Long & Skinny boats - British Seagull racing

Discussion in 'Boat Design' started by ber1023, Oct 20, 2012.

  1. ber1023
    Joined: May 2008
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    ber1023 Junior Member

    I know i have posted on here before about this.. but I almost know what I want to build now. I just need peoples opinions on some things

    here are some pictures of this years boats

    https://www.facebook.com/media/set/?set=a.10151902646280637.876667.677075636&type=1&l=b730656d11

    the yellow, blue and white "twisted tea" boat is mine its 24ft, all straight cuts with sides being 10" and bottom being 14" so at the stern its 28" wide. It seems to be fast but I would like to get .5mph more (currently doing 14.4/14.5).

    One of the things I don't know is about the boats twist, in the attached files I circles the areas I mean. Do you think i want these areas to have a tight twister or more gradual? I built my boat by letting the wood just bend as gravity wanted it too.


    Another thing is the length and width. Some people think skinniest is best or is this that till a point? and length, I think my next boat should be about 28ft and the same width I am currently, I feel like my boat is on the verge on wanting to sort of plane at the moment and would benefit from some length to keep it more of a displacement hull. I would also like a square straight up and down bow as thats whats fastest (read that when looking into kayaks)

    what do you think? it would be built with 1/4 ply.

    some videos of how they run

    http://www.youtube.com/watch?v=uzcHqkPtmtU

    http://www.youtube.com/watch?v=pPzrtYuzDY4 - 52secs, 1:41 (I copied that boat), 2:20, 2:50

    also the angle of the sides could be talked about, at the stern it ads buoyancy so i think they should be flared but im not really sure

    https://fbcdn-sphotos-c-a.akamaihd.net/hphotos-ak-ash4/484100_10151964970205637_2120169115_n.jpg
     

    Attached Files:

  2. whitepointer23

    whitepointer23 Previous Member

    now that looks like fun.
     
  3. PAR
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    PAR Yacht Designer/Builder

    Forget what you've learned from kayaks, the information is irrelevant at 2.5 S/L ratios, which is what you're doing a 14.5 MPH, assuming a 24' LWL.

    The V'd entry and midship sections are wasting some energy, as the water is thrown to each side, rather than functioning as a pure lifting element. Improve plane mode efficiency with a flat bottom and of course the narrow beam. There's also no advantage to flaring the sides. It's just extra weight and potential drag, leave them plumb. Lastly a sliding seat might be helpful to get trim right at different points along the acceleration curve. You're on the edge of full plane mode, but you'll need to be much closer to 20 MPH to be completely up on plane, with a 24' LWL craft.
     
  4. ber1023
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    ber1023 Junior Member

    so are you saying to force the wood to bend faster and have more of the bottom of the boat flat? which is turn is making the "twist" im talking about tighter and sooner? how would you have the entry?

    forgot to mention, race is around bermuda and the flared sides help with stability also
     
  5. PAR
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    PAR Yacht Designer/Builder

    Flare to the sides will not help with stability, unless you are experiencing heel angles similar to sailboats, which seems quite unlikely. A good example of this is racing sailboats of similar shape, like the sharpie. The fast ones had plumb or nearly so topside planks. This was discovered fairly early in their development and flare was decreased progressively until they became plumb.

    As to the "force the wood to bend faster" comment, I'm not sure what you're referring to, but you can get rudimentary resistance predictions from several few or low cost programs, though realistically, these bits of software will not be helpful without a good grasp of the dynamics involved. Your entry isn't as important as you might think. At the speeds you're currently reaching, you should be "carrying" your bow and most of the entry anyway, so the entry hasn't much to do with top end ability. This said you do need an entry fine enough to cut through chop, but not so fine it can't "keep her head up". Personally, I'd use a fatter entry so get some lift and offer some buoyancy to prevent plunging, a dead flat run from midship aft and enough surface to plane off on. How much surface? Well, that's the butt kicker and you'll have to rely on experience and some testing, possably some of the prediction programs I mentioned will help. In the end, you can just forget about all the canoe and kayak stuff, as these boats are trapped into speed limitations, governed by their LWL. You've already exceeded these usual limitations and are now running into a dynamic lift issue. If you want to plane faster, use the most efficient form, with the least amount of wetted surface and wave making drag.

    Back to the twisting wood. If the boat's topside planks are all but plumb, you will not have any twist. Simply put, draw up a flat bottom shape with plumb sides and enough rocker forward that will get the bow clear once you approach 12 MPH. At this point you should have a 3 or 4 degree stern down trim which will be just about right for generating lots of lift. If you're not doing a lot of turning or maneuvering, then some very shallow vertical strips, say 1/4" tall by as thin as you can make them (1/8"), in the last 1/2 of the hull, spaced outboard as far as possible, will help contain the pressure waves under then hull, before separation, adding to the lift potential. It's real easy to go overboard with these types of "gimmicks", but it could offer a 1/2 a knot or more, compared to an exact hull without them.
     
