canoe (respacing stations, reducing length by 1 ft)

Discussion in 'Boat Design' started by prathab, Mar 28, 2014.

  1. prathab
    Joined: Mar 2014
    Posts: 19
    Likes: 0, Points: 1, Legacy Rep: 10
    Location: india

    prathab Junior Member

    i have 17' 1" canoe temporary station position & drawing . i need to reduce by 1 feet , that means i would like to do 16' wood veneer canoe ,.. i have tried different station position , but still the fair lines is not pleasing ,,, looks little unshapy. where i have to reduce that 1 feet length , each station one inch is better or center is better , give me perfect solution to get nice shape .
    i am going to do 45/45 two layer wood veneer of 2.5 mm thickness
    thank you
     
  2. alan white
    Joined: Mar 2007
    Posts: 3,731
    Likes: 121, Points: 0, Legacy Rep: 1404
    Location: maine

    alan white Senior Member

    Convert 17' 1" to inches (205"), then subtract 12" (193").
    Divide 193 by 205 (.94 is close enough).
    The distance between each station is now multiplied by .94, as is the overall length and any other fore/aft measurement.
    Therefore, if station three, e.g., is 20" from station four, multiply 20 by .94 (18.8").
    No matter how much you shorten the canoe, it should end up fair when lofted.
     
  3. gonzo
    Joined: Aug 2002
    Posts: 13,570
    Likes: 375, Points: 93, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    When you shorten the station spacing, usually the last station needs to be re-faired. In the case of a double ender, first and last.
     
  4. alan white
    Joined: Mar 2007
    Posts: 3,731
    Likes: 121, Points: 0, Legacy Rep: 1404
    Location: maine

    alan white Senior Member

    Do you know why? Especially when the reduction is only about 6%? I imagine holding a model of the original length flat on the plan view and then angling it to my view, which would make it appear to the eye to be slightly shorter. At which point would the bow change its aspect relative to the rest of the boat?
     
  5. gonzo
    Joined: Aug 2002
    Posts: 13,570
    Likes: 375, Points: 93, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    Usually the lines at the curve of the forefoot is where the lines need to be re-faired.
     
  6. alan white
    Joined: Mar 2007
    Posts: 3,731
    Likes: 121, Points: 0, Legacy Rep: 1404
    Location: maine

    alan white Senior Member

    Right. I meant, at which point in the reasoning does the forefoot or bow not maintain the same proportional relationship with the rest of the boat, or am I missing something?
     
  7. Tiny Turnip
    Joined: Mar 2008
    Posts: 605
    Likes: 87, Points: 28, Legacy Rep: 743
    Location: Huddersfield, UK

    Tiny Turnip Senior Member

    If the length of the canoe is reduced, without reducing the depth (or beam) of the hull, (as I presume will be the case here) then the prow and stern would become steeper, and possibly become unsightly and need attention.
     
  8. alan white
    Joined: Mar 2007
    Posts: 3,731
    Likes: 121, Points: 0, Legacy Rep: 1404
    Location: maine

    alan white Senior Member

    Yes, that would make sense. I can see how it would potentially cause such a problem. I imagine it would happen when a greater reduction in length was done, say 20% for example, but I see the point.
    I don't see why this particular small reduction would have "unfaired" the canoe hull however. The steepness of the ends would be almost unnoticeable and in any case, would be as fair as ever.
    I think the poster's problem may have to do with something other than the aforementioned unsightliness. I can't tell from his post whether the molds are actually set up or still on the computer or what so maybe he could fill us in.
     
  9. Tiny Turnip
    Joined: Mar 2008
    Posts: 605
    Likes: 87, Points: 28, Legacy Rep: 743
    Location: Huddersfield, UK

    Tiny Turnip Senior Member

    Yes, Alan, I quite agree. If I read the OP correctly, he has tried reducing the distance between all stations by one inch, and then by taking all the reduction out of the midships; either approach might result in unsightliness, and is much cruder than the proportional method you describe.

    I take Gonzo's comment to be a refinement on your method, when length is reduced but not depth, and yes, I agree the prow/stern fairness is unlikely to be a huge issue in this example, as the reduction in length is relatively small, but will become more so as the percentage reduction in length increases.
     
    Last edited: Mar 29, 2014
  10. alan white
    Joined: Mar 2007
    Posts: 3,731
    Likes: 121, Points: 0, Legacy Rep: 1404
    Location: maine

    alan white Senior Member

    In his case, he should multiply every fore/aft dimension by .94 to arrive at 16 ft. The problem is that one usually sets up the fore/aft dimensions first. Then it's easy to adjust each station to match, say, an even number of whole inches. The stations follow whole numbers and this makes lofting easier.
    Now reduce dimensions to arrive at 16 ft even. The stations now fall at odd dimensions such as (example) 76.0936 inches. This works fine I suppose by itself though it requires conversion to fractions, which is not easily done without calculator in hand.
    It would be better if the station positions were reduced by an inch each, allowing them at least to be accurately spaced from each other. Or at least some reasonable dimension like falling on the half-inch.
    So he's done this but he won't know his LOA until he tries the same percent reduction on the ends. The problem is that the end reduction dimension is part of the information needed in order to arrive at the new LOA, meaning a trial and error procedure where station spacing is reduced by maybe an even fraction at a time.
    If this is what has happened, he needs to do this trial and error and each time he guesses again to find out what his actual percentage reduction is. So a 20" reduction to 19" is 5% and he needs to take his end dimensions and reduce them by the same 5% exact percentage and then measure the LOA to see if he's close enough to his target. If he misses his target LOA, change by some reasonable station spacing like 1/4" or 3/8", until the LOA is as close as is practicable.
    If I read him right, I am beginning to understand why fairing the ends becomes necessary if you don't do the math. Not because the true reduced dimensions create a problem, but because one has started with station spacing belonging to a longer design. Originally, the shape by fair battens created the station mold shapes and not the other way around.
    It's not hard to do this reduction, but it seems trial and error is the easiest way to go to arrive at the target LOA.
     

  11. ThomD
    Joined: Mar 2009
    Posts: 542
    Likes: 13, Points: 18, Legacy Rep: 111
    Location: TO

    ThomD Senior Member

    My first boat involved reducing the USCA boat by 1.5 feet to 17 feet, and I redesigned the upper part to be flared which was at the time a popular form. It was suggested by someone on the phone that I pull the mid form, which leads to a more rounded section overall. Worked OK. I had to fair the forefoot because the strips at that point gain thickness due to the acute shape and there was plenty of meat there to just plane in the shape I needed.

    I was later interested to discover that a renowned designer had made the same adjustments in producing his respected 17 footer. The only large difference being that he had differentially spaced the forms so that the PC would be higher, and there would be a fraction more meat in the ends. Made sense to me. An experience I have had over time is to see behind the curtain of the experts and discover that the differences are not that acute.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.