Senior design project help

Discussion in 'Boat Design' started by Chikokishi, Sep 10, 2012.

  1. Petros
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    Petros Senior Member

    that boat of yours in not a bad little boat, much better than I would have thought based on your earlier description.

    Usually small boats use simple design rations rather than a lot of caculations. With a fixed size keel what happens is you risk either stalling when you are trying to point high at low speed (high Cl on the keel sufrace), but than in all other conditions you have more keel area than you need and it add weight and drag. So there is no "ideal" size of a keel/centerboard/dagger board, it is a compromize. Usually for small boats they use 5-6 percent of sail area, and about half that for rudder size. But it can vary all over the place.

    Since you want fixed keel, and shallow draft, and ability to beach it, why not use two small keels or fins about 3 ft apart. this will allow you to beach it, just make them strudy so you can rest the weight on them. Use a NACA 8 percent foil section (symtrical 000x series). Make them on the larger size will give you good control and allow for a lot of mistakes without getting you into trouble.

    Good luck.
     
  2. PAR
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    PAR Yacht Designer/Builder

    The NACA 0008 series is a pretty flat foil, particularly if you have two of them. Maybe a 0012 through 0016 would be a better choice. If they're shallow, low aspect bilge keels, the foil shape isn't really all that necessary. A slab sided foil section would be as effective, considering the speeds this boat will travel at and these are a lot easier to make, especially make symmetrically. You only need a few square feet of appendage area, which will be hard to do in two bilge keels. I estimate you'll have nearly twice the surface area necessary, with it's associated drag, plus you'll be a bit compromised on windward performance because of aspect ratio (and the extra drag).

    One solution would be to have a stub keel that completely houses the centerboard. This frees up the interior of the boat of a case, you can get a desirable aspect ratio, for windward performance and it retracts into the stub keel for beaching.
     
  3. Petros
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    Petros Senior Member

    Par,

    how do you figure a 12 to 16 percent thick foil is less "fat" than an 8 percent one?

    Does not matter, perhaps you are correct in just using a slab keel(s). I just thought since he was an engineer student his professor might appreciated the attempt to improve the performance of a floating box with something looking more "modern" hanging off the bottom. :)
     
  4. NoEyeDeer
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    NoEyeDeer Senior Member

    So tie a bit of rope to your daggerboard. Simple and effective.

    If it's in the way and you don't like leeboards, offset the daggerboard slightly to one side. It'll still work just as well.
     
  5. PAR
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    PAR Yacht Designer/Builder

    Oops, I missed a letter, now corrected. I'd use a 0010 or 0012, maybe pigeon toed a bit, possably asymmetrical, but again on the low aspect shapes I think the original poster is intending, likely a huge waste of energy (making them), considering the preformance envelope involved.
     
  6. DCockey
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    DCockey Senior Member

    Given the very low aspect ratio which the OP seems to be considering, probably on the order of 0.2 or less, I doubt a foil section would make much difference. The flow past the keel will be much more 3D than 2D when the keel is generating side force. Round the front, taper the back with a 10 deg or less half-angle, and put a radius on the longitudinal edges and be done with it.

    Such a low aspect ratio keel will be less efficient at generating side force relative to its area at low leeway angles than a typical vertical daggerboard/centerboard/leeboard, but will have a considerably higher "stall" angle.
     
  7. Chikokishi
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    Chikokishi Junior Member

    I can see that most of you have been dealing with such questions for a while. Im still just getting into it, so i had to look up 3/4 of what was said. If it changes matters any, this boats max hull speed recorded is 9 mph, and averages about 5 mph. So as far as using air foils and such, they need to be effective at epic slow speeds. But at the same time, because of such slow speeds i cant imagine making a huge impact between using a fancy foil as opposed to a rounded off piece of wood. Of course, iv sanded foils into the keels that iv made thus far, more of a generic shape than a precision cut. Or would it be that because of such low speeds a very high foil would be needed to give the wood any chance of countering wind?

    Also, here's a couple more images of what we built, for those who are interested:
    [​IMG]
    [​IMG]
     
  8. Chikokishi
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    Chikokishi Junior Member

    Hello again everyone. I was reading some posts i made on here while working on my first couple of boats - I sounded like a complete idiot. I hope i am not giving off that vibe again. =(

    My current idea for my boat is that i want the sail mast placed as far to the now as i can possibly get it to allow for room int he boat. For the keel i want a full keel type rear so that i can have something to mount the rudder to - i want an internal rudder control, not a tiller type. Reason being that some of this boats time in the water will be as a forward sitting pedal boat with steering wheel. Having a keel to mount the rudder to makes construction much simpler than trying to build a strong enough mount for the rudder by itself. Now i say a full keel rear, as i dont know what i want the front to be. I want the rudder as far to the rear as possible to allow it to have good leverage on the rest of the boat.

