Deck sweeping sails and effective aspect ratio

Here are two examples based on the Spindrift dinghy mentioned earlier (a 10ft yacht tender with a 68sq ft cat rigged triangular sail, AR = 2.7). It is no racer but is reputed to be one of the better sailing tenders, so I will use it in its original configuration as my "good pointing" reference. It is the boat at the bottom of the attached image.
All values were calculated on the spreadsheet alluded to earlier. The aerodynamic drag - and lift - of the hull and crew was calculated with cfd.

As you will see, all the ratios that you mention as well as their significance in the Course Theorem have been considered and all loads are in equilibrium.

For the comparison case (the top boat), and in order to illustrate the benefit of a deck seal for boats with lots of parasitic aerodynamic drag and "cruising spec" keels, I made the following modifications:

• Keel aspect ratio decreased from 2.2 to 0.9.
• Keel area increased from 1.96 to 3sq ft.
• Hull and crew aerodynamic flat plate areas doubled
• No gap between sail and deck, i.e. entire sail assumed to have been lowered by 2ft to put the boom flush on the deck. Apart from that the sail is left unchanged.

True wind = 10kts. The small triangle at the bow represents the True wind, Apparent wind and boat speed vectors.

To eliminate any possible inaccuracies in my hull-vs-speed formula (based on Gerr and Wyman), the modified boat was pinched until the boat speeds were equal.

A detailed breakdown of all the vectors and other results are tabulated in the second image attached.
Just to summarise: the modified boat points 4.3 degrees higher than the original.

Some assumptions made to simplify the model:
Neutral rudder, i.e. no lift or induced drag on rudder.
Hull drag is not affected by leeway angle.
Zero heeling.

 

So Will, shown in your spread sheet result, if air L/D improves 6% and hydro L/D improves by 10%; exactly as I said, the boat points higher.

No one is going to argue against the fact that if you theoretically improve the hull and theoretically improve the sail, the boat will theoretically point higher.

That being affirmed, the pertinent question is about how much of the theoretical improvement is actually seen on the real boat.

A separate issue:
Maybe Tom Speer could step in here and explain how, in his spread sheet, the hull L/D performance got better when it was changed from a fin keel to a shallow full keel. Tom are you reading this?

 

Was searching for some other links to put in a private email to you (so as to not hi-jack your primary discussion), and I ran across these references that more directly apply to your present discussions:
http://www.boatdesign.net/forums/sailboats/sail-cfd-help-interperate-my-results-7123.html (but some of the links no longer work?)

http://www.boatdesign.net/forums/hydrodynamics-aerodynamics/sail-aerodynamics-457-3.html (are you familar with this rather lengthy subject thread?)

 

The example is to illustrate just how much the sealed gap contributes. If there still exists a 1" gap then the total aero L/D is 13% less than that of the original and it will point 1deg lower for the same boat speed.

The L/D ratio for the shallow keel itself is in fact a good deal lower - 10.6 vs 17.5 for the fin keel. The value in the table includes the hull drag. Note that the improvement of total hydro L/D is the result of the significant increase in magnitude of the loads on the keel. The contribution of the hull drag to that ratio becomes proportionally less.

One fault that I have detected is the aerodynamic lift component of hull and crew which increased when I doubled the flat plate areas. I have corrected it (reduced it from 3.9 to 1.9). It still points 3.2 deg higher.
The increase in magnitude of the sail lift component - over and above the increased L/D of the sail itself - also goes a long way to overcome the additional hull and crew drag.

Just for what it is worth: the L/D of the sail itself improves from 5.7 to 8.4.

 

Yes Brian, I remember it took me quite a while to work through that lengthy thread but it was well worth it.
I have learned a lot by just browsing the comments of some of the more knowledgeable forum members and still consider myself a student.

 

Thanks for the interesting report. I finally got to the part where it mentions the deck generated vortex -

"These figures show that a vortex generated by the deck edge which increases with heel, but that doesn't affect substantially the flow under the boom... (pg 25)

I agree that it must have at least some influence, most likely as you point out because of the opposite direction of rotation. The report however is clear about it not being a significant effect.

 

To have a good idea of induced drag and effective lift coefficient, I use the basic formulas with Oswald coefficients (available at Tom Speer home page).
For an A-cat the induced drag dropped from 44 Newtons to 27 Newtons, and the Effective lift coefficient from 0.74 to 0.84.

