Catamaran Comparison (SA/D vs D/L)

[Heath68]

How can one work out where the Neel 51 would be or even the yet to be launched47.? They say its "fast" but i have no context to place the claim in.

[Escape_Velocity]

Really interesting to learn about Lwl/Bh, but the problem I have with Multihull Dynamics is I cannot be sure they're numbers are right. Take, for example, the Helia 44 (which you have in your chart too). If you look it up, according to MD the SA/D ratio is a weak 17.3 whereas my calculation is strong 23.8.

It looks to me that they are wrong. They report a "displacement" of 31,382 lbs and a "sail area" of 1,076 sq/ft for the Helia 44, whereas FP's website specs claim a displacement of 21,600 lbs and a sail area of 1,237.9 sq/ft. Likewise, this article from Sail magazine reports a Helia 44 displacement of 23,800 and sail area of 1,230 sq/ft (those were the numbers I selected since they were more conservative).

There seem to be similar discrepancies on the site for other boats and it doesn't have the latest models. In addition, the formula for calculating Lwl/Bh is, as they admit in their definitions, an "approximation" unless it's reported by the boat designer. I've no idea if this approximating is accurate, but given the other discrepancies, I'm leary of it.

Anyway, I appreciate where you're coming from and am learning from this exercise. I hope you got your car safely unloaded!

p.s. I know MD limits its calculation of SA to main and jib, but that still does not account for the difference.

[Cadence]

Originally Posted by Escape_Velocity

Really interesting to learn about Lwl/Bh, but the problem I have with Multihull Dynamics is I cannot be sure they're numbers are right. Take, for example, the Helia 44 (which you have in your chart too). If you look it up, according to MD the SA/D ratio is a weak 17.3 whereas my calculation is strong 23.8.

It looks to me that they are wrong. They report a "displacement" of 31,382 lbs and a "sail area" of 1,076 sq/ft for the Helia 44, whereas FP's website specs claim a displacement of 21,600 lbs and a sail area of 1,237.9 sq/ft. Likewise, this article from Sail magazine reports a Helia 44 displacement of 23,800 and sail area of 1,230 sq/ft (those were the numbers I selected since they were more conservative).

There seem to be similar discrepancies on the site for other boats and it doesn't have the latest models. In addition, the formula for calculating Lwl/Bh is, as they admit in their definitions, an "approximation" unless it's reported by the boat designer. I've no idea if this approximating is accurate, but given the other discrepancies, I'm leary of it.

Anyway, I appreciate where you're coming from and am learning from this exercise. I hope you got your car safely unloaded!

p.s. I know MD limits its calculation of SA to main and jib, but that still does not account for the difference.

It's probably like any boat design, a good eyeball is the secret.

[BigBeakie]

Originally Posted by Escape_Velocity

Really interesting to learn about Lwl/Bh, but the problem I have with Multihull Dynamics is I cannot be sure they're numbers are right. Take, for example, the Helia 44 (which you have in your chart too). If you look it up, according to MD the SA/D ratio is a weak 17.3 whereas my calculation is strong 23.8.

It looks to me that they are wrong. They report a "displacement" of 31,382 lbs and a "sail area" of 1,076 sq/ft for the Helia 44, whereas FP's website specs claim a displacement of 21,600 lbs and a sail area of 1,237.9 sq/ft. Likewise, this article from Sail magazine reports a Helia 44 displacement of 23,800 and sail area of 1,230 sq/ft (those were the numbers I selected since they were more conservative).

There seem to be similar discrepancies on the site for other boats and it doesn't have the latest models. In addition, the formula for calculating Lwl/Bh is, as they admit in their definitions, an "approximation" unless it's reported by the boat designer. I've no idea if this approximating is accurate, but given the other discrepancies, I'm leary of it.

Anyway, I appreciate where you're coming from and am learning from this exercise. I hope you got your car safely unloaded!

p.s. I know MD limits its calculation of SA to main and jib, but that still does not account for the difference.

