Before the whole list goes off on an episode of Bernie-bashing let me say
that Bernie's "calculations" look quite sound to me. Hard chines maximize
initial stability but as the boat approachs the horizontal, they suck. Power
boats have then because there is little heeling force and what they need is
maximized initial stability. Offshore powerboats will have a deeper vee and
less pronounced chines so that they reduce the chance of being capsized due
to wave action.
Stanley Smith knew this and he says again and again that the Potter is to be
sailed at very modest heeling angles on the order of 15 degrees. This is not
at all unusual in dinghies since most are built, even those with rounded
chines, to have high initial stability at the expense of ultimate stability.
You never seem to get to have your cake and eat it too. Rats!
I wish I had a reference handy but since I get most of what I read from the
public library I don't have a large collection of my own - anyway, I've seen
several books that show how a stability diagram is derived. Basically this is
a plot of the righting moment vs. angle of heel. In order to have one that is
always positive, the cross-section of the boat will be very round and there
will be a big chunk of ballast located some distance from the centerline.
Boats of a more practical shape have a portion of the graph which will dip
into the negative - that is to say that at some point as they are being
rolled they will prefer to be upside down. Boats that are not "self-righting"
may reach this crossover point before they have even-heeled to 90 degrees.
I've never seen a stability diagram for a Potter but I picture something
like this:
The curve rises very quickly and reaches a "soft" maximum spanning the 30 to
45 degree range and then slopes back down crossing the zero point at about
105 degrees ( 15 deg. past horizontal) and remains negative (wanting to
turtle) from that point on. Thats for the boat - as we have all observed,
where the skipper and crew situate themselve in a P-15 effects things quite a
bit. That's where the 15 degrees of heel comes in. Picture the boat heeled
about that amount , one side of the hull is carrying the weight and the
leeward chine is digging in. The center of buoyancy has moved way off the
center line and is probably just past the midway point between the keel and
the chine. The weight of the centerboard now has a lever arm to to pull
against the center of buoyancy but it is not great when compared to the
weight of the skipper (and crew?) sitting to windward who have a longer lever
arm and a lot more weight. As the boat heels further, the centerboard becomes
more effective while the crew's lever arm starts to get shorter. When the
boat reaches 80 degrees or so the crew are situated directly above the center
of buoyancy (if they haven't been tossed into the water by now) and have no
leverage at all, the centerboard, nearly horizontal, is providing the
righting moment. But, at only 70 lbs., we have far less righting moment than
we did back at 15 degrees, right? Hence Stanley's advice.
I'll probably live to regret this next remark, but the P-19 is going to have
a "better looking" stability diagram since it's ballast to displacement ratio
is greater. With almost 400lbs. of daggerboard acting on a 1200 lb boat
(about 30%) compared to 75 lbs (or 100 depending on who you believe) acting
on a 475 lb boat (about 15%) the righting moment at angles approaching
knockdown is going to favor the P-19. On the other hand, the 19 has to carry
that extra weight everywhere she goes and that helps those of us with 15s
keep up despite our shorter waterlines.
For those of you who aren't interested in this stuff, I apologize for
burdening you with it. For the rest, I hope sharing what little I know about
this helps clear things up.
Dave Kautz
P-15 #1632 Tilly Lucy
Palo Alto, CA