SA/D ratios

[Kukri]

I have got myself into a muddle trying to work this out.

Sail area in square feet, squared
____________________________________
displacement in pounds/64, cubed

???

I keep getting silly answers!

 

[michael_w]

Try this one.

 

[Kukri]

It’s the 2/3 that gets me. I cannot make it work so as to give numbers between 10 and 25

 

[michael_w]

Try here someone might have done the sums for you!

 

[stranded]

Try here someone might have done the sums for you!

Not sure that is very reliable - puts our Sunbeam 44’s SA/D at 23.81, which Wikipedia says puts her in the high performance racing yacht class - love her to bits, but racer she ain’t.

 

[Roberto]

I have got myself into a muddle trying to work this out.

Sail area in square feet, squared
____________________________________
displacement in pounds/64, cubed

???

I keep getting silly answers!

It should be sail area in squared feet / displ ^2/3
That formula gives a (sort of) adimensional ratio, l^2 on the numerator, (l^3)^(2/3] --> l^2 on the denominator

 

[Laminar Flow]

I have got myself into a muddle trying to work this out.

Sail area in square feet, squared
____________________________________
displacement in pounds/64, cubed

???

I keep getting silly answers!

Sorry, its not cubed, but to the 2/3 power - see Roberto's post try 0.66 0n your calculator.

Also, SA/D used to be calculated using all sail that could be carried to windward, i.e, a 150% genny. Since the days of furlers, as headsails have gotten smaller to enable more effective reefing , it is now calculated using the fore triangle only (J and I values). This leaves older designs to appear under powered. I think one should use the largest truly reefable head sail for a fair comparison.

The SA/D and D/L data published is often thoroughly unrealistic as the displacement figures quoted are mostly and at the very best aspirational (I'm being polite here). The figures given in Sailboatdata are particularly unreliable in this respect. If one were to use real displacement figures a lot of contemporary "performance" cruisers would be a lot less spectacular, on paper at least.
In case anyone thinks that high SA/D numbers are a recent achievement: Colin Archer's yachts consistently had SA/D numbers in the low 20's.

 

[AntarcticPilot]

Surely the Swallows, Amazon's and Dee's ratio is 4:2:2?

 

[Kukri]

Not sure that is very reliable - puts our Sunbeam 44’s SA/D at 23.81, which Wikipedia says puts her in the high performance racing yacht class - love her to bits, but racer she ain’t.

Mine comes out as 16:1 and 23:1 in the same page in Sailboatdata. I think the 23:1 relates to a lightweight alloy “racing sled” version, which won the Sydney Hobart, shortly after the dinosaurs met the meteorite, and the 16:1 takes the 100% foretriangle and the stability book departure displacement.

 

[Kukri]

Surely the Swallows, Amazon's and Dee's ratio is 4:2:2?

4.5:2:2 - remember Bridget!

 

[michael_w]

Nah!
2 - 4 - 3 is the winning ratio

 

[Kukri]

Nah!
2 - 4 - 3 is the winning ratio

It’s 1:1.61803398875! See also Fibonacci…

 

[Kukri]

What I really need, for the purposes of a controversy in another place, are the SA/D ratios of …

a) the Cutty Sark . Sail area 32,000 sq ft, laden displacement 2,100 long tons

and:

b) The Preussen. Sail area 73,260 sq ft, laden displacement 11,150 long tons.

 

[michael_w]

With ships doesn't it depend on what cargo they are carrying? From what I've read, The Master could push the ship harder if he knew the cargo wouldn't shift. i,e, timber, bales of tea, sacks of grain, when compared to coal, nitrates and ballast? Steel hulls and masts helped too.

 

[Kukri]

With ships doesn't it depend on what cargo they are carrying? From what I've read, The Master could push the ship harder if he knew the cargo wouldn't shift. i,e, timber, bales of tea, sacks of grain, when compared to coal, nitrates and ballast? Steel hulls and masts helped too.

I think that is certainly true, because at the time when these ships were trading the modern self trimming bulk carrier hold with hopper and saddle shaped ballast tanks had not been invented. I think both ships would have relied on shifting boards for bulk cargoes (the Cutty Sark was not above carrying coal, whilst tramping!)

The Preussen was all steel with all steel masts yards and rigging; the Cutty Sark was framed like an iron ship of the day but with wooden planking to permit copper sheathing, and with steel masts and lower yards. Both ships will have had steel wire rigging.

 

[Laminar Flow]

What I really need, for the purposes of a controversy in another place, are the SA/D ratios of …

a) the Cutty Sark . Sail area 32,000 sq ft, laden displacement 2,100 long tons

and:

b) The Preussen. Sail area 73,260 sq ft, laden displacement 11,150 long tons.

Cutty Sark: SA/D 19.86
Preussen: SA/D 15.10

Average speed of a clipper ship (Cutty Sark) 6kts, average relative speed 0.38
Average speed of the Preussen 7kts, average relative speed 0.35 ( Marchaj, Sail Performance)

Average relative speed of sailing yachts 0.9, i.e. yachts are on average over two times faster than the clipper ships.

 

[Kukri]

Cutty Sark: SA/D 19.86
Preussen: SA/D 15.10

Average speed of a clipper ship (Cutty Sark) 6kts, average relative speed 0.38
Average speed of the Preussen 7kts, average relative speed 0.35 ( Marchaj, Sail Performance)

Average relative speed of sailing yachts 0.9, i.e. yachts are on average over two times faster than the clipper ships.

Thank you very much indeed! In a shipping industry publication, I described a proposal for a “cargo carrying sailing ship” intended to maintain 11 knots on a Transatlantic service, as “an exercise in greenwashing”, as the SA/D is 10, with a ro/ro (huge above the waterline) hull. The promoters say they will save 90% of emissions. I very much doubt that, and think this is a diesel ship, prettied up with masts and sails.

 

[Laminar Flow]

Thank you very much indeed! In a shipping industry publication, I described a proposal for a “cargo carrying sailing ship” intended to maintain 11 knots on a Transatlantic service, as “an exercise in greenwashing”, as the SA/D is 10, with a ro/ro (huge above the waterline) hull. The promoters say they will save 90% of emissions. I very much doubt that, and think this is a diesel ship, prettied up with masts and sails.

To be sure, anything below a SA/D of 13 is a motorsailer. The best average speed on passage for the Parma ( one of the famous Flying P line) was 9kts (Best Passage).
Not even yachts, given the general propensity towards motoring when keeping a holiday schedule, manage a 90% reduction in emissions.

 

[Kukri]

To be sure, anything below a SA/D of 13 is a motorsailer. The best average speed on passage for the Parma ( one of the famous Flying P line) was 9kts (Best Passage).
Not even yachts, given the general propensity towards motoring when keeping a holiday schedule, manage a 90% reduction in emissions.

Thank you.