Planing hull question

Discussion in 'Boat Design' started by graham, Dec 8, 2001.

  1. graham
    Joined: Dec 2001
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    graham Junior Member

    Hi Guys
    Can any one give me some insight to this question.
    Thanks in advance.
    I am designing and about to build a 17ft aluminum speed boat with extra long range capability.
    It is relatively heavy for its size with around 2274pounds displacement.
    It has full length planing strakes on the chines aprox six inches wide.
    It has fairly sharp entry following through to about 17deg dead rise at the transom.
    The design I like has a shoe on the keel which sees a flat section begin gently near the stem following all the way to the transom were it is a little over a foot wide.
    Question Is this heel realy going to make the ride that much harder or have any undue sea keeping qualitys.
    The advantages seem high to me.
    It reduces my draft and should give me good dynamic lift and help to reduce HP required.
    My theory is that most of the wave piercing is happening at about one third to half way along the keel line so with the keel deep in the water.
    I think the industry also calls this a shoe.
    So whats it to be Shoe or conventional keel???
     
  2. Guest

    Guest Guest

    I'm not a naval architect, just an interested amateur builder, but I'm very curious as to what you're designing. At first when you said "keel shoe" I was thinking of a more traditional full length keel to add protection when grounding, etc. which would protrude down beneath the hull. But then when you also said it would reduce draft and would widen to 12" at the stern I realized it sounds more like a keel pad like on a typical high performance boat. In the case of the later, I don't think a 12" pad would be horrible (with the assumption that the goal isn't to go as fast as possible though the most difficult sea possible - probably in a 17' boat you'll be riding any substantial waves or breaking them with the bow rather than trying to plane over the top as a larger raceboat might do). Of course as a tradeoff for a potentially ever so slightly increased bumpiness, what do you gain? At 17 degrees, we're only talking about saving less than 2 inches in draft, which isn't a huge deal unless we're talking about powering it with a jet drive. But since you're looking at a heavy boat, the slightly faster planing time would itself probably make it worth it. I'd be interested to get a better idea of what you're designing...
     
  3. graham
    Joined: Dec 2001
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    graham Junior Member

    Thanks for the response
    Who ever you are??

    I need to give a little of the intended use of the boat to shed some light on what I am trying to achive.
    It was my intention to have a small but extremly sea worthy boat self draining design which I could explore the coral islands of the great barrier reef off the east coast of australia with.
    I wanted a boat which I could go ferrel on for up to a week (Limit of my beer cooling capacity).
    I was hoping to use one of the new suzuki 70 hp 4 stroke fuel injected outboards.
    I know this sounds under powered but I am hoping to average 20 litres/hr at 25 knots on an extended trip.
    With a fuel payload of 350 ltres should give me a range 440 n/miles.
    This was based on a starting weight of 2500lbs
    I have tried to have the chines entering the water as gently as I could at the water line because for general running arround
    the weight will be more like 1900lbs which has very little of the chines in the water.
    As For Top Speed, I am not to concerned.
    My overall length was 17ft with a beam of 7'6" and a draft of 10".
    My primary concern with posting the question is:
    1/ the rough ride at the 20-25knot range.
    2/ Will the boat keep on track at slow speeds on the side of large swells, and not side slip due to the flat keel section.
    3/ Because the under water profile at the rear 50% is now not parralel and actually pushing water outwoulds all the way will that consume a lot of my precious hp.
    4/ I would like to have a hull that will get up on a slow plane as soon as possible.
    This is extremly important being in places were you shouldn't in such a small boat.
    If any one knows any tricks to do that you could let me know but naturally I don,t want it looking like a flat bottomed punt.
    I have attached a zipped up Bitmap of some snaps from the design software.
    It will have a different transom and upper deck but you will get the idea.
    By the way If you think my HP or fuel figures are crap do speak up??
    You asked me why worry about 2 inches.
    If you saw how low in the water my outboard was getting useing a 20" leg and a boat that varys in draft alot between full load and empty you would be concerned.
    Trouble is they don't make that model in a 25".


    Cheers Graham.
     

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  4. Stephen Ditmore
    Joined: Jun 2001
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    Stephen Ditmore Senior Member

    I don't see any problem with the flat along the centerline. In fact, I think it's probably a good idea. I note, however, that your sections are straight line. While this may appear to be the "simplest" shape when drawing the boat, sheet material doesn't think so when you try to twist it. I suspect that a "developable" bottom would not only be easier to build, but would result in a better boat.

    You can make the bottom developable by making it a subsurface of a general cylinder, of a general cone, or by using software that used a more complex algorithm and can draw a developable surface between the chines as you've drawn them. Using either of the first two methods you might have to adjust the chine line in profile some, but perhaps not that much. As you've drawn the chine line I suspect the simplest method - that of using a general cylinder, might work out nicely. What that means in practice is that your straight ruling lines must be parallel in all views.