Yes EV, I hear what you are saying re MD. It does not seem to be as active as it was in the past, which is a shame, particularly wrt being out of date with newer designs.

I can say with some confidence that they do try and validate their data from multiple sources, or at least they did at the time I did this exercise. I know of 2 designers they spoke to at great length and detail, as one of them was the designer we chose. His impression was they knew about catamaran design, in detail and context. And at the time someone questioned their Helia Bh value, so I went and measured it myself, & MD were correct.

I can also say I spoke with a crane operator who lifts alot of different boats, and he checks his lifting weight calibration, and he told me not to believe any of the published weight data about common production designs, Outremer excepted. Often they are wildly "optimistic", so yes the displacement figures are a worry in this exercise.

Wouldn't it be a great thing if more owners asked for a weight certificate from the manufacturers?

But hull shape is something, as has been mentioned, that at least can be eye balled when these boats are out of the water, like at The Boat Works in Coomera. A very interesting & educational exercise.

[cwjohm]

Please let none of us suffer under the delusion that any of this is scientific.

First, Lightship displacement is specified as :-

the weight of a ship with all its permanent equipment, excluding the weight of cargo, persons, ballast, dunnage, and fuel, but usually including the weight of permanent ballast and water used to operate steam machinery.

but only in the case of commercial shipping is this taken seriously and the manufacturers play fast and loose with these numbers.

Second, the performance of any given boat will vary as you load weight depending on hull shape, displacement distribution, and drive characteristics either motor or sail.

Third, the type of drive potentials being motor or sail will have a bearing on the performance of the boat, the most common being the material used in the sailing wardrobe.

In regard to MD or any other data it is simply a matter of BS in gives BS out. We can make general inferences based on the results, and hopefully those inferences correlate to how any boat performs in reality, but none of it is really scientific.

This is a characteristicsof the pleasure marine industry in general. The only real data upon which anyone can rely is actual performance of any given boat or equipment given a variety of conditions. It stands to reason that the more instances you have the more reliable is the data, so we can speak with some assurity as to how a production boat which has been replicated many times will generally perform. Not so, necessarily, for any boat.

Yes, I can hear now the purists saying that Marine Engineering is a precise science and using historically acknowledged math and flow testing and so on and so forth the actual performance of the boat can be precisely predicted but I am calling BS on that as I have yet to see a Marine Engineer's Polar that actually represents reality and there are good scientific reasons why this is the case, and no scientific reason why they should be used in misleading marketing about any particular boat.

[cwjohm]

Originally Posted by Heath68

How can one work out where the Neel 51 would be or even the yet to be launched47.? They say its "fast" but i have no context to place the claim in.

I have tried and you cant, and talking to Eric Neel he states that based on his enormous experience sailing trimarans the expected performance is blah,blah, blah. All just marketing hype.

You can however, infer that it will probably have sailing potential beyond most private sailors and that it will perform well in light winds. Will you get performance beyond that of any other fast cat - possibly. Would this override other considerations like where in the hell do you situate this enormously wide beast - personal decision.

[Heath68]

Originally Posted by cwjohm

I have tried and you cant, and talking to Eric Neel he states that based on his enormous experience sailing trimarans the expected performance is blah,blah, blah. All just marketing hype.

You can however, infer that it will probably have sailing potential beyond most private sailors and that it will perform well in light winds. Will you get performance beyond that of any other fast cat - possibly. Would this override other considerations like where in the hell do you situate this enormously wide beast - personal decision.

I hear what you are saying and I found the 51 to be very wide.. but the 47 will be only 2ft wider than a Leopard 48...

[Heath68]

Does anyone know if in the real world you would notice any difference? I mean the St Francis 50 is missed on this but in reality do you think you would notice any difference in performance between say the Xquisite/Dean 5000 and a Knysna 500 and a Privilege series 5? I'm not sure if you would but I'm also not too experienced.

