Giant Dinghy

Discussion in 'Sailboats' started by Omeron, May 21, 2009.

  1. Omeron
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    Omeron Senior Member

    Lookig at the way, race boats evolved, many many years ago i had envisaged that one day, pure raceboats would be nothing but giant open dinghys. It nearly has gotten there, but not exactly to the point i expected.
    What i mean by that is, all the boats being built still have decks, and an enclosed space underneath. Most of these boats (except long distance racers) spend their race career in day races, or a few overnight races.
    Most of these boats do not really need an enclosed space for the protection of their crew, and really not much gear to be protected from the elements.
    So my question is; why do a raceboat still have a full deck, rather than having an open shell?
    Ofcourse you need to hold the hull together, and support the mast. But that can be done by a few transverse beams where necessary. Other than that all the deck material which is not there is weight saving at a height where you want it most.
    The problem remaining is the discharge of water taken in. These boats have such a shallow profile below the waterline that an appropriate level of floorboards with an open transom would probably take care of it. And the space underneath the floorboards would be used for tankage etc,as well as providing a platform to work on and bolt all the gear.
    I know it would be one hell of a wet boat, but who cares!
    Other factors which come to my mind are the effect of sea and spray coming in contact with the full deck,windage when beating, which obviously retards the hull, as opposed to running through and exiting aft with minimal loss of inertia. What do you think?
  2. CT 249
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    CT 249 Senior Member

    But if you have a few transverse beams, how much reinforcing do you have to do to spread the loads transmitted by those beams sufficiently? If your beams are say 2m apart, the hull at the gunwale will be able to flex a lot in the 2m gap unless you spend weight reinforcing the gunwale in the gap, won't you?

    And how does the bowman get to and from the bow, etc, when they've got to get over the beams? Where do sails go?

    What is the weight saving (if any) of having no deck compared to the weight gain involved in ensuring that every bit of water that hits the deck has to run all the way down to the leeward bilge, rather than just simply washing over the deck?
    While the water is "running through" to exit aft, it is surely a very significant amount of weight, in a very bad part of the boat. And the free-surface effect of all that water could be a major problem.

    Won't the floorboards add weight just like the deck does? The problem is that water sits on top of the floorboards until it's pumped out - more weight, and a lot of effort in pumping.

    How does a crew get up to gunwale level quickly if they have to cross the boat at floorboard level? There's a reason raceboats have shallow cockpits. Last thing you want when tacking is to have a big step up and down as you cross the boat.

    How much gear (winches etc) can you place at a convenient operating height, without adding so many beams that you end up with a very heavy structure?

    And if this is so effective, why did sailors in classes that banned decks (Int 14s, 18 Foot Skiffs, 16 Foot Skiffs, R Class skiffs) introduce foredecks and double bottoms (which are high enough to be effectively like a yacht deck)?

    With respect, many an old IOR boat and a Kiwi AC boat had very little decking aft, but it wasn't an advantage.
  3. Omeron
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    Omeron Senior Member

    All valid points CT, but worth exploring.
    The floorboards would be positioned higher than normal, to drain water off the open transom without accumulating anywhere. This raised position would help stiffen the hull transversly, and the crew would be shoulder height at the gunwale.
    No need for your bowman to practice acrobatics on a wet deck. All the control lines can lead to mast base and within reach of him. Even the foot of the jib is no longer limited by deck level, and can be lower. In fact the foot of all the sails may be lower than customary, as there would be no restriction such as having to clear the crew when tacking. All you have to make sure that the boom clears the gunwale.
  4. gonzo
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    gonzo Senior Member

    An open deck has more windage. The induced turbulence will slow down the boat. Structurally, a deck is a very light compared to beams. Also, the hull can be lightened because the sheer is supported along it whole edge.
  5. ancient kayaker
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    ancient kayaker aka Terry Haines

    sailboat hulls are stressed in many directions, especially torque loading via the mast and keel or centerboard. A deck on top of the hull converts it into a closed box. Box beams are used in engineering and construction where strength in all directions and resistance to torque is required; in aircraft wings for example. It's a very efficient ITO strength vs weight.

    Sailboats also heel more than other boats: the deck keeps the boat from flooding when the gunnel is pressed below the surface. A decked boat can be built with less freeboard (lighter, less windage) than an open boat. In a small sailboat, the crew find the side decks handy for hiking out to keep the boat more upright.

    Finally, when you capsize, it's a real boon to have it shed most of the water when you right it again, saves a heap of bailing.

  6. Sail IC
    Joined: May 2009
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    Sail IC New Member

    As this is my first post, I extensively raced 505 and International Canoes. With age I know need something less athletic and just bought an RSK6 which I'm sailing single handed with a trapeze added.

    Intersting topic. Firstly, the crew-work cannot be jeopardized as its unmeasured. Given that, the layout of a 10m boat sailed by a crew of 5+ has to look different to a 6-7m sportsboat sailed by 2-3.

    Paul Young (owner of Rondar boats building 505s) once told me that he makes the smallest 505, ie 505 with least surface, hence its the lightest/strongest given similar construction materials. With this in mind and as an example building a 6-7m sportsboat I would do the following.

    Freeboard hight as low as possible. The RSK6 is lighter than other comparable sportsboats mainly (I think) due to its low freeboards. This might be taken to a further extreme aft of the shrouds.

    Forward of the mast I would deck watertight with no opening hatches. Sails (ie gennaker) would be stored in bags behind the mast.

    To save weight and add strength I would potentially watertight deck aft where the tiller moves. This would depend how far aft the crew needs to move downwind.

    Big question is cockpit floor. The 505 floor is the hull which saves weight. Old fashion suction bailers take care of any water getting in which is not much and I dont see that more water would get into a 6-7m sportsboat. On the RSK6 water in the cockpit is a now issue so why have it doubled floored. Strengthvice, two twarts at the front/back of the keel and that should really be it.

    In the cockpit I would not do any side tanks but the crew would sit on small one foot wide wings.

    Above I think would be considerable lighter that any 6-7 sportsboat on the market.

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