Deck sweeping sails and effective aspect ratio

Are you referring to actual sailing tests?

You make an important point about broad reaching which is well worth exploring further.

Keep in mind that not only racers can do with a few more degrees pointing at times:
Joe Daysailor in his lugger sharpie gets caught off-guard by a windshift and finds himself on a lee shore. He has to put a reef in his low aspect ratio sail, reducing his pointing ability from marginal to almost nothing. Now had he known that he might be able to save his skin by jury rigging the boom-tent tarp that is stashed in the hatch into a makeshift gap seal...I think you see what I am getting at.

 

I disagree. If closing off the gap allows the boat to points slightly higher or gain a little bit of speed then it could cut hours off a long beat when cruising.

 

My point is that closing off the gap will not provide a significant improvement in a cruising boat's ability to point higher. The windage drag on the hull and rigging plus the large hull drag of a cruiser would obscure any improvement available from closing the gap. It's kinda like the guy who puts aerodynamic mirrors on his old Jeep.

I am curious though. What percentage of the cruising crowd does long beats? Don't they just fire up the cast iron tailwind?

 

Re "There is no mystery in the fact that closing the gap between the deck and the foot of the sail has aerodynamic benefits. For the record, I am not just referring to headsails, but any sail in general.

What baffles me is the fact that the concept receives so little attention from designers - especially those that get to start with a clean sheet of paper. Land- and ice yachts in pursuit of speed records seem to appreciate the benefits, as do some of the latest AC72 multis. "

With respect, as others have mentioned the concept has NOT received little attention from designers - in fact as long ago as the 1930s (and perhaps before) designers were very conscious of the end plate effect. Boats like the Js, 12 Metres and Ben Lexcen's IOR 45 footers were specifically designed to utilise the endplate effect. But time and time again, the end plate effect has been proven to have so little effect that designers and sailors from the testbed of the hot development classes have largely abandoned it.

In the real world, the problems are major. You can't tack well under a very low boom. If the boom is too low at the clew, it hits the water when the boat heels and that can easily cause a capsize or broach. Vangs and mainsheets become more complicated. Minor aero gains are easily lost when the boat hits the drink. So despite being a well-known factor for many decades, closing the gap only works in a small number of classes. When something has been attempted dozens of times over decades and has failed to work very effectively it seems that the theories must be incorrect.

There are some classes that still close the gap, of course. Windsurfer racers close the gap, to be sure, but only those with racing sails since even in boards the hassles are not seen to be worth it. And even windsurfer racing sailmakers are no longer as determined to "close the gap" as they were in the '80s, when sails from Rushwind etc had huge foot panels.

C Class cats also close the gap, but not As, F16s or F18s. Not 12s, 14s or 18 Foot Skiffs, Moths or Canoes, despite the fact that in Canoes you normally go around the back of the main and therefore have no need for room under the boom. Julian Bethwaite uses "cuffs" to create some endplate effect on the 9ers and Byte, and the MPS has a very low gooseneck and a high clew for (IIRC) a similar reason - but none of those boats are significantly faster than comparable boats with higher boom heights.

The RS700, for example, has a medium-height boom but is close in speed to the MPS which has a very low tack. The Byte CII is a redesign of the Byte (a 'baby Laser" with a Laser-style rig) with a modern carbon-masted fully-battened mylar sail designed by Bethwaite, complete with a "cuff" like the Sprindrift in your post 4. Despite having a 20% bigger rig, the CII is just 2.6% faster than the original Byte. As a comparison, the standard Laser also has a 20% bigger sail than the Radial, and is also 2.6% faster than the Radial. When the same SA difference gives the same speed difference to two similar hulls, despite the fact that the CII has a newer rig AND the claimed advantage of a "cuff" to lengthen the luff, it appears that there is no evidence for any big advantage.

So why is there an apparent disconnect between the science and the practical applications? Years ago Frank Bethwaite once look at the pics of spray flying over a boat and mused that the spray's movement indicated that the flow low down on the deck may have been too turbulent to make the end-plate effect work. One other point is that nothing comes for free. If you add sail area down low it costs weight, complexity and some drag, surely - and if you're going to pay those costs why not use them to make the rig taller?

Re the Dashew article - with respect, one article about one boat does not outweigh 80 years of experiments with hundreds or thousands of boats.

Petros, re "so you are either stuck with rules that limit the innovations in sail design". Sorry, but with respect there are thousands of sailors who race classes where there are no such rules.... sure, there aren't very many in the USA, but that doesn't mean that they do not exist elsewhere and have not been around for many years. It's a bit frustrating when so many posters from the USA appear to be determined to ignore what happens elsewhere.

 

Single-masted Ketch

Hi Will,
You appear to have a good mastery of computer analysis technics, certainly much more than mine....ha. So I would ask if you have any interest in analyzing an unusual single-masted ketch I've tried promoting for a number of years. And it is on a non-heeling catamaran.

Too many of today's modern cats have the booms for their mainsails so high off the decks to clear their flybridges,...its almost criminal in terms of sealing their bottoms. In the drawings I present you will find some deck sweeping attempts, plus some streamlining of the cabin structure.

Over the years I've had many naysayers come to the aft-mast forum discussion (including Tom Speer) and suggest that the drag of my rigging will be excessive to any potential gain in performance. I've now got my rigging sizes and amounts down to just about what might be seen on a normal sloop rig, and certainly less than would be found on a normal ketch with its second mast.

I sure would like to hear what you might have to say about this drag factor of my rig design??

Here are just a few postings that might help present my case in the briefest form:
Origination & Justification

Sails in Combination

http://www.runningtideyachts.com/sail/

Over the years I have defended and then ignored this subject on a number of occasions. It was only just recently when I unboxed a model of this vessel I had taken to the Miami show back in 2005 that I gathered a new energy to prove the possibilities of rig design.

