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Cast Iron Keel vs Welded Steel Box

Posted: Tue February 12th, 2013, 10:03 pm
by Creidiki
Apologies if this has been answered allready.

Doing feasibility study on building a new Waterwitch and while it does not seem too bad, the cast iron keel is proving to be the biggest questionmark. The times are such that small foundries which would serve backyard builder have died off and bigger ones either will not even consider casting a one-off keel or the price is out of this planet.

So, next best thing is welded steel box, filled with scrap metal bound by concrete or what?

Re: Cast Iron Keel vs Welded Steel Box

Posted: Mon March 4th, 2013, 8:04 am
by Kevan
I too have checked out this concept when reading George Buehler's book and contemplating a Riptide build (still on hold as I have bought another boat). I think though that I would chase the foundries a little harder and (with me providing the appropriate form in timber) get quotes for them to make the mold in thier sand, and in at their convenience in conjunction with their other work, cast it and put to one side with me to organise delivery. (Basically, make it as simple as possible for them!)

However if that isn't practical George B makes a good argument for the correspondingly less dense concrete mix version - in or out of a steel box. George's ballast keels are pretty rectangular in section though - the same width all along the keel - and he designs his boats to suit. Maurice preferred a more accepted "foil" design - wider at the fore and narrowing aft.

But, if you can make that shape issue work, George suggests the concrete is 250 lb/ft3 versus iron at 400 lb/ft3. With an eventide ballast keel at 2360lb, that is 6ft3 of iron, or 9.5ft3 of concrete mix. Approximating the plan surface area of the keel at about 8ft2, that would add about 7 inches to the proposed 10 to 12 inches. For me that is getting a bit deep and a marked departure from the plans. While it lowers the overall mass (by about 3 inches) it increases the wetted area and would have other impacts - longer bilge keels for example. You might consider internal ballast to reduce the volume/depth increase required - or make the keel a slight bit wider as well?

Needless to say (but I will!) this is a casual glance at the complicated issue of modifying a design, and you have to make your own assessment etc. Professional advice is always warranted. Happy to discuss further. Cheers