Design for DSS-Foil Assist for Keelboats

At a given speed a foil with fixed pitch should deliver fixed lift. Much the same as a crew of given weight cannot deliver more than a certain righting moment. The need to adjust sail to suit conditions shouldn't surprise anyone in a boat, DSS-equipped or not.

Heeling moment tends to rise faster than the resulting speed for a displacement hull. I believe foil lift is proportional to the square of the speed so if there is adequate DSS lift in a moderate wind there should be adequate lift at the higher speeds obtained with a heavier wind. However, heeling moment builds instantly, speed and foil lift build rather more slowly, so sail drive needs to be tapped gradually. But surely any crew would know that?

Of course this is not a rigorous analysis: maybe the foil angle is incorrect.

 

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No, it's not(at least it is very,very unlikely)-the boat has won races with the foil as is and did pretty well in this regatta. There was ,obviously, a crew judgement problem in that the boat was overcanvassed for the conditions-as stated by the SA poster.

 

The pic is of MC2 which is a Frers design that Wally built. Has it been fitted with DSS?

 

The boat was successful, no argument, but what I meant to say was, it sounds like the crew were piling on the canvas in an attempt to push the boat past the limits imposed by available DSS lift, and more lift might have allowed them to succeed in what they were attempting. Perhaps they didn't have sufficient understanding (training) of the requirements of a DSS-equipped sailboat?

I always get suspicious when plane crash investigators conclude it was caused by pilot error.

 

Dss

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Good question, Terry.

 

This post is somewhat confusing. First you indicate the crew was using excessive sail area for the conditions, and then you are suspicious of pilot error as a cause. Too much sail is definitely preventable pilot error.

There isn't enough experience with DSS in race conditions for anyone to have all the answers yet, especially those of us constructing post mortem analysis from the Internet. As in most debatable situations you have to defer to first hand reports, unless incontrovertible evidence is available to counter them.

If the anecdotal reports are to be believed, the confusing rough sea state appears to introduce a lot of variables to predictable benefits of DSS. The ability of the crew on the Shaws to independently watch, anticipate and instantly make righting moment adjustments in the conditions may have been a factor - as well as eliminating the work of handling the DSS foils. The results seem to indicate that the predicted benefits of DSS may be mitigated somewhat by race conditions. As conditions get more challenging, the crews that can keep their heads outside the boat generally do much better.

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CutOnce

 

Dss

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Any boats' results may be mitigated by race conditions. There is no factual evidence, whatsoever, that handling the DSS foil was, in any way, responsible for the problems the boat had in race one where the crew was sailing with the boat overpowered for the conditions, according to the SA poster.
Again, the boat got third overall despite a DNF in race one.

 

In the pics of B3, in the SA link, sailing upwind the DSS board is sticking out to weather. Do they tack the board or does it just stick out both sides? Pic would indicate that it sticks out both sides, so there isn't really any handling of the DSS system

 

Again, this is speculation on my part, but a significant logic "law" called Occam's Razor postulates that the simplest explanation is most often the right one.

DSS foils DO move from side to side - and the reasons for doing so are pretty obvious. Failure to move a DSS foil (as in the picture) is most simply explained by "It wasn't worth the effort or time, given the circumstances". This provides a plausible train of thought that may (or may not) be worth following: If 50% of the righting moment benefit can be gained without crew action, the crew may defer moving the DSS board because they are too busy, may be anticipating a