Stability - from whence?

Or should I have headed it can-of-worms?

While contemplating my om this morning I had a BFO about boat stability, righting moments etc.

I can calculate the righting moments resulting from a girt great lump of lead at the longest arm we can engineer in the box - relatively easy.

My BFO related to the fact that the stability of the hull itself (without a great keel/bulb) is very low indeed (by comparison). The numbers and righting moments from the hydrodynamics of the hull must be in the small range

Whereas the righting moments from the keel must be in the hundreds (if not thousands) of

Hence the ability to use circular x-sections with low inherent stability (they keep the same section as they roll) - cos the hull shape matters very little to overall stability.

Am I thinking right about this in general? Was my BFO valid?


Well, kind of. Having made a series of tubular hulls in my “Eel” design line in the M Class I can attest that what seems logical about your BFO is a simplification of the forces at work on the hull. I pursued the tubular idea to make skin friction constant on all angles of heel to see if there was any impact on acceleration, as I sail mostly in light winds here in NYC. I didn’t really achieve the spectacular results I had hoped for but I learned a bit about how a hull “rolls” in relation to the wind.
Simply stated, on different points of sail the force exerted on the hull via the wind in the rigging applies both forward and downward pressure on the hull. As a boat moves forward through the water it must push some of that water out of the way and as the wind moves more forward in relation to the direction the boat is headed the hull will drift a bit sideways as well (“side-slipping” or “crabbing”).
If the motion that a hull goes through when sailing on different points of sail were confined to rolling around a fore-and-aft axis then everyone would be sailing a tubular boat. Because hulls yaw as well as roll a lot of care must be taken to control as much as possible the movement of the various centers so that the boat “balances” as it sails. Tubular hulls need some finessing to achieve balance. They are also more practical for longer canoe body type boats like the M Class where the displacement is distributed over 50 inches, not 12.

Niel, thanks.

I realise that I was thinking and BFOing entirely in the roll axis - and all of what you say is applicable to the real world.

My (simplistic) view seems to me to explain quite well much of the movement towards lean drainpipe boats - and I found this very satisfying.


Perhaps my Litefoot is in the very forefront of modren thinking! Normally I cling on to outmoded things until they come back into fashion.

marginally stable in all axes