Hope it can help

Regards

EK

 

I still consider myself a student as well ...and even more so as my age has begun to cloud my memory somewhat. Glad I posted some of my postings when I was a little more clear headed. Here's one I just found:

 

BTW, here is just a brief example of that 'criminality' I referred to with modern cats placing their mainsail booms so high in the air....and this is just a few of them

 

Yes, I know the theory. The other jib is deck sweeping and the other one is about 30 cm from the deck.

 

Groupama performs very well, but slow C Class cats also had sealed gaps.

Mountain Lion, Dulcinea, Helios and Splice are all C Class cats that appear to have extremely closely-sealed gaps in illustrations. Helios' sealing device can be seen here;

http://www.boatdesign.net/forums/at...tralian-c-class-cats-cclasshelioscontrols.jpg

None of these boats were competitive against boats like Sliepner and Miss Nylex which had much larger gaps. Sure, the "sealed gap boats" had other problems but if sealing the gap was as much a bonus as claimed then many of these other issues would have been irrelevant.

Some drawings show that Canaan, the Canadian boat that topped the C Class before Groupama had a larger foot gap than much older C Class cats. So while Groupama had a sealed gap, one could say that there is overall no trend towards winners having sealed gap, which indicates that the claims that sealing the gap creates a major advantage cannot be correct.

 

I agree that the cuffs would not appear to have the same effect as a fully-sealed foot, but they are examples where a claim for such an effect are arguably not borne out in reality.

Yes, the crew weight differences and other aspects of the CII rig also have an effect. A lot of the available comparisons are not testing apples against apples, but arguably that's not really necessary given that the claims made for the improvement performance are so vast.

For instance, it seems extremely unlikely that closing the gap would create a 7% speed improvement, since that is about twice the difference you get in all round speed when you add a spinnaker to a fast cat. There's only about 9% speed difference between a Farr Mumm 30 and a Farr 40 OD, or between a First 31.7 and a First 40.7, a Laser v a Contender, or between a 3' wide plywood, dacron and alloy Moth from the '70s and a 1' wide carbon/mylar Moth from the 2000s.

A performance difference of that size not the short of thing that will normally get hidden by the "noise" of other design differences- it's the sort of thing that will see a boat blow the competition away.

In rating boats, an overall boatspeed improvement of 1% or less was enough to have people acclaiming that a revolution was at hand; a difference of 7% was never seen. It's about the difference between the early alloy or timber masthead IOR boats, and the last lightweight IOR boats or a modern Benny, J Boat or X Yacht of the same length.

Advantages of such magnitude would not be tested (as it so often has) and then discarded, so the theory must be wrong.

 

That pic appears to be Paka or Biele from the early '90s. Around that time, there was so much attention being paid to closing the gap that we had downhaul cleats that were mounted so that they actually projected underneath the mast, to reduce the gap as much as possible. The way the foot drops down at 90 degrees to the boom at the clew was also (IIRC) an attempt to close the gap.

Pics these days show that the gaps are wider, and the boards are going 30% faster. To me that indicates that the boards these days are not dramatically less efficient, as some theories would claim, ergo the theories must be wrong in this situation.

Windsurfer COULD close the gap more, but it comes at a significant handling cost as the foot is affected by the waves, footstraps, sailor's feet etc. Although windsurfers have been very aware of the theoretical effects of sealing the gap for decades, and have experimented with it for decades, they have found that there is no real-world gain from closing the gap more.

 

The spreadsheet uses lifting line theory, which is linear, so the larger the AoA the greater the error will be.

 

I have to agree with you CT, those are very convincing examples.

I would not go so far as to declare the theory wrong, otherwise it would not still be taught to aeronautical engineers after more than a century of testing and validation. But I agree that there could well exist large gaps in our understanding of how to apply it to sailing as such. I guess that is what you meant in the first place.

I am now even more curious to investigate the matter and to find an explanation for why such a discrepancy exists between our understanding of theory and its application in practice. I would like to find answers to questions such as:

Is the same discrepancy which applies to uni-rigs such as C-class cats or windsurfers (that arguably operate at relatively low CL values) also the culprit in high CL applications such as a sloop?

Does rig hight and wind gradient play any role? I suppose the agreement between C-class cats and windsurfer results suggests not.

Are there unexpected secondary results of flow interaction with the hull, and would these be the same for mono vs multihull?

The effect of heeling was also raised. My Ragtime model is progressing nicely. I have done some test runs on the stock configuration and still need to get the sail sheeting right to get rid of a few patches of separated flow on the jib, but otherwise it nicely shows the individual vortexes, with the one caused by the windward rail apparently content to lay cradled between the side deck and the side of the deck-house. Makes one wonder how different the flow would have been was it not for the boxy deck-house...colourful fluid dynamics indeed!