[Morph]

Hi, Have been reading this with interest.
My boat is a Pescott Whitehaven 11.7m catamaran. She also does not seem to fit with the numbers. Normal weight was 4500kg and probably 5000kg with full tanks. Hull/beam ratio was around 12.5:1, real length was 11.85m (that's where the cedar finished and the builder put the transoms there). Sail area is not huge, or rather not that out of proportion for a boat of this size.

Performance wise, however she is fast--and this is with old white sails. Normally we could do 10knots upwind at 42 deg (apparent) GPS speed. When I lived on her and she was down on her lines, that top speed to windward dropped to 8 knots.

Off the wind she was obviously faster with regular 12-16 knots.

This is still a bridgedeck cat with two queen berths, a double, full shower and toilet, galley etc and storage for 500 litres of water and 150 litres of fuel.

when I asked the designer why his boats sail so well especially against all the standard numbers his reply was something about the prismatic coeffecient...(I'm not really sure what this is and he wasn't about to enlighten me...)

Because she is a handbuilt boat built light of western red cedar and foam with carbon reinforcing on certain areas (there are about 13-14 of them in and around Australia and New Zealand), she doesn't fit with many of the commercially built boat's numbers. I have been urging the builder to start moulding her because it is such a good sailing boat--alas he has retired.



Certainly being a daggerboard boat makes a huge difference however, once you start loading with over 2000kg of 'stuff' the performance does start to drop.

Refering to one person's comments, once you take off the weight you still have a great sailing boat--this was exactly the case--once I moved off, we were back to normal speeds and would sail in windspeed from 3-4 knots upwards.
Cheers
Phil

[BigBeakie]

Good post and you've highlighted an important hull design point. There is MUCH more to performance hull design than just the simple numbers of SA/D, L/D, and even Lwl/BH, although that at least has something to indicate rough hull shape.

But shape is everything, as Mark Pescott was alluding to with his Prismatic Coeeficient statement. He designs slippery hulls, and that's why your boat moves so nicely in light winds & why you don't need a big rig and sail plan to do really good numbers in a fresh breeze.

The CP is how the hull shape changes shape from the pointy end, through the hull shoulder to the belly and then back up to the stern. Whether the laminar flow of water is smooth and transitions to a clean exit at the transom, or causes turbulence, and parasitic drag.

It is a beautiful thing in a good performance cat. You need less rig & sail area, you get more boatspeed with less, and probably get better sea kindly motion through chop and short swell when the hull shape is just right.

[Tornadosailing]

Interesting conversation and charts. My 2008 FP Salina scores pretty well using the light ship numbers, and actually sails reasonably well for a condomaran. SA/D 27.3 D/L 102.3 That's of course using the unrealistic light ship number of 23,900 lb. I suspect after the generator, air conditioner, dive compressor, full fuel and water, Yanmar 75's (instead of Volvo 40's), and a storage room full of tools and spares, I'm much closer to the Max Displacement of 31,960. I did find out a couple weeks ago that under 15-20 kt conditions an Outremer 45 is only slightly faster. I'll gladly trade the comfort of my boat for a few tenths in speed and a few degrees of pointing. We didn't get out the spinnakers, and I suspect I would have been in for a proper horizon job if we did, but my AC feels way better than his fan when the sailing is done.

[Just Another Sa]

