In a message dated 9/28/01 4:02:48 AM, Burkhard Becker writes:
<< I intend to install a get home and anti roll device (in former times
called:
sail). Is there any rule of thump which indicates the area I need at least,
if I want to reach the next harbour in an emergency case? Something like
4.5KW/1t (6hp/2,222lb) as estimation of enginepower.
Burkhard Becker (D) >>
In the sailing community there is a rule of thumb that implies that 27 sq.
ft. of sail in a 20 kt. wind is equivalent to one hp. at the propeller. You
would need about 162 sq. ft. to meet your requirements. This is the amount of
sail that you would find on a 20' daysailer. I don't know what kind of boat
you intend to put the sail on but if it is a typical trawler it would be best
to keep the center of effort low and just forward of the point of lateral
resistance of the hull. That's the spot where you could drag the boat
sideways without it turning in either direction.
Now about the details. For simplicity I would suggest a lugsail. This type of
sail works best off the wind and actually develops more drive than triangular
jib headed sails. True, they can't point into the wind worth a damn but I
doubt that you will heading upwind with a sail rigged trawler anyway. I
suspect that the best upwind track you could achieve would be about 20
degrees into the wind. You would have the sailing ability of a 15th century
caravel - but Columbus made it across a large body of water in one of those.
Rigging would have to be quite heavy for the sail area since a typical
trawler weighs considerably more than a daysailer.
A sail of this size, or even a bit smaller, would work fine as a steadying
sail in beam winds. Sheeted tightly amidships it would damp roll almost
completely.
We covered a lot of this stuff in the past so check the archives. If you
seriously intend to use a sail for get home or steadying purposes, talk to a
naval architect, preferably one who designs cruising sailboats, since mast
placement and support are critical.
Larry Z
In a message dated 9/28/01 4:02:48 AM, Burkhard Becker writes:
<< I intend to install a get home and anti roll device (in former times
called:
sail). Is there any rule of thump which indicates the area I need at least,
if I want to reach the next harbour in an emergency case? Something like
4.5KW/1t (6hp/2,222lb) as estimation of enginepower.
Burkhard Becker (D) >>
In the sailing community there is a rule of thumb that implies that 27 sq.
ft. of sail in a 20 kt. wind is equivalent to one hp. at the propeller. You
would need about 162 sq. ft. to meet your requirements. This is the amount of
sail that you would find on a 20' daysailer. I don't know what kind of boat
you intend to put the sail on but if it is a typical trawler it would be best
to keep the center of effort low and just forward of the point of lateral
resistance of the hull. That's the spot where you could drag the boat
sideways without it turning in either direction.
Now about the details. For simplicity I would suggest a lugsail. This type of
sail works best off the wind and actually develops more drive than triangular
jib headed sails. True, they can't point into the wind worth a damn but I
doubt that you will heading upwind with a sail rigged trawler anyway. I
suspect that the best upwind track you could achieve would be about 20
degrees into the wind. You would have the sailing ability of a 15th century
caravel - but Columbus made it across a large body of water in one of those.
Rigging would have to be quite heavy for the sail area since a typical
trawler weighs considerably more than a daysailer.
A sail of this size, or even a bit smaller, would work fine as a steadying
sail in beam winds. Sheeted tightly amidships it would damp roll almost
completely.
We covered a lot of this stuff in the past so check the archives. If you
seriously intend to use a sail for get home or steadying purposes, talk to a
naval architect, preferably one who designs cruising sailboats, since mast
placement and support are critical.
Larry Z
