[MHml] Re: Diagonal Stability

[John Shuttleworth]

Big snip
>
>For the discussion of different proa configurations, and their comparison
>with
>cat's and tri's,  a footprint for, say, 20 kt, would tell us a lot about the
>boat's stability.  If you move the real c.g. (by moving crew or adding water
>ballast), the whole footprint just shifts with it.  Actually, the added
>weight of water ballast would shrink the curves some, but you get the idea -
>the shrinkage could be conservatively ignored for a first cut.
>
>The footprint is very easy to work out once you have the applied moments.
>And
>I don't see how anyone can design a boat unless you're willing to estimate
>those moments.  So I wonder why we don't see more of these footprint plots
>for different boats?
>
>Cheers,
>
>--
>Tom Speer

Yes of course I do this type of calculation, and I do provide this
information to my clients, but I think the diagram you show is too hard to
understand for most people, and I prefer to give a stability number for
each reef point. This is the apparent wind speed that the the reef should
be put in. The percentage of actual static stability is reduced at each
reef point to account for the decreasing stability of the boat as it heels
in waves.

I define diagonal stability as the point when the whole displacement is
taken by one hull with the deck at the point of burying. The downwind
stability is the point when both hulls are taking the whole displacement of
the boat, with the bows about to bury.

In a heavier cruising design like the Aerorig 52 the diagonal stability
will be in the order of 15 % higher than the sideways stability, and the
directly downwind stability will be 23 % higher than the sideways
stability.

In a cruiser racer like the Shuttle 40, where the displacement is lower,
and the boat is likely to be sailed closer to the stability limit, the
diagonal stability is 38% higher than the sideways stability and the
downwind stability is 28% higher than the sideways stability.

These calculations are done for when the bow is about to go under. If the
bow does go under, then the hull drag goes up, and there is a component of
downforce on the deck right at the bow. This means that the stability will
decrease much more sharply when the bow buries on the diagonal and
downwind, than for the sideways stability where bow burying is not a
problem. So the 15% and 23% extra stability represent a reasonable reserve
to account for the bow burying effect.

Therefore I give the sideways stability figures for various sail
combinations to my clients, and I know that that includes a safety factor
for diagonal and downwind sailing, to account for bow burying. I also give
a guide to how much that stability should be reduced as the wind strength
increases and the effect of waves is taken into account. I also give the
following explanation about the relation of true wind speed to apparent
wind speed on various points of sail.

Tek 35. Fully loaded.
The wind speed at the first reef is 25 knots . Upwind in flat water you
will be able to sail in higher apparent wind speeds but if you bear away
suddenly the boat will be overcanvassed.

On a reach this is apparent wind. Again you will be able to sail with
higher wind speeds because the sail is eased off. The stability number
represents the wind that would capsize the boat if the sails are sheeted in
hard and the wind is dead abeam. Also waves are taken into account in
relation to the sea state at the wind speed. If you are in flat water you
can keep full sail up until the apparent wind speed is 29 knots.

Sailing downwind the apparent wind will be less than the true wind. Keep an
eye on the true wind speed and bear in mind that should the boat stop, by
burying into a wave, the wind speed will increase to the true wind speed.
The safe sail area will be the amount of sail that the boat cannot capsize
with if it were sailing with the true wind speed. Bear in mind also that
downwind the main cannot be depowered. If you are sure that you can release
the jib, or the spinnaker then the max wind will be determined by the
mainsail area. However the boat is more stable fore and aft than it is
sideways, so as a good guide the static stability  of 29 knots true wind
when sailing downwind with full sail would be very safe, until you are sure
of the way the boat performs, and your own ability to judge that.

Having said all this I think it is likely that you will reduce sail sooner
than this on a reach. The performance figures show that in 25 knots of wind
you will be doing 17 knots. That will probably be too fast in the open sea,
particularly if you are cruising, and I would think that when you are
cruising you would reef at about 14 knots boat speed which will be at 18
knots apparent wind.

Regards

John Shuttleworth