    I've drawn lines on your bitmap file approximately where I think these ruling lines should be drawn. If my lines aren't precicely parallel please chalk it up to bad eye-hand coordination - they should be.

    That's as much explination of developability as applied to your boat as I'm going to give here, but if you were to contact www.atlcomposites.com there in Australia they could help with this.

    I think designing your boat for slow planing, and to transition easily from semi-displacement to planing speeds, makes sense, and I'm working on a semi-displacement launch myself because I think this speed range is the right one for comfort and safety much of the time. It does require that the center of gravity be farther forward than for a boat operating at higher speed, however, and since you're going to have a four cycle outboard on the transom you will need to look for ways to bring weight forward. One thing you might consider is building the bottom out a little on either site of the outboard, giving you more bottom area aft to support the weight of the engine.

    Also, you might want to look into a pitch-shifting propeller. I think Land & Sea makes one, and Quicksilver makes one for Mercury/Mercruiser. Those things get you out of the hole fast!
     

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  5. Stephen Ditmore
    Joined: Jun 2001
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    Location: Smithtown, New York, USA

    Stephen Ditmore Senior Member

  6. Guest

    Guest Guest

    Hi Stephen

    You have me confused now.

    I am only a novice and this is the first attempt with computer aided design.

    In the software I am using there is a setting where I asked for a developable surface and I scaled this boat down and nested it and printed it to scale on semi ridgid paper.
    I know that is very rough but I wanted to see if it was giving me what I wanted.
    It seemed to be correct when I formed the hull.

    I was rolling the angle of the veiw arround to get a good angle for the screen print I posted.
    Could it be thats why the lines are so, do you think???.

    I agree totally with your thoughts on semidisplacement hulls.
    I recently owned a roberts 42ft semi and was very pleased with the vessel.
    Getting back to my little one I will have three fuel tanks along the keel line with a fuel shift pump to adjust trim.
    I am thinking in those slow plane situations I can move weight foward.

    Cheers Graham
     
  7. Stephen Ditmore
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    Stephen Ditmore Senior Member

    Sorry for the confusion. Please let me try to clarify.

    If your software has a developable surface function, what I'm suggesting is that you force it (how you do this would depend on what program you're using) to use the bottommost/aftmost of the lines I drew, running from the inside chine line at the second section forward of the stern (on your .bmp file) to the edge of the keel at the fifth section from the stern, as one of the ruling lines. You can let the program take it from there if you'd like.

    This line is intended to be approximately the "stagnation line," i.e. the leading edge of your planing surface. The result should be that the sections become convex at the transition between the bow and the planing surface. This shape is essencially the modern equivalent of the "bell shaped" bow sections recommended in the '60s era literature.

    Good luck with the project!
    Stephen
     
  8. graham
    Joined: Dec 2001
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    graham Junior Member

    Hi Stephen

    Thanks for your input.
    Me thinks you are right.
    This is me being being humble by the way!
    I must have rolled that boat over so many times and I thought those surfaces were developable.
    But I forced the lines as you sujested and walar, now Ive got a big brooker tiny.
    The sweet little shape I had has gone into never never land.
    Question for you, If you focus totally on having all the lines parralel the boat looks like it feel out of the ugly tree.
    When you roll your plate around you can always get a little twist in it as it comes around.
    Can you cheat ever so slightly with your drawing lines to acount for the fact.

    I started messing with that drawing just to see how convex I could get all surfaces with no real regard for any thing else.
    Just before I was about to delete the wild looking thing I started staring at it and could not think of any paticular bad feature of the under water profile.
    Would you look at it and cast your opion please.
    I always hated the gull wing style of design because getting the profile of the wing right at all angles of trim looks impossible.
    But this would solve that, Its a reverse angle chine at the extreme.
    All dead rise angles are good and you should get a cushioning from the entrapped air passige through under the hull.
    I think the surface area has increased but that should consume less horsepower than pushing a lot of water out of the way like a deep vee.

    Looking forword to your comments.

    Cheers Graham.
     

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  9. Stephen Ditmore
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    Stephen Ditmore Senior Member

    I have work to get done today, so I'll have to look and respond later, but in the meantime take a look at http://www.delphion.com/details?pn=US04193370__
    This is Schoell's "delta-conic" hull patent. It's expired now, so it's public domain. The Alsberg 26, the subject of another recent thread in this forum, is (to my knowledge) the most recent non-stepped delta-conic hull by Schoell.

    Stephen
     
  10. Stephen Ditmore
    Joined: Jun 2001
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    Location: Smithtown, New York, USA

    Stephen Ditmore Senior Member


  11. Stephen Ditmore
    Joined: Jun 2001
    Posts: 1,389
    Likes: 44, Points: 58, Legacy Rep: 699
    Location: Smithtown, New York, USA

    Stephen Ditmore Senior Member

    Well, after looking at your modified boat I have the same question I did concerning the first one. Why are the sections all straight line? Is that a limitation of your software?
     
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