The weather says launch

Discussion in 'Powerboats' started by Wayne Grabow, May 8, 2011.

  1. Easy Rider
    Joined: Oct 2009
    Posts: 920
    Likes: 46, Points: 0, Legacy Rep: 732
    Location: NW Washington State USA

    Easy Rider Senior Member

    Thank's very much Wayne. Unless one is going to plane fully I feel that a straight run kills good semi-planing performance and should be avoided. It sounds like you've got about 3" (or close to it) of rocker. That's quite a bit and sounds like you're original power of 25 to 30 hp should have been fine but at 30 mph I would think the bow would be sky high and you'd probably experience lots of porpoising but you do'nt speak of it so I'm assuming you actually have less rocker. This is my favorite semi-planing hull:
    http://www.atkinboatplans.com/Cruisers/Tang.html
     
  2. Dirteater
    Joined: Oct 2010
    Posts: 203
    Likes: 21, Points: 0, Legacy Rep: 300
    Location: Canada

    Dirteater Senior Member

    great looking boat Wayne!
    I'd say your sunday afternoons just got
    a whole lot better :D

    nice.
     
  3. Wayne Grabow
    Joined: Aug 2003
    Posts: 251
    Likes: 17, Points: 18, Legacy Rep: 297
    Location: Colorado

    Wayne Grabow Senior Member

    Easy, I appreciated the previous feedback that you gave me on your 40 hp ETEC. I agree that 25-30 hp would have been enough power at sea level; here 40 hp would have been enough. But the dealer had a 50 hp in stock so I went with it. I had read on threads here about problems with porpoising and was also aware of the tendency of semi-planing hulls to stick their nose in the air at speed; so I had that in mind when I designed the hull.
    The initial idea for such a hull came when I was examining the powerboats at the Clayton, NY, antique boat museum. A hundred years ago they tried to achieve speed with heavy, low power engines using a variable deadrise design. I thought that some of the same principles may apply. I didn't do research, but did weight and displacement calculations and built a structural model to scale the weight and do simple flotation tests. I avoided a sky-high bow by having a light displacement hull and minimizing the rocker angle. Porpoising is somewhat connected to weight distribution from what I have read. The intention of this hull is that when the bow rises about a foot, the immersed underbody shape changes from semi-planing to a planing shape. There is a long, triangular area of straight run aft. Power trim on the engine also helps. The waterline beam is not quite five feet, giving a L/B ratio of about 3.5. I would guess that a slender shape should reduce the tendency for longitudinal oscillations.

    I like Atkins designs, they encompass a lot of common sense concepts, but am not keen on 800 pounds of ballast as mentioned. It would seem better to either reduce displacement by changing the bottom shape, increase scantlings, or add creature comforts instead of just adding inert weights.
     
  4. Wayne Grabow
    Joined: Aug 2003
    Posts: 251
    Likes: 17, Points: 18, Legacy Rep: 297
    Location: Colorado

    Wayne Grabow Senior Member

    Thank you. You are absolutely correct. Most importantly, my wife loves the boat. Second, we get lots of admiring looks and compliments when launching and retrieving. Outside of websites like this, there are very few people who can conceive of building their own custom boat.
     

  5. Easy Rider
    Joined: Oct 2009
    Posts: 920
    Likes: 46, Points: 0, Legacy Rep: 732
    Location: NW Washington State USA

    Easy Rider Senior Member

    Wayne,
    That explains a lot. I have in mind to power the Tang w my 60 hp Suzuki OB.
    Yes .....the banana shape w the stem fwd. Tang and especially Handy Billy have that feature of changing from a disp hull at slow bell and then mostly a planing form at speed with about 8 to 10 degrees angle of attack. My old Albin 25 was a bit like that too. I'm glad you mentioned the E-tech feedback as I could'nt remember if I'd passed it along. On Atkin I also like "Marcia" and feel she would do well w the Suzuki 60. I love your picture w the rainbow spray.
     
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.