designing a fast rowboat

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

  1. NoEyeDeer
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    NoEyeDeer Senior Member

    Got a question for anyone who has rowed guideboats, and been able to compare them with a range of other boats.

    How good is their directional stability?

    The reason I ask is that I know the Herreshoff 17 footer has (IMO) poor directional stability. The 19 footer I built has better directional stability than the Herreshoff, but a still not as good as I would like.

    I put this down to the way the water has to flow around the aft sections. Basically, with the flat bottom and two sides at the stern, the boat has to be at an angle to the direction of travel before the skeg/bustle/whatever starts biting.

    The bottom on this boat is approximately 330 mm wide (about 13"). Guideboats generally have a bottom about 150 mm wide (6"), or slightly less than half the width of my boat's. They are also effectively flat bottom and two sides at the stern. Being half the width on the bottom should be better for directional stability, but how good?

    Short version is that for practical reasons (beaching, etc) I would like to keep a narrow flat bottom, but it that is not going to be all that great for directional stability then I would just go for deadrise right the way through.

    So, guideboats: how good is their directional stability?
     
  2. flo-mo
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    flo-mo Junior Member

    I don't have any first hand experience but here is a quote from a very competent person, Ben Fuller on the WoodenBoat Forum:

    We have been there before -- see post #1514 but as this is such a monster thread it is no wonder we occasionally repeat ourselves.
     
  3. NoEyeDeer
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    NoEyeDeer Senior Member

    The Herreshoff has barely any stem immersion when trimmed level. Draft is about 85mm and rocker is about 75mm. If trimmed by the stern significantly, the bow would be out of the water, which would make it even more of a pig in a breeze. You could only get about half a degree of trim before the heel of the stem started lifting clear of the water. At this point you'll still have about 50mm of rocker aft.

    This is assuming it's built light, and carrying a normal-sized human. If it was built really heavy trimming by the stern might still keep the bow immersed a bit, but who would want it that heavy?

    Anyway, how much trim is supposed to be required for decent tracking?
     
  4. flo-mo
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    flo-mo Junior Member

    I understand your doubts about getting the trim right for the Herreshoff 17.

    With the Guideboats it should be easier but still it seems quite tricky and there is a lot to consider.

    Here you see Paul Neil in one of Steve Kaulback's Guideboats in very calm conditions:

    https://www.youtube.com/watch?v=Ef17_CLb0yQ

    Paul successfully raced for a long time in boats like this so he should know all about trimming the boat properly (Paul's achievements: https://adirondack-guide-boat.com/race-results/).

    At first the trim seems to be perfectly alright but as he starts to accelerate he produces so much lift that the bow comes out of the water and LWL is reduced considerably.
     
  5. Clinton B Chase
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    Clinton B Chase Senior Member

    H17

    I rowed a Herresshoff rowboat that had a windsurfer skeg on it. One of the fastest rowboats I've rowed and great tracking compared to the original.
     
  6. NoEyeDeer
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    NoEyeDeer Senior Member

    Yup, I remembered that. Fins are a pain for beaching and for shallow water though. Can make them retractable, but still a bit of a nuisance.


    I don't think there is any significant lift involved. Far more likely is that the boat is simply squatting due to the speed it's being driven at. For a 16' waterline, the crest will be at the stern at about 5.3 knots. If you drive it to 6 knots, the crest will be a long way back and the stern will be in the trough of the wave.

    The Herreshoff boat does this far more severely than the guideboat. It'll sit up at a quite remarkable angle when driven hard, but it's still not due to lift. It's just squat.

    Edit: Just thought of something. Due to the way squat changes with speed, from effectively no squat at Fn < 0.4 to maximum squat at around Fn = 0.57, the amount of static trim to get a double ended hull tracking properly will be dependent on the desired speed. So for lower speeds you might want 1 or 2 degrees of static trim, because it won't change at all once you're under way. For sprint speeds you might not want any static trim at all, because the boat will be squatting while in motion anyway, and this will automatically mean the bow is far less immersed than the stern.

