designing a fast rowboat

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

  1. flo-mo
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    flo-mo Junior Member

    Here is a quote from the ProSurf 3 manual regarding the resistance calculation the program uses:

    That's the reason why I used the results from the Sulkava race as reference.
     
  2. NoEyeDeer
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    NoEyeDeer Senior Member

    Yup. I just tried KAPER on a hull I'm currently building, and it gave results pretty close to Michlet, if allowing for the fact that Michlet underestimates slightly. However, KAPER seems to miss some of the humps and hollows in the resistance curves. It seems to be a rougher approximation, in general. I doubt it would pick up fine adjustments to lines.

    KAPER may also be coded to simulate a rougher surface, since even allowing for the known underestimate in Michlet, KAPER still gives higher indicated resistance. Michlet uses the old ITTC line by default, which isn't for a mirror finish by any means. I'm not sure what KAPER uses.

    How did you use the Sulkava results? I mean how did you get the relative speeds of the boats, compared to the Sulkava boat?
     
  3. NoEyeDeer
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    NoEyeDeer Senior Member

    Aha. Just tried something. Checked more closely (only with one hull, mind) and it seems Michlet and KAPER are in very close agreement up to speed/length ratios of around 1 (Froude number 0.3) but start diverging above that point.

    The pic shows the KAPER prediction from Delftship overlaid on top of the Michlet prediction for the same hull (green line, not orange) and same speed range.
     

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  4. flo-mo
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    flo-mo Junior Member

    Simply by a bit wearisome iteration.
     
  5. NoEyeDeer
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    NoEyeDeer Senior Member

    Sounds like fun. :D I've done that sort of thing before. Very tedious.

    ETA: Just added a comparison between Michlet (orange line) and KAPER predictions (black and white overlay) for the Herreshoff 17.
     

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  6. Clinton B Chase
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    Clinton B Chase Senior Member

    Annie data

    SO, at this point I STILL see no reason why the SLRS can't be as fast as a GB...the only major difference is weight...the GBs are lighter and their Cp may be higher. According to Flo-Mo's data...SLRS and GB are about the same in Cp.

    So what the heck?
     
  7. DCockey
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    DCockey Senior Member

    For vessels with the proportions of a guide boat or Saint Lawrence river skiff (and no immersed transoms or similar) viscous drag predominates over wave making drag up to a Froude number of 0.3 or somewhat higher. Drag will depend mostly on wetted surface area. I would expect most drag prediction methods to be in reasonable agreement for total drag of these types of boats at Fn of 0.3 or lower.

    As the Froude number approaches 0.4 wave making drag will predominate and become much greater than viscous drag. Drag is much more difficult to accurately predict. But this is the speed range which is of interest for those interested in competitive rowing (racing or bragging rights) in the type of boats discussed in this thread.
     
  8. NoEyeDeer
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    NoEyeDeer Senior Member

    31.6/29.2 = 1.08 wetted surface. Still aint gonna be faster, unless you dramatically cut wave drag. :)

    At a very rough guesstimate, that difference would be enough to drop you back around 2 minutes in every hour of rowing. Another way of looking at it, roughly 1 boat length every minute.
     
  9. NoEyeDeer
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    NoEyeDeer Senior Member

    Not for these types of boats, AFAICT. I know single sculls are around 10%/90% even at Fn approaching 0.6. I'm getting results of around 25% wave drag and 75% viscous even at 6 knots on an 18' waterline length (Fn around 0.42).

    Example:

    U (m/sec), Rr , Rt

    2.551204, 0.002463, 0.027184

    2.813653, 0.006307, 0.035877

    3.076102, 0.010423, 0.045236

    DWL 5.426 m (17' 10")

    Subtracting Rr from Rt at 3.076 m/s-2 (close enough to 6 knots) gives Rv of .034813 kN, which is 0.770 of the total.

    The other interesting thing is that although Kaper matches Michlet well for this hull at Fn < 0.3, it doesn't match nearly as well for the Herreshoff/Gardner 17 footer in the same speed range. No idea why, but that's what they spit out.
     
  10. Clinton B Chase
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    Clinton B Chase Senior Member

    I am appreciating the subtleties more and more.

    OK, so if one were taking the SLRS hull form....

    1) reduce WSA as much as possible...perhaps start by pulling in the BWL a little amidships. Because the boats are a lot lighter than they used to be, maybe pull the keel up a bit?

    2) Reduce wave drag.....how? Is this where getting the longest lwl comes in? I can see stretching it a bit on the water.

