low wake design

Discussion in 'Boat Design' started by halfrhovsquared, Jan 9, 2020.

  1. rxcomposite
    Joined: Jan 2005
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    rxcomposite Senior Member

    Length is always beneficial but there is a limit. Length will increase wetted surface area, increasing resistance. There is a point where it crosses and any further increase in length will be detrimental.

    The same goes for cats. High separation to length is beneficial in reducing wave/hull interaction making it beneficial. High s/L ratio however increases weight because of the crossbeam structure connecting the two hulls wherein weight becomes a penalty.
     
  2. Dolfiman
    Joined: Aug 2017
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    Dolfiman Senior Member

    ** Be care that this study is focused on low speed, Froude 0,1 to 0,3. Your SOR is L ~ 22ยด and speed cruising 10 to top 15 mph, meaning in metric units Lw ~ 6,7 m at V ~ 4,5 to 6,7 m/s >> Froude 0,55 to 0,83, you are in the range 0,4 to 0,9 of the study I posted.
    ** For a catamaran made of slender hulls (L/B > 7, with a square off rear transom, high Cp > 0,6) in such Froude range, the wave drag component responsible for the wake is mostly function of Lw/D^(1/3) (D of each hull) and of the space ratio S/Lw (Space S of the hulls axis).
    ** with your displacement max of ~ 450 lbs, i.e. 204 kg for 2 hulls >> 102 kg for each hull >> D ~ 0,1 m3 >> Lw/D^(1/3) ~ 14,4
    ** On the fig. 1B of my study posted previously, you are on (or slightly below) the pink curve for Lw/D^(1/3) = 11,5 to 13,5 , where the hump of the wave drag at Froude 0,4-0,5 is quasi not visible, meaning you effectively minimise the wave drag component responsible of the wake.
    ** So in my opinion a cata made of slender hulls of let'say Lw/Bw ~ 9 (> 7) , Cp > 0,6 , S/Lw ~ 0,4 and for a total displacement in charge D such as Lw/(D/2)^(1/3) ~ 13,5 or over (i.e. D < 550 lbs when Lw = 6,7 m) , can be a minimum wave drag solution. The last (but not least) point to clear is the relationship wave drag component / far field wave height.
     
  3. gonzo
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    gonzo Senior Member

  4. rxcomposite
    Joined: Jan 2005
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    rxcomposite Senior Member

    Dolfiman practically designed it.
     
    halfrhovsquared and bajansailor like this.

  5. BlueBell
    Joined: May 2017
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    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    Then it has to be a catamaran.

    Ya, Dolfiman has summed it up nicely.
     
    halfrhovsquared likes this.
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