    Looking at the YouTube video closely, when the boat is stationary at 0:08 the trim is close to level. The line between the black bottom and varnished topsides makes this clear. It might be 1/2" up at the bow and 1/2" down at the stern, but probably not more than that. So it looks like Paul Neil runs his guideboat with hardly any static trim.
     
  7. Manfred.pech
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    Manfred.pech Senior Member

    I can understand people who like old designs of famous designers. They are mostly well known for their merits and made of wood they are sweet boats.

    But there must be modern and more effective row boats like the designs of HJS: http://sassdesign.net/Design of a fast rowboat.pdf .

    Does anybody know of scientific research like CFD or tank tests of different hulls with different shapes to optimize older designs or to create new and better concepts? Thanking in anticipation, M.
     
  8. SukiSolo
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    SukiSolo Senior Member

    I think you will find the racing sculls still use fins for the very reason of directional stability. Note they are not that flat bottomed either so unless you reverse rocker the hull locally creating a sort of skeg, a retractable fin may be a good idea. A tiny daggerboard type might be good so you could have a couple of different sizes to optimise depending on wind and wave conditions.

    You will also find that the old flat brass fin has been superceded in some cases by a properly shaped section in carbon...;) at least for some of the racing boys and girls.
     
  9. NoEyeDeer
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    NoEyeDeer Senior Member

    I know sculls use fins. In fact I have been surreptitiously eyeing off some of the ones available from Sykes. ;) I was thinking that, with the greater windage of a "real rowing boat" it would probably make sense to use a fin from a four, or something like that. Their older, pre-2000 four fin looks good simply because its shape would be less likely to catch weed. And water lilies. And umpteen other vegetable octopi that frequent the waterways around here.

    So I could easily add an upper extension to that fin to turn it into a small daggerboard. And build a small case for it. And cut a hole in the boat and stick the thing in. Problem is that, for several reasons, it's not really an appealing prospect and I'd prefer to not have to put up with it.


    More modern and effective than what? The example boat he was testing against was a Bassett Liz. They have a very short waterline for a sliding seat boat and do pitch rather a lot. So if you want a sliding seat, and if you are restricting yourself to the same waterline length, I'm not that surprised that it would be possible to come up with something better than a Liz.

    OTOH, sliding seat boats that aren't restricted to such a short waterline still seem to optimise at the classic fine ended shape.


    The Savo racing boats have had extensive optimisation done on them. Leo has also done a fair bit of work on modern racing sculls.
     
  10. Manfred.pech
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    Manfred.pech Senior Member

    Thank you NED, that`s what I thought. M.
     
  11. Matt Gent
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    Matt Gent Junior Member

    Can anyone point me to a good reference on setting up the seat / oarlock / oar length geometry for a recreational fixed seat rowing boat?
     
  12. Matt Gent
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    Matt Gent Junior Member

    OK a bit of googling results in this calculator & reference dimensions, though I don't have the article that goes with it: http://www.woodenboat.com/geometry-rowing-craft-0
     
  13. jarmo.hakkinen
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    jarmo.hakkinen Junior Member

  14. BenH
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    BenH Junior Member


  15. NoEyeDeer
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    NoEyeDeer Senior Member

    That one is for sliding seats, and for racing (scull) hulls. I'm not sure any of it would be applicable to fixed seat, or to wider and more stable hulls.


    One thing I find odd about these last two links is that they both give rowlock positioning relative to the aft edge of the seat. That's fine if you also say what width you are assuming for the seat, but they don't give any indication of this.

    People tend to sit with their butts centred on the seat, longitudinally as well as laterally, so variations in seat width could make a big difference in rowlock positioning relative to your torso, and relative to your torso is the really important measurement.

    If you have a narrow thwart around 6" wide and are sitting so your torso is centred over it, a rowlock pin 13" aft of the aft edge of the seat (Welsford's recommendation) will be 16" aft of the centreline of your torso. If the seat is much wider, like say 14", the same 13" aft of the seat edge would be 20" aft of the centreline of your torso.

    One of these is going to be wrong. Come to think of it, depending on Welsford's assumptions both of them may be wrong. Same goes for the WoodenBoat page. There's not enough information provided to really use the suggested dimensions.
     
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