Designing part for 2" to 8" cut plate... how to keep cost low?

[jinboston]

Background:


I am designing a steel keel for a sailboat. I want to cut a hydrodynamic shape - an airfoil. I plan to do this as a 2D shape in plate steel. I am aiming for 1200 - 1500 lbs of steel, and *rough* dimensions of 90" (X) 8" (Y) and 4" - 8" thick (Z) It will be epoxy coated for corrosion resistance.

Obviously I'm thinking CNC plasma or oxy flame cutting.


I want to have this cut out (and maybe welded together if stacked) for a reasonable price. I could make this out of various combinations of steel thicknesses.


The question:


Any suggestions on thicknesses that are the least expensive to buy and cut?


I know plasma can go to 2" thick.


It seems plate steel over 6" is rare. 3" plate steel seems much more common. Is oxy cutting two 3" plates cheaper than one 6" plate for the same profile?


Should I limit the job to 2" so I can get quotes from shops that have either plasma or oxy flame?

Will the cost of the steel or the cost of the cutting dominate the total cost?


Thanks for any suggestions you can give!

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[vmax549]

Having it CAST would be the way to go. (;-) TP

[peteeng]

Hi Jin - Having been involved in yachts for some decades I'd advise to be very careful with welded keels. They fatigue and welds fail and keels fall off more often then you think. When they do fail it can be catastrophic. Casting is a good idea.... Peter

[jinboston]

Peter,

I agree that fatigue would normally be an issue. But this is for a sailboat I am converting to a displacement speed electric powerboat. There won't be a sail. There will be some wind load from the canopy / panels but nothing like a sail. There will be *very little* dynamic or side load on the keel. Some options I'm looking at include welding on only some of the weight so it wouldn't be catastrophic if it did fall off, or bolting it on.

-Jeff

[routalot]

I agree with peteeng that casting is a good idea and ,I suspect,very likely less expensive.How about having an accurate pattern CNC cut in wood and then cast?Not only would you get a better surface finish and fidelity to the design,but you could core the bolt holes and no have to drill them.

[jinboston]

I will give casting another look.

In the meantime I've used some online steel supplier pricing systems to find that there can be a huge price difference (per pound) in different thicknesses. So far 4" plate looks the best on a per pound basis.