Ski boat project questions

Discussion in 'Powerboats' started by aussiebushman, Sep 22, 2012.

  1. aussiebushman
    Joined: Oct 2009
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    Location: Taralga NSW

    aussiebushman Innovator

    There is a project hull available locally - looks like an honest boat and the guy has genuine reasons to abandon the project. The price is only a few hundred dollars. I need a new project to keep me sane while I'm at home in the bush and away from the coast.

    I have 4 previous boats worth of building experience and have just finished building and launching a 6.4 metre sailing trimaran - before that a 30' cruising cat. All my previous builds have been sailing boats mostly in strip timber/epoxy but this is the first time I have considered a power boat, so please excuse several silly questions.

    The ski boat was originally set up for a marinized Ford Windsor inboard engine, but having spent too many years head down, bum up in the engine bay, I would much prefer to strengthen the transom and fit a 4 stroke outboard, with remote tilt, electric start etc. Comments on this please and suggested HP - how about 115?​

    The hull is glass and the gelcoat has been sanded - no cracks or osmosis. Is a 2-pack finish feasible?​

    The interior has been stripped out and this seems to offer the opportunity to replan it, with fwd and aft facing seats. The picture of the white boat is pretty much what sort of layout I have in mind​
    . The boat would then be used as a pleasure craft, not a skiboat.

    Considering I'm semi-retired, with plenty of time, lots of shed space, good tools but little ready cash, this project (like the Tri) could achieve a worthwhile result and with the cost spread over a couple of years.

    Any suggestions before I take this on?

    Alan
     

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  2. keysdisease
    Joined: Mar 2006
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    keysdisease Senior Member

    So this is a production boat that's being refit? Was this model ever offered in a outboard version? The shift in the CG is something to think about and that I'm not qualified to address. Having said that this kind of repower is done all the time, mostly with no problems.

    Refit the interior how you want. Maybe get her running and use weights (sandbags) to replicate the weight of the planned interior with guests to see how she runs before making and fastening interior.

    Painting gelcoat is recommended, sand, fill, prime and paint her, no worries there.

    Looks like fun, Steve :cool:
     
  3. PAR
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    PAR Yacht Designer/Builder

    The outboard conversion is a good idea. The transom will need a cutout and a splash well. I'd recommend a 20" (510 mm) shaft and the transom should be a minimum of 1 3/4" (44 mm) thick for a 115 HP engine, with 2" (51 mm) being preferred. Put some knees in the corners of the transom where it meets the hull sides, well tabbed. The CG location wouldn't be a big concern, unless you plan on pushing this puppy past 50 knots. At these speeds, you'll want to get her balanced, but below these speeds, you'll likely be just fine. Place a below floor fuel tank about where the inboard would have lived, which will help with CG balance.

    Of course the transom cutout and splash well, will require modifications to the deck cap, but this looks to have been cut up already, so . . . Frame everything up with 3/8" (9 mm) plywood, well covered in fabric and also well tabbed the hull shell at contact areas. She'll be fun with a 115 HP outboard and you'll have a lot more interior room to work with too, without the engine box in the middle of the boat.
     
  4. aussiebushman
    Joined: Oct 2009
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    Location: Taralga NSW

    aussiebushman Innovator

    Thanks guys - most helpful and I'm comforted by comments on the suitability of the outboard. I'm not really interested in the "go fast" meaning anything close to 50 knots. In that case, it sounds as though I might even use a slightly smaller engine - say 90 HP or so.

    Par, remember I only have sailboat experience. so please provide a bit more detail about the splash well and cutout. I'll look on Google, but if you could please provide a rough sketch, it would help.

    Thanks again

    Alan
     
  5. aussiebushman
    Joined: Oct 2009
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    Location: Taralga NSW

    aussiebushman Innovator

    Got some more details looking through Google images. The first picture shows a small electric bilge pump to drain it - it that the usual method?

    In the second picture, the cutout looks very low, leaving little freeboard and not much room for the engine mount. I presume I must select the engine first and get the dimensions of the mount before deciding the shape and depth of the cutout. Can't find the picture again, but one guy decided to raise the transom (no cutout) rather than use a jack plate so that the steering gear would fit inside the splash well. That sound like good idea to me but your comments would be much appreciated

    Alan
     

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  6. keysdisease
    Joined: Mar 2006
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    keysdisease Senior Member

    There are industry standards for motor well sizing so all engines will fit all boats. You don't need to look them up, just take a tape measure and get some sizing from a variety of similar sized boats.

    This will take into account the turning and tilt ranges for an outboard so you don't have to figure it out yourself, not that you couldn't, but a lot of smart industry types have already done the figuring.

    What PAR was talking about re transom strength is that your transom was never intended to take thrust, now it has to. You need to reinforce the entire transom and then install knees (triangle bracing) to help transmit this thrust into the stringers and hull sides.

    Steve
     
  7. PAR
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    PAR Yacht Designer/Builder

    It's probable the hull came with an outboard option, in which case you may have sufficient transom beefiness, but don't bet on it. Again, measure the thickness of the transom. If it's thick enough and the tabbing looks good, you probably will not need a lot of additional reinforcement. Of course if it's undersize, it'll need structure added.

    115 HP doesn't seem out of the park for this hull, though post it's general dimensions and transom width, just to be sure.

