designing a fast rowboat

Discussion in 'Boat Design' started by nordvindcrew, Oct 13, 2006.

  1. Clinton B Chase
    Joined: Mar 2005
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    Clinton B Chase Senior Member

    Good info thanks!

    I'm happy with how my Drake raceboat is turning out. Pretty well done with the hull, now just getting the strakes lofted. Its been fun really paying attention to the numbers on this design, more than others which have been rather intuitively approached.
     

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  2. petersont
    Joined: Dec 2010
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    petersont Junior Member

    racer drake

    Looks sweet - what are your dimensions and estimated weight? Maybe you posted them before but I could find any.

    And judging by your last question, I'm guessing you'll have it ready by the next one...?
     
  3. NoEyeDeer
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    NoEyeDeer Senior Member

    Dimensions (at the time) were back in this post.

    I generally like the plank lines a lot in the latest shot. Only thing that concerns me is the sheer strake might look slightly odd compared to the gunwale. It's increasing in width from the stem to 1 or 2 feet back, then narrowing again. This doesn't quite match the way the other three stakes are shaped. Maybe tweak the sheer slightly?
     
  4. NoEyeDeer
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    NoEyeDeer Senior Member

    BTW, I have some paint (undercoat only) on mine now, and all the Deks Olje on the inside. There's more painting to do, but I'm thinking I might just concentrate on making the oars for now. I'm keen to try it out even if it's not fully painted. :D

    The other thing is that I'm really liking the soft matte look of the undercoat. I don't really want a gloss finish on the enamel top coats, so may look at finding a flattening agent for the enamel.
     

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  5. Clinton B Chase
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    Clinton B Chase Senior Member

    Thanks for thoughts. I'm going to spend more time on that sheer strake. It is not final. I'll make a quarter scale model of the boat before going to CNC/kit phase.

    It will be all 4mm Okoume plank and WRC for any stem/backbone. I assumed 75lbs for final weight. Here are the specs to date (inches)


    Volume Displacement = 8784.93
    Center of Buoyancy = -7.9788, 2.1534e-14, -1.50343
    Wetted Surface Area = 3953.94
    Waterline Length = 203.501
    Maximum Waterline Beam = 27.7637
    Water Plane Area = 3557.27
    Center of Floatation = -6.07636, -2.08053e-14,0

    Draft 3.9"
    Cp = 0.58

    NED: it looks GREAT! FAST!
     
  6. NoEyeDeer
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    NoEyeDeer Senior Member

    I'm seriously thinking I should do a scale model next time I build a boat. These graphics apps never get real life perspective quite right, and if building the boat upside down you have no idea how it'll look right side up (even though everyone stands on their heads in the shed to try and see what it'll look like, and pretends this is sensible behaviour :D ).

    Anyway, those specs of your boat in inches are a bit hard on the brain. I had to convert them anyway to make sense of them, so here they are for humans.

    Salt water displacement = 325 lbs (148 kg)
    Center of Buoyancy = 53.9% aft
    Wetted Area = 27.5 sq. ft (2.55 m2)
    Waterline Length = 16' 11.5" (5.169 m)
    Waterline Beam = 27.8" (0.706 m)
    Draft = 3.9" (0.099 m)
    Cp = 0.58

    Interesting thing there is that BWL and WSA and prismatic are almost identical to my boat (difference is a few poofteenths of not very much) but you have a lot more displacement (yours is 325 lbs compared to mine at 253) while I have more waterline length (yours is 16' 11.5" compared to mine at 18' 8.1").

    If mine was sunk down to 325 lbs displacement, wetted surface would increase by around 10%, but then BWL and stability would also go up a fair bit. OTOH, waterline is 10% longer, and wave drag on these sorts of boats around race speeds is roughly inversely proportional to the cube of the waterline length (or 6th power of speed/length ratio), which would give mine about 3/4 the wave drag at the same displacement, all else being equal (which it probably isn't).

    That wouldn't be quite enough to offset the wetted surface penalty, meaning my boat should have a bit more resistance at the same displacement. That's ok, since it was never meant to be the fastest thing around anyway. It was only intended to be fairly quick, but still good for picnics.

    But yeah, it does look pretty good and will keep me happy for a bit. :)
     
  7. Clinton B Chase
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    Clinton B Chase Senior Member

    Real #'s

    I'll have a new set of numbers after the strakes are developed. This was the round bilged model. New displacement should be about 300 and I predict a slightly lower prismatic.

    Will repost and I will not be lazy this time. I'll post in feet and metric!

    ned, can you elaborate on the whole low WSA vs long DWL and their effects on drag. I felt 17' was more than enough for a single rower in open water. It seems that in rough water, low WSA can be a real benefit.

