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Photographic evidence!

I think we have a break in the case of the missing speed.


click to enlarge

I was thinking more about Cyrille's comments (and others) about the hull skin deforming under the pressure of the water. The hull is made from 2 layers of 5.8 oz carbon and 1 layer of 6 oz carbon/Kevlar weave plus a final layer of 4 oz fiberglass. It's pretty thin - you can cut it with scissors. To add stiffness, I added 6 bulkheads running down the length of the hull:


You can easily indent the hull between bulkheads with a bit of pressure from your fingers. I didn't think that the water pressure could depress the skin because the pressure is distributed evenly around the hull. Imagine pressing one finger into an inflated balloon and then imagine evenly distributed pressure happening from all around the balloon.

I decided to see if I could find any evidence of deformation from the photos, and low and behold - I think I found something. It is very, very slight, but everywhere I thought I could see the water line moving away from a smooth, gradual curve was exactly between where the bulkheads are. I believe that the water is indeed depressing the skin between the bulkheads.

I have an idea how to easily confirm this. I can pressurize the hull with enough air to keep the sides from depressing in (any idea how much air pressure would be required?). First, I need to add some fiberglass tape and epoxy around the edge of the deck flange to seal off the leaks. When it is choppy, water washes over the bow and water leaks into the hull from gaps at the edges of the deck. Once this edging is on, and a few very small pin holes are filled with epoxy, the hull should be air-tight.

I would think that I could pressurize the inside of the hull with a bicycle pump, then quickly hop on for a speed test.

Wait a sec.... If the air pressure isn't enough to stop the deformation, or if I can't get it air-tight enough to hold pressure permanently, how will I get back under the deck to add additional reinforcement to the skin if I have already taped up the edges? I guess if I determine that air pressure would be enough to stop the skin from indenting, then I am sort of committed to that solution once I add my edge tape. Any other ideas?



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11 Responses to “Photographic evidence!”

  1. # Anonymous Murray

    Greg,

    You would need to match the pressure at best which presents an issue. Water pressure increases with depth, whereas the internal air pressure will be equal everywhere. If the hull is submerged in the water 1 foot at the waterline, then the maximum pressure on the hull is 12 inches of water at the lowest part of the hull. If you pressurize the hull with air to 6 inches of water (0.217 psig, 1 inch of water = 0.03612 psig) the hull could potentially depress at depths below 6 inches and bulge at depths above 6 inches. Also, if hull depresses in the water under water pressure, I would guess that the hull will bulge out when the boat is out of the water with only internal pressure on the hull. I like the idea and it might be worth a try. I would consider doubling or tripling the number of bulkheads. The bulkheads could be very thin and hollow, or more of an internal support ring. If the hull still depresses easily between support rings, maybe thats a sign for another layer of carbon over the hull or longitudinal supports between the bulkhead and support rings.  

  2. # Anonymous Russell

    Greg, I really doubt that the deformations you are seeing would account for the 12% speed loss. Protuberances are much worse than concavities in creating drag.
    That said, it is important not to over pressurise the hull if you do manage to seal it and pump it up with air. I attempted to do the reverse once to a sailing boat, in an attempt to draw some epoxy into a inaccessable leaking centreboard case, using a vacuum cleaner. The entire foredeck collapsed in a shower of fibreglass splinters!, and I was frantically trying to rip the vacuum hose off the boat. When I mentioned this to a boat repairer, he said that the same thing had happened in reverse to someone attempting to check for leaks by pressurizing the hull, and they blew the deck off with too much air pressure.
    If I now want to check for leaks I use a camping mattress pump that connects to a car power outlet, it produces a much lower air pressure, as it uses an impellor that can't get to high pressures.
    Also if you do seal the hull be very wary about leaving it in the sun, as your boat has a relatively small air volume to surface area, and it could force the deck off.  

  3. # Anonymous Anonymous

    You should be able to use float bags from any kayak store. Put 'em in the hull, inflate them.

    --Kelly  

  4. # Anonymous Anonymous

    Greg,

    Why don't you fill it with some water?  

  5. # Blogger Giles

    Hi Greg,

    I agree that the easiest way to test is to use bags and to use a foot or hand pump to get pressure so you can do it on the spot. You can test by changing pressures and of course seeing how much power was required to maintain the same velocity. If you want to test with different displacements just haul it back to the peer measuring the force required to maintain constant velocity and load with different dead weights up to your own weight (live weight!). This way you can also test with and without propulsion in the water (can it be made to free spin?)
    Also if you want to see bow and side patterns can't you mount a camera on a pole looking down? On the still water you had there the effect of flexion between bulkheads should show up pretty clearly.
    Some suggestions anyways.
    Also could some clever person explain to me what the difference in drag is between what is effectively a bi-hulled vessel (one outrigger almost always in the water and one long main hull) and a potentially much shorter catamaran with the same displacement?
    - Giles  

  6. # Blogger fhe

    Why don't you fill it with packing peanuts or some other material? Air will move around being pressed out of the deep end and push on the shallow parts.

    I also doubt the deformation matters that much, but a test will be conclusive.  

  7. # Blogger biff

    The answer is simple, expanding polyurethane foam. Go to Rona, its called "Great Stuff" or "Mono Expanding foam"

    I have used it before to increase the strengh of a fibreglass steering wheel.

    If you want to get fancy, shape some pink foam to roughly fit the hull, spray some expanding foam in the bottom of the hull, then push the pink foam in and put a little weight on it (like 1kg on each section) then spray the more expanding foam. The foam has very small pressure when expanding so it won't deform your hull, when it hardens it will really increase the strenght.

    I assumed you already had a foam filled hull for rigidity and protection from sinking from a puncture. or I would have suggested this before. I don't think it is the final answer to all your speed problems, but probably a significant contributer. Also to buff the skin properly you probably want to increase its stiffness.  

  8. # Blogger "the Dude"

    Which cuts faster through dense viscous material, a dull butterknife or a serrated bread knife?

    If the bulkheads are equidistant apart approximately, the flow should be continuous along the hull unless the "divots" are really deformed inwards, in which case the turbulence is too much.

    The humpback whale's fins are serrated.  

  9. # Anonymous Anonymous

    Its got to be the prop!  

  10. # Anonymous PoiterH

    It might be the "nut" on the end of the pedals?  

  11. # Anonymous Anonymous

    My boat had no bulkheads the top deck was strengthened and to hold the air pressure. An aluminum U channel was used around the edge and the cross bar of the seat structure also was clamped to it. The deck was sealed with marine RTV stuff that I have used on my car radiator that has 15 lbs. in it. I would have to cut the seal to open the deck. It worked for years. I made the hull bottom as smooth as glass, a lot of sanding and buffing.  

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