Converting a Sailboat to a powerboat

Discussion in 'Boat Design' started by fpjeepy05, Sep 22, 2021.

  1. Mr Efficiency
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    Mr Efficiency Senior Member

    I seem to recall there was a thread about an attempted conversion of a 16ft Hartley trailer-sailer to a boat that could do the speeds you wanted, one factor that was mentioned that when you attempt to increase speeds beyond the original sedate progress, and also head directly upwind, which also isn't in the original role, you expose the structure to slamming loads it was not designed for, so don't leave that out of your calculations.
     
  2. philSweet
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    philSweet Senior Member

    The least worst solution is to add boards to the stern quarters to make a wider riding surface aft. Sailboats are designed to be powered from above and see a lot of bow down moment underway. Powerboats are designed to be powered from below and see some bow up moment underway. Add an outboard on the back and you have really messed things up from a balance point of view. Expect the boat to trim up bow high a lot, like 20 degrees a lot before transitioning to a plane. You will need at least 25 hp to get this over the hump with any kind of load in it. There is a discussion of these traditional lifting transom mods that date back to the first engine retrofits in the Med. You can still find modern examples of the type, although I really don't know why they still exist (okay, they are cute).

    how about a 25hp outboard on a planing sailing dingy? https://www.boatdesign.net/threads/how-about-a-25hp-outboard-on-a-planing-sailing-dingy.39732/

    Gozzo boats https://www.boatdesign.net/threads/gozzo-boats.35772/#post-425591

    Gozzo type
    [​IMG]

    There are certainly better starting points than some of those you listed - 505, Etchells, Flying Scot, Highlander, Thistle, Lightning, Star.

    [​IMG]

    [​IMG]
     
    Last edited: Sep 23, 2021
  3. Mr Efficiency
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    Mr Efficiency Senior Member

  4. fpjeepy05
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    fpjeepy05 Senior Member

    They don't look like they should work, but they do. Google Humphree or Zipwake.

    Good points. I agree a little bouncy would be nice. There might be a version somewhere between the interceptor version and the wedge version.

    With the interceptors or wedges I don't think the squat would be much different than a typical planning hull, but still something to consider. And your point about a high propulsion is a good point. More the reason to think that modification of some sort is going to be needed.

    Agreed. The interceptor version seems easier to fabricate than the wedge version.
     
  5. fpjeepy05
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    fpjeepy05 Senior Member

    A lot of good information there. Thank you.

    I don't really get the point of the Gozzo boat either. I agree adding board to make a flat wide aft would help with planning, but I don't know if I agree that it is the best solution.

    As for better starting points...
    The 505 style boats would definitely plan easier being a planning hull, but I would like to use the boat as a launch/shuttle and some fishing. Low freeboard isn't great for either of those and the cockpits aren't a very functional shape for anything other than sailing. Also not having much of a transom makes adding power difficult. With a boat like the O'Day daysailor, mods would need to be made to make the cockpit a little friendlier, but I don't think it would be a huge undertaking.
     
  6. cracked_ribs
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    cracked_ribs Senior Member

    I have never seen a Zipwake project more than a couple of inches and that on pretty big boats...nor do they take up the whole transom.

    I wouldn't expect that to translate well at all to your concept, personally. I think you will have massive drag issues that will require more power to overcome and then you need more structure to support the motor and the boat gets heavier and needs more power...

    Considering your requirements are moderate speed on a 17+ foot boat with a 25 I don't understand why you want to convert a hull designed to do something else instead of starting with a hull form that does that already. It's worst-of-both-worlds territory.

    I'm a big fan of outside-the -box solutions but you have to understand why the box is shaped like it is first. I'm on board with SoCal's narrow design because what he wants it to do is so specialized and I think he understands that he's making big trade-offs in terms of how it will behave. As far as I can tell he's also actually setting up to build the thing, so he's putting his money where his mouth is, and if it turns out not to act like a scaled-up version of the boat he already uses, which isn't identical but shares a few big traits, well, it's his asterisk.

    I'm less enthusiastic about this because I think there are existing boats that will easily do what you want, and I think this concept will not, but unless it gets built I think you're going to just keep insisting that it would work.

    But there is a reason that the boats that would do this, don't have hull forms like your proposed starting point.

    That's my take, anyway.
     
  7. fpjeepy05
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    fpjeepy05 Senior Member

    Zipwakes don't protrude much from the hull because they are used at higher speeds and aren't trimming much.

    I agree that it's not the best place to start, but the other options aren't great either. I would love a little ROS 18 or another Downeast/Novi lobster boat, but they aren't cheap. The typical 18fter these days is wide, heavy, and built to handle 150hp and go 50 mph. These sailboats on the other hand are narrow, light, and can be bought for pennies. If nothing else I'm saving a boat from going to the landfill. I like the lines of the sailboats; more like the Downeast boats, and the cabins are a great base to build a pilothouse on. Also, some of them have pretty good freeboard which isn't as common in the powerboats. What I like in a boat most people don't and what most people like in a boat I don't care about. It's a boat for myself. The resale value will be $0 but I've excepted that.

    I found a boat for free on craigslist and I'm in the process of getting a trailer for it. If I go through with it I'll do my best to document it. Worst-case scenario I'm out some money or I have to drive it slower than I want. Trawler Building Procedures This guy has been doing them (albeit slow-speed versions) for a while.
     
