Dec 18, 2012, 04:32 PM
Registered User
Joined Dec 2012
17 Posts
Question
drop keel

Hi, I am scratch building a 43" shallow "V" hull sail boat, she is 12" across the beam. My drop keel will be aluminum flat bar of 2" x 3/16" x (L ?) any suggestions as to how deep the keel should be or how much weight I should attach? I am planning on using a torpedo style ballast. Is there a formula for determining these things?

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 Dec 18, 2012, 04:43 PM Useful Idiot Asturias, Spain Joined Mar 2001 3,522 Posts The area and height of the rig should be mentioned but the boat is obviously for relaxed sailing with probably a smallish set of sails. I'd go for a fin no longer than maybe 8-9". The weight is more a question of the designed water line. Put a weight in the hull equivalent to the rig, the radio etc. and do a flotation test in the bath to see how much additional weight is needed and where.
 Dec 18, 2012, 04:47 PM Registered User Canada, BC Joined Jul 2011 408 Posts Good luck with your project. It is similar in size to the Star45 class boat and that weights 12 lbs, of which about 10 lbs is in the ballast. See the Star 45 forum http://groups.yahoo.com/group/Star45/ Here is a reference for bulb calculations http://www.onemetre.net/Design/Bulbcalc/Bulbcalc.htm I would recommend against just a flat plate for the fin. The hydrodynamics are really bad. I suggest that you plan to glue on two balsa cheeks and sand them to an airfoil shape, then cover with fibreglass cloth and epoxy resin. John
 Dec 18, 2012, 06:14 PM Registered User Joined Dec 2012 17 Posts Thanks Martin, I still have to decide on a fixed keel or drop keel. I'm building by guess & by gosh and was hoping to eliminate some of the guess work. Dave
Dec 18, 2012, 06:29 PM
Registered User
United States, MA, Wenham
Joined Jan 2007
1,193 Posts
Quote:
 Originally Posted by martin richards The weight is more a question of the designed water line. Put a weight in the hull equivalent to the rig, the radio etc. and do a flotation test in the bath to see how much additional weight is needed and where.
Good luck.
 Dec 18, 2012, 09:09 PM Registered User Joined Dec 2012 17 Posts Thanks, I sort of figured it would come to that...Gramps1
 Dec 19, 2012, 08:16 PM Registered User Joined Jan 2006 548 Posts flat plate keels ie less than 1 or 2% thickness can work well in the Re numbers our models deal with. Problem is 3/16 is pretty thick and 2" is too narrow for the boat you are doing. You need to be more in the 4" wide range and .090 thick. A star 45 fin is 6" wide at the top, 4" at the bottom and about 11" deep for reference.