What are the structural/stability implications of converting yacht to powerboat?

Discussion in 'Boat Design' started by Kit_L, Aug 18, 2018.

  1. Kit_L
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    Kit_L New Member

    See here:

    2008 CUSTOM CUSTOM 38' CRUISER https://www.tradeboats.com.au/detail/custom-custom-38'-cruiser-628300?utm_source=findads.com.au

    What are the implications of removing the rigging? And are there any downsides to going down this road if you are perfectly happy running at an S/L of 1.1? I can cope with a 1.8m draft, but assume that this vessel will roll significantly more in bigger seas than if she were rigged, but what do the experts think? I am not thinking about buying this particular vessel; I am exploring the concept of converting a used yacht to a power-only boat.

    WRT structural integrity, what effects does removing the mast and rigging have, in general terms?

    I have been looking for some time now for the right boat for me; I want to go power, not sail, and the basic hull shape of most yachts looks much closer to Beebe's "Passagemaker" concept (long, narrow, ballasted hull) than modern trawlers. Is this a silly idea?
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    Maybe bribe someone to take you for a ride on one like this, to see if you like the motion, which is going to be a bit snappy. Some like that kind of motion, others not.
     
  3. Wavewacker
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    Wavewacker Senior Member

    Nice Strawler, trim tabs might help, ask a designer/engineer about proas mounted aft for a flatter ride when needed. You could go for a smaller trawler with more room and similar efficiency for the same money or less. Check out Albin trawlers. A pilothouse on that might be nice too. :)
     
  4. Tad
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    Tad Boat Designer

    Removing the rig will reduce the structural loads. The ends are no longer being pulled up by the head and backstay, and middle isn't being pushed down by the mast compression. If the ballast keel has been left intact, rolling will be very quick, uncomfortable for many and likely to induce motion sickness. The big downside is lack of fuel capacity and power to buck a lot of weather or current.
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    The pitching motion probably won't be the best, either. Remembering that sailboats don't sail "close to the wind", when you head upwind under power you could be getting a bit more hobby-horsing than you liked, and as Tad says, the small engine will be working overtime.
     
  6. MikeJohns
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    MikeJohns Senior Member

    The problem is a high righting moment coupled with a large reduction in Roll Gyradius (roll inertia). Roll motion mightl be a bit quick in a seaway, and more prone to being knocked down in heavy weather.
    Sailboats more motorsailer than Sailboat with auxilliary would be better for a full conversion.

    A general rule of thumb that's a good ready guide for many displacement hullforms, is that the vessel roll period in seconds should be equal to it's beam in meters. If it's less it's going to be a bit stiff, if it's much less it'll be intolerable, if it's a lot more it's dangerously unstable.
     
    Last edited: Aug 18, 2018
  7. Alik
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    Alik Senior Member

    The first problem is usually performance, as sailboat hull is rarely suitable for speeds expected from displacement/semi-planing powerboats.
    Say, the hull of sailboat on photo is optimized for 5kts; on powerboat Your cruising speed should be, say, 8kts - large difference in Froude numbers requiring different hull shape.
    There is no way to cope this other than to heavily modify the stern.

    Rolling, pitching, etc. are secondary issues and yes they won't be perfect.

    Layout of the boat needs re-thinking, say visibility from steering station.

    PS Sailing catamaran is more suitable for conversion into power cat, compared to monohull craft.
     
  8. MikeJohns
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    MikeJohns Senior Member


    Kit said Kit was happy with a " S/L of 1.1 ":)

    Good point about the catamaran.

    Swinging a decent prop probably the biggest issue on a lot of sailboats.
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    One assumes the structure will take it, but slamming tendencies going up-sea might emerge, that formerly as a sailboat, did not, as those headings were not available, largely.
     
  10. Kit_L
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    Kit_L New Member

    Wavewacker wrote, "trim tabs might help"

    Retrofitting these to that hull would be major surgery, I assume.

    Tad: this is what my intuition told me the result would be (and I assume this is why this kind of conversion is not done more often).

    MikeJohns wrote, "the vessel roll period in seconds should be equal to it's beam in meters": that will definitely not be the case in the example I showed, I think. This sounds like an excellent rule of thumb, both for stability and comfort.

    Thanks to everyone who replied; I am grateful.
     
  11. Squidly-Diddly
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    Squidly-Diddly Senior Member

  12. Kit_L
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    Kit_L New Member

    :) Agree completely. I was using this converted yacht to explore the idea; I had no intention of buying her. Definitely would need a pilot house.
     

  13. Mr Efficiency
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    Mr Efficiency Senior Member

    Trim tabs, 100 % waste of time, would only produce drag, and minimal stern lift.
     
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