designing a fast rowboat

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

  1. NoEyeDeer
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    NoEyeDeer Senior Member

    No idea, but it can't be good. The thing is that if I want a reference, or if anyone else wants it a starting point for changes, having a clean control grid would be a huge advantage. You couldn't use the current Delftship file as a basis for building anything, because you wouldn't know where you were with regard to which points were accurate and which weren't. It's all an inpenetrable maze.
     
  2. NoEyeDeer
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    NoEyeDeer Senior Member

    Just as an example, this is a 6.5 metre recreational scull I was playing with a while back. The shape is quite sophisticated, with flare in the bow, etc, but the control grid is really simple. It's all faired to a stupid standard of accuracy too (Delftship does control curves down to 0.1 mm adjustments).
     

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  3. DCockey
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    DCockey Senior Member

    It won't have any effect on Kaper or similar prediction methods which are based on overall parameters. I expect the effect on Michelet results would be minimal but that would need to be verified.
     
  4. NoEyeDeer
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    NoEyeDeer Senior Member

    Yeah true. I'm just ridiculously fussy. My 2c is that the whole point of CAD is that the plans will be accurate enough that you wont be able to build the thing more accurately than the plans.

    That doesn't mean that you have to be stupid about it, just that I probably will be. :D

    Anyway, now that we have some numbers for a "good normal guideboat" my curiosity is pretty well satisfied for the the moment. The numbers are telling me that the boat I want to build for myself will be about as slick as a guideboat of the same length, but with better stability and rowlock spread. That's good enough for me. I'm gonna build the sucker. :)
     
  5. NoEyeDeer
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    NoEyeDeer Senior Member

    Ok, just to get it out there:

    taking the current Saranac Laker file (lumps n all) and overlaying its resistance prediction graph with those of the 16 foot Grant Virginia, and Virginia stretched to 18 feet, we get this. :)

    One interesting thing is that the Rushton boat apparently lacks the wave drag hump around four knots, which I'd put down to not having the "container ship" shoulder effect in the waterlines. Apart from that, not a lot of difference, but the Rushton boat does appear to have a slight edge overall compared to the original Virginia.
     

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    Last edited: May 16, 2013
  6. Clinton B Chase
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    Clinton B Chase Senior Member

    Firslt, N.E.D. thanks for that last graph. I like what I see about the Rushton on the theory side. It is "guideboaty" which is fine for now. It might also be made to look St. Lawrence River "Skiffy" above the waterline. Imagine the Laker at 17, 18'...hard to beat?


    1) Absolute musts; won't even look at the design if it doesn't meet these requirements.

    pretty to look at is #1. I am a Dad and teach full time (and get caught up in projects like this one!) so I look at my boat more on land than ever. I want the boat to be very different from Drake so there is no overlap. Oars-on-gunwale, no less than 8' oars. The thing has to have the potential to beat the best Adk. guideboat rowers, assuming I am in shape to do so, and get around Cape Ann safely.

    2) Essentials; some wiggle room if not 100% meant

    -very light construction, strip or lapstrake
    -fast in the >4kts range. Be able to average 5kts around Cape Ann...or was it 5.2kts the winning speeds in the BBC.

    3) Really want to have

    -something that remains seaworthy....mainly for the BB Challenge and similar races
    -something that harkens back to the traditional boats but is clearly updated using new design technology
    -something cartoppable for getting into Lakes, Rivers etc at hand launch sites.

    4) Would be nice​

    -if it met the GB race rules
    -love the oarlocks on the Savo's...I can get those shipped over. They are designed for unique gunwales...22mm thick max. But I also like the GB oarlocks.


    Where does being a "double ender" fall?

    Not a must...willing to look at a narrow, tucked up transom. Kinda fond of double enders though.

    Any interest in shapes which are not "traditional"?

    Sure. I am open to it, but as I mentioned in #3, I would like to have something that is reminiscent of traditional boats, but been "updated". The GB, Skiff, Rangely builders did not have the advantages of CAD, NURBS, Resistance and hydrostatic tests, CNC cutting and the like. I am not bashful about employing these fully. I second what N.E.D. said, that we have the capability in CAD to do some outstanding fairing and analysis and cutting. So we should. I think we took that approach very much with the St. Law. River Skiff project.
     
  7. NoEyeDeer
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    NoEyeDeer Senior Member

    In "skiffy" or in "guideboaty" form? Point is that an 18 footer in "skiffy" form would be getting close to a 16 or 16 1/2 foot waterline, which is very different to an 18 foot waterline.

    For what you want to do, and given your size, my preference would be to go for an 18 foot waterline. I know what Ben Fuller says, and I'm not saying he's wrong within his own terms of reference, but you want to race this thing. I know Paul Neil went from a 16' GB to a 17, and I don't think he's as tall as you are. A 17 should be about 5% better than a 16 in this speed range.