On the subject of ratios like SA/D, L/D, and Lwl/Bh predicting performance,
https://eprints.soton.ac.uk/46442/1/071.pdf
would be source for understanding factors affecting hull resistance of displacement catamarans.
Since sail area and rig height do effect sailing performance, but not hull resistance, it is of course not even mentioned.
How does model 4a compare with 5c?
Or 5a with 6c?
Table 2 on page 27/84 gives the ratios of those models.
All are 1.6 meters long at lwl.
Lwl/Bh is 10.4 for 4a, 9.9 for 5c, and 12.8 for 5a, 11.7 for 6c.
lwl/Displacement^(1/3) given as L/D is 7.4 for 4a, 8.49 for 5c, and 8.51 for 5a, 9.50 for 6c.
For separation length ratio S/L = 0.5, realistic for sailing cat, resistance coefficients are given in:
figure 15d for model 4a on page 32/84, figure 20d for model 5c on page 35/84
figure 18d for model 5a on page 34/84, figure 23d for model 6c on page 36/84.
Max total resistance coefficients are approximately:
10.2 for model 4a, 8.6 for model 5c
8.8 for model 5a, 7.6 for model 6c.
Conclusions: 5c is wider for its length and have less resistance than 4a, and 6c is wider for its length and have less resistance than 5a, therefore Lwl/Bh does not dominate hull resistance, but lwl/Displacement^(1/3) clearly does as stated in page 16/84 heading 7.2.

Length/Displacement ratio was found to be the predominant hull parameter, resistance decreasing with increasing Length/Displacement ratio ...

That remains a fact for all kinds of hull shapes used in displacement catamarans, including rounded bottoms just as well as v-bottoms.

[Escape_Velocity]

Very interesting resource, thank you!

[cwjohm]

Originally Posted by Just Another Sa

On the subject of ratios like SA/D, L/D, and Lwl/Bh predicting performance,
https://eprints.soton.ac.uk/46442/1/071.pdf
would be source for understanding factors affecting hull resistance of displacement catamarans.
Since sail area and rig height do effect sailing performance, but not hull resistance, it is of course not even mentioned.
How does model 4a compare with 5c?
Or 5a with 6c?
Table 2 on page 27/84 gives the ratios of those models.
All are 1.6 meters long at lwl.
Lwl/Bh is 10.4 for 4a, 9.9 for 5c, and 12.8 for 5a, 11.7 for 6c.
lwl/Displacement^(1/3) given as L/D is 7.4 for 4a, 8.49 for 5c, and 8.51 for 5a, 9.50 for 6c.
For separation length ratio S/L = 0.5, realistic for sailing cat, resistance coefficients are given in:
figure 15d for model 4a on page 32/84, figure 20d for model 5c on page 35/84
figure 18d for model 5a on page 34/84, figure 23d for model 6c on page 36/84.
Max total resistance coefficients are approximately:
10.2 for model 4a, 8.6 for model 5c
8.8 for model 5a, 7.6 for model 6c.
Conclusions: 5c is wider for its length and have less resistance than 4a, and 6c is wider for its length and have less resistance than 5a, therefore Lwl/Bh does not dominate hull resistance, but lwl/Displacement^(1/3) clearly does as stated in page 16/84 heading 7.2.
That remains a fact for all kinds of hull shapes used in displacement catamarans, including rounded bottoms just as well as v-bottoms.

So I am trying to get my head around this. In a practical sense L/D obviously does not dominate over all conditions. You cannot make an 51
sail at windspeed at 35Kn AWS. I suppose you could argue that a better L/D number allows any given boat to sail at a higher percentage over rated hull speed comfortably. What that maximum number is I am not sure. For example we have seen fast cruising boats sail at over 20kn in flat conditions. Let us assume for the sake of argument than in average sailing conditions it is reasonable that a yacht with an excellent L/D number sails at 150% of rated hull speed comfortably.

The question remains as to how you effectively calculate the low wind speed performance of any given yacht given that hull speed has not been reached. Plainly the data given says that the hull performance is determined predominantly by the L/D parameter, but some heavier boats use larger sail area (e.g. catana) to overcome this.

My question is as follows. From this data we can assume that a yacht with a lower L/D will perform better in light airs irrespective of hull shape. What therefore is the relative effect of carrying more sail in such conditions. Put another way how would you calculate the performance of any boat in light conditions taking into account L/D and SA/D, assuming of course that the boat is equipped with manufacturers specification sails only.

[44'cruisingcat]

I would think that in light wind SA/D would be more important.