T&T: Steadying sails

[Lawrence Zeitlin]

On Oct 2, 2008, at 12:00 AM, Marin wrote:

> First, given the
> typical boom and mast dimensions and hardware configuration of a
> trawler like a GB36, you'll be lucky to carry as much as 30 or 40
> square feet of sail.  Second, the semi-planing hull with its wide,
> flat after-body and hard chines doesn't have the long, swoopy roll
> you'll get in a round-bottom boat like a Willard.  So there will be
> less movement of that small sail through the air to generate any sort
> of dampening force.


I agree with most of what you have said. Many semi-planing hulls are  
poorly suited for sail because the mast is located too far aft and  
they neither have enough keel area or rudder area to maintain control  
with the sail's center of effort located so far from the ideal position.

But I fear that you have misunderstood the thrust of my post.  
Steadying sails work best when there is considerable wind pressure  
against the sail. This is typically the case when motoring in a beam  
wind with the waves and the wind coming from the same direction. In  
this case the steadying sail tilts the boat to leeward and inhibits  
the return to windward. In a 20 KT beam wind, the wind pushes against  
a 100 sq. ft. steadying sail with about 200 LB of force. If the center  
of the sail's effort is 15 ft. above the waterline, the lever arm  
provides a force of 3000 lb. resisting the rolling motion.

In the absence of a beam wind, if you want to use a steadying sail as  
a large fan blade whose aerodynamic drag damps the boat's roll, you  
will need a much larger sail or a very tall mast. Certainly a 30 sq.  
ft. sail will not do it on a GB. Air is a pretty tenuous medium. Water  
has 800 times more density. If you want to damp the roll, put your  
small steadying sail under water. But that's a flopper stopper or a  
paravane - another topic entirely.

Larry Z