    Yup, as a basic estimate based on overall elapsed time. No corrections made for the effects (if any) of currents.


    If entering GB races I suspect they'd require GB rowlocks, but personally I think the Savo ones are better. They wont lose pitch control if they wear a bit, and they're less likely to wear.


    IMO the big bonus is not having to waste time and knees lofting. You can just trust the offsets and go for it.
     
  8. Clinton B Chase
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    Clinton B Chase Senior Member

    Good point about the lwl...it would lose its "skiffiness" and only become "guideboaty". I could live with a 17' boat but I see that the GB rules allow longer, so I am definitely going 18'.

    [/QUOTE]If entering GB races I suspect they'd require GB rowlocks, but personally I think the Savo ones are better. They wont lose pitch control if they wear a bit, and they're less likely to wear.[/QUOTE]

    I've used the Savo's and the oars felt so good with those locks.

    [/QUOTE]IMO the big bonus is not having to waste time and knees lofting. You can just trust the offsets and go for it.[/QUOTE]

    That has been my experience thus far. Occassionally, you get a garboard that unrolls and isn't quite right because of the twist in the GBD, but that has happened only once. I'll never forget fitting Drake's garboard on the kit version. For hull #1 that wide garboard gave me fits. But the unrolled plank developed in Rhino was perfect.

    So, I'll be shooting for something 18' x 44". And I'll likely need to use GB oarslocks. If I throw away GB compliance, I might shoot for 48" Bmax and fit the Savo (Sarana) oarlocks.

    I have time to think on this as a dory project and skiff kit get finalized for a couple customers. Then I can play designer. Maybe someone will draw something in the intervening time.

    I look forward to hearing how your boat goes.
     
  9. DCockey
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    DCockey Senior Member

    All waterline lengths are not equal. As an extreme example the stem could be extended forward with a thin plate to lengthen the waterline length. I doubt it would decrease overall resistance and would increase viscous drag.

    The attachment shows the waterlines for Ghost, a guideboat, at the stem. Blue is per the plans with the wide outer stem. Red is what happens if the waterlines are extended straight from the rabbet. Difference is about 2" and my guess is the shorter version would be quicker due to less viscous drag. This would be interesting to investigate using Michlet if sufficient resolution was available.
     

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  10. DickT
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    DickT Junior Member

    No, I didn't follow through on that. I tore out ACLs and MCLs on both knees xc skiing downhill way too fast around that time. It set me back a bit and that project disappeared off the list. I agree with you that its worth trying. I still have a stack of those 13"x60" 3mm birch cutoffs I wanted to use. I'm curious what the effect of the vertical chines would be on laminar flow at rowing speeds. My son has been doing graduate research in slamming loads with a faceted carbon fiber panel over steel frame 30' test boat, but the speeds are way up there and the transitions don't seem to matter.
     
  11. flo-mo
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    flo-mo Junior Member

    The wide outer stem is a good example for the subtleties in boat design. Compared to the version with the waterlines extended straight from the rabbet the original version has lower cp, longer waterline, more wetted surface and a finer entry. My guess would be that in spite of the higher frictional resistance the original version with the outer stem is the faster one.
     
  12. NoEyeDeer
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    NoEyeDeer Senior Member

    Yup. If there's no volume in it, it wont affect wave drag.


    One of the quirks of Michlet's coding is that it assumes a zero width at the stem anyway. It will handle stern cutoffs, but not bow. In practice, no sane person is probably going to try and make the stem face less than 1/4", so Michlet will introduce a very slight distortion of the lines there. Not having any way of quantifying the effect on resistance, I think it should be consigned to the same area as inponderables like seagulls farting in the wrong direction in your general vicinity.

    I'd go for the red version myself in your example. I can't see any point in introducing a kink in the the waterlines. A fair line just seems more sensible to me.

    Re resolution: Michlet will handle up to 81 stations and 81 waterlines (including the ones at 0, so 80 intervals). That's pretty good resolution. Good enough for practical purposes, I think. :)
     
  13. NoEyeDeer
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    NoEyeDeer Senior Member

    You seem to be assuming that a lower CP and finer entry are always beneficial. They can be worse for wave drag under some circumstances.
     
  14. flo-mo
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    flo-mo Junior Member

    Not basically. My guess is that 4" more waterline will compensate for the difference in wetted surface - I am not sure about the effects of the difference of CP and the angle of entry but there is a difference.
     

  15. DCockey
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    DCockey Senior Member

    So do you believe that the thin plate extending the stem and therefore the waterline length would decrease drag?

    If the plate doesn't decrease drag then how thick does the hull extension need to be to cause a reduction in drag?

    Models of complex systems and phenomena based on overall parameters and regression have limitations.
     
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