the concept of unsinkable yachts
With all due respect I think that your post is way too much of an over- simplification and in some ways is a bit misleading in some of its apparent implications.
For example it is a bit of a non-sequitor to say that "You don''t have to be a rocket scientist to understand that any pressure at the bottom of the keel translates into a lot more pressure at the top with fin type keels. It is all to do with leverage"
and then to assume that means:
"Maybe it is time ETAP too considered longer keels";
The reason that I say that is that in most long keel boats the ballast keel of the boat is placed at the leading edge of the keel and for any given hull form the ballast keel itself rarely extends any further aft of along the keel than it would in a fin keeler. That happens because in designing two boats of equal design except for the keel, there is only one right place to place the ballast and that is set by the 'statics' of the boat.
In a frontal impact on a long keel, given the rigidity of the ballast keel, the ballast keel itself tries to rotate up through the bottom of the boat, independent of the ''deadwood'' portion of the keel, with pretty much the same impact force that a fin keel generates as it tries to rotate up through the bottom of the boat.
In other words, a more accurate conclusion is that you might then argue that deep draft is a bad idea from an impact standpoint, but of course that would be counter to what is ideal from a motion comfort and seaworthiness standpoint.
And if puncturing of the hull is your primary concern then I would also caution against boats with encapulated keels because they are especially vulnerable to puncturing the watertight envelope in a frontal impact to the keel.
I also think that your statement "Remember steel has about 27 times more impact strength than fibreglass of the same dimension"; is also very misleading. That is only true if the dimension is thickness. If the dimension is weight, that is totally eroneous.
To more realistically represent the reality of the situation, the statement should have said something to the effect of "Remember that pound for pound a moderately densely cored, vinylester resin glass hull with a minimum of non-directional material is roughly 8 times more resistant to puncture than a A36 steel hull, and a vinylester resin and glass sheathed cold molded cedar/fir hull is roughly 11 times more resistant to puncture than an equal weigh A36 steel hull."
It is often neglected in discussions of the relative strength of materials that the density of steel is so high compared to glass (roughly 4 times denser) or typical wood planking species (roughly 17.5 times denser), that steel plating and structure of equal weight is way thinner than either fiberglass or cold-molded construction, and so despite steel's superior cross sectional properties, steel is actually way weaker than the other two materials on an equal weight basis. Of course this last point somewhat complicates the discussion a bit because few glass or modern wooden boats are constructed with hulls of an equal weight to a steel hull.
Lastly, I also think that the discussion of full floation in a cruising boat is an extremely complex one. In my mind the real issue with trying to get full floatation is the weight and placement of that weight of the floation material, as well as the volume of the boat that is given over to that create that floatation. If sinking is so important to you then it probably makes more sense to look at the puncture issue rather than the floation issue. Using modern materials and careful engineering, it is possible to achieve puncture resistance at any impact speed that a sailboat is likely to sustain in collision at sea. If you really are concerned with seaworthiness then it would make a lot more sense to push towards engineering for a higher impact load resistant design on both the hull and keel. That can be achieved with less of a cost, weight and carrying capacity penalty than trying to achieve full floatation.
Last edited by Jeff_H; 04-16-2006 at 08:59 PM.