Cat Sponson Modification idea to improve fuel economy

Discussion in 'Boat Design' started by Bullshipper, Jun 3, 2015.

  1. Mr Efficiency
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    Mr Efficiency Senior Member

    Use some contact adhesive, if they fall off, use a stronger adhesive !
     
  2. Jim Caldwell
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    Jim Caldwell Senior Member

    Trade for a boat that fits your needs. Adding lift is not likely to increase economy as it will also increase drag.
     
  3. Bullshipper
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    Bullshipper Bullshipper

    The light weight cats in the 26' range that get 3.5 mpg run $140K USD. I have $27K spent on this one after 2 seasons, and my gas savings is less than $5K a year against an increased depreciation cost of $10K per year on the new hull and I am 64. They are so popular that there are NO used ones around for less than $85K.
     
  4. Bullshipper
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    Bullshipper Bullshipper

    So you completely discount the idea of a foils use on a heavy semi planning hull?
     
  5. groper
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    groper Senior Member

    i would consider installing HYSUCAT type foil on this boat.

    So a foil made of a steel type material mounted across the tunnel, usually with 2 stabilizing fins near the transom. You will have alot of reading to do if you do not simply pay someone to design it for you and you will also have to accurately find out the center of gravity of your boat somehow. Sottorf is a member here - he is one such professional contact in this field.

    The lift drag ratio of a planing hull is about 10:1 IIRC. The lift drag ratio of a decent high aspect ratio foil is at least 20:1. This means despite the extra drag of the foil, the total drag of the boat becomes lower as more of the lift is carried by the more efficient foils.
     
  6. Bullshipper
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    Bullshipper Bullshipper

    This symetrical hull design is not suitable for a foil in the tunnel imo. It would make it too unstable. That is why the angles are proposed for the outer chines of the sponsons, and serve the same purpose.

    But it appears that no one is willing to help with the lift calculation that they would create so we are still treading water.

    Hysucat has a light asymetrical 22 Cat with a foil and they claim they can get 74 knot. But they have twin 200's on that hull as opposed to the 115's on mine getting 36 knots. As I would be happy with a 25 mph cruise with better gas milage I am proposing more surface area on these angle to lift this heavier hull at a lower speed to decrease her draft and wetted drag.

    So I hope I havent given the wrong impression here. 3200 lbs for a stout tall hull with inner bracing is not out of line. But when you add 1000 lbs for the motors batteries steering and pumps, then 700 lbs of fuel, 100 lbs of ice the 300 lb tee top and three passengers, 5800 is the fishing weight yeilding 25% less gas milage and a deeper draft than advertised for the light boat test with a total of 4600 lbs with less wind drag without the top. The dry hull is only about 400 heavier than the newer technologies which is insignificant against a total displacement of 5800 lbs but these other hulls have wider chines and flatter planning areas to help lift along with 4 extra feet of length to draft less.
     
  7. Bullshipper
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    Bullshipper Bullshipper

    Pictured is a 263 Cacutta that gets about 3+ mpg loaded out. It is a slimmer design that rises more with its wide (think angle iron) chines. It is light cas it has no cross bracing. It uses a vacummed bagged 1.5" thick nida core deck as its primary structural backbone in place of the cross bracing.

    Light boat tests with no top push this hull over 40 mph with twin 90's.
     

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  8. Mr Efficiency
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    Mr Efficiency Senior Member

    Groper's idea of the foil in the tunnel has to contend with there being a lot of aerated water in there, the effects of which might make it a different propostion to one mounted in a box-tunnel. As for it making the boat unstable, that I would not be concerned about, and in any event the boat is supposedly well over-weight already. I'd go the groper foil if the highly aerated water isn't an obstacle, but I can't find much information about the effects of it on the net. But propellors don't work properly in aerated water, and a foil cutting through the water may lose lift, depending on how solid the water/air mix is.
     
  9. Bullshipper
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    Bullshipper Bullshipper

    I understand that Prof Hope did 100's of pull tests to get his foil/hull design right. I have ridden on one of his, a corsair, a Haas Spartain and an M1 that were unstable with their foils, and needed a lot more than 25 mph to make a difference on efficiency. I also felt that the forward placement of the foil held the boat down in a following sea, so as stated before a cross beam foil inside the tunnel is a no go.
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    I put a foil in the tunnel of a 21' box-tunnel boat I made, just one around the COG, and I did not gain a great deal of extra speed, although it did give noticeable lift, evident in the increased bank in turns. Obviously the drag of the foil was cutting into whatever extra speed came from the additional lift. There is no reason why a foil should make a cat inherently dynamically less stable, though, the inertial characteristics do not change, even if the COG is higher. But you have made up your mind to stay out of foils, OK, but in theory applying additional dynamic lift to two separated demihulls, which meet different conditions at the same time, is more likely to create dynamic instability, though your boat is both too heavy and too slow to encounter drastic problems as a result. No-one can give a figure for whatever lift you might get from the angle extrusions, it will vary with the angle of attack considerably, you will just have to suck it and see, but I'd say 3" wide would be about the max, and I would try just the two inside the tunnel to start off.
     
  11. Bullshipper
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    Bullshipper Bullshipper


    I stated a fixed 3 degree angle of attack to make the problem easier but I guess I will have to figure out that part by myself as no one has come forth.
    Really appreciate all the advice.
    Thanks
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    I mean the lengthways angle of attack, and the lift you obtain will vary a lot with that, and the effects of the hull adjacent to it, it has to be trial and error.
     

  13. Mr Efficiency
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    Mr Efficiency Senior Member

    I have seen a few contrivances inside tunnels of cats that were overweight, and underperforming, with pronounced hooks and the like built in to them, also large trim-tabs from the "roof" of the tunnel, none of these things appear to work, probably because what they are acting on is more air than water.
     
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