Help with a Simple Problem

Discussion in 'Boat Design' started by DiningShip, Aug 17, 2009.

  1. DiningShip
    Joined: Feb 2009
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    DiningShip Junior Member

    This will be an easy answer for you more learned Sailors, but I'm having trouble finding the information I seek on the net. Any links, or direct knowledge is very appreciated.

    I'm presently constructing a 15' flat bottomed sailboat, with a modified sail design in a reverse triangle. Imagine a Bermuda sail, upside down. I'm sure there is a name type with this, but I don't know it. Sail mast is roughly 13'.

    At any rate, this boat is a weekend camper with cabin, and has a beam of 4 feet max. I had to maximize interior space and have nixed a bit in centerboard, or dagger board. Lee boards are also not an option. Now I know there should be some type of keel, but I'm having a hard time figuring out the depth I need to go. Could I also substitute dept of keel with weight?

    I'm on the west coast of FL, and want to use this boat to sail in skinny water, so draft is an issue. Help is appreciated! Thanx.
     
  2. marshmat
    Joined: Apr 2005
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    marshmat Senior Member

    DiningShip,

    I'd be curious to see a picture or drawing of your sail- it sounds like it might resemble a crab-claw rig?

    If you've nixed centreboard, daggerboard AND leeboards, you might be in a bit of a pickle. It might work downwind, but on anything higher than a broad reach, a boat with no keel/board could be pretty hard to keep moving in the desired direction. A sailboat's board or keel is intended as a lifting foil, to counteract leeway when reaching or pointing.

    How big that keel/board should be is one of the many tradeoffs a boat designer must work out on his/her drawing board. But there has to be something to counteract leeway if the boat is to be useful for sailing. Ballast won't do that- you need either lateral area or lifting foils.

    Are you really, really sure that leeboards are not a possibility?
     
  3. DiningShip
    Joined: Feb 2009
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    DiningShip Junior Member

    Hey Matt,

    I'll have to sketch something out for you. Crab Claw is close, but with a single mast and boom. Boom is on a 45 angle from the mast. Sail has two battens built in to keep sail form. Sail is made from Tyvek and originally started as a Bermuda rig, design changes necessitated some drastic changes to the already started sail. Sort of stumbled on this solution. My build now and engineer later approach keeps me on my toes. The reason for the change is the cabin is going to have a roof that opens like a VW bus camper top for cabin access. This sail will fold up toward the mast to provide room for this feature.

    As for the keel, I'm ready to build anything I have to do. I guess to clarify my position, I would want to use a permanent keel (but I’m not ruling out leeboards as a last ditch option), but am having a hard time figuring out minimum depth of this. Is there a way to calculate that? Presently my boat will draw a couple of inches of water, so I can work with anything. It would be nice to sail right up on the beach, but not all that necessary since I have a built in self draining anchor storage box.

    Please keep in mind that this boat is not for performance, but rather simple cruising and camping. Don't need to get anywhere quick.

    Any suggestions will be investigated. Thanx!
     
  4. DiningShip
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    DiningShip Junior Member

    Addendum

    Has anyone ever worked with, researched, or considered a V tail style lifting foil for a sailboat? Is that possible? It would provide lift as a centerboard does, but also work as a permanent keel, providing some lateral area while not taking up to much depth.

    Of course, I might be a little crazy.
     
  5. messabout
    Joined: Jan 2006
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    Location: Lakeland Fl USA

    messabout Senior Member

    Try to avoid high tech thinking for a low tech boat. The v shaped foil or foils of various other cofigurations do ,indeed , work. You have a beach cruiser type boat. The flattie that you describe is ideal for that. (It will almost certainly draw more than "a couple of inches" of water however) Use lee boards. They work, they do not intrude into the cabin space, and they are the best option for thin water gunkholing by a large margin. If you insist on using some oddball sail rig the lee boards have the advantage of being movable fore and aft, because they wont be in the right place the first time. The kind of boat you are thinking about will probably work with ....oh.... maybe 4 square feet of immersed board. Dont worry about the depth of the boards, think about wetted area. If the boat was a race boat then we'd be concerned with depth as a function of width for technical reasons but is of little interest for this kind of boat.

    You need to explain, more clearly, your reason for turning the sail upside down. Why not use a conventional sail? You have some latitude about where to place the mast. You can put it near the stem like a catboat, you can offset it to one side or the other, you can split the rig into smaller pieces like a ketch or yawl and so on.

