Area and volume

Discussion in 'Boat Design' started by Saqa, Jan 3, 2015.

  1. groper
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    groper Senior Member

    Just a data point for you saqa, pour foam in a 4lb density costs $1000aud per cubic meter. That's $3500 just for the foam in your 2 hulls. This is why you don't often see people building cat hulls like your thinking, it's too expensive.
     
  2. Saqa
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    Saqa Senior Member

    Thanks for the heads up mate. I been quoted $300 FJD which is around AUD $150 for drum kit by a local resin outlet at wholesale price. I need to work out how much the drum will cover and ultimately cost
     
  3. groper
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    groper Senior Member

    It's not just the cost...

    It's an inefficienct weight structure also. You need a high density near the skin for compressive strength and resistance to damage and high slamming pressures. Then you don't need any density in the center (it can be hollow) , but if you have the entire thing built from 4lb foam, it's a lot of material and weight for no structural merit.

    For comparison, a thin plywood hull, of your said 22.5m2 surface area, would weigh 43.875kg done in 3mm ply assuming a density of 650kg/m3. Only need a few frames and some glass tabbing, glass sheathing, you would probably pull up each hull for under 70kg at a rough estimate.

    The cost all this material to do both hulls would not even pay for your foam to do 1 hull...
     
  4. Saqa
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    Saqa Senior Member

    Yeah there are a few things I need to reconcile. A primary requirement is that it can take a beating. This is just as important as traveling through bad chop

    To put things into perspective, I have expanded my horizons quite a bit to two boat builds. I am awaiting on Jacques at Beteau and Richards Woods collaboration on my SOR, a few other guys have voiced their desire for an efficient cat and we are providing information on all that we would like to see on anything they design in this style. Thats the boat I will be building in either foam sandwich or honeycomb sandwich or a combination of both. The boat will be moored in good water and will be demountable for cyclone season. That will be the commercial charter boat

    That still leaves me wanting for a personal boat that is a paddock basher. Will be moored in bad water, shallow bay with barely enough water to float the boat at low tide and prolly get pounded on broken reef rubble during low water. Its at an entrance of a creek so will get beaten up by debris during the wet. I need a boat I can take out almost every day to work out new grounds so it still needs to slice chop

    I am getting close. You are correct, foam will be heavy so one of the things I need to reconcile. 123kg for the hull is not too shabby if I can smash it with an axe and have to bounce off. The foam can wait. The shell will have 4 sealed chambers so just hollow can be ok, if need be can still be foam filled by cutting ports to pour and weld back without too much pain
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    Getting regularly pounded on coral rubble, rules out just about anything except aluminium.
     
  6. Saqa
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    Saqa Senior Member

    HDPE will cope well, I have been bashing my old boat on that type of rubble, driving up it to ground it during low water. Mooney Mooney ramp as an example, the pontoon with ques waiting to get to their trailers, I would just drive up the rocky side and run get the trailer :) But seriously, I am playing with a few pieces of 6mm HDPE and it looks promising. The prewelded 90 degree seams are amazing
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    HDPE is too flexible to be suitable to make boats from sheet material, all the boats i've seen ( moulded ones) have a lot of ridges etc designed into it to stiffen it. Even then, they have a reputation for distortion and losing shape.
     
  8. groper
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    groper Senior Member

    Mate, hdpe is too heavy... Your boat will not perform with low horsepower and high speeds if it's heavy. This is what adhoc was trying to drum into you. You need an SOR, a list of what's important to you and the order of priorities. You then need to decide on compromises during the design because you simply can't have it all. Well, not without NASA's budget that is.

    If it's lightweight, it'll be good on fuel and require small engines, but it will be more fragile .

    If it's heavy it will use more fuel, bigger engines, but will have more toughness if designed with that in mind.

    You can't have both, unless your prepared to pay for super exotic materials such as Kevlar and carbon reinforcements and high strength foamed resins as core material - both of which cost a bomb.
     
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  9. Mr Efficiency
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    Mr Efficiency Senior Member

    Well, alloy certainly offers lightweight toughness, but as for cutting through chop comfortably at speed, not so much.
     
  10. Saqa
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    Saqa Senior Member

    Thats what I was talking about reconciling wants with achievable, maybe all I can end up with is a solid 10knots instead of the 15 I am aspiring too. It will still work for me in working new ground

    I am targeting Silver 29s dry weight. My hope is I can get that on hulls of similar lines. If I can make that happen ( lets assume it can happen), do you reckon I could look at performance numbers close to that if running the same power?

    Mr E, the sheet stuff is actually quite rigid. Once its bent and welded its even better and looks like I can pull two bends in it. I am sure what flexibility remains will cause drive losses but I wonder if it might be a good thing in the chop

    I currently have a very very narrow monohull that I put 3" of rocker. I made that for the same choppy conditions and I have been playing with wing foils too. I am getting a very comfortable motion under way and at rest but only under 15 knots and its not very good for standup fishing as its very tippy
     
  11. Saqa
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    Saqa Senior Member

    And the really exciting part!! New Saltiga coming out this year!!!

    [​IMG]010 by jonny.toobad, on Flickr

    I can finally finish making this rod as there is reel dressed for the occasion
    [​IMG]blue by jonny.toobad, on Flickr
    [​IMG]wrap by jonny.toobad, on Flickr

    I did the deco butt wrap then put it aside as life got in the way. GTs beware :D
     
  12. waikikin
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    waikikin Senior Member

    Looks nice Saqa, maybe thats what I need to convince the fish to bite my hook....
    Jeff.
     

  13. michael pierzga
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    michael pierzga Senior Member

    I used to make fishing rods when I was a kid. Never liked foam grips...cork always felt better in my hands.
     
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