Design and build plywood fast cruiser with lifting keel, 36 feet

Discussion in 'Boat Design' started by Petr Podobsky, May 8, 2024.

  1. Petr Podobsky
    Joined: May 2024
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    Location: Prague

    Petr Podobsky Junior Member

    Hello,

    I want do discuss and share here design and build process of boat I want to build in comming years. But first little introduction of myself.

    I have boat building experience with composite boats. There was a time I wanted to start a small boat building company, but I quickly realised that was a silly idea and went back to real estate building. With my friend we made some boat designs.

    [​IMG]

    I have almost 10m long garage where I can build the boat (in parts). I have also freind – welder. We made some rather comlicated steel products already.

    I like to work with wood/plywood. My wife prefer building with plywood because of enwiroment, neighbours and our childern. Therefore the choice of plywood.

    Unfortunately I do not have huge sailing experience. In next 18 months I have to finish some projects and then I want to return back to boat buiding and sailing.

    I want to say I have experience, resources and will have free time. I want carefully design and plan the building process. Good plan is the key of success.
     
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  2. Petr Podobsky
    Joined: May 2024
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    Location: Prague

    Petr Podobsky Junior Member

    The boat

    I like the boats like RM 1080, JPK 39FC and JPK45. I really like the cockpit ergonomics of JPG39FC. The boat will be a combination of three boats mentioned.

    I want to build in wery simmilar way the RM yachts are build including the steel frame. The lifting keel case will be a part of the steel frame. It will be lifting keel, not a swing keel. Draught about 0,9m to 2+m.

    Lets say JPK39FC is the base for design. What will be different? Of course the building method. Sligtly smaller. I don´t like the curve of roof. Fixed, clear sprayhood, Lifting keel. If possible I want the boat to be able to sit on keel and rudders. Main purpose is fast cruising, not racing.

    I want to discuss many details in the future but now I want to put a key question:

    Is that even possible to build such a boat from plywood? I mean, in the real enviroment how easy is to get RM yachts planing?

    I don´t want to be stressed about boat weight and then have the boat performance under my expectations. Most of the time the boat will be sailing in the mediterranean sea with family and friends. In the future, maybe, I want also start circumnavigation.
     
  3. skaraborgcraft
    Joined: Dec 2020
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    skaraborgcraft Senior Member

    Build in plywood, yes. How often do RM yachts plane? That will depend on the design, the willingness to hold onto sail when some may reef. There is a quote from the builders that they build "fast cruisers" not "planing yachts". I got a V bottom plywood boat to plane, but it required waves in excess of 20ft with breaking crests.......otherwise it was a displacement hull.
    Fast cruising in my opinion means a light displacement boat with a high sail area to weight ratio, and not overloaded. If you load it for a circumnavigation, it will be naturally heavy ( increase in waterline displacement) for its sail area. Piling on sail area helps, but then stability has to be considered.
    Fast, is not always comfortable, but there are merits to sailing a fast boat throttled back.
     
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  4. CarlosK2
    Joined: Jun 2023
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    Location: Vigo, Spain

    CarlosK2 Senior Member

    "in the real enviroment how easy is to get RM yachts planing?"

    Well

    This is the Question: "light displacement boat with a high sail area to weight ratio"

    The really planing sailboats are bugs < 100 D/L (easy to achieve by increasing the Length) AND > 40 (!) square meters per Ton, something very difficult to achieve: most cruisers carry about 10 square meters per Ton, and the lighter ones 20.

    Above 6 meters length "Displacement / Length" ratio (D/L) improves but "Sail / Displacement" ratio worsens, in the end the great sailing ships of the XIX century hoisted 1 square meter per Ton.

    In short: an RM does not plane, unless we call "planing" to exceeding hull speed or surfing with waves.

    Cruising sailboats, exaggerating a little but not too much, are hippopotamuses and planing sailboats are insects that handle enormous forces in relation to their weight, like an ant.
     
  5. Petr Podobsky
    Joined: May 2024
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    Petr Podobsky Junior Member

    I was thinking something simmilar. Cruising sailboats are nor real planning boats they just somehow exceed the hull speed. I just do not understand the physics behind it.

    I remmember I sailed Feeling 326 downwind. Suddenly the speed exceeded 8,5 knots, the rudder become somehow lighter and it was seen from behind the boat that we are sailing faster than usual. It created smile on my face. That was for sure different mode of sailing.

    Can someone explain more exceeding hull speed?
     
  6. skaraborgcraft
    Joined: Dec 2020
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    skaraborgcraft Senior Member

    Carlos will explain better than me. Lightweight with flattish sections give more lift and if combined with a large sail area, it will exceed "theoretical" hull speed more often than the usual types. The other option is long and narrow, with less wetted area, but again with good sail area. Nearly all boats will get a surge of speed with some wave/swell action behind them.
    You could have a long skinny boat with a faster displacement speed than a shorter skimming type. Is the 36ft a length restriction?
     
  7. Petr Podobsky
    Joined: May 2024
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    Location: Prague

    Petr Podobsky Junior Member

    Yes. I want to have relativelly small boat with not too much expensive mooring costs. Wider boat also provide a lot more space which is also my requierement. I also see that wider boat is the current trend.

    They have more righting moment. When the flat bottomed boat is not too much heavy and its heeled, the bigger part of the hull is lifted out of the watter and BWL ratio is getting better. Maybe therefore the flat bottomed sailboats are faster? Am I right?
     
  8. skaraborgcraft
    Joined: Dec 2020
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    skaraborgcraft Senior Member

    Within reason. [​IMG]

    But keep them light and have enough sail area AND stability to carry it.
     
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  9. Rumars
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    Rumars Senior Member

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  10. Petr Podobsky
    Joined: May 2024
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    Petr Podobsky Junior Member

    Interesting. Especially the lifting keel. I wonder if the lifting keel has been konstructed. Because on the pictures it seems to me, that there is only fixed keel.

    Long keel rised to the boom , is interesting but construction may be very complicated ...
     
    Last edited: May 13, 2024
  11. Rumars
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    Rumars Senior Member

    The foil is welded from sheet steel on longerons and the bulb is bolted to it. Lifting is via hydraulic jack or a cable with winch. Lifting past the boom is only necessary for beaching or very shallow water situations, normally you lift just enough to accommodate the harbor (normal draft is 3m, many harbors will necessitate this). On the photos with the bermudian rig you can see the keel lifted just below the boom and the hydraulic piston between the foil and the mast. Balta Patrick Architecte naval, architecture navale, Motor yacht, yacht design, catamaran http://balta.fr/enfant%20perdu%20bermudien.html

    The keel case is constructed like a daggerboard case and goes from keel to deck, it's a very strong way of doing it, no steel floors needed like in the RM's. It also has an inspection plate on the side for inside cleaning.
     
  12. Petr Podobsky
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    Petr Podobsky Junior Member

    I wonder what happent if such a long keel will hit a bottom. Is there any technical sollution for that? It is better that the keel will be bended, than the hull damaged ...
     
  13. Rumars
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    Rumars Senior Member

    The better question is not what happens, that we know and can calculate with enough precision, it's when do we give up and let it break.
    Boats are engineered for a "normal" use case, and designers define what that normal is according to the client's wishes.
    The forces involved in hitting the bottom depend on speed, so we define what the "normal" speed is where everything remains intact. Example: at 2kn everyone usually expects the boat to remain intact even after multiple events during the life of the boat. If you want the same for a speed of 20kn the question is if the client is willing to accept the weight and bulk of the additional structure. Very low speed groundings can be common in certain waters, so even very lightly buildt boats should be capable of handling it. If 20kn is a very infrequent speed engineering for it is questionable, because the added weight hurts performance so that speed becomes even more improbable. Is the client willing to accept the performance penalty for a once in a lifetime event, or is it better to apply other solutions like making sure the boat doesn't sink instantly so the crew has time to launch the emergency raft?

