Sailboat to trawler / tug conversion

Discussion in 'Boat Design' started by cthippo, Jun 29, 2020.

  1. brendan gardam
    Joined: Feb 2020
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    Location: east gippsland australia

    brendan gardam Senior Member

    no idea what would happen with a powerboat , just wanted to share what happened with that sailboat.
  2. KeithO
    Joined: Jul 2019
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    Location: Michigan

    KeithO Senior Member

    Dynamic Stability of a Monohull in a Beam Sea

    We may summarize this article in a few take-home points:
    The dynamic behaviour of a yacht in waves is at least as important, if not more so, as its static stability curve.
    • When we're taking heavy weather from abeam, we want to keep roll accelerations low.
    • Roll accelerations are reduced if we have a high roll moment of inertia or if we reduce the overturning forces.
    • Deep ballast and heavy rigs increase the roll moment of inertia. Dismasting greatly reduces the roll moment of inertia, and lightly rigged or dismasted yachts are generally easier to capsize than those with sturdy (and intact) cruising rigs.
    • Reefing the sails and retracting the underwater appendages will reduce the overturning forces. Anything that presents a lot of lateral resistance in the water can "trip" the boat if it gets hit beam-on by a breaking wave.
    • All of this is less of a problem if we can keep the boat bow-on to, or running with, the waves.
    Powerboats generally derive their stability from the geometry of the hull. Wide stern, sharp chines, possibly no keel at all or possibly a shallow keel supporting the rudder. There is a significant performance penalty for the keel when the speed is intended to be in the semi displacement speed range, which is often the case. Whereas a sailboat hull is generally wineglass shaped or an approximation of that by using single or multi chines but by no means equivalent in form to a powerboat hull. Weight has a large impact on performance so if there is any ballast, its just the absolute minimum to meet static stability requirements. The engines often constitute a significant part of the ballast. While with a sailboat, often half of the displacement is ballast, all below the bilge.

    Will Gilmore likes this.
  3. Will Gilmore
    Joined: Aug 2017
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    Will Gilmore Senior Member

    Keith, thank you for that article. Very well written and explained.

    I can definately see that happening.

    As for my own experiences, the schooner was very shallow draft for 56' and a wave that size wouldn't likely have the subsurface counter movements at only five feet of depth. A deep fin keel would perhaps not have stayed as upright. I never thought about the problems of counter subsurface water movement.

    It seems like modern high performance sailing yachts are starting to look more and more like powerboats under the waterline, as their bottoms get flatter and the rocker is shallower. The deep ball canting keels obviously make for a different behavior on the water.

    -Will (Dragonfly)
  4. KeithO
    Joined: Jul 2019
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    Location: Michigan

    KeithO Senior Member

    Will, that was primarily the capsize concern with a deep keel and no mast.

    In terms of making the crew seasick, it is the accelerations that the crew is subject to that are the determining factor and the heavier and deeper the keel, the quicker the restoring motions will be. Some papers suggest that cats are up to 50% worse for seasickness than monohulls when having beam seas to quartering seas, because they too have a very stiff motion. Bottom line is that when doing this type of conversion, one is going to have to make design changes to the keel and ballast. Most likely one is going to want a long shallow keel to help with tracking (displacement speeds - not much drag penalty) but raise up the ballast compared to where it originally was in the keel bulb... Im also guessing that most people doing a conversion like this wouldn't be able to afford one of the modern wide "planing style" sailboat hulls....
  5. cthippo
    Joined: Sep 2010
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    Location: Bellingham WA

    cthippo Senior Member

    The trade-off there is you would be adding a substantial amount of mass in building a new cabin, probably enough that by playing with the ballast you could get it back into reasonable trim.

  6. Ross Whitlock
    Joined: Oct 2020
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    Location: Nelson, New Zealand

    Ross Whitlock New Member

    I have done that successfully with a Hartley 18
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