Sail area/Displacement #s

[Wpg3ut]

I have a question regarding different calculations for data that influences the potential speeds of sailboats. I have been on sailboatdata.com and Ocean waves.com, etc. and have compared several boats and the calcs that are shown on some of comps seem bizarre. I did a quick comparison between a Outremer 51, a Nautitech 46 and a Leopard 46. the disconnect is when I look at the SA/Disp calc and see that the Leopard shows a a score of SA/Disp# of 30.56 which means it is in the extreme racing category versus the 51 Outremer with a SA/Disp of 25.33 which seems perfectly normal in the blue water cruiser category. Sorry to be so naive but this is not correct right? When I look at Displacement #s for the Leopard 46 in other resources they are in the 37k to 42K displacement range. In Sailboatdata it states that dislplacement for the eopard is 24is this the fallacy in these #s or am I missing something? And yes, I know there are multiple factors that determine speed etc. but I am just trying to get my head around this one.

[donradcliffe]

Bad numbers. Leopard is on the heavy side.

[SailingHarmonie]

The formula and the numbers are very easy to find. Just do your own calculations.

Also, be really careful with these kinds of numbers. Not so true for SA/D but most of these were generated with monohulls in mind and don’t work well for cats. Make sure you do enough research you know you are using the right comparison.

[John_Trusty]

I didn’t check your numbers, but would be cautious with the comparison ranges, “...which means it is in the extreme racing category ...”. Those ranges were originally stated for keeled monohulls. You can use the ratios, such as SA/D or the slightly more inclusive Kersell (includes a factor for waterline length) when comparing multihulls, but ignore those ranges.

[Tin Tin]

When I type the L46 stats into the SAD calculator of 1593sq ft and lightship 24206 lbs = 30.46 If you use a realistic displacement of 14T then the ratio is 25.63. The fully loaded displacement is 17T and this results in a ratio of 22.51. The displacement numbers are from the owners manual.
This is using the sail area as sum of genoa and main. If sail area used is sum of main and fore triangle only then the SA would be 1388sq ft and this would change the SAD considerably.


The fully loaded displacement of an 051 is 13.7T and results in SAD of 22.67 At LS it is 26.52 on SA of 1389.

SA/D Calculator - see note at bottom of the site


The Outie will be faster because of LWL and to windward because of daggerboards and overall with finer hulls, hence lower load capacity.

[Tupaia]

Originally Posted by John_Trusty

I didn’t check your numbers, but would be cautious with the comparison ranges, “...which means it is in the extreme racing category ...”. Those ranges were originally stated for keeled monohulls. You can use the ratios, such as SA/D or the slightly more inclusive Kersell (includes a factor for waterline length) when comparing multihulls, but ignore those ranges.

I think you are referring to the Kelsall formula.



Spf = 0.5 x ((SA x LWL) / (D x 2240)) ^ 0.5


This give a Speed Factor indicating the expected speed on a reach in 10 knots. 0.5 = 5 knots, 1.0 = 10 knots (never understood why he never included a x 10 in the equation).


But this and other caluclations have many flaws. It doesn't take account of all sailing conditions. Sailing to windward requires good hydrodynamics (boards or keels). Wave interaction between widely or closely spaced hulls. Hull drag coefficients when downwind, wide sterns tendency to surf compared to narrow sterns less drag and faster when not surfing. The effect of large or small rudders to maintain control when running. Drag from fixed props, folding props or lifting outboards. Effect of hull flare on immersion, while catamarans sail upright for all practical purposes a small degree of heel can lead to very different hull displacement ratios (additional drag) for different hull shapes and the ability to resist flying a hull. Of course a racing cat may well aim for flying a hull as easily as possible a cruiser not so much. Hull rocker and the consequences it has on hobby horsing.


Lots of variations to consider. Helmsman, good boat lazy helmsman terrible boat attentive helmsman. Often a perfromance boat sailed badly is less rewarding than a simple boat sailed well. Sail trim, mast rotation, board position.


Wpgt3ut



The only real way to find out is to "sail" as many different cats as possible in all shapes and sizes not just the size you think you "want" and you will learn what aspects best suit your "needs".

[GordMay]

See:
Ted Brewer Presents A Primer on Yacht Design
The Numbers (More Than You Ever Wanted To Know!)
➥ https://www.tedbrewer.com/yachtdesign.html

“... SAIL AREA/DISPLACEMENT RATIO: The SA/D ratio is the sail area in sq. ft. divided by the displacement in cubic feet to the 2/3 power, or SA/D.667 .

Ratios below 14 are suited for motor sailers, from 14-17 for ocean cruisers and from 16-18 for typical coastal cruisers. Ratios over 18-20 are seen on racing dinghies, inshore racers and ocean racing yachts. The more extreme screamers can have very high SA/D ratios indeed; My 60 foot design, WILD THING, had a SA/D ratio, based on 100% foretriangle, of well over 30, depending on her displacement at the moment. Her displacement varied widely as she could carry 8,000 pounds of water ballast in tanks on the windward side ...”

And:
Carl's Sail Calculator ➥ Sail Calculator Pro v3.54 - 3200+ boats

“... Sail Area to Displacement: The sail area is the total of the main sail and the area of the front triangle. I cannot be sure that this datum was entered correctly for each listed boat. A racing boat typically has large sail area and low displacement. A number less than 13 probably indicates that the boat is a motorsailer. High performance boats would be around 18 or higher ...”

For Multi-hulls:
Multihull Dynamics ➥ https://multihulldynamics.com/

[Wpg3ut]

Thanks for the reply SH, much appreciated.

[Wpg3ut]

Thanks for the Detail TinTin.

[Wpg3ut]

Wow Gm that's moving! That's why I thought it was so odd to see a Leopard 46 with SA/D # @ 30+.

[Wpg3ut]

Thanks John for the links and detail. very helpful!

[JustMurph]

Generally, when comparing the performance of designs by the numbers, the first place I look is the DLR (displacement to length ratio). Make it longer and make it lighter. That will have a much much larger impact on performance than anything else. By a large margin.

The effect of SA/D really does depend a lot on the DLR, so isn't the best primary indicator of performance. Boats will a low DLR can get away with a small SA/D and still be very fast. A high SA/D would only improve light wind performance, as they need to reef early. Boats with a high DLR need the big SA/D to move and can generally carry that sail area into higher winds.

Next is waterline, which has a huge effect, especially on average speeds in real world mixed conditions. The Kelsall performance index is nice for balancing up all three, but does seem to normalise for ideal conditions. I found another performance calculation called "base speed" that puts more weight on the water line length, that I think gives more realistic numbers for an ocean crossing.

Hull shapes and all of the other factors trail a long way behind these, but are certainly interesting to understand!

[Wpg3ut]

Thank you Murph!
"Next is waterline, which has a huge effect, especially on average speeds in real world mixed conditions.
Isn't the waterline factored into the D/LR equation already?

[JustMurph]

Yes, but the water line length still has an effect independently.

For example, if you take a 40' boat and a 60' boat with the same DLR, the 60' boat will be faster, especially in mixed conditions. Eg, a Balance 580 has a worse DLR that my Mumby, but the 580 will be faster everywhere.

[Wpg3ut]

Thanks Murph! Is the point that the LWL on the longer of the boats will make it faster?