Seabright (box keel) info needed

Discussion in 'Powerboats' started by Bob S., Jul 28, 2006.

  1. Bob S.
    Joined: Jan 2006
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    Bob S. Junior Member

    I read with interest the rather long thread started by Guillermo on pocket cruisers as well as Mark's on the PG designs as there was quite a bit of discussion involving variants of the box keel. My interest in this is attempting to determine what types are strictly displacement boats and what types will efficiently respond to increases in power.

    Specifically, anyone who has knowledge in this area would be asked to please provide comment on the boat in the following photos. It is a 25' pilothouse/crabber type rig built out of marine ply/glass/epoxy of about 4500 lbs dry weight. Beam is 8'6", draft 2'6". Power is a 60hp diesel nestled down into the keel so as to provide a total below decks install and shaft straight out the back. Fuel & water tanks are also in the keel ahead of the engine.

    With current powerplant she cruises at 11 knots @ 1.25 gph. The big question I am attempting to resolve is whether or not a repower with perhaps 100hp would succeed in developing a comfortable 14-16 knot cruise or would she simply burn up the extra fuel and resist any urge to see the world at a slightly faster pace?

    [​IMG]
    Exposed wood on the bottom of the keel is an oak skid plate (for beaching) which is attached over the glass/epoxy bottom. The fuzzy looking thing in the photo is a piece of carpet on the trailer support beam.

    [​IMG]
    A view of the chine and the widest point in the keel

    [​IMG]
    View aft with rudder on edge
     
  2. Tad
    Joined: Mar 2002
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    Location: Flattop Islands

    Tad Boat Designer

    Bob,

    At 1.25 gph you are only utilizing about 20HP. What happens when you push the throttle forward? If the boat just trims more and makes bigger waves but does not go any faster, chances are that more power (and weight) won't help.

    There could also be a problem with your reduction gear and prop being inappropriate for the power or speed of the boat.

    Tad
     
  3. Bob S.
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    Bob S. Junior Member

    Tad,

    Thanks for your response.

    I have not run this boat but have chatted quite a bit with the builder/owner who now has it for sale. It was designed by a Glen Fredholm in Washington State about 15 years ago.

    In a discussion just today the owner revealed that when the boat was new the hull would lift to the point that it would raise the chine areas of the hull higher than the waterline and the boat would lean over as a result. The owner then added about 500 lbs of lead to force the boat back down into the water while underway and he was satisfied with the results. I found that to be a remarkable bit of news. He went on to say that the designer commented at the time that more power may require the addition of even more lead. I looked in the hatches and sure enough, there are lead ingots distributed around below decks.

    What do you make of this? Where would that lift be coming from with such modest power? The substantial flat keel bottom?
     
  4. Tad
    Joined: Mar 2002
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    Tad Boat Designer

    Bob,

    I am not familiar with the work of Mr. Fredholm, which is neither here nor there.

    The lift would definitely be caused by the hull form moving through the water, and the flat section would be a large part of that. But why did they want to weigh it back down in the water? Perhaps dynamic instability? Or trim problems, perhaps the bow headed for the sky? Is all the ballast forward or aft, or all over?

    Something is not right if you have to buy fuel to haul lead around.

    Tad
     
  5. Bob S.
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    Bob S. Junior Member

    Most of it appeared to be under the cockpit sole, just aft of the engine.
     

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  6. mark424x
    Joined: Feb 2006
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    Location: Seattle, WA

    mark424x Junior Member


    Hi Bob, I think the key is the owners comment that the boat "leaned". That would suggest to me that it was generating enough dynamic lift that the "bottom of main hull" were lifted just clean of the water, and thus the boat leaned to one side - since it can't just stand up on it's keel. By adding weight they could push the boat faster without lifting so far, but incuring much more drag as a result. It seems that the boat was designed to efficient but to have a limited usable top speed - or designed to carry much more load.

    First a caveat, I'm not a boat designer or hydrodynamics expert, my one fluid dynamics class >20 years ago as an undergrad gives me no insight here. All that being said, I have a hunch that one might be able to analyze the box keel using decomposition. That is, think of the keel as a slender displacement hull and figure the drap/power required and then the rest of the submerged section of the hull as a semidisplacement or planning hull form whose displacement is reduced by the displacement of the keel. Having a lower displacement meens it would have less drag and plane with less power. Of course there will be coupling and surface effects that I can't account for in my crude thought experiment. This boat is a little tougher to think about since it's not a traditional round bilge hull with a box keel bolted on the bottom.

    The challenge with the boat in the photos is how to stabilize it laterally as you add power. Imagine you added 500 hp or whatever it took to plane completely on the foot of the keel. If there is a slight roll to port (or starboard) there is nothing about the hull form that exerts a counteracting force to keep it upright - lack lateral stability. So you'll end up speeding along at a 45 deg angle riding on one edge of the keel foot and on one chine. (and going in a circle) :)

    So you have a couple choices - the brute force way - add more lead and more power so you get to the desired speed without the chines lifting out of the water. The other option is to take the lead out and add some sort of righting force - perhaps an Ama, like on an outrigger canoe. The weight of the ama itself (or perhaps with an enclosed tank) might make only one necessary - it would move the CG far enough out board to keep you from floping to the opposite side at speed. Perhaps a set of small paravanes. I haven't really thought about a problem like this so perhaps there are better ideas - a wing on the keel, etc - I have no idea. The good news is that you might hit your target without changing the engine.
     
  7. Bob S.
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    Bob S. Junior Member

    Well its all academic now as the boat just sold to someone else before I could decide whether or not it was a viable project.

    It is an interesting hull none-the-less and I can't help thinking that there is something worthwhile in that form that a competent designer could tease out.
     
  8. mark424x
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    mark424x Junior Member

    Yes, thanks for posting it. It certainly is an existence proof that there is something here. Although, I'd love to know what the consumption numbers would be without the lead - I'd bet it slides along nicely before the chines come out of the water.
     

  9. Bob S.
    Joined: Jan 2006
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    Bob S. Junior Member

    One point worth looking at is typically hulls with broad flat sections lift nicely but are pounders as these sections come into contact with waves and chop. On this hull I would submit that much of the lift is generated deep enough in the water column so as to avoid a rough ride in lumpy conditions.
     
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