angle of exit for planing dinghy

Discussion in 'Sailboats' started by sawmaster, Jul 12, 2010.

  1. sawmaster
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    sawmaster Senior Member

    I 'm looking for some information about how much fore and aft rocker a 15 ft flat -bottom skiff could have and still reasonably be expected to plane in a 10 mph breeze.The boat as currently configured,has the stem/bottom point about 1/2 inch above the LWL,greatest draft of three inches [board up] about two thirds of the way back from stem to stern,then sweeping up rather suddenly to the transom at exactly lwl,Waterline beam is 50 inches and occurs at point of greatest draft.Sail area is approx 100 sq ft. Will she plane?
     
  2. wet feet
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    wet feet Senior Member

    Not really enough information to give an answer,but the sail area seems modest for a 15 ft boat.How heavy is the boat and what do the occupant(s) weigh?
     
  3. sawmaster
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    sawmaster Senior Member

    the boat weighs in at about 130 ,add about 250 for 1 fat occupant
     
  4. Doug Lord
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    Doug Lord Flight Ready

    I raced a 15.5' Windmill that had 119sq.ft. of SA and weighed 190lb.+ 320lb. crew and it planed in just under 10 knots of wind. Generally, a planing hull sailboat should have a fairly straight,flat run aft.
     

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  5. sawmaster
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    sawmaster Senior Member

    the general intent was to be something similar to a flat bottomed, plywood megabyte but I think my boats bottom sweeps up where it joins the transom,where the megabyte has a flatter exit
     
  6. sawmaster
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    sawmaster Senior Member

    I know that in general flatter exit is better for planing,but I have seen some boats with upswept bottoms plane,I guess I'm just being paranoid wondering how much is too much,but tomorrows forecasted 10-20 mph breezes will give me an opportunity to find out first hand.I guess time will tell.
     
  7. Doug Lord
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    Doug Lord Flight Ready

    SD, how does your hull compare to the 'Mill? Got any pictures from the side or quarter?
     
  8. sawmaster
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    sawmaster Senior Member

    Doug,I only have the scale drawings I made to build the hull,I drew them on 18 by 22 (and larger ) graph paper.I am using the computer here at the public library,and I don't know what,if any scaning capabilities they have.The hull form was inspired sportboats like the i 550,but I couldnt get quite as much flare in the gunnels.The upswept bottom at the transom was necessary because of the point of max beam being so far aft,and the desire for 16 inches of legroom (an improvement over the laser I used to own-sheer torture for a big man).If I had used a flatter exit,the LWL would have been almost 3 inches up on the transom.Anyway,picture if you can, a scaled down sportboat,but with a canoe stem,instead of a plumb stem,slightly reversed sheer, and a transom instead of open stern.And here's the kicker-Its two plank lapstrake!
     
  9. Doug Lord
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    Doug Lord Flight Ready

    Sounds very interesting. Best of luck! Let me know how it goes....
     
  10. tom28571
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    tom28571 Senior Member

    The aft underbody of a sailboat is a major compromise when it is expected to both plane and sail well in light wind. No rocker at all is best for planing and keeping the transom just clear of the water is best for sailing below planing speed. Heavy weight is also a major detriment to planing and much less so to at lower speed. The devil is in the details and no one can give a definitive answer to your question. In a properly designed boat and rig, 100 sq ft is adequate for planing but not optimum at the weight you give.

    A Sunfish will plane this weight on much less than 100 sq ft and it has some rocker.
     
  11. sawmaster
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    sawmaster Senior Member

    thanks for the input of everyone who responded to my post.I took my new design out for trials tues on lake palestine and was able to get into planing mode several times,thanks to the force 4-5 whitecap conditions on the lake.I have a bit more weather helm than I want and am now concentrating on cutting down an old catamaran mainsail to make a working jib,which hopefully will both reduce weather helm,and provide enough added power to plane in lighter conditions.
     
  12. ABoatGuy
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    ABoatGuy Member

    Take a look at a 49er or 505 bottom. Both are planing boats in the same size range. You can probably find a lines drawing or at least a profile online. Sail area of a 49er is over 200ft2, and a 505 is around 175ft2 (as I recall) and it takes a little more than 10kts to get them planing.
     
  13. messabout
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    messabout Senior Member

    The boat you describe has its' bottom on backward. If the point of deepest draft is far aft, then the angle of the run with respect to the waterline is steep. If the point of deepest draft is forward..say 45% of LWL then the run angle can be made smaller. The smaller the angle the more likely that the boat will plane. That is one of the factors but there are others. Size of the bottom at the transom among other things. Also whether the boat is sailed flat or not. If you study the profiles of some really quick small boats you will notice that most of them are kind of "chesty". The deepest part of the bottom is forward of the middle of the boat and the following part is nearly straight as it rises to meet the transom. Typical run angle for hot rod boats is in the range of 2 degrees. Whereas typical run angles for boats, not intended to plane, may be 5 degrees or more.

    A cast iron stove will plane if you supply it with enough power. As a generality, bottom loading a very big influence on planing potential. It is easy to see that lighter weight will provide smaller bottom loads for a specific bottom dimension. Looking at it in the other direction, a wide flat bottom is good for planing but maybe not so good for non planing regimes. But the width of the bottom influences the possible initiation of a plane in a specific wind strength therefore bottom area versus weight is not the only consideration. Put all this stuff together and you get a real clear view of the complexity of good design work and you also begin to see that each design type is suited for a specific application.
     
  14. sawmaster
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    sawmaster Senior Member

    thanks messabout,for your input.

    If you read my post of 7-15 you will see that I did manage to get my boat with the "backwards bottom" up on a plane.I am aware of the "chestier" profile of most planing craft,however,it was my hope that by placing the deepest point further aft,planing would still occur,it would just be a more gradual transition from displacement mode to planing mode,rather than "popping up" on a plane once the initial resistance of the more conventional profile was overcome.This is what seemed to have occured in my sailing trials.By the way,you seem to speak with a great deal of authority,are you a N.A.?
     

  15. messabout
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    messabout Senior Member

    Not an NA. Not an authority of any kind either. Just a lot of years dinghy racing in all sorts of small boats.

    At some point, in the distant past, I began to contemplate the differences in various racing dinghys. Why would my Windmill best a snipe off the wind but not on the wind unless the wind was piping and my crew could and would hike like a demon? Why would my Thistle not plane as readily as that FD that just blew by me? How come my state of the art, full house, Sunfish planed only when there was a half gale? The Sunfish would happily surf down the front side of a big wave but fall flat when the wave had passed. After pondering, reading technical stuff, and observing a lot of different boats I began to see certain characteristics that appeared to define planeing tendency.

    At some point in time I discovered that my new A-cat would smoke almost any of the planing dinghys. But the A is a different boat altogether. It would go very fast but not plane in the usual sense of the word.

    Having grown long in the tooth, I have abandoned the regatta circuits and now satisfy my sailing compulsion with a super simple 16 foot flattie. It is very light, has only 60 square feet of sprit boomed sail. It will plane, readily (off wind) in 12 Knots of breeze. The lowest part of the botttom is forward of the midpoint but the widest part is aft of the mid point. You get to fiddle with area distribution, influence entrance angle, and Prismatic when you move these two maximums to selected places.

    Post a picture of your boat. There are lots of flattie and sharpie enthusiasts who hang around the forum.

    Fair winds to all.
     
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