manlyp
Member
How do I calculate the ratio?
Sail Area : Displacement Ratio
- Thread starter manlyp
- Start date
Salty John
Well-Known Member
There is a calculator here:
http://www.sailingusa.info/cal__sad_ratio.htm
Sail area is 100% foretriangle.
http://www.sailingusa.info/cal__sad_ratio.htm
Sail area is 100% foretriangle.
aitchw
Well-Known Member
Just used this on my little tub and got 14.4 which it described as a cruiser/racer. Very funny as she would have difficulty outpacing a Topper /forums/images/graemlins/grin.gif
Salty John
Well-Known Member
Maybe you're sailing a wolf in sheeps clothing!
aitchw
Well-Known Member
I wish! /forums/images/graemlins/laugh.gif
Lakesailor
Well-Known Member
I tried it and got 18.99 which is exactly what the specs say.
snowleopard
Well-Known Member
24
Who needs a lead mine anyway /forums/images/graemlins/smile.gif
Who needs a lead mine anyway /forums/images/graemlins/smile.gif
bumblefish
Well-Known Member
Elizabethan 30 with furling headsail, 17.39, sounds good to me with a capsize formula of 1.86!
AngusMcDoon
Well-Known Member
My son's DF920 37.2 (not including asymmetric). Scares me!
rwoofer
Well-Known Member
I've noticed a number of boat specs don't actually use 100% foretriangle, but full sail area.
Given some boats have self-tacking jibs and others have 150% genoas, this may not be a bad thing. As an example my father has a self tacking jib and a high sail area/displacement ratio, whereas I have a 150% genoa and a low SA/D ratio. I power up in much early wind strengths, the opposite of what the numbers suggest using 100% foretriangle. Boats are within 3 ft loa of each other.
Given some boats have self-tacking jibs and others have 150% genoas, this may not be a bad thing. As an example my father has a self tacking jib and a high sail area/displacement ratio, whereas I have a 150% genoa and a low SA/D ratio. I power up in much early wind strengths, the opposite of what the numbers suggest using 100% foretriangle. Boats are within 3 ft loa of each other.
Lakesailor
Well-Known Member
Can't quite follow that. Are you saying your ratio is numerically lower than your father's?
The % size of your headsail doesn't matter, it's the sail area versus the displacement. Specifying the headsail size doesn't take into account rigs designed as headsail or main driven. If you're saying you have a lower ratio perhaps even with your 150% headsail you still have less area or that your boat is heavier.
This formula page has been criticised before, especially for Hull Speed and Capsize Formula.
The % size of your headsail doesn't matter, it's the sail area versus the displacement. Specifying the headsail size doesn't take into account rigs designed as headsail or main driven. If you're saying you have a lower ratio perhaps even with your 150% headsail you still have less area or that your boat is heavier.
This formula page has been criticised before, especially for Hull Speed and Capsize Formula.
rwoofer
Well-Known Member
Following the correct formula (using 100% foretriangle for the sail area, typically I*J/2) means that my SA/D ratio is 16.1, whereas my fathers is 20.2. This would suggest that his boat would power up much earlier than mine (ie do better in light winds) when in reality the opposite is true. This all comes about because his can only have a self-tacking jib and so he cannot use more than 100% foretriangle.
If I use my full area for the 150% genoa, my SA/D is 20.2, which given the fact I would have a more efficient slot(greater overlap), fits with observed performance. That's why I think the incorrect use of the formula reflects reality better than the correct use of it.
So the direct answers to your questions are:
"The % size of your headsail doesn't matter, it's the sail area versus the displacement" - the 100% rule means that you should not use total sail area
"Specifying the headsail size doesn't take into account rigs designed as headsail or main driven." - The headsail size just determines how much extra area you have over 100% and therefore gives you a clue as to whether your boat with the standard rig can perform better than the formula suggests.
If I use my full area for the 150% genoa, my SA/D is 20.2, which given the fact I would have a more efficient slot(greater overlap), fits with observed performance. That's why I think the incorrect use of the formula reflects reality better than the correct use of it.
So the direct answers to your questions are:
"The % size of your headsail doesn't matter, it's the sail area versus the displacement" - the 100% rule means that you should not use total sail area
"Specifying the headsail size doesn't take into account rigs designed as headsail or main driven." - The headsail size just determines how much extra area you have over 100% and therefore gives you a clue as to whether your boat with the standard rig can perform better than the formula suggests.
Lakesailor
Well-Known Member
I think we are saying the same thing.
If your rig is designed to be foresail driven with a weeny main the formula is making you opt for an unrepresentative ratio.
If your rig is designed to be foresail driven with a weeny main the formula is making you opt for an unrepresentative ratio.