Box Keel

Discussion in 'Boat Design' started by Brands01, Feb 22, 2007.

  1. PAR
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    PAR Yacht Designer/Builder

    John continued the development of his father's efforts, on these hull forms into the late 50's that I know off. Several were larger boats, but most were small sheltered waters craft. Many of these hull forms aren't available on the Atkins and Company web site, including the one I mentioned. Without discussion with Billy or John, the extent of further development is speculation and rehashing of previously covered work, though there are sources available, such as Jay Benford who sat directly behind Billy's drafting table in the early 60's during his tenure on the design team. I haven't spoken with Mr. Benford in a couple of decades, but his recollections and input could be quite enlightening.
     
  2. Willallison
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    Willallison Senior Member

    Yes - as you say, we are all really just speculating - it may indeed be that this is the hullform of the century and we've (nearly) all just been overlooking it!

    FF - It stands to reason that the SBS would have good load carrying capability. I wonder though, how its economy is affected by the additional weight.....
     
  3. Oyster
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    Oyster Senior Member

    There are two hulls being built as we speak, that I know of, but none are in the water. One is in Wash. state and one is in Maine, both with plywood.
     
  4. FAST FRED
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    FAST FRED Senior Member

    Although "only" 20K -25K or so is desired as top speed , I have been reading Lindsy Lords book on plaining hulls.

    I notice that lots of plaining hulls do negative work due to hull suction.

    With the prop filling the aft cavity , I would imagine these R-M style hulls do not suffer from this suction drag at speed at all.

    Also Lord thought many plaining boats were far too light on their waterplane for good efficiency. While he preferred .35 lb ratio the light skinny hull of the RM should be as loadable at speed as the military vessels he designed for.

    Unhappily many tables bottom at 10 ft beam , and water plane areas way higher than a boat the size of the Sea Bright I'm dreaming of.

    However the Surge tables (for a good sea ride ) in Gerrs book say that the hull should at least be comfortable in ocean work.

    FF
     
  5. sal's Dad
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    sal's Dad Atkin/Bolger fan

    Three, I think - one in Massachusetts/Maine, in aluminum.

    The form did get developed a bit further, with "Shoals Runner", a lovely, curvy, sleek shape.

    fcfc wrote:
    This is an assertion I made, on the WoodenBoat forum, in the hope that somebody could document another (I have heard rumors, but have been unable to track one down). When I say the hullform is "not developable", it is based on my experience trying to get very large sheet materials to bend to shape; it is clear to me that some CAD work to tweak the design would be a big help.

    This summer, there will be some hard data, with a heavyish RM running on 18HP, with a wide variety of loads. Robb White reported that a full load of roofing materials didn't substantially effect the performance. My wife will be sorely disappointed if it takes more than 45 minutes for a grocery run (11 miles).
    Sal's Dad
     
  6. Willallison
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    Willallison Senior Member

    FF - you have to remember that Lord's book was written rather a long time ago now. Much of the thinking has changed - suction being one of them. Imagine drilling a hole in the bottom of your boat, just fwd of the transom. Does water run out - or shoot up in the air? It's the latter, of course. The pressure at the back end of the boat is indeed lower than at the front - but it's still in an upward direction.
     
  7. Oyster
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    Oyster Senior Member

    I see several are discussing speed. Well folks, speed and shallow draft doesn't always go hand in hand. The angle of the boat that is required to get a normal boat on a high speed and planing requires some addtional angle and water. A conventional planing hull does not get on plane without the need for some additional draft , what it takes for even a tunnel hull to run on top or on cruise. Many of the flats boat require you to place it in a huge turn creating a swell to get the boat on plane . Even with those hulls which are marketed to really shallow water folks, still require hydraulic lifts to raise the running gear.

    I think all this must be kept in perspective. A tunnel hull, is not the fastest hull, on plane with the same hp of any given propulsion. RW seemed to be very comfortable running the boat at a nice leasurely pace which gave the boat the ability to reach a decent speed with the reduced hp, which in turn keep the angle of the hull to a minimum. This also gave him the feature we have come to know and thats RW running along the beach.. BUt his hull did not jump up on plane. If you actually look at his hull, and if I recall correctly, even though he had a huge tumblehome, the bottom was left fairly broad to the transom, giving the boat some additional lift. In a lot of tunnel hulls, to make up for the loss of planing surface, we let the bottom run full from midship aft to the transoms.
     