    3) The Skiffs are quite fine aft, I thought about filling the sections out a little aft...that would increase prismatic, no?

    How does prismatic coefficient relate to draw? I understand how lwl will impact it.
     
  11. NoEyeDeer
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    NoEyeDeer Senior Member

    The subtleties are probably going to be where it's at. Racing really involves getting terribly serious about trivia that sensible people (who just want to get out and have fun) wouldn't care about too much.

    Yup, reducing BWL is where I'd start. Not sure about the keel. My inclination would be to see how it all looks and sort it out as you go.

    Length is always a good way of cutting wave drag. Prismatic is trickier, because the best prismatic is going to depend on both the length and the required stability. That last bit may seem stupid, but it's to do with trading off length against wetted surface.

    Warning: here be long and boring stuff (no dragons)

    I'd be looking at the problem like this: basically, you want to go in the Blackburn and beat guideboats. So, first thing I would be doing is finding out how long the Blackburn course really is. Their site says 20+ miles, but doesn't say what "20+" actually means, and doesn't say if it's statute or nautical miles (big difference). So, I'd be getting myself a pretty accurate distance and be knowing which units it was measured in.

    Once I had that, the next thing I'd be wondering about is currents. When the winning times were set, were the boats significantly aided or held back by currents anywhere on the course? If they had a 2 knot current behind them for half the race, that would make a big difference. It would effectively make the course shorter, meaning their average speed through the water would be significantly less than what you might think if you just took race distance and time elapsed.

    Once all that information is available, you're in a position to work out how fast they were going through the water, which is the important bit for figuring out resistance.

    The next thing would be to get a reference figure for "Clinton in a guideboat". I'd pick the lines of the best 17 foot guideboat I could get my hands on, select a DWL that realistically accounts for your weight, plus the boat's weight, plus the weight of any gear you will have to carry in the Blackburn, and run those lines through Michlet at the relevant speed. That will give a figure for resistance.

    You want to have something that will beat that, so I'd then take that figure and knock 10% off it. I'd call that the target. Any hull that can equal or better that target is a hull that should do the job. Any hull that wont equal or better that target is a hull that aint really gonna do the business.

    That 10% reduction in resistance should give you around five minutes up your sleeve at the end of the race, which is what I'd call a satisfying win that should help you sell boats. :D
     
  12. Clinton B Chase
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    Clinton B Chase Senior Member

    Interesting approach. I think the SLRS and GB and a couple others will all be weighing in. I will likely sketch sooner than later. Especially if Drake sells.
     
  13. flo-mo
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    flo-mo Junior Member

    John Winters' (who developed KAPER) articles "The Shape of the Canoe" are a good read about the various factors that influence the performance of a canoe (should also be relevant for fast rowboats).

    The downloadable Free Study Plans Catalogue by Bearmountainboats is an excellent source of lines plans and specifications for a large number of canoes and kayaks and two rowboats. Especially the underwater shape of the modern asymmetrical canoes and larger kayaks might be worth looking at when you are searching for a design that could beat the Adirondack Guide Boats.
     
  14. ancient kayaker
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    ancient kayaker aka Terry Haines

    Thank you Flo-mo for that tip: I hadn't noticed that deal before. It looks like a great reference book. I look forward to studying it in detail.

    I think the underwater shapes of the fast rowboats are more slanted toward speed than most canoes which need more compromise for stability. Non-outrigger rowboats have the advantage of those wide gunnel beams with all that reserve stability, a luxury that canoes with their commitment to paddling cannot afford. There are a coouple of rowboats in the catalog that show this rather well.
     

  15. NoEyeDeer
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    NoEyeDeer Senior Member

    Thing is, with the sort of boat required to beat guideboats, you don't really get much choice for midship section. It's a tightly constrained problem, so all solutions are going to look much the same.

    You require a certain minimum freeboard, but you don't want much more than that because of weight and windage. You require a certain spread at the rowlocks. You have to keep BWL pretty close to 2 feet. There has to be a certain amount of immersed area.

    The only real variable here is how much rowlock spread you regard as acceptable. If you're happy rowing a guideboat with 3'3" of beam, you can reduce topsides flare quite a bit. If you want a "real rowboat" that has a confortable 4 feet at the sheer, then you end up with something like the picture attached. You can do minor variations (ie: round bilge, slight deadrise in the middle, etc) but the shape wont vary much. The Savo/Sulkuva boats look like this too, and for very good reasons.
     

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    Last edited: May 11, 2013
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