Barrel Back Ski Boat - Engine Setup Design

Discussion in 'Boat Design' started by thomaschilly2, Feb 2, 2017.

  1. thomaschilly2
    Joined: Feb 2017
    Posts: 3
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Auckland

    thomaschilly2 New Member

    Hi Guys,

    New to the site, pleasure to be here.

    About 8 months ago during the university holidays a mate & I decided to jump in the deep end and purchase a run down old boat with the thoughts of converting it into a barrel back ski boat or similar.
    We are brand new to boat building scene and have been on a steep learning curve ever since (YouTube & forums are wonderful things).
    It has come down to the stage where we need to start exploring engine designs & engine positioning. We have run the generic calculation for the size of the engine, though we are limited by height as the hull design is very shallow (please see below for hull side height & boat width measurements) therefore any help on suggesting engines or engine designs will be very much appreciated.

    It would be great to get an idea of where the engine should sit/the optimal drive system/strut angles etc. As this is a barrel back look alike we are looking to have an inboard with a straight drive or V drive (or any other inboard propulsion system that would work).

    The main reason for posting on here is because us 2 muppets don't know the first thing about Naval architecture and we need to find out the optimum position for an engine in our boat. Below are all the measurements of the boat (measured from 4 points on the boat) & photos.

    1. O cm (Transom) - Height = 41cm Width= 122cm
    2. 157cm - Height = 64cm Width= 122cm
    3. 314 cm - Height = 65cm Width= 122cm
    4. 471 cm - Height = 60cm Width= 122cm
    5. 628 cm (Bow) - Height = 0cm Width= 0cm

    Note: These height measurements have simply been taken from the bottom of the boat up to the top rail, there has been no allowance for a potential rise in engine cover/housing (which is slightly possible)

    As we are students our budget to build the boat is next to none so we would greatly appreciate any help anyone can offer.

    Thanks for taking the time to read this.

    Will try and attach photos.

  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.

    You'll need to do a weight study and have an accurate set of hydrostatic figures, so you can figure out where to place the hunk of iron.

    Typically the engine is mounted amidship and there's a fair bit of latitude possible, which can be addressed with trimming ballast, though this is less ideal than having the engine right where it needs to be, without the ballast.

    Engine placement is often governed by prefered shaft angle(s) too. This is where a V drive can come to play. The engine can be moved aft, into the most buoyant portion of the hull and the drive placed, to permit a better shaft angle. Most of this stuff is worked out on the drawing board, rather than on the fly during the build. This isn't to say it can't be done, as it often is, but this is with well experienced builders, having some (considerable) understanding of the type and typical arrangements.

    Basicly what it boils down to is placing the engine and other heavy stuff, like tanks, batteries, etc., in locations that let the boat sit where it's designed to sit. This way, you're not bow down or stern down come launch day. Of course, nothing ever works out perfectly, so you might need to move something or even add some trimming ballast, but ideally, you like to have the boat sit in proper trim, when you slide her off the trailer for the first time.

    One way around your dilemma, is to launch the boat and place weights (sandbags, concrete blocks, whatever) in the locations you figure stuff should live. Take some pictures and have a look at where she's sitting. Maybe you can still see a previous waterline on the boat, which may offer a hint at the proper trim for the boat. I've been hounded by the local harbor patrol many times for launching a restoration, still a long way from being completed, with a stack of concrete blocks in it, just to get an idea of how she floats. Most of them know me now, so not as big a deal. I even have a plywood bracket, designed to hold sandbags and concrete blocks on the transom of a boat. I look up what the engine weighs, hang the bracket and toss as much in weight as necessary on the bracket, to simulate the outboard's weight. One thing I've learned (the hard way) is to try to preserve or at least mark the known waterline, before it's sanded off or the panels are removed. Guessing sucks, but it looks like you're already in the shallow end of the pool.
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