Sail Area vs. Keel Weight

[Shane and Sheryl Wilson]

Last week, or even the week before, I asked about keel weight versus sail
area and received a couple great responses. Last Wednesday my hard drive
crashed and I lost everything. If you know the answer to the question could
you either repost or send me a message?

The questions were:

Is there a formula that determines the required weight of a keel based on
sail area?

Is there a variable for expected wind velocity?

Does locating the weight of a keel further from the waterline produce more
ballast?

[Rich Hampel]

there are several:

SAIL AREA / DISP RATIO = sail area/(disp/64)^.666 This is
a ratio of power to weight, calculated using a 100% jib. Most monohull designs
range between 16 - 18. Racers can be much higher, motor
sailors lower. We want a cruiser with enough power to sail well but not
so much
that the crew is fatigued by constant sail changes or
worried about having a fragile oversize rig. I selected 14 as the
minimum, 15 - 17
optimal, and 18 maximum.

VELOCITY RATIO = 1.88*lwl^.5*sail area^.33/disp^.25 /
(1.34*lwl^.5) The numerator of this equation is an empirical
formula that relates
high speeds to a long lwl, large sail area, and small
displacement. The denominator is the traditional Hull Speed term. Their
ratio is a
measure of how well our ideal cruising boat will perform
under sail. A well designed boat (adequate sail area and light weight
hull) will have
values are between 1 and 1.1. All out racing machines will
be as high as 1.8. I selected 1.0 as the minimum, 1.04 - 1.08 as
optimal, and 1.14
maximum.

[Marty Browne]

These are interesting questions. Usually naval architects and yacht designers
are interested in these questions, not us, the end-users.

I will restate these questions into the form that most of us, the end-users
might think about these things.

SAIL-AREA and DISPLACEMENT are connected because the SAIL-AREA is the
potential horsepower of the boat where DISPLACEMENT is the theoretical weight
of the water the boat displaces. (yup, and I can't explain the variable of the
identical boat in salt vs fresh water.)
There is a formula for computing the relationship of SA:D. I won't quote it.
But it produces a number, frequently in the teens.
A boat whose SA:D is ...14, should require a lot more wind to sail fast, than a
boat whose SA:D is 18. the higher the number, the more power or sail-area to
displacement.

The formula doesn't care if the weight is ballast, hull material, liquids such
as fuel, water, or beer, or humans. Weight is weight.

I don't know what you mean about "variable for expected wind velocity." I will
guess that you want to know that the force of the wind increases by the square
of the speed.

The lower the center of gravity, in a sailboat, the greater the effect of the
weight of the ballast as the lever to keep the boat from heeling over. For a
given weight, the deeper the weight, the greater effect that weight will have
in preventing heel. (But, under many circumstances, a boat sails better when it
is heeled over.) Center of gravity doesn't care about ballast or anything else.
Again, weight is weight and the center of gravity is based on weight.

Most boats have a preferred angle of heel. I did three MARION-TO-BERMUDA RACES
on a boat which liked to sail upwind, "rail down." That is about thirty degrees
of heel. I now own a boat that seems to sail upwind at 10-15º angle of heel.
Most boats, sail downwind best, at 0º angle of heel. Most boats get an
important part of their stability from their form. A designer creates the
stability for a boat by combining all the effects of the hull, keel, rudder,
mast height and weight, and windage of the above the water-stuff to create the
best design for the purposes within the mandate of the person or company
commissionning the design.

Now, stop thinking about keel weight and GO SAILING.

Shane and Sheryl Wilson wrote: