Most stable dinghy hull form

Discussion in 'Boat Design' started by Owly, Jan 18, 2017.

  1. Owly
    Joined: Oct 2016
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    Owly Senior Member

    I'm planning to build an 8' sailing dinghy / tender, that will be a 2 piece nesting boat. I want this to be as stable as possible for use in rough water, and serious sailing. Something I would not be afraid to take out into the ocean to sail to the next bay or a nearby island to explore, a tall order for an 8' dinghy.
    I've concluded so far that the shallow V bottom is probably the best all around, though a flat bottom should offer a bit more stability. I'd like to incorporate removable outboard inflatable floatation tubes for additional stability when heeled over, similar to what Walker Bay offers, though not necessarily all the way to the bow....... though I haven't gotten far on the subject of how to build them. Weight of course limits what I can get away with doing without building as "tank". For capacity, it will be a double transom pram design, and it will probably be built using stitch and glue. A centerboard is a nuisance, but so is a center bulkhead for splitting the boat in two, I'll probably split it forward of center significantly instead of shooting for the shortest possible length. I'll also move the centerboard case forward of typical, as I plan to build a simple light junk rig rather than the typical sprit rig for reasons of my own, and the sail will have about 15% forward of the mast.
    I'd be interested in any constructive thoughts and ideas on this. I'm looking at the B&B catspaw 8 as the "foundation" / starting point, as it pretty much meets my criteria. There are simpler boats, flat bottom prams, and prettier boats with double chines or plywood lapstrake and lovely curves, but function takes priority over form here, and I don't want the pounding of a flat bottom. The net offers many dinghy designs, some of them even free plans, but good well supported plans can be a real benefit.

    H.W.
     
  2. Richard Woods
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    Richard Woods Woods Designs

    8ft is pretty small for an adult, do you really need it nesting?

    I assume you have seen lots of videos of Optimists sailing in rough weather. Keep the water out and you can sail anything anywhere

    I started sailing in a Poole AB, check it out. More seaworthy than an Optimist as it has built in buoyancy

    I assume you have also seen my nesting 2 sheet ply Duo dinghy. It also has inflatable collars but I fitted them near the bows, not transom, so that they made a bumper when coming alongside, deflected more spray and added more buoyancy and thus stability forward, where the boat is narrowest

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  3. upchurchmr
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    upchurchmr Senior Member

    A catamaran.
     
  4. Owly
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    Owly Senior Member

    Richard:
    This will also be a tender, and while 8' will fit on the foredeck, it pretty much fills it up. The extra few feet will make a difference.
    I know 8' is marginal for a sailing dinghy, about as small as you can go realistically. My plan is to build in some flotation of course, and I hope to build that is using 3/16 plywood creating some narrow side deck and foredeck area in the process which should also help in keeping water out when heeled. Again, weight is a major consideration, and I may abandon this idea in favor of the tubes. I'll look at the Poole AB. I have looked at your Duo....... very much like what I want to build, but larger. Blunt the bow on the duo and you have almost exactly the same thing I'm looking at. I don't expect an optimal sailor. Anything like this is a compromise, and everybody has to choose the directions in which to compromise. This will be a tender for a pretty small yacht (31').
    Do you purchase or build your flotation tubes?

    H.W.

     
  5. Owly
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    Owly Senior Member

    a photo

    Richard
    Here's a photo of pretty much what I would like to end up except obviously it won't nest. I believe it is a build of the Poole AB. With narrower side decks, no rear transom deck, and a split forward of center, along with a bit different shape, it could be made to nest. It's obviously just a rough idea....... Here's a link to the Cat's Paw 8 which is quite similar, and which I'm looking at using for my starting point. The Poole AB does not seem to be readily available as a set of plans.
    I'm about to the stage of trying to develop some scale models at an inch to the foot, and ultimately at about 25% scale just to get a feel for it. It's winter here, and a good time for that kind of project (scale model)

    H.W.

    http://bandbyachtdesigns.com/sail/catspaw/

    [​IMG]

    EDIT: The image I tried to include is protected, so here's the link to it: http://www.harbourviewphotography.com/keyword/ab cap/i-dt9Z5Sb/A

    H.W.
     
  6. Owly
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    Owly Senior Member

    It would be about the only 8' catamaran tender out there ;-) I think I'll go with a pram.

    H.W.
     
  7. Richard Woods
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    Richard Woods Woods Designs

    You can get plans for the AB if you check with the class at Parkstone. The tubes I bought for my first Duo were inflatable pvc boat rollers, now we have "dinghy dogs" which are tapered hypalon. About the same price as Walker Bay tubes, which you can buy separately BTW, not sure how you attach them to a wood boat though

    my other design is the 8ft Crayfish, a better load carrier and more stable than a Duo, but it uses 2 1/2 sheets ply

    RW
     
  8. Owly
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    Owly Senior Member

    Richard:

    Parkstone Yacht Club does not accept Email contact from non members it seems. If you do not enter a membership type it rejects your email.

    From what I've seen with regard to flotation tubes for the Walker Bay, the cost of tubes exceeds the cost of building a dinghy by a significant amount depending on how economically you can build.

    I don't see any real attachment problem if one plans for it. A wooden strip well below the gunnels spaced out from the hull, as well as gunnels designed for lashing would allow easy lashing of flotation tubes.

    Thanks for the additional information you sent me on how you split the Duo. It is a simple and rather ingenious system.

    I'm also impressed with the strategy you used with the mast, using a sort of gaff that stands vertically and is retained by a pin forward of the mast that allows it to drop and fold down parallel to the boom. An excellent and simple way along with the jaw system to quickly remove or attach the sail, and to keep the mast short enough to stow inside the boat.

    The single piece bottom panel with splits to allow it to form into a V in the bow and stern makes more sense than two complete panels, and should create enough V to reduce the pounding one gets on a flat bottom.

    Your plans leave a lot to the imagination, yet provide sufficient information to build the boat. I'm curious as to what material you used for the mast. With only 28 square feet of sail area, it shouldn't require a lot. I'm assuming probably 2" diameter aluminum tubing if your drawings are close to scale.

    I think you made some excellent choices, as any light weight small dinghy of this kind will entail a lot of compromises. I intend to build a mini junk rig with about 15% balance area forward of the mast which will throw off the CLR / CE relationship somewhat, but this is a dingy and very light, so it's not a huge issue. What is an obvious issue is mast height. Your lower mast is clearly designed to stow, and is not tall quite ,enough for my purposes, as the block at the top must peak up the yard which will be significantly less than vertical. To avoid having a sawn off stumpy appearance and get the sail as high as I want it (lower than yours) while maintaining the foresail area, it will be necessary to place the block higher than the existing mast top.......... A telescoping section will be necessary, or some other "creative solution".

    Thanks again for your input and suggestions.

    H.W.
     
  9. Richard Woods
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    Richard Woods Woods Designs

    Thank you for purchasing the Crayfish plans. As my website says, they are very basic, hence the low price. The Duo plans are far more comprehensive, 23 sheets of drawings and instructions, but then I charge more. So you get what you pay for.

    Yes a 2in wood or aluminium tube is fine. As you can see from my Facebook page I use a 2 piece windsurfer mast on my Duo. I measured the deflection and posted the result, so you can use that as a guide for a stiff mast

    https://www.facebook.com/richard.woods.designs#

    I have never understood why everyone makes the bottom panels in two pieces, not one, as I do on the Crayfish. It is so much easier to line up, and of course uses less glass/epoxy

    We used 1in webbing straps to hold our tubes in place, hopefully less chafe and looks neater. I really meant I don't know how to make the slide in (presumably) Walker Bay system in wood, lashings are an obvious but not an elegant answer.

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  10. Owly
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    Owly Senior Member

    Richard:
    I wasn't complaining about the plans, I'm sorry if it came across that way. Interestingly the bottom, sides, bow and stern transoms, all seem to be in scale.... what scale I don't know, so I got out the scissors, and cut them up, copying them to heavy tag board / card stock this morning, and put together a really basic scale model just using tape to hold it together, as I was in the process of brewing a special beer for the occasion called Black Friday Pale Ale. I didn't put the deck on or the seats in, as that drawing was obviously a different scale, but I did make a gunnel spreader to scale.
    I plan to build a serious model at 20 cm per meter (20% scale) our of better materials before getting serious about building the real thing.

    As far as nylon straps versus actual rope lashings, it's kind of a non-issue. It would be nice to be able to custom build the float tubes and simply incorporate fastenings into them, but apparently nobody home builds such things though one would think with the proper rubberized fabric and glue it would be more than possible.

    H.W.


     
  11. portacruise
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    portacruise Senior Member

    "Most stable dingy form" =Catamarans. There a number of inflatable sailing catamarans (since you will be attaching side float tubes anyway) that turn up on a search that are self bailing, collapse to smaller and weigh much less than rigid nestling boats. Here are some other approaches at this shop, which you might be able to copy depending on your skills: https://nestawayboats.com/shop/dinghygo-sailing-inflatables/ The smallest inflatable sailing dingy listed holds 3 and is a little over 7', some of these have good reviews. Nautiraid and portabote collapse almost flat.

    PC
     
  12. Owly
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    Owly Senior Member

    I have zero interest in inflatables. The inflation tubes would be an optional, not always used feature that might be used for dinghy sailing, or as part of the package which would make this into a "life dinghy", as with the Portland Pudgie. Small diameter tubes which normally never support weight.

    H.W.
     
  13. Owly
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    Owly Senior Member

    Richard:
    Any thoughts on a skid/keel, or beaching skids to protect the bottom? In real life use, this is likely to be used landing on beaches, etc, not just at docks. It seems that either skids or multiple layers of sacrificial kevlar or something of the sort might be in order.

    H.W.
     

  14. Richard Woods
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    Richard Woods Woods Designs

    I would add a skeg aft on a Crayfish to aid directional stability when rowing. And I usually put wood runners underneath to protect the hull

    But the whole idea is to have a lightweight boat that one person can carry up the beach. Too heavy and you have to drag it, which means you need to make it heavy with extra reinforcing, which means... the usual spiral. My first Crayfish (built with 2mm ply sides, 4mm bottom) blew away up the beach and was wrecked. But it did carry 3 adults and we used it as a liveaboard tender for 4 years before then

    RW
     
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