Making A Fast Rowboat Faster

Discussion in 'Boat Design' started by Tallman, Jan 20, 2013.

  1. Tallman
    Joined: Jun 2011
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    Tallman Junior Member

    I've gotten to the point in building Ken Bassett's Firefly Double where I need to decide if I can make the hull shape faster. It's not an ideal speed shape to begin with, but it's what I have, and I want to optimize it. The boat is 23' long (stretched a bit from plans, per Ken) and 39" wide at the sheer; the bottom planks -- it will float essentially at the chines or slightly above -- are 6" wide at stern and 14" wide at widest beam. I hope to keep it under 100 lbs before adding the two Piantedosi sliding seat rowing rigs.

    As two strong, experienced oarsmen will be powering this, hopefully in open water races that can last 3 hours and cover 20+ miles, even a few percentage points of speed will make a huge difference. My understanding (and I've tried and have zero ability with hull programs, hence my posting with primitive cardboard models) is that while skin friction drag goes up with speed, the wave drag goes up much faster. With two rowers, I'd like to maximize waterline length and -- I don't know the term here -- bring the water smoothly together at the stern, per a racing shell, Guideboat, etc. Given the power moving the boat, I expect it to hit max wave drag speed pretty easily and so wish to raise that bar as much as possible.

    The original plans called for a wood skeg per the shape labeled #1. It's unclear to me if the skeg is thick enough to count as added waterline length in terms of adding a little top end speed.

    I was wondering if a fuller stern section -- #2 cardboard piece -- would work; the mock-up is messy but is meant to approximate grafting a single racing shell stern piece to bottom so that the flow is continuous from where the rocker curves down through to the stern.

    Intuitively, #3 seems faster: a tapered "fuller" skeg shape that makes the end slipperier, like a Whitehall or wherry stern.

    The #4 is hard to see but is essentially a smaller version of #3 and might not be in the water as much given the rocker of the bottom.

    In all cases the fuller skegs would be very light/buoyant foam/glass/ply constructions to minimize added weight and ideally add a little buoyancy. This is not a workboat and will be carried to and from water, and treated like, a delicate racing shell.

    A racing shell fin will be installed about four feet in from the stern so it's not important that the tail section provide guidance.

    I hope this is clear and would love to hear any opinions on what might work best.
    Thanks much,
    Tallman

    http://www.flickr.com/photos/slowslide/8399756130/in/photostream
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  2. messabout
    Joined: Jan 2006
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    messabout Senior Member

    We have a very long term thread that deals with this subject extensively. Use the search function to find that one and several more. You will find much input there and you can ignore my commentary that follows.

    Your boat does not look like my vision of a racing shell. OK you have what you have. SInce you are looking for small increments, you could start with the chine. The hard chine is an eddy maker, but if it was rounded, there would be a small improvement. I am at odds with the apparent hollow at the entrance. There seems to be some concavity near the stem. Never mind. That's just a personal quirk but I think that some of our more brilliant members might not like it either. Leo, Rick, where are you?

    I can not view some of the links because Yahoo demands a password which I do not have. I can see three of them however. I like that boat and I expect that it would go very well with small power input. But I am not a competitive oarsman.

    The skeg in one of the pix is enormous and adds an absurd amount of wetted surface. That thing has to go. If you are to add a fin the skeg is unneccesary I'd like to see a lot more deadrise as the bottom approaches the transom. That would get your exit down to a very narrow dimension. If the bottom planks had a bit of convexity, that would be nice. A tad less wetted surface would result.

    The picture with the add on aft bottom displays an idea that could have some merit but I do believe that a whole new new build would be more appropriate.

    According to your numbers, the boat has L/B ratio of more than nine. That is probably getting there in terms of minimized wave drag. I think that you should concentrate on very precise hull fairing along with a superb immersed surface finish. That is especially so at the forward sections. Wax the above water surfaces carefully but never the underwater sections. The idea is to shuck off topside water and its attendant weight. Every pound counts on a long race course.
     
  3. Tallman
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    Tallman Junior Member

    That other thread didn't talk about this issue in terms of the options and the cost/benefit of the different stern/skeg treatments so I'd thought I'd start fresh.

    Thanks for you input -- it's been hard enough to get where I am now in the build that I'm really only concerned with the stern flow in terms of what I can do to tweak for a little more speed. It sounds like the #3 shape -- sort of a Whitehall end -- is what you are recommending. I'm still not clear though whether the long flat skeg creates actual waterline length or not in terms of wave drag.
     
  4. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    I only opened a couple of your drawings, but if you want some speed improvements, lose the skeg all together. The least amount of stuff hanging down in the water, is what you want. Don't "build down" the hull as seen with the one marked with a 3 on it's flank. The boat appears to have enough deadrise to offer reasonable directional stability. A small rudder is an option, if desired/necessary, but lose the skeg or the variants.

