Modify Sailboat to Semi planing

Discussion in 'Boat Design' started by Doug Meyer, Jun 14, 2013.

  1. messabout
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    messabout Senior Member

    If you are going to do all that work and risk a poor outcome, then why not just build a new boat? Maybe not much more work to start anew and maybe not even a lot more cost. For sure a lot more fun.

    When you start adding all sorts of material to the existing boat, you will make it heavier and that is going in the wrong direction for speed increase. Huge modifications to a perfectly good sail boat will diminish its market value too.

    The 20 HP motor, that you mentioned, will be enough to get the 12-15 MPH that you want.....if the hull is made for the job in the first place. A long skinny flattie, I would venture, will get the job done. It'll be fast enough to run from approaching bad weather, but not blindingly fast with the 20. A far sight faster than the sailboat.
     
  2. Doug Meyer
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    Doug Meyer Amateur Boat Designer

    You are right she runs that way at the moment hence the modification to get the angle of attack opposite to what it has now and warped V from amidships back.Attached 2 pictures one photo shopped.It will be closer to a planing hull shape with the chine running below waterline at the transom and not above as it does now. The waterline beam is only 6ft and waterline length will be about 17ft.I believe will create a certain amount of lift but will be running through the water not fully on top at about 10 to 12 knots.
     

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  3. Doug Meyer
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    Doug Meyer Amateur Boat Designer

    Sorry can't find photo shopped picture.
     
  4. tunnels

    tunnels Previous Member

    and ozzies are even worse !!:mad:
     
  5. Doug Meyer
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    Doug Meyer Amateur Boat Designer

    I am originally from Zimbabwe lived in NZ for 10 years must have rubbed off.
     
  6. tunnels

    tunnels Previous Member

    Damn it arnt there any genuine Kiwis left in nz ?? seems they learned to fly and left the place . I hope the last one turned the light off !!:):p:D:p
     
  7. PAR
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    PAR Yacht Designer/Builder

  8. Doug Meyer
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    Doug Meyer Amateur Boat Designer

    Thanks for your input.I have looked at both and modifying this hull shape is a lot less work and materials than building a Ninigret from sctratch.I am not going to add a huge amount of glass.Going to cut hull on the chine and across the transom from transom to midships.Drop the bottom down to required shape.Then glass from the inside and carry the shape a further 2 feet aft of the transom.I will fit high gloss panels to the sides and the bottom as formers for the glass.Then after sea trials fit new self draining deck and finish off topsides.Its still a lot less work than starting from scratch.
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    Knew you'd bite ! :p
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    My feeling is adding bouyancy aft, and having a straightish bottom aft, won't get you the result you want, because the front half of the boat is the wrong shape for it. It will be more akin to a planing hull rather than semi-planing, and 20 or 25 hp won't pop it out. Or if it did, it would be flat strap. There are remarkably few examples of trailable semi-displacement craft to be found. Rather than tinker with the full size version you could make a scale model and experiment with changes to that, to see what happens when the transverse wave system falls behind the stern.
     
  11. Doug Meyer
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    Doug Meyer Amateur Boat Designer

    I have seen a wing on the Apreamare and Gozzo Gecko.

    https://www.youtube.com/watch?v=jLSuJWAkxxU

    I just think whatever is added on will just be more wetted surface area resistance. Might help with trim. Any mod to the hull shape towards a planing shape will give me a boat able to go faster than capable at the moment and might have to up the HP at the end of the day. I am changing the bottom to submerged transom but will follow the lines of the chines in plan view slightly tapering off to a narrower transom than at amidships.Also maintaining a soft chine?These last 2 items will keep the boat efficient at low speeds.Still haven't decided in hard or soft chine yet.
     
  12. philSweet
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    philSweet Senior Member

    Okay, so I had this 16' warped bottom sailboat. (photo 1). GF wanted a boat that was HERS. We had access to a dozen boats any time we wanted them, but they weren't HERS. So rather than get another one, I converted it to take a 15" shaft Yammy. Then it went from wheel steer to tiller steer, and the 15" shaft wouldn't work, so it ended up with a 20" shaft 25hp Yammy.

    When sitting on the transom it was pretty nose high, but I moved the battery forward, and with two people it levelled out ok. The stock prop should have been swapped for a flatter one. It would do 20 knots with just me, cruise at 16 knots at about 1000# total, and would barge along at 12 with 1600# total. Used a tiller extension most of the time. It retained its good seaworthy manner and we would be five feet shorter than anything else out oceanside pulling traps in a blow. (that varnish job didn't last too long after she started hauling traps over the side, but I ate real well for a few years)

    So get the motor hung and splash her and you can fiddle with the hull over the next couple of years. But nothing is required. You can add boxes aft and graft a sheet of plywood onto the bottom. But I wouldn't spend more than two hours doing it the first couple go rounds. Just screw the leading edge of some 3/8 ply up to the hull about three feet in front of the transom and fiddle with different stringers wedged between the hull and the ply. You can sort it all out in a day. Then you can make it pretty.
     

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  13. Doug Meyer
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    Doug Meyer Amateur Boat Designer

    Hi Phil,thanks it sounds like a good idea. I will mull it over while I finish off the Tunnel Cat.
     
  14. Doug Meyer
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    Doug Meyer Amateur Boat Designer

    PAR Love your Cooper Jr nice lines and almost straight stem.

    When you say wing what do you mean.
     

  15. PAR
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    PAR Yacht Designer/Builder

    I mean a horizontal foil, under the boat's butt, so she does squat when pushed past 1.5 S/L. I don't think you have enough entry volume to make this an easy task, but with enough power, you can make a Nimitz class plane off.

    Phils little sailor/power conversion worked, because it started with a hull that could plane off. The run on that boat is relatively flat, so some wedges and trim would make it scoot. Your boat on the other hand, has steep buttock angles, so she'll never get up without redesigning the entire aft half of the boat. If you're up to it, I thinks Phil's idea of just trying some reshaping with 3/8" panels is a good idea. You'll quickly find out if it'll work and if cutting up the hull is worth the bother. You don't even have to seal it up tight, just glue the leading edges down and duct tape the rest, with the back wide open. Underway, the partly flooded sections of the hull extensions will evacuate and you'll get a good idea of how she'll be at speed. I do think that lowering the aft sections, to flatten the run, will cause huge trim issues, as you'll be adding a lot of displacement, some offset with an engine installation. You might be able to iron these out, with moving things around, but then there's still the entry issue. If it was me, I'd build a model and look at the wave train, but a few sheets of 3/8" CDX doesn't cost much either.
     
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