    Splash wells are roughly standardized, so lets get the dimensions, so we can figure out how big a motor she can live with and the appropriate well dimensions too. No, a bilge pump isn't a reasonable way to empty a splash well. A drain hole or two will work much faster and have no moving or electrical parts to wear out too. In fact, both photos above show poorly designed splash wells.
     
  8. rxcomposite
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    rxcomposite Senior Member

    Notice the outboard in tthe white boat? When the outboard is stowed horizontally, the head of the outboard should have clearance in the splashwell. Not enough clearance and you will not be able to stow the outboard.

    The depth of the transom cut out is that when the outboard is mounted, the "cavitation plate" (that wing like structure) in the shaft housing lined just above the upper propeller tip should extend past 1" from the bottom of the boat. It is a general reccomendation by the manufacturer but enthusiasts vary that clearance in order to get different behavior from the boat.

    You can always drain the transom well by using a drain plug. It comes in many types. Some even self bailing. Water drains downward and out but will prevent water rushing in by a flap valve or a floating ball. You can buy this in any boat supply store or marine catalouge.
     
  9. PAR
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    PAR Yacht Designer/Builder

    I don't use splash well drain plugs, just holes with a sloped well floor, for natural draining.

    Each engine manufacture will tell you how high they like the cutout to be on the transom. A new engine will have an installation guide which will cover this. A used engine can be as simple as downloading the manufacture's user's guide.
     
  10. rxcomposite
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    rxcomposite Senior Member

    Agree. Lots of info in there, including width of the splashwell and thickness of the transom at the mounting plate. Saves you a lot of trouble by having the info beforehand before committing to buy the engine.
     
  11. rasorinc
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    rasorinc Senior Member

  12. PAR
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    PAR Yacht Designer/Builder

    One way drains are unnecessary on splash wells. A single 1" drain is common with duel 1" drains a better idea. Even with two drains, boarding water through these drains, isn't significant enough to warrant them, let alone the cost compared to a length of 1" pipe which is the usual route for a splash well drain.
     
  13. tunnels

    tunnels Previous Member

    Hi
    You semi retire! so am i 68 years old and still building boats !!! i love having the wind through what hair i have left its a fantastic feeling !!.
    Outboards the 90 hp and 115 are the same motor and weight and size so go bigger and have the extra ponys if you going to use it for water skiing!! not a silly idea but if you come across and 25 inch outboard would be worth a looking at ! the motor will be up higher , the slash well is smaller to almost nothing and if you do have a splash at all the well can drain inside and use a big bilge pump !! On my boat i dont have any holes in the hull at all ,so is never a worry "did i put that plug in?""i have never had water even look like coing into the very small splash well i have !
    An under floor tank for sure and use that whole area as storage with floor lift outs all the way though and just a glassed in bulkhead between where the tank is and your storage at the back the sides of the longtitudinal stringers is a goog place to exstend the knees up onto the transom to strengthen it !!the tank and the seats shoud be almost in the same place with the fron of the tank about leven with the front if the seat The boat woll always ride nose up so the fuel weight will always be behind the seat . yamaha have excellent powertrim so can use that to get the nose up or down with !!
    Transom for skiing id go for 50mm thick glassed over if you going to be pulling a couple of skiers !!!
    Make and use a steel towing pole inside the boat and keep the toweye above the height of the motor about 300mm it can be slotted into the floor and a plate welded and bolted to the front of the splash well .
    I have done all what you doing with a older boat (1975) a few years back but i totally built a new transom with 450mm step back for the motor . 115 yamaha 6400rpms 12.7 dia prop and 21inch pitch is off the clock full out so well over the 50 you were talking about . its just a fishing boat with a semi cabin had a good hull and has no wood at all any where .
    Set up for the motor is very important !!! to use all it power and get the power to the water the underside of the plate above the prop on the motor should be close to 10mm plus above the bottom line of the hull and if you have a small keel of any sort at all you need to loss at least 1800 mm of it from the back of the boat .
    Its enough to give your outboard a bad time with airated water and cavitation even going in a straight line and really bad when cornering either way !!.
    Been there done that !!:p.
     
  14. PAR
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    PAR Yacht Designer/Builder

    I agree in the motor size and if available the extra long shaft. They're not very common, but if you order a new motor, the extra long shaft is the safer way to go. I disagree about going with out the splash well. Bilges pumps are notoriously slow to remove water. A coupe of 1" holes and gravity are pretty reliable and bit faster then the best bilge pump.

    As to ventilation plate location, you'll have to sort out where it likes to be, for best preformance, but that seemingly long, lean hull will probably want the ventilation plate pretty much even with the bottom of the deepest portion on the V at the transom. You can fudge it up or down a 1/2" either way but, this level of fine tuning usually isn't necessary, for most folks.

    It's unlikely that type of boat will have a skeg. If it does, the best thing you could do is whack it off and patch the bottom.
     

  15. keysdisease
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    keysdisease Senior Member

    Just to clarify what Par is saying about the drain tubes as there seems to be some confusion: these are self bailing as the bottom of the splash well is above the waterline even on short shaft engines. 2 - 1" holes through the transom into the bottom of the splashwell. KISS

    His description of cavitation plate placement in regards to the hull bottom is the outboard rigging standard, its where you should start and probably where you will end up.

    Steve


     
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