Convert a steel multi-chine to hard chine?

Discussion in 'Boat Design' started by Jack1.0, Jan 4, 2021.

  1. Jack1.0
    Joined: Aug 2018
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    Location: South Coast, UK - Originally Stono Ferry SC -

    Jack1.0 Junior Member

    Happy 2021 to all!

    I'd like to convert an existing plan from Multi-chine to Hard chine (single chine)
    I've built several multi/radius and hard chine boats before but, always working from a design.
    Roberts, Dix and Ganley...
    In no way am I any kind of designer and as I just passed 67, I don't want to hit the books if I can avoid it. I'm just a sailor who has also built steel boats for fun and profit. Well not always that much profit to be honest!...
    I have an unbuilt set of plans from Roberts for a Tom Thumb 24 that I bought for my brother 20 years ago. But he changed his mind and we built a Tahiti Rover (Merrit) for him instead.

    So as a retirement boat for myself. I'd like to build a hard chine version of the TT24 and equip it with twin keels. My sailing is on the UK south coast and Brittany. Both have no end of drying harbors and twin keels are perfect. But twin keels with hard chine are perfecter ;-)

    I don't want to argue the merits of hard v multi or that Shannon designed it and Roberts is a Dick etc.

    What I'd really appreciate is if one of you bright designer types can suggest any easy way to tweak the offset table to produce a hard chine version that would float to her lines more or less.
    Any simple method?

    As an example: To determine the CLR of a dutch boat with no paperwork, no known designer.
    I cut out the shape of the boat hull from a photo and folded it into a concertina and balanced it on a rulers edge. It may not have been a professional way, but it gave me a ballpark CLR for a twin keel conversion.

    I originally thought to just add an inch or so to the beam of the 1st and 2nd chine but realized I have no idea what I'm doing yet again... so thought I'd check here?

    I'm okay calculating the twin keel placement as I've done that successfully a couple of times as above...

    DogCavalry likes this.
  2. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    So the idea is to simplify the build ? If not, how does it open the door for bilge keels, that the original shape does not ? Stiffer in the water with the single chine ? It would seem easier to just get plans for a hard-chine bilge-keeler.
  3. Jack1.0
    Joined: Aug 2018
    Posts: 11
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    Location: South Coast, UK - Originally Stono Ferry SC -

    Jack1.0 Junior Member

    Mr E
    Simpler, Quicker, less potential distortion, (3mm plate) Easier on my bad knees during construction, stiffer and less roll at anchor.
    But above all - I've built 2 standard tt24 and have an interior in mind that benefits from the hard chine hull form on the interior. They are pretty big inside for such as small boat and I plan to carry the coachroof out to the sheer (raised coachroof) I can just squeeze in a stand up shower.
    I put one in a normal tt24 but the 2nd chine meant that you couldn't stand flat in the shower.
    Remember it's my retirement boat- I wasn't particularley lithe at 35, much less so now ;-)
    Plus I just like the salty look of the TT (eyes of the beholder etc) It's always easier to find another boat, design etc but that's not what I want...
    It's not for racing around the cans or the Jester Challenge. Just an old guy's campervan by the sea and much better among the rocks in Brittany.
    I hope I explained well enough? Any ideas about my original question ?
  4. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    Maybe you could just loft the lines, if you can find a big enough floor, or loft it to half size, and interpolate and extrapolate from there. It might give you a fair single chine. When you say you have plans, are they just tables of offsets, or full size sections ?
  5. Jack1.0
    Joined: Aug 2018
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    Location: South Coast, UK - Originally Stono Ferry SC -

    Jack1.0 Junior Member

    I have the offsets and lines plan. The problem is I have very little space. I have enough room to loft the frames from the offsets, but only just.
    My plan is to make the frames in my driveway/yard and then transport them to an outdoor building site in the summer.
    I gave up my boatbuilding space (just a barn on a farm) but with the current rules against sandblasting. I now have to take it to a site where I can blast after building the hull.
    I will use pre-primed steel, but I can't bring myself to forego a complete blast at least on the inside.
    But even if I could loft the thing out full size, I lack the skill to calculate how much I need to adjust the beam and or rocker to regain the flotation. If push comes to shove, I'll build it in multi chine, but I'd like to find a work-around if I can.

