Join Date: Feb 2000
Location: Annapolis, Md
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more on old IOR boats
I want to start by saying that I am finding this to be a great discussion, and in many ways I think that, at least to me, this represents what is the best of what a discussion group should be. I do think that my comments were not as defined as they probably should have been and that has to the very interesting kind of grappling for a definition that seems to be taking place here.
I would like to start with my comment about plumb bows having more reserve buoyancy than a raked bow. Let me try to explain this with an example. I am suggesting that we visualize two boats with the identical deck plans, and the identical static buoyancy forward of the point of maximum beam, but one boat has a plumb bow and the other a ''destroyer bow'' (by which I mean the straight raked stems found on most IOR era boats). Both bows hit an identical wave. On the boat with the plumb stem, the forefoot of the stem will be in contact with the rising portion of the wave and will begin to feel upward forces causing it to start to rise at a point several feet before the stem at the deck is in contact with wave. The stem of the destroyer bow will only feel the lift of the wave several feet later, somewhere around the point that the waterline contacts the wave and it is only at that point that the bow starts to lift.
In the case of the plumb bow, as the boat''s momentum carries it into the wave, the buoyant volume represented by the flare of the topsides between waterline and the deck would represent the reserve buoyancy and that volume is beginning to lift the bow several feet before the volume of the topsides on the destroyer bow would come in contact with the wave. It is that difference in location of the reserve volume that gives the plumb bow its increased reserve buoyancy.
What complicates this discussion are a number of other factors which I think Jack alluded to. Modern race boats often have not only plumb stems, but also have minimal flare to their topsides. When you have a plumb stem and a plumb topsides there is less reserve bouyancy than is ideal. When you further couple that with the very fine bows on the more extreme racing types, you end up with a bow that is very intolerant of weight forward. That said, old IOR boats were far less tolerant of weight forward than any other type of boat that I have ever sailed.
Which brings us to other pieces of the puzzle. If we go back to our original example, the plumb stem would tend to have a sharper entry angle. This means a much more gentle collision with waves because while the bow starts to feel the lift of the wave sooner, the actual upliftinf force is acting over a smaller surface area, so instead of slamming into each wave the plumb bow tends to start to slice into the wave. That feeling of slicing into the wave at first gives the feeling of ''submarining'' into the wave, but the deck is less likely to slice through the top of the wave because ultimately there is more reserve buoyancy to prevent that per the above. To some extent it is that softer impact that gives the newer plumb bowed boats a more comfortable ride.
Here is where I need to abandon the model described above because as has been noted, newer boats have their center of buoyancy further aft than was popular during the IOR. When combined with a plumb stem angle of rotation when passing through a wave is reduced. I know this sounds a bit counter intuitive if you think that a plumb bow has greater reserve buoyancy but I can explain it this way. I think that we can assume that all other things being equal, the bow of a boat with greater reserve buoyancy would rise further out of a wave than a boat with less reserve buoyancy. (This is somewhat mitigated in our discussion because the reduced impact of the finer stem imparts less impact energy to push the upward than the fuller bow of the destroyer bow.) So while the bow might rise higher, the angle of rotation would be measured to an axis of rotation that is further away and so the actual angle of rotation is reduced. And because of the greater damping of the longer waterline length (mentioned by Phil I think), there is less of a tendancy for the pitch to continue as the pitching energy is disipated.
Anyway, those are my thoughts.