    Cheers
     
  8. NoEyeDeer
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    NoEyeDeer Senior Member

    Well for the drag predictions I'm currently going off Michlet, and for what intend to do I can assume flat water, or near enough to it, when I'm trying to go fast. That means I don't have to worry much about what happens in waves.

    Anyway, Michlet can't go far wrong on the wetted surface drag. That's well understood, and mathematically very simple (it's a very basic function of WSA, coefficient of friction and Reynold's number). Predicting residuary resistance (wave drag and any other odd bits) is trickier. Leo doesn't claim 100% quantitative accuracy for this stuff. If Michlet is out by a substantial amount that could screw up my predictions, since what boat works best depends on the balance between skin friction and residuary resistance. That's the caveat, and I'm ok with taking the risk given that Michlet has generated useful results for people in the past. It's still a bit experimental though.

    My boat was designed with stability as the limiting factor, for general versatility (read "friends and relatives who are useless in boats"). If I just wanted fast, I would have done something narrower on the waterline. I wanted some fast though, to keep me happy. This means I ran comparative resistance calcs based on something too tubby for "real speed". It still turned out predicted to be faster at 19' LOA than at 18' LOA.

    The 19 foot version has about 5% less predicted resistance for the same stability, despite having more wetted surface, because the reduction in wave drag more than offsets the increase in skin friction.

    I ran another calc at 20' LOA and that dropped another 2.5% off the resistance for the same stability. It would have to be a little bit heavier though, which would nullify some of the advantage. I felt I was getting into the region of diminishing returns if I went longer than 19' LOA, and it's a handy size since it will just fit inside a standard carport or garage (6 metres external, over here). It just feels right to me too.

    Still, if the predicted resistance drops 5% from 18 to 19 feet, and another 2.5% from 19 to 20 feet, that implies the minimum should be around 21 feet, and should be around 4% less than the 19 footer. So if I was serious about racing, and if conditions could be assumed to be close to optimal, I'd have to think seriously about building a 21 footer.
     
  9. ceproof87
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    ceproof87 CE Boat Certification

    For the fast rowboat you have to make the Centre depth of a boat 13,5", Beam on waterline 35", Length on waterline 15'6" and weight of the boat is approximately 70 lbs.

    [​IMG]
     
  10. DCockey
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    DCockey Senior Member

    How did you determine those dimensions?
     
  11. NoEyeDeer
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    NoEyeDeer Senior Member

    Yeah, sure. :p That would be slow, by the standards of this thread.
     
  12. NoEyeDeer
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    NoEyeDeer Senior Member

    Extracted from posterior, I suspect. ;)
     
  13. NoEyeDeer
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    NoEyeDeer Senior Member

    ceproof87: I already have a boat that has waterline beam of 27.5", waterline length of 18' 8", and weight of 60 lbs.
     
  14. SukiSolo
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    SukiSolo Senior Member

    As it happens I really like turquoise against reddish timbers :) so finish my own boats at something close to your undercoat colour. Nice boat, she looks quite quick, well done. I'd be interested to see how she pitches or rather lack of, hopefully you can feed back on this after trials.

    Although gloss is faster ie less resistance, if you want a nice way of getting a semi matt after the top coats, try very fine 000 or 0000 degreased wire wool after flatting with fine wet and dry.

    Looking forward to the splash, all the best.
     

  15. NoEyeDeer
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    NoEyeDeer Senior Member

    Yes the colour turned out really well. There's a story there. I had some good oil-based undercoat and enamel sitting in the shed. I have no idea how old it is, since it came with the house, but when I opened the tins to check it was in perfect condition. Did a couple of small test patches, and it behaved like good paint should, so I decided it was good enough for a rowboat.

    Both enamel and undercoat were tinted to a Dulux colour called "Cambric", which is basically cream. I thought cream was a bit boring, so talked to the paint bloke at the local hardware store and came up with the idea of throwing in some green and blue tint. This was the maximum amount of tint recommended for that amount of paint, and it came out as you see it in the pictures. I'm very pleased with the result. The only drawback is that not being a standard colour it might be difficult to match in future, but that's not a big deal IMO.

    Anyway, top coats will be the same colour. I had thought about rubbing down the enamel. I'd be a bit wary about using steel wool since it's likely to leave tiny rusty bits in the surface. Bronze wool is hard to get around here. Stainless only seems to be available in very coarse. I'll have to sand between coats anyway, so it occurred to me that I could knock the gloss off the final coat with 1200 wet and dry or something.
     
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