designing a fast rowboat

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

  1. NoEyeDeer
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    NoEyeDeer Senior Member

    Hey just to kick things off, I opened up Google Earth and plotted the Blackburn course. Now bear in mind that I have zero local knowledge of the area, so I'm just assuming that you'd be taking the shortest possible course regardless. If it's necessary to hug the coast all the way, then the distance would be longer. Same for any other obstacles you might want to avoid (underwater rock shelves, etc).

    Anyway, if the shortest course is feasible, then the distance is 15 nautical miles. Since Google Earth is basically taken from satnav, this should be accurate enough.

    Shortest times I could find on the Blackburn site, for fixed seat single men's class, are around 3 hours 30 minutes. If currents are largely neutral for effect on the boats, then average speed for the 15 mile race is just under 4.3 knots.

    That's a basic starting point. Adjust for local knowledge, etc. :)

    Now, if this is the sort of speed you're targeting, you are not going to need a particularly long boat. DWL in the 16 to 18 feet range should be plenty (check with calcs, of course).

    ETA: Dug a bit more and found a record time of 3:20. That would bring average speed to 4.5 knots.
     

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    Last edited: May 11, 2013
  2. rowboat70
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    rowboat70 Junior Member

    Blackburn times

    I did the Blackburn in 5 hours, in round numbers, both years.
    The winners did it in half that time.
    2 young corn-fed gorillas in a 28-foot double skull are twice as fast as I am.
    An Olympian in a Surfski is twice as fast as I am.
    Conditions are so variable that even though my boat for the second try was faster, the elapsed time for me came out the same.
    I finished about 150 out of 200 both years.
     
  3. NoEyeDeer
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    NoEyeDeer Senior Member

    Yup, but at the moment we're talking fixed seat single rowboats. :)

    Anyway, I just ran stuff through Godzilla to see what it said for a 290 lb dsplacement and 4.5 knots. It said the best would be around 22 feet DWL with a really low prismatic. That's not really a manageable shape for the purpose, and the extra length would add weight anyway, which would nullify some of the advantage.

    So I ran it again with a maximum length restricted to 5.5 metres (near enough to 18 feet). That gave a result which only had around 3% more resistance than the unlimited monster (assuming exactly the same displacement). Indications are that a prismatic of 0.54 and a BWL of 27" would be a very good place to start, with wetted surface around 26 square feet.

    Output file is attached as a rough guide.
     

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  4. Clinton B Chase
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    Clinton B Chase Senior Member

    I won!!!

    Two things:

    1) I won the Essex River Race today. It was a great row. I finished first in the fixed seat rowing class, quite a bit ahead of the 2nd place boat....something like 4 minutes.

    [​IMG]
    ERR2013_course by Clint Chase Boatbuilder, on Flickr

    I averaged 4.37 kts this year, 4.65 kts in 2009. The wind and tide were killer, so I am happy. That is about as good as Drake will do with me is decent shape. I want to hit 5kts over this course, but it will need to be capable of handling some pretty serious chop and wind. There were 2' waves in some parts where wind and tide were opposed. Today I felt like Drake was great, but a "race" version might be all I need. 4 strakes perhaps with 4mm ply...very different keel and a bit more beam to carry 8 1/2' oars.

    2) I think the midship section presented is a nice section for a lighter rower and protected water...I think the bottom shape forward may not do so well in the conditions I rowed int today. But the overall shape seems right on. I sketched something like this on Friday. Instead of a flat bottom it is a flat curve that turns quickly up to the waterline and then flattens almost reverses to get me 50-52" of spread at the locks.

    THanks for your posts, NoEyeDeer. Definitely thought provoking.
     
  5. Clinton B Chase
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    Clinton B Chase Senior Member

    Hmmmm....I could do that in Drake.

    I thought the BBC was 20+. ANd are we talking SM or NM?

    Lets do nm and kts when we chat.
     
  6. NoEyeDeer
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    NoEyeDeer Senior Member

    Nautical miles and knots it is. That's what I was talking about. It even says nautical miles in the Google Earth screenshot. :D

    Anyway yes, I thought the BBC was 20+ too, but according to Google Earth it isn't. If someone wants to do a few checks with known distances in that area against distances based on Google Earth, that might help.

    After listening to you Yanquis banging on about your infernal guideboats, I've decided I'm gonna model one just so I have some point of reference. Since my interest is limited, I'm not going to purchase plans. I'll just use the small scale drawings from Nick Schade's site as a reference for doing something in Delftship.

    Don't worry. I'll re-fair the lines properly. Tiny changes in lines usually give tiny changes in resistance, so any differences shouldn't be significant for modelling resistance. Schade's boat is basically a Grant, so should be representative of a good guideboat. To save time, I'll model the thing to just above a reasonable DWL, and wont worry about all the topsides.

    I'll post it here to give everyone a good, quantitative reference point. Anyone with a free copy of Delftship and Michlet will be able to spit out Michlet and KAPER comparisons against "a guideboat" to their heart's content. :)
     
    Last edited: May 13, 2013
  7. NoEyeDeer
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    NoEyeDeer Senior Member

    Hey check this: chart from the Blackburn Challenge site. Note the scale for nautical miles at upper right. Ran a ruler around the thing roughly, and it's looking very much as if 15 nautical miles is about right.

    http://www.blackburnchallenge.com/images/BBCchartLarge.jpg

    Re the guideboat modelling, I had a quick play with that too. Not finished yet, but is already very close to an H.D. Grant Virginia in terms of all areas, coefficients, blah, blah, blah.