 

Thanks for all the examples CT. I am much newer to sailing than I am to aerodynamics, so I do not have a "big picture" when it comes to the development history that has led up to where things are today. So please bear with me while I prod some more. In doing so I want to achieve two things:
Firstly: Separate the tests or historical examples from which relevant deductions can be made from those that cannot (if any). For example -
a test with a lowered boom, but still leaves a significant gap, cannot be used to make deductions about actually sealing the gap. As demonstrated in the Trillizas ferry earlier, a gap that is a mere 0.4% of the luff length still has a big enough effect to influence results.

Secondly: I would like to make sure that the results found on any particular boat is interpreted correctly when applied to boats with very different configurations and applications. For example -
a test performed on a racer with an already high aspect rig might show proportionally lower gains. To further compound things, if the gains are expressed as a speed increase one has to remember that a racer typically operate at higher speed-length ratios, and that it takes a lot more additional drive to gain 2% at the top end. The small percentage increase does not necessarily mean an equally small reduction in, say, the possible healing moment on a Solway Dory canoe.

To facilitate discussion, I will break up the case studies.

Let me start with the Groupama C-class cat. It was the only example I could find that REALLY seals the gap - how do they perform compared to the others?

 

Rushwind

This is the best picture I could find that shows any attempt to close the foot.

Would this be considered as a "sealed foot" to be used in comparison with more conventional gap sizes?

 

Byte CII

The cuffs on the 9er and Byte might have some effect of increasing aspect ratio, but it does not come anywhere close to the effect of actually sealing the foot. The cuff on the Spindrift in post 4 has its issues too - the sharp reduction in chord causes a big jump in lift coefficient which causes the lower sail region to stall unless sufficient twist (washout) is used. So if any of the previous testers did not take that into account, their results would not be a true reflection of the real benefit.

One question about the comparisons between the originals and those with 20% more sail: are those speed differences for similar heeling moments?

I ask because it is nevertheless an interesting comparison and I just want to ensure that we compare apples with apples. If the Byte CII is faster with the same crew weight as the original, but on a Laser you need a larger crew weight to realise the speed difference, then one cannot ascribe the similar speed differences to just the sail area increase. Leach control on the Byte CII is also mentioned as one of the more significant improvements over the original Byte.

The benefits of adding rig height and area up top also quickly diminishes if heeling moment is a limiting factor.

 

You are quite right about the hull and rigging drag reducing the proportion of pointing and speed benefits. The benefit of reduced heeling should not be influenced by hull drag or even that of rigging.

For the purposes of this discussion, we would like to be able to say exactly by how much, to quantify it by test or simulation data. Someone new to the field should be able to read this and make an informed decision based on his application.
Many cruise boats also have arguably much more induced drag to potentially get rid of than racers, so it would be improper to simply state as fact that the benefits will be obscured by hull drag.

 

Brian, it would make an interesting case study for sure.

I do not have the benefit of multiple parallel processors for quick cfd solutions, so I am limited to how much detail I can include in the model. I will contact you by email and we can take it from there.

 

Having done a bit of land sailing where you basically go everywhere close-hauled I started thinking about whether I could come up with a boomless rig where the foot of the sail is attached to the centreline of the vehicle.

Since my criteria are ultra-simplicity and ease of dropping the rig (rather than having to tip the whole landsailer on its side) I have come up with a idea which is essentially half a rogallo hang glider placed on its side. 'Sheeting' would be achieved by pivoting the (short) mast fore and aft to alter the amount of 'billow' in the sail. It will probably work OK from a reach to a beat, which is all I require.

Obviously this would be a very low aspect ratio rig with the foot length approximately equal to the mast height and therefore not particularly efficient but it may be a cheap, simple, fun option for a knockabout landsailer. I am thinking that being able fix the foot of the sail might mean that the endplate effect can be exploited to claw back a little performance.

 

Will, this is what I have been trying to point out to you. Maybe if you look at it in the following way: There are two determinants of a sailboats ability to point. The L/D of the air side and the the L/D of the water side. To point well, they both need to be good. if either one is bad, it will dominate and degrade the pointing ability.

 

This report suggests that the vortex created by the upwind rail of a heeled boat has a significant effect on the flow under the boom. That makes sense partly because the vortex off the rail would rotate in the opposite direction of the vortex created by the flow around the bottom of the boom.

http://www.hiper08.unina.it/cd hiper 08/HTML/Papers/2 - Fossati.pdf

So that hints at a possible reason for a more noticeable effect from closing the deck-sail gap on a catamaran vs on a monohull. The cat does not have an elevated windward rail to generate the vortex that would influence cross flow under the sail.

 

I don't know what kind of closing off the gap are you talking about, but I have on deck furler and furling jibs + racing jibs without a furler. I have also raced with one of my furling jibs and haven't really noticed much of a speed or pointing difference to my racing jibs. The racing jibs are a bit larger (LM jib 7%) in area and may be a bit faster on a light beat, but not something you could notice from instruments nor from sailing against a known competitor for a short time. I would say the difference is not more than 2% on a beat, which is close to what VPP sees as the difference due to larger area.

Here are photos of those two jibs (light brown sails with spinnaker partly visible and white furling sail)
http://sailpix.pp.fi/bow12/#N73_1707_DxO.jpg
http://sailpix.pp.fi/wbsails11/#N70_7170_DxO.jpg

 

Joakim, we are talking about well and truly sealing the gap under the sail.

Theory suggests that only a small portion of the potential benefits will be realised if there is still the slightest gap under the sail.