  8. cracked_ribs
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    cracked_ribs Senior Member

    Well, although I think this will be an exercise in frustration for you and intuitively I think it's not going to work out well, and that you could instead spend the money on supplies that would get you rolling on a build that would do what you want with a high degree of predictability, I hope it works out.

    If it were me, I'd mount the motor on the free boat as-is and run it at hull speed, and just start building a planing boat with a pretty shallow V that will plane easily with that motor. To me, that's maximizing the results for the spend.

    But of course we all have our own methods of getting where we want to go.

    For the record, the boat I am working on right now - I mean I am standing beside it and just took off a pair of nitrile gloves - is 17' long (a tiny bit more if you include the stem), powered by a 1979 Johnson 20hp, and I think all-in I have around $3000 CAD in her and definitely much less than $5000.

    And it's for fishing and commuting to an island place I own, and will break 16 knots, no question about it.

    So I understand the requirements and how to get there pretty well. I think there is an easier way. But naturally you are not obligated to take it just because I think it's more practical.
     
  9. duluthboats
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    duluthboats Senior Dreamer

    Back from the dead! A while back I bought a Catalina 22, in fair condition, with a great trailer. I'm not a sailor kind of guy but love to be on the water. I have been selling off the sailing bits and now I almost have a free, very good displacement hull. My plan is for a very small electric motor, about 5hp, some solar panels, and some batteries. I plan to end up with a boat that will be as fast as it was intended to go when built, without the work of a sailboat, or the noise and smell of an ICE. Going slow is not bad. You can shoot me again now. ;-)
     
  10. tlouth7
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    tlouth7 Senior Member

    I would start with a planing dinghy, perhaps a 470 though that is a little shorter than your spec.

    First I would put a smallish (maybe 10hp) outboard on the transom, with a nice long tiller extension, and see how it coped.

    The only modification I would make to the profile would be to fill in the turn of the bilge to make a sharp chine, thus increasing the area of the planing patch.

    Rising buttock angle is easily counteracted by trimming the bow up, as that is how these boats are designed to plane. Having a heavy outboard on the back will naturally help with that.
     
  11. fpjeepy05
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    fpjeepy05 Senior Member

    Building my own would definitely be the best solution, but also expensive and time-consuming.

    To get a hull and deck/cap for an 18x6ft boat I think would be between 150-1000 hours. 150 would be bare-bones done by a professional. Giesler Lake Nipissing sells for ~$6000. $500-$1000 in materials. $45 shop rate would mean they aren't making much if they can't build them in less than 150hrs. Most amateur builders I have seen are around 1000hrs for boats in this size range.

    If modifications can come in less than $1000 and 1000 hours. I think I come out ahead. Also, I value my time a little more than $ in this situation so if it cost more but saves some time, and is still cheaper than paying something new/used. I'd still call that a win.

    Also what island?
     
  12. cracked_ribs
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    cracked_ribs Senior Member

    ...but you may not end up with a boat that does what you want, in which case, $1000 and possibly hundreds of hours for nothing. Anyway, it's your time and money. I am around 500 hours in on my 17 footer but every build is different. I began using it when I had about 300 hours on her and was around $2500 CAD into the build.

    The island my cabin is one is a very small one in the Gulf Island chain of the west coast of Canada. The closest one that would show up on most people's radar would be one called DeCourcy; we're close to that, but on a much smaller island with no hydro etc. It's good fishing and boating territory. Inside the channel there is pretty protected; Georgia Strait can be a bit rough but generally manageable aside from a few spots that get really nasty. Fishing off the west coast of the island, of course, can be as rough as any stretch of open ocean. My main house is on Vancouver Island, so I have the option of fishing the open ocean or the more protected inside passage depending on the weather, which is a convenient setup.

    Although most years by about November I've had all the rain I can stomach, with three more months of it in front of me.
     
  13. fpjeepy05
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    fpjeepy05 Senior Member

    Sounds like you are doing pretty well on your 17fter. DeCourcy Island looks like a pretty cool area.

    If my project failed it would just be slower than I wanted it to be.

    I was looking at pictures and drawings of the MacGregor 26M. That boat planes with a 50hp motor. The buttocks aft look a little flatter than a typical sailboat, but not a huge amount. Looking at the MacGregor 17 and 21, I don't think a lot would need to be done to get them to plane out.

     
  14. container
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    container Junior Member

    You could always extend the hull either side of the outboard, 600-800mm is ample length to turn the round bilge into a hard chine, curving down as it nears the transom to flatten the buttock lines and espouse the stern wave. Zoom in on the a** end of this mainship trawler, that is a perfect example. They are fully round bilge below the first chine ( the one on the watetline) but they can also plane at 20+kts
     

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  15. fpjeepy05
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    fpjeepy05 Senior Member

    I think this same thing can be done on the bottom, not just as an extension

    There will be "hook" in the buttocks in this situation. I think this typically causes dynamic instabilities (like porpoising) but I think because I'm targeting such low speeds I don't think it will be an issue, but I don't know.

    Here is a quick model of a 2"x6" wedge added to the bottom of a hull. Only adds 23# of bouncy.
     
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