    Unless anyone has plans for a salty looking 24' hard chine steel sailboat plan with twin keels ;-)

    Yes, I'm opening up to your original response ;-)

    Anyway, it's just about 02h00 here so bedtime for me...

    Attached Files:

    Last edited: Jan 4, 2021
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    You gain volume by going to a single chine, not lose it ? But you also "gain" additional surface area and drag from the sharp corner.
  7. Rumars
    Joined: Mar 2013
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    Rumars Senior Member

    It's not really possible to make the conversion without redrawing the boat and going trough the math. And in the end she will not be a TT24 anymore.
    The actual process of converting from two chines to one would be to extend the bottom plate up, and the top plate down, until they intersect, and that would be your new single chine. Then you play with the deadrise angle until you get your target displacement. Next step is to check if the resulting shape is still developable and tweak the angles until it is.
    Since you also want bilge keels instead of the original long keel, you will need to change the frame scantlings.
    Basicly you are designing a new boat from scratch to the dimensions and displacement of the old one.

    What you can do is loft the boat to scale, say 1:10, then apply the changes to the chine and build a model in plywood to see how it looks. But honestly, if you really want a single chine TT24, hire someone to redesign the boat. They will do it on the computer and provide you with cutting files wich will enable CNC cutting of all components, greatly simplifying the build. No lofting, no patterns, no grinding, no crawling around on your knees, etc., you can go straight to tacking the thing together.
  8. Will Gilmore
    Joined: Aug 2017
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    Location: Littleton, nh

    Will Gilmore Senior Member

    This isn't necessarily true.
    To address your original question, just calculate the volume of each cross-section below the waterline and match the material weight to maintain the trim. You will end up a bit more shoal of draft, but the difference shouldn't be that big a deal.

    As for the properties of stiffness and rolling, a single hard chined boat will be stiffer against forces above the water, but more sensitive to forces below the water. You'll feel the waves more and the wind less, at anchor.

    Good luck with your project, I hope you keep us all posted.

    -Will (Dragonfly)
  9. Jack1.0
    Joined: Aug 2018
    Posts: 11
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    Location: South Coast, UK - Originally Stono Ferry SC -

    Jack1.0 Junior Member

    Thanks for your replies guys.

    You're both clearly thinking along the same lines, where as I was thinking to reduce the volume.

    If you view the attached generic frame. I'll call the points of interest from top to bottom
    a) The sheer
    b) Chine 1
    c) Chine 2
    d) The keel

    My idea was to go direct from chine 1 to the keel; thereby reducing the volume by bypassing chine 2.
    Then letting out the sheer line and chine 1 a certain distance (?) to regain the lost volume in beam.

    to extend it out to meet the extended line coming from down from chine 1 and thereby increasing volume with a repositioned chine as correctly
    stated by Mr E. (Sorry for the confusion E)

    I simply wanted to remove a chine, not reposition it.
    This is clearly not the way it's done in the real world of Yacht design.

    Another problem is that my total experience has been building from simple straightforward plans, and repairing many steel boats using what I hope was common sense. This site has a lot of people who have a strong interest and substantial knowledge in the correct way to design or redesign a boat.
    But my technical theory is poor. It's a bit like trying to explain US politics to Rain Man.

    Paying for a pro or semi pro redesign is outside the retirement budget, so I might just go ahead with the multi-chine as drawn.
    I'll swing by again if I do go ahead, and let you know how it's going.
    Thanks very much all for your advice.
  10. Jack1.0
    Joined: Aug 2018
    Posts: 11
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    Location: South Coast, UK - Originally Stono Ferry SC -

    Jack1.0 Junior Member

    Hi Will
    Sorry but I was writing ;-)
    All I can say is that I sailed around the world in a hard chined sailboat and at sundowners, many people said the boat was much calmer than theirs in various rolly anchorages, but with an occasional slap on the quarters. It's good to know why!
    As for calculating a cross section...Need I say more ;-)
    Thanks for your input Will...
  11. rangebowdrie
    Joined: Nov 2009
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    Location: Oregon

    rangebowdrie Junior Member

    Actually that is quite a "professional" way of working that out.
    That method was used for many years.
    Francis Kinney, who for many years was a designer with S&S describes doing that in his 8th edition re-do of the book "Skene's elements of yacht design".
    THE book that all aspiring designers should have and read before anything else.
    Will Gilmore likes this.
  12. Jack1.0
    Joined: Aug 2018
    Posts: 11
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    Location: South Coast, UK - Originally Stono Ferry SC -