  8. Willallison
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    Willallison Senior Member

    Oyster - The SB skiff appear to differ from most boats in the way it behaves regarding angle of incidence. As you suggest, hulls of NORMAL form require an angle of attack in order to get on the plane. This angle is greatest at 'semi-displacement' speeds and reduces as the boat goes faster. The SB skiff, however already has this angle of incidence built into the bottom, in the form of the downturn at the aft end of the tunnel. Indeed, this hullform reaches a point where the upward pressure on the tunnel is so great that the boat is in danger of burying its bows.
     
  9. tom28571
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    tom28571 Senior Member

    Will, that argument which is also erronously used by Dave Gerr to prove that suction does not exist under the aft part of a hull is not a proof at all. All it proves is that the NET pressure is positive, not that there are no negative partial pressures generated.

    It is well established that aft rocker in the hull bottom results in a stern down attitude. Any fore and aft convex shape in the aft hull bottom must create negative pressure unless Bernuli's theorem is invalid, which it most certainly is not. Whether it is called negative pressure or suction is irrelevant. Lord's use of the term suction may be unfortunate but he was still correct in his analysis of the events.

    In addition to hull shape, most aft appendages such as skegs, struts and prop shafts do create at least some of ths offending shape and do cause suction, or negative pressure if that term offends.
     
  10. Willallison
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    Willallison Senior Member

    Tom - I accept your statements up to a point, but at the end of the day, the nett pressure acting on the aft sections of a hull are indeed in an upward direction. I would suggest that the main reason rocker results in greater angles of trim is because as the buttock lines sweep up toward the transom, so too must the angle of incidence increase in order to present an angle of attack to the waterflow.
    It's not that Bernoulli isn't there - it's just that Newton is stronger!

    I didn't know that Dave uses that as proof of upward pressure - he must be a smart fella!:D Actually it's my bastardisation of of what I can remember from the Westlawn text (written long before Dave came on the scene). I'll dig it out again and see what additional detail is given...
     
    Last edited: Mar 8, 2007
  11. fcfc
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    fcfc Senior Member

  12. FAST FRED
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    FAST FRED Senior Member

    Although Lord was written long ago he does point out that the Sea Sled , which is a form of inverted V does not suffer from the negative work aft of many flat bottomed plaining vessels.

    Lords complaint about the Sea Sled is it has too much lift forward , and is a very hard ride.

    Perhaps the R-M form combines the normal bow sections , with roughly the sea sled stern to get around the hard ride?

    A remarkable concept.

    Since the drive of the prop stream is responsible for much of the lift aft, would towing a model work at all?

    I plan on building a 1 inch to the ft. model to be sure the proposed interior will function as desired.

    EZ enough to waterproof the model to tow , but not if an operating engine is required for lift.

    FF
     
  13. tom28571
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    tom28571 Senior Member

    Will,

    Actually I was amazed that Dave Gerr misinterpreted the results of his test of bottom pressure and chose to claim that Lord was wrong.:confused: This sort of thing occurs throughout engineering studies. That is, the summation of several factors to make up a final result. It is called the superpersition theorem and it allows the solution of some complex situations that are impossible to solve when considered as a whole.

    To deny that negative pressure exists under the aft bottom of some boats because of longitudinal convexity is to deny that lift results from the convexity of the upper surface of an airplane wing. These ideas are identical and Bernulli lives.

    The total pressure in Gerr's experiment was positive as shown by water squirting upward, but it was the summation of all the pressures, both negative and positive, if both existed on that particular hull. The opposite of aft rocker (negative pressure) is a hook (positive pressure) on an aft hull bottom and the results are also opposite.

    I hope this I am not belaboring the point but it does seem important to me to see the distinction .:)
     
  14. tom28571
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    tom28571 Senior Member

    I suspect you are correct Fred. The prop first raises the water (along with rebound from the forward hull) and then directs it downward on the aft curved bottom.

    I have had this problem with all jet propulsion systems. That is, lifting the water upwards from the inlet must result in a down force (from Newton) which effectively increases the displacement and reduces the efficiency of the jet system. The tunnel stern is not the same as a jet system but does show some of the same factors.
     

  15. moTthediesel
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    moTthediesel Junior Member

    I had the same thought -- so why not buy a cheap RC model boat, remove the controls and drive and mount it in your hull? Far from perfect from a controled conditions standpoint; - true. But using the same powerplant in different or modified hulls might result in some worth while information.

    Or if not, at least it would give you a good excuse to play with toy boats :D

    moT
     
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