    But as i am designing this boat, i could easily come of with a design that allows me to have a full keel for when im sailing straight and need the lateral resistance, and allows me to lift the front of it up into a modified keel for turning. Really the sky is the limit isnt it?
     
  9. Petros
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    Petros Senior Member

    you want the mast "as far to the now" as you can get? Is that as opposed to a one placed "later", or rather "sooner"? Some actually like the sooner rig, personally I do not care for it, too complicated. I am with you, the now rig is the way to go, it always good to have sails at the "now" at all times, less time to rig as compared to the later, or the sooner for that matter.

    The now rig should always be ready to sail now, rather than later. When one consider it, you can see the benefit in such a rig, more sailing now is always better than later.

    Oh, and BTW, I highly approve of autopilot pictured above attached to the helm. Though be careful some of that type can be temperamental and can cause unending frustrations, especially on long trips. but overall very beneficial to have along.
     
  10. Chikokishi
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    Chikokishi Junior Member

    HAHAHA

    Petros, you made my day. The "now" is a typo that originated from "nose" I want it as close to the nose as i can get.

    Man, that was funny.

    Chiko
     
  11. hoytedow
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    hoytedow Carbon Based Life Form

    You might want to use a kick-up centerboard that works like a kick-up leeboard but is hidden inside a centerboard trunk.
     
  12. hoytedow
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    hoytedow Carbon Based Life Form

    Ha! I thought you were going to say bow instead of nose, since b and n are neighbors on the keyboard. :)
     
  13. Petros
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    Petros Senior Member

    That means you will want a cat rig (single large sail with the mast all the way forward with large single boom). You might also consider the junk rig, easy to operate and control, the forces are low so it can be very light, very efficient as well. It puts the mast a bit aft of where it would go for a cat rig, but usually worth the benefit. Also, a balanced lug rig would be a good choice for a forward placed mast.

    None of those rigs have jibs, so the sail rig has fewer parts. the junk usually is supported by a canatlevered mast, the cat and lug can also use the cantalvered mast as well. this means no shroud lines or fore stay so rigging is faster and simpler, though the mast usually ends up a bit heavier.

    Consider to that the dagger board or center board will have to be placed further forward as well to match the sail location. I would not recommend a full keel, not necessary and it adds weight, best to stay with a simple sharpie type hull with a dagger board or swing keel/center board. with the dagger/center board up you can beach launch and land.
     
  14. ancient kayaker
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    ancient kayaker aka Terry Haines

    A full keel is not appropriate for a camping boat, it will make it dificult to get close to the beaches and banks where you will wish to camp. In tidal waters it will flop over at low tide.

    If you don't like movable lee/dagger boards you can use external chine logs (also called chine runners) to get more bite on the water. Chine logs are long battens that reinforce the joints between the side (or sheer) and bottom planks. They are usually inside, you may have used them for your boat. There's no reason why they can't be put outside: they would then be attached to the bottom edges of the sheer planks before attaching the bottom plank, forming a kind of shelf. They would considerably increase the resistance of the boat to moving sideways through the water - called leeway - when sailing. A leeboard or daggerboard will give better performance but if that is not really important . . .

    One caveat; ECLs can mess up the flow of water past the hull, slowing the boat, so it would be advisable to buy a design unless you have the software that can analyse flow. Phil Bolger published several designs with ECLs.

    Also, with ECLs there's no reason why you can't carry a leeboard for better performance whre depth is unlimited.
     

  15. Chikokishi
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    Chikokishi Junior Member

    My boat is being designed to be class legal for the PD Racer racing class. (http://pdracer.com/) it wont be of racing grade, but we are using the hull supplied by this website for the base of our design.

    As for the flow analysis, that is a requirement of our design project... so i will be doing flow analysis and such.

    And as for the ability to bring it into tight spots and camping areas, it is a 8' by 4' (also it has pure right angles, so it has good balance.. nice and sturdy) boat, so being able to move it around shouldnt be too hard with just paddles (and it isnt in my previous design)

    Thanks for the ideas!
     
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