of energy due to the shaft and gearbox. P(brake) = P(shaft)/E(s) = P(prop)/E(p) = P(effective)/E(hull), where E(i) is the efficiency of the respective items i.e. E(hull) is the efficiency of the wake produced, E(p) is the efficiency of the prop (around 50% to 70% depending on the design), E(s) is the shaft and gearbox efficiency (around 97%). Note that the brake power from the engine will be somewhat greater than the effective power!"

    So, if I was wanting say a maximum speed of 22 knots, the "P(effective)[kW] " column shows I theoretically need around 65 HP.

    My weight table allows me up to say 160 kilos for the engine ( 350 lbs )

    One this excellent outboard comparison site, http://outboards.findthebest.com/

    I find the Suzuki DF70A or the Yamaha F70 ( both 4 strokes ) which will fit the bill.

    Hmmm, on thinking about it, it could be good to take this exercise over to the Calculation thread to thrash out the implications.
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