    The boat you describe will, when every thing is working just right, make about 5MPH and very likely no more than that. So don't try for speed. If you have an auxilliary motor, it does not need more than two HP. Any more than that is just a waste. The boat will not go more than 5 MPH no matter what you do within the bounds of reason. That speed will get you a mile in twelve minutes so enjoy the scenery
     
  6. alan white
    Joined: Mar 2007
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    alan white Senior Member

    That sail has been successfully tried but not copied as far as I've seen. Diablesse was the name of the boat, which was featured in Woodenboat magazine. The sail was used to fill in the space created by normal burmudan sails (I think the boat was either ketch or schooner rigged).
    As said, the rig worked well. I actually saw the boat about 15 years ago at a wooden boat regatta.
    Anyhow, the upside down aspect was not the whole rig. The sail was secondary, a light weather sail similar to a topsail in utility.
     
  7. DiningShip
    Joined: Feb 2009
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    DiningShip Junior Member

    Messabout and Marshmat,

    That was EXACTLY the answer I was looking for. I've tried to KISS this boat (keep it simple..) as much as possible, and in doing so might have gotten carried away with the design. Leeboards, as you both pointed out, are practical and the best solution in this case. Thanks for adjusting my course. The benefits far out weigh the astetic look.

    The sail solution was not to 'try something new' for kicks. I have hand sewn a tyvek sail and refuse to abondon it to start new. I've built every aspect of this boat aside from the mast which is recycled off a windboard. It was designed with comfort in mind from the start. I found that I didn't like the low boom interferring with cabin space (in this case, a raisable center section) while anchored. By turning it upside down (this may be a misleading phrasing) the boom becomes a triangle, which can be pulled into the mast when anchored. It's pretty simple.

    5 MPH sounds real nice. My furthest destination will be an island 5 miles from launch point. This boat is more camper than boat. I'm not placing a motor on this boat, it will only use oars.

    I promise to provide pictures as this boat is nearing completion.

    Thanks everyone. This forum is excellent and always helpful.
     
  8. lewisboats
    Joined: Oct 2002
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    lewisboats Obsessed Member

    Instead of turning your sail upside down...why not angle the foot upwards ALA Leg o mutton...giving you both the angle at the foot and more conventional (and predictable) sail handling characteristics with an added plus of having the boom higher on the mast giving more hatch room. What you suggest (seems to me) is going to give very large amounts of heeling moment from having the greater part of your square footage at or near the top of the mast as opposed to the bottom. A good gust just may lay you on your side faster than you can react to it. You only have 4 ft (max) beam so your waterline is probably less...say 36-40"on a 14 or so ft waterline length...not a lot of stability there IMHO.

    I think you are not being honest with yourself. A few trips to that island and you will be searching the horizon for other conquests me thinks ;) .

    Well...a box with the dims and draft you quoted would give about 640lbs displacement in salt water. Introduce rocker and a real boat shape and you can pretty much half that displacement so...say 320 lbs at your couple of inches draft. The boat with cabin and rigging will easily exceed 200 lbs (despite the best intentions it usually weighs more than estimated) Camping kit can easily hit another 50 lbs or more. I think you should re-consider your draft estimates by at least 50% but doubling it would be more like it.

    Steve
     
  9. lewisboats
    Joined: Oct 2002
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    lewisboats Obsessed Member

    If you are weight conscious I would go with a cloth cabin...a canvas cabin-ette would run you 30-50% of the weight of what a full out wood cabin would and if designed right would be transferable to shore as a tent-ish shelter too. Overlapping cloth panels on a light wood frame would shed well and be easy to store as well as set-up. This also will cut down on the weight of your finished boat structure as well as just WHERE that weight would be situated. Properly designed you could still incorporate your pop-up center section...just make it more like a skin on frame structure than a permanent wood panel cabin.
     
  10. DiningShip
    Joined: Feb 2009
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    DiningShip Junior Member

    Lewisboats,

    This link is the boat design I used: http://www.simplicityboats.com/LCB.htm

    The cabin is only 2 ft high, until you raise the center section. After raising it, the cabin uses curtains to fill in voids. Even loaded, the boat will be very light. One guy can pick up and flip it as it stands now. The reason for the hard top is that the front section of the boat will serve as an air tight compartment for equipment and flotation in the event of a capsize.

    The sail does present the gust problems you speak of. I have thought of that. Luckily, the weather here on the west coast of FL is pretty tame most of the time. This boat isn't an all weather machine, she is realy designed for a slow trip to the island and some comfort when there. I have several other sails which do fit mast and can be used. I just want to try my home made one first. If she presents a problem, I can be honest and replace it. I will be trialing this boat soon, so we will see how she does.

    Further than 5 miles? Maybe, one day. But initial design is mission specific in this case. Being home built means modifications can be made at a later time.

    Thanks!
     

  11. HakimKlunker
    Joined: Aug 2009
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    Location: Thailand

    HakimKlunker Andreas der Juengere

    Hi.
    When suddenly a thunderstorm is coming up and the water is shallow around, you will certainly want to get somewhere quickly :)

    The Dutch have (long ago) invented a sort of centreboard at the side of the hull (each side of course). Maybe this principle gives you a suggestion. One of the ship types made like this was called 'EWER'.
     
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