    Jack1.0 Junior Member

    Well there you go RangeB, I knew I read it somewhere...
    Just joking!
    I looked at it on a bookstore years ago, but went with 'Bury my heart at wounded knee'
    Skenes was too heavy ;-) A very serious tome...
  13. surfaceclutter
    Joined: Jan 2021
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    Location: Fair Oaks, CA

    surfaceclutter New Member

  14. Phil_B
    Joined: Mar 2019
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    Location: New Zealand

    Phil_B Junior Member

    Two points:

    1) The 24 foot Eventide can be built in steel and the plans are available from the Eventide Owners Group (THE EVENTIDE OWNERS GROUP in the UK for £5 (£10 overseas) so you can't complain about the cost. >};o) The plans set includes the 26 foot Eventide too, with various options (transom hung rudder vs trunk rudder, deck houses, construction material - steel, plywood, planked/strip planked, deep central keel or shallower draft) so you can mix and match the features to tailor the vessel to your tastes. It has either a deep keel or a shallow keel, both with bilge keels as standard. The Eventide is ideally suited to both the type of sailing you want to do and the waters you intend to sail. Like this one:

    2) If you are determined to redesign the TT24, then get a copy of Weston Farmers book "In my Old Boat Shop" where he fully describes how to build a scale model of the preferred design and how to scale the weights needed (such as an engine, fuel and water tanks etc.) so you can see how the full size boat would float and behave before you spend time, effort and money on building it only to be disappointed in its performance. Loft the plans as others have suggested above and project the topsides and bilge plates and see what the finished vessel looks like, build the model as described in the book and see how it floats.

    Designing a chine hull is not as easy as people think and I would very much suspect that the reason the TT24 was designed with double chines was for two reasons - one, to reduce drag of the turbulent water flowing across the chine (the chine should, ideally, follow the flow of the water around the hull to avoid this) and secondly and most importantly, to allow the steel plates to develop compound curves and conform to the shape of the hull without too much difficulty. Also, the TT24 was specifically designed to be virtually frameless and again, I suspect that the 2nd chine will act as a semi girder and contribute to longitudinal strength.

    The cost of having the plates either shaped and rolled professionally or having to bodge the frames to get them to support the plating after erecting the framework would likely outweigh the cost of consulting a Naval Architect. The design fees usually amount to less than 10% of the cost of the finished boat so I'd personally not risk ending up with a dog of a boat for that amount of cash.

    I would hate to think that you altered the design to find out that the boat is virtually impossible to build, perform poorly or even dangerously and be impossible to sell afterwards.
    bajansailor and Will Gilmore like this.

  15. rangebowdrie
    Joined: Nov 2009
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    Location: Oregon

    rangebowdrie Junior Member

    Jack1.0, at one time I contemplated an interesting idea that was similar to yours.
    Having been, (and still am,) a fan of William Atkin, I wanted to build a design of his named "Sunnie", a V bottom 36' ketch.
    A perusal of the design shows a wide flat keel, that changes its siding over a large degree.
    What I wanted to do was to extend inward the lines below the chines, (the bottom,) so that they would "meet-up", so to speak, at a keel of constant siding between stations 2>12.
    This would increase draft, deepen the ballast casting, (lowering its CG,), and give a deeper bilge.
    Also would give the boat more lateral plane, (should "hang-on" the wind better,) but would increase wetted surface/drag.
    I re-drew the lines to show the change in profile of the rabbet line,, and ended-up with a general hull form, (below the chines,) that could be compared to another Atkin design, "Island Princess", which has a keel siding that's more or less constant.
    Did not build either of them, but looking at the two of them you can see the pedigree.
    Sunnie; Atkin & Co. - Sunnie (
    Island Princess; Atkin & Co. - Island Princess (
    Try re-drawing the profile/plan view of your boat, you might be surprised. It might not be such a "horror story".
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