    Examples of what can be done technically (up to a point, with enough force everything breaks):
    1. Engineer for the lightest possible solution and accept its downsides. For example the Enfant Perdu uses the bulkhead and deck as keel partners therefore avoiding the weight of steel floors. The owner has to live with a big daggerboard style case in the saloon and can't have a central door to the forepeak. X Yachts offers a carbon fibre floor structure instead of a steel one, the owner must pay for it.
    2. Provide an alternative solution for force dissipation. For example install a hydraulic folding mechanism for the keel as used by RM or Pogo. You hit bottom the overpressure valve trips, the keel folds and a lot of the force gets converted to heat in the hydraulic system. Downside: one has to pay for top notch custom designed hydraulics and accept the weight and complexity of the whole installation.
    3. Provide a completely different safety mechanism like making the boat unsinkable. Downside: you loose a lot of inside volume reducing living space.

    In the end it's all about compromises. Having a 3m draft instead of the more common 1.8-2m means less ballast needed for the same righting moment. The designer then chooses what to do with this fact, have a lighter boat for more performance, use the freed weight to make a stronger hull, change the hull material to something heavier but cheaper (like wood instead of carbon fiber), etc.
    Every decision the designer takes has some downside. Some race boats have the keel foil machined from a single piece of high strength steel, do you need to go there? Is a lifting keel a real necessity or something you use once every 10 years?

    Other aspects can be debated to death:
    Is okoume plywood embalmed in epoxy really more eco-friendly and healthier then a balsa cored polyester build? Shouldn't you use hungarian plantation paulownia and black locust as your main woods instead of something from the rain forest? Why can't you use czech fir hauled by horses and cut by waterwheel for your build?
    Isn't it better and healthier to substitute some of the soybean based eco-epoxy with casein glue from the local cow when it comes to glueing wood? Is carbon fibre made with hydroelectric energy in the USA greener then glass fibre made in the Czech Republic with polish coal?
     
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  14. Paul Scott
    Joined: Sep 2004
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    Location: San Juan Island, Washington

    Paul Scott Senior Member

    IMG_1628.jpeg IMG_1626.jpeg How about something like the Lawley 224 (second pic)? The precursor to the Int 110 (first pic is a turboed version)- plywood, light, fast, not very deep, put a blister on the Lawley for sitting headroom. 510 might be good- I’ll post a pic / info if I can find some

    aha!

    C. Raymond Hunt's "TEN" Series (110-210-310-410-510-1010) - Classic Sailboats https://classicsailboats.org/c-raymond-hunts-ten-series-110-210-410-510-1010/

    C. Raymond Hunt - The Lawley 225 - Classic Sailboats https://classicsailboats.org/c-raymond-hunt-brumby/

    C. Raymond Hunt Associates Spirit of Tradition 512 Daysailer - Classic Sailboats https://classicsailboats.org/c-raymond-hunt-512-spirit-of-tradition-43/
     

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    Last edited: May 14, 2024

  15. Petr Podobsky
    Joined: May 2024
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    Location: Prague

    Petr Podobsky Junior Member

    1) Maybe I should mention that I have more important requirements than speed and that is accomodation space. I want to have three cabins. Therefore wider hull and designs like RM or JPK are sollution. Slim rockets are not for me.

    2) I´m discussing this topics because now I look on things like lifting keel design from point of skilled amateur. I can imagine how labour intensive or complicated it is but I have never constructed it. So maybe I will put silly question but the reason is to get information I´m not aware of. Regarding to keel I have seen sollutions that has been simply not good enough for me. I mean finished keels.

    3) We really needn´t to discus "Why can't I use czech fir hauled by horses and cut by waterwheel for your build?"

    In conclusion I´m looking for sollution that is possible to build by myself, enjoy the building process and get the results that I´m expecting. Regarding the hull it will be most likely "RM yacht construction method".

    Regarding the keel will it be lifting keel or weighted swing keel? I don´t know yet. I see pros and cons on both sides. I think lifting keel is easier to construct and maintain but I´m worried about hitting bottom. The "small speed does nothing" is obvious. The "more robust construction will bear larger hit" also. But I highly doubt that designer who is selling few plans is making such a precise structural analysis that he knows what breaks where and in which speed. Therefore my question was: Is there any formula that these keels are constructed in therm of breaking. Some designing convention. Maybe there is, maybe not. I don´t know.

    Hopefully I explained it well.

    Note: I´m still interested in building plans of simmilar boats. There can always be some interesting sollutions.
     
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