    Rounding the chines will help from a technical stand point, but it's not something you'll really notice or see on a speed chart. Of course, as Michael, suggests, a really fair and smooth bottom will help too. As far as the shape, well you have what you have and without lines, it's difficult to say, but she looks about middle of the road in stability. Which simply means they've traded wetted surface and some speed potential for stability.
     
  5. Tallman
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    Tallman Junior Member

    Thanks for your answer. I'm still a little unclear about the notion that a shorter waterline length here would be better. With nothing else in the water for the last couple of feet -- aren't I losing hull speed as two rowers can push this boat to the point where the wave drag becomes overwhelming?

    I guess what I'm thinking is that the negative of wetted area of a certain shape stern piece (shaped to minimize skin friction) would be far outweighed by the benefit of waterline speed gain...
     
  6. PAR
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    PAR Yacht Designer/Builder

    Again, without lines to look at, we're guessing, but with a crew aboard, what percentage of the boat's length is overhang?
     
  7. Tallman
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    Tallman Junior Member

    Stern overhang is about 5" and the bow curve adds about 17" so 22" total overhang which is about 8% of length. This assumes the rockered stern bottom is in the water which it will be only, intermittently, in heavier water. Otherwise it'll be out of the water a linear foot or more at the stern so much of the time the total overhang will be more like 13-14% +.
     
  8. 805gregg
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    805gregg Junior Member

    Add an engine or sail.
     
  9. PAR
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    PAR Yacht Designer/Builder

    The difference between a 20' LWL and an 18' 3" LWL in real boat speed, will be about a 1/4 of MPH at best. I'll bet your boat does what most do and drives in, wetting her down, then accelerates and clears her ends, before then next stroke drives her in again. So, the best you could hope for is, the extra LWL length across 50% of the time underway, so now the net speed difference is more like 1/8 of a MPH. Ouch, that's working way too hard for these speed increments.
     
  10. Tallman
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    Tallman Junior Member

    Thanks for the calcs -- so, If I have this right, in a 2+ hour race, pressing it hard the whole way, I'm looking at a .25 to .50 mile margin over where I'd be without the modifications? Not too shabby.
     
  11. rwatson
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    rwatson Senior Member

    Assuming no other competitors have multi-chine, less rocker, lighter built boats - you should be laughing.
     
  12. PAR
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    PAR Yacht Designer/Builder

    Agreed, the 50% potential is optimistic at best, though a few hundred yard lead, in a 2 mile pull still isn't anything to sneeze at.
     
  13. HJS
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    HJS Member

    How much is the total wetted surface?
    How long is the waterline?
    What prismatic coefficient has the hull?
    What is the metacentric height transverse or longitudinal?
    How big is the total displacement including crew?
    What speeds in practice?
    And so on .....

    If you intend to compete with the boat, so required a little more than guesswork

    JS
     

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  14. Tallman
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    Tallman Junior Member

    HJS--
    Man, I wish I had those numbers but I don't nor the chops to use a hull program to figure them out (Ken's design is from like 1978 or something). At this point just trying to optimize what I have within reasonable parameters (as in time to work on it). That link is cool - the "new" hull shape looks a bit like my boat, minus the rocker which I wish it had less of. The stern bottom wetted surface comes to a more tapered completion at the transom than mine. If I can shape the last few feet to mimic that it might be a good compromise. I see that new shape also has a racing shell type fin which confirms my plan on that.
     

  15. kvsgkvng
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    kvsgkvng Senior Member

    Tallman,

    I am not a pro here, but if I face the problem with speeding up the boat, I would try to analyze every aspect of the whole matter. You are concentrating on adding stuff or changing the shape of your already pretty heavy and done boat. It may work but there is a limitation to changing the shape. Another aspect of speeding already finished hull, I would say, is to make it lighter thus reducing wet surface. I understand it is easier to say than to make it happen.

    I am not sure about your competition rules, but if I would be so determined to win and rules would allow it, I would use your already finished hull as a mold for a new foam sandwich build. Making it lighter would reduce the weight, this would lift the boat a bit out of the water and this would make the wet surface lesser. I would lose all cross frames, beams, spars, etc. and make a simple thin skin-shell with stiffening ribs, again made of laminated foam. If you want, I could draw you something of this matter.

    One more thing, it may be very trivial, but I would try to limit water intake by adding a deck and some rather lose fitted skirt around the rowers' waste. Extra water in the boat's bilge equals extra weight and extra wet surface, no matter how small it may be. In addition, it would take out a little bit out of wind drag, as the closed deck would provide a better sliding for the air mass over smooth surface. The deck may be just a thin fabric shell. Use air bladders to support it.

    If you want to lighten your boat even more, I would go extreme and use Skin-On-Frame construction, using light polyester fabric coated with polyurethane compound with aluminum spars on the same foam sandwich bulkhead frames. This would make your boat feather-light.

    Short of building a new boat and limited with already set shape, the only opportunity to gain something is to lighten the boat. If your team would lose extra 10 pounds prior to the race, it may help as well.

    JMHO -- Regards.
     
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