    The good news is that, as long as you're careful, it should be possible to get that close to that 10% less resistance around 4.5 knots, without going to a totally stupid midship section and without going past 18 feet DWL.
     
  8. Leo Lazauskas
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    Leo Lazauskas Senior Member

    1. It would be better to "optimise" over a range of speeds.
    For example, use 2.0, 2.5, and 3.0 m/s (or similar).

    Even better, try to estimate what fraction of time you will spend at each
    speed. For example, in the input file, use
    2.0, 0.3
    2.5, 0.5
    3.0, 0.2
    to simulate spending 30% of the time at 2.0 m/s, 50% of the time at
    2.5 m/s, and 20% at 3.0 m/s.

    2. One poster mentioned that the ITTC line is the "default" in Michlet.
    There is no default, and I personally favour the LL08 line or Grigson's line.
     
  9. flo-mo
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    flo-mo Junior Member

    Congratulation Clint!

    Great achievement - good for you and Drake.
     
  10. NoEyeDeer
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    NoEyeDeer Senior Member

    Sure, better if you can get figures that are an accurate reflection of how boat speed will vary, but how easy is that going to be for a race like this? If anyone can come up with such figures I'm happy to use them, but at the moment I'd only be taking wild guesses. That's why I'm just using an average speed at the moment.


    Ok, I'm using the export from Delftship, and that defaults to ITTC, with the other options of Grigson or none. I've always just left it at ITTC. Do you think Grigson is better for fixed seat rowboats?
     
  11. NoEyeDeer
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    NoEyeDeer Senior Member

    Ok, can't be bothered getting it closer at the moment. This is pretty close to a Grant Virginia from the baseline to 200 mm (8") up. I doubt the difference would add anything significant to the resistance.

    Displacement is currently set at 290 lbs in salt water. Delftship hull file and Michlet output file attached. Delftship free version will also export the lines to DXF, if anyone wants to do that.

    Total resistance (kN) from Michlet, in the 4 knot to 5 knot range, in 0.1 knot intervals:

    4.0 - 0.019276
    4.1 - 0.019768
    4.2 - 0.020222
    4.3 - 0.020786
    4.4 - 0.021568
    4.5 - 0.022636
    4.6 - 0.024013
    4.7 - 0.025689
    4.8 - 0.027634
    4.9 - 0.029800
    5.0 - 0.032137
     

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  12. Clinton B Chase
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    Clinton B Chase Senior Member

    Translate, please!

    NoEyeDeer, I use Excel and Rhino...

    can you export any file formats for me?

    thanks for that work.

    I have a few midship sections to sketch, too.

    I think making this a Forum process would be great...we can use more fixed seaters in the races.

    RE: BBC....this whole time I thought the race went through the length of the Annisquam canal.

    The race looks a lot easier than when I thought it was 20+ miles :)
     
  13. Clinton B Chase
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    Clinton B Chase Senior Member

    I'm going to download the programs...
     
  14. NoEyeDeer
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    NoEyeDeer Senior Member

    I don't know what formats Rhino uses. I don't have Excel and Rhino. I only do a bit of amateur designing, so it's not worth my while to fork out good beer money on apps like Rhino. For the cost of something like that, I could actually build a boat. :D

    I don't have Excel on this box either. I figured the only thing I wanted from MS was the OS. I wasn't really interested in any of their apps. Open Office will probably export in something Excel-compatible, but I'd have to get the data into it to start with.

    -------------------------------------------------

    Ok, about the Blackburn. I have good news and bad news. The good news is it's not 20 nautical miles. The bad news is it's not 15 either. I think I was looking at the wrong bridge in the Annisquam River. Told ya I had no local knowledge. ;)

    I think the map on the Blackburn site is misleading too, because AFAICT they are only including waypoints once out of the Annisquam River. They're probably assuming that anyone who needs waypoints inside the river is too dumb to be let out on the streets anyway.

    So, starting from the double track rail bridge in Gloucester, which I'm pretty sure is the right bridge, the distance comes in at 17.22 nautical miles. Google Earth pic attached. :)

    IF that is right (someone better double check that), that means the average speed for a winning time of 3 hours and 20 minutes comes in at just under 5.2 knots.

    This is getting up into the range where I'd definitely go for at least an 18 foot waterline. It'll probably come down to how much boat you think you can handle comfortably around the course. I'll run a few more things through Michlet, but at those speeds getting something slicker than a standard guideboat actually gets easier.

    I've found this interesting, because although we hear a lot about guideboats I've never seen any definitive figures for them. Having the close-to-Grant hull as a reference is handy. If anyone can get me the real offsets for it I'll make another Delftship model.
     

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  15. DCockey
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    DCockey Senior Member

    Perhaps I should start on my often postponed "guideboat" project. I have lines and offsets for:

    Warren Cole, built 1905 16' 2-1/2" LOA

    "Ghost", circa 1882, 16' 4-7/8" LOA

    Ben & Ira Parsons, circa 1926, 12' 8-15/16" LOA

    Blanchard, 1935, 13' 1" LOA

    "Virginia", Grant, 1905, 16' LOA These are in The Adirondack Guide-Boat

    "Saranac Laker", Rushton, 16' LOA

    I assume the longer boats are of primary interest on this thread.
     
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