  6. ber1023
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    ber1023 Junior Member

    thats the thing, im not sure if i should be trying to plane them or not. The rave involves 2 people per boat with cooler and 6 gallons of gas and a engine thats probably 4hp if your lucky and tuning well
     
  7. ber1023
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    ber1023 Junior Member

  8. WestVanHan
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    WestVanHan Not a Senior Member

    That is cool racing those boats,however I think they should run at least 20 hp.
     
  9. messabout
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    messabout Senior Member

    No not a hooked afterplane. Par has told you that the bottom should be dead flat and straight from mid section all the way aft. The rocker is to be only in the forward end of the bottom. and probably not a lot of it.

    The boat that you have shown is painted prettily but not an efficient design. You are wasting a lot of boat by making it toothpick narrow in the front. Why do you want to increase the length of the new boat?

    You may very well find the other half mile per hour by tinkering with propellors and leaving the boat alone. You will almost surely do better with a more correctly designed boat however. Then experiment with props and you may be able to embarass your competitors.
     
  10. PAR
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    PAR Yacht Designer/Builder

    If you want to go fast on limited power, you need to keep it as light as practical, decrease the wetted surface within sufficient buoyancy to hold everything up and provide the most efficient running surface you can. If you want to V the bottom, reverse V it and try to hold the pressure waves as long as possible. A long narrow boat is just fine if you intend to stay within displacement mode speeds, but if you'd like to "bust out" then cut it's length in half (or more) make it from 1/8" plywood, don't paint it, because this weighs too much and provide a flat plane surface for her to "get up" on.

    Take a flat bottom canoe and cut it in half, just aft of midship. Place a transom on it and hold on the best you can. The resulting boat will be very light, have very little wetted surface and a flat bottom, with maybe enough rocker in the forward sections, to get the bow clear when you throttle up. If well thought out, you should be able to run mid, maybe even high 20 MPH range.
     
  11. messabout
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    messabout Senior Member

    You can run in the 20s with 4 HP as Par has explained. WIth a Seagull I am not so sure. Those engines may not be capable of produceing enough revolutions at the prop shaft to get to 20+. Aside from the boat itself, attainable speed is a function of prop RPM, prop pitch, and slip rate.

    If I recall correctly (probably not, I havent seen a Seagull in ages) the gear ratio is fat and intended for big loads not high speed. That is to say the ratio is such that the prop shaft turns only moderate revs at WOT. Revs at WOT of a seagull is not an impressive number even at the crankshaft. My sneaky mind might form a plot to fudge the gear ratio to get the prop turning faster.

    I think a useful experiment would be to borrow a small Jon boat (flat bottomed tinnie) and strap the Seagull on to check whether the boat will plane. Check the speed to
    evaluate the potential if it does plane. Light weight is the aim. That means no gear or passenger other than the driver.
     
  12. PAR
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    PAR Yacht Designer/Builder

    My old Silver spun 4,500 to 5,000 RPM with a 2.5 reduction. You might be right about them not having enough "legs", but if the boat is light enough and has a sufficient plane patch . . . I'm think 14' long max, 24" wide max, 1/8" plywood hull, a transom and grab bars.
     
  13. Froudian Slip
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    Froudian Slip Junior Member

    Don't be so quick to dismiss kayaks as "trapped by their LWL" PAR. I suggest you look at Olympic sprint kayaks, which for shorter distances of 200m or so exceed S/L of 2.0 with a paddler output of over 500watts. This outstrips nominal "hull speed" (yes, just an arbitrary fig. i know) by 100%. I have personally have attained speeds just shy of 15mph as measured on GPS in a K1, albeit for very short distances. These boats have Cp's approaching .68-.69 and do not really even experience a wavemaking hump in the resistance curve as is typical with "normal". kayaks. Surfski's racing downwind can go well beyond this even, admittedly with assistance from surfing following seas. Now you are vastly more knowledgeable in the area of boat design than myself, but it would astonish many on this forum to crunch some numbers on racing kayak design and the velocities they can attain with world class athletes. espescially considering that stability is practically nonexistent and supplied largely by the kinematics of the stroke and not the hull.
     
  14. Froudian Slip
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    Froudian Slip Junior Member

    Also. pertaining to the OP, my eye and intuition agrees that the superfine half angle of entry on this boat severely limits the amount of volume that can be carried forward. In other words, I concur. Obviously the ply construction limits the sectional shapes available but this design is just needlessly giving away useable length, it would seem.
     

  15. PAR
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    PAR Yacht Designer/Builder

    There are exceptions to every rule, but generally this type of craft (kayak) is designed to make operation relatively easy on the paddler. Yeah a special case can be built for short duration blasts, but these wouldn't be considered the usual lot of the breed.

    I disagree in the limitations with plywood. You can use building technique to get nearly any shape desired with plywood. In this discussion, using mostly developed panels makes sense, though slitting some of them and filling the resulting "pie wedges" with fabric and goo will yield much better shapes. On the other hand 1.5 mm plywood is available, as is 2, 2.5 and 3 mm, which means a double diagonal hull of about any set of shapes is possible.
     
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