Need Help with Sponson design

Discussion in 'Metal Boat Building' started by finmanfish, Jul 10, 2014.

  1. finmanfish
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    finmanfish Junior Member

    hi, I'm looking for someone to help me design a small center sponson made from aluminum to improve flotation at the rear and provide a place to mount a large transducer on my 26 foot Glacier Bay Island Runner Catamaran power boat. If interested contact me at bitemwahh@gmail.com. Looking to add about 500 lbs of lift at the rear.
     
  2. yofish

    yofish Previous Member

    Just curious, why aluminum and not glass? Where are you thinking this added floatation will go?
     
  3. finmanfish
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    finmanfish Junior Member

    Easier to build. It will go in the rear between the engines. Similar to the typical assembly used to mount the engine on recreational pontoon boats. It should be something like 96 inches long 10 inches wide and 24 inches tall. It will mount to the main hull but protrude rearward between the engines about 24 inches beyond the transom. It shall be designed to provide needed flotation at the stern and provide an effective mounting location for an Airmar CM599 pocket mount transducer. I need help with the hydrodynamics of this to ensure a clean water flow over the transducer face and to provide some lift while underway. The Glacier Bay is considered a displacement hull and in my view the ride at speed would benefit from a little lift at the rear because these boats tend to be stern heavy. Many people put Doelfins on the motors or some even put hydrofoils across the bottom to do this so I think my thinking is sound. Thanks for the reply.
     
  4. gonzo
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    gonzo Senior Member

    It would be fairly easy to make the sponson adjustable. You could use it as a large trim tab.
     
  5. Hoodling
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    Hoodling New Member

    From your description of what the new addition (sponson?) will look like, I assume you tend to mount it mid tunnel, fixing it to the roof of the tunnel, right? In other words, of the total 8' length, 2' will be beyond the transom and the balance inside the tunnel with the top of the sponson fixed to the tunnel roof.

    From a structural point of view, the fixed portion measuring 6 of the 8 feet should be sufficient - assuming of course you make a good and secure job of the mounting. By this I also mean including beefing up the boat's tunnel roof to take the additional loading. You could also provide cantilevered compression struts from the top of transom to the top surface of the extension. Every little bit of added strength will help.

    I'm not a fan of doelfins, hydrofoils or any other underwater appendage - they simply increase drag. I have experience with the hydrofoil supported catamaran system, and have yet to be convinced it results in a good all round improvement to some boats which, in their own right, have good designs and performance anyway. Sometimes we try to achieve too much, and gain little.

    Your option seems a good one given the situation you are faced with - which is, an existing well built and designed boat, but with what in your mind's eye is a "heavy stern'; bearing in mind just about all boats with engines at the stern have a stern heavy feel and appearance. The Glacier Bay is a high speed displacement boat - achievable because the sponsons are slender.

    There is a considerable difference between your boat and that of pontoon boats. But, having said that though, what you want to do is doable.

    The only draw back I see, if it can be considered that, may be an increased air pressure at higher speeds in the boat's tunnel - because of the reduction in the cross sectional area of the tunnel, and a higher level of water running through that portion of the tunnel where you will have mounted your "third sponson", effectively making the aft section of your boat a trimaran. No problem with that - I already have an advanced design of this type of hull configuration, so watch this space.

    As for the "additional hull", my suggestion would be to duplicate the forward part of one of your boats sponsons (If I remember correctly they are symmetrical, right?); just a scaled down version of exactly what exists. The hydrodynamics have already been figured out, tried and tested. Make yours exactly the same, only proportionally scaled down.

    But, getting the same shape wont be achievable in aluminium I'm afraid - not without the right presses and bending machines. So, I'll suggest a very basic optional method that you may want to consider.

    Get a block of polyurethane foam and "shape" it, much the same as is done with hand shaped surf boards. Then coat it with layers of glass fibre (mat, cloth and woven roving), rub down and fair, with a top coat of flocoat (a gelcoat that air dries), matching the colour to your boat. The custom made surf board builders have been doing it this way for years. It's a bit of work, but it will look right and work well. I could go into more detail of how to do this if you wish, but for now, this is the basic idea of what I think you should do.

    Then, hollow out the inside of your new "sponson" and reinforce it with ribs to stiffen it up. The next step would be to make the "deck", or top, of your sponson; i.e. that part which will mate your new sponson to the roof of your boat's tunnel. This will have to be fairly substantial, and I'm not sure how you intended to do this in the first place, but presumably by bolting your aluminium sponson to the tunnel roof (which would also have to have been reinforced). There are other ways of doing this with your fibreglass version of course, but that's up to you after considering this as a preferred option.

    Hope this helps. Good luck.
     
  6. finmanfish
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    finmanfish Junior Member

    Wow ....thanks so much for your time and suggestions .... I will consider them all and post progress here. Great input!
     
  7. JSL
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    JSL Senior Member

    If the sponson 'fills' the space between the hulls it could act like a sea anchor or 'plug' and your performance could suffer. On a couple of occassions I have seen this happen on asymmetric cats (tunnel hull).
    Instead of more buoyancy could you try less weight?
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    Are the engines transom-hung, or sitting on elevated pods ? ( I'm assuming outboards )

    A possible option would be to add pods that are a continuation of the bottom, bolted on through the outboard mount holes. Work out what the engines weigh, and what extra bouyancy you would get with the add-ons, to figure out if it has prospects.
     
  9. yofish

    yofish Previous Member

    finmanfish, to be clear, is this the boat we are talking about? Do you have any good views or drawings of the hull?
     

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  10. finmanfish
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    finmanfish Junior Member

    JSL, the tunnel is about 24 inches wide and the new sponson plan would be for a maximum width of 8 to 10 inches and tapered in the front to cut through the water like the existing sponsons.

    Mr. Efficiency, That is an interesting idea basically lengthening the boat ..... Extending the engine riggings and cables might be a problem.
     
  11. finmanfish
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    finmanfish Junior Member

    Yofish, yes That is it. No I don't have a drawing but I can get pics.
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    I have seen a lot of power cats with outboards fitted with the Permatrim bolted to the cav plate, and some with trim tab-like appendages at the rear of the tunnel, which seems an unlikely solution given they are not acting on solid water.
     
  13. finmanfish
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    finmanfish Junior Member

    The permatrim only made mine porpoise allot so I removed them.
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    Porpoise when the engines were trimmed down ? That is odd. The tunnel width is not great, so placing a 'sponson" in there is going to create a choke situation, and certainly if you are hoping for 500 lbs of lift ! I recently saw a smaller cat that someone had decided to pod the outboards, ( raised pods ) and then fitted an unusual "object" in the tunnel aft that was aggressively "hooked", obviously to counter a problem that arose from the new engine placement. They had just wrecked the boat, in the name of gaining more internal space by extending the cockpit back into the area formerly occupied by the engine wells. The only thing that might have worked was the kind of pods I described earlier, but if the boat has a tendency to drop the bows in when running downhill, in its original set-up, that would likely just worsen.
     

  15. Mr Efficiency
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    Mr Efficiency Senior Member

    It is possible that a foil across the tunnel aft could be a solution here, but it would need to not foul the trailer (if there is one). The structural aspect of attaching safely to the hull would be something that need close attention, but it has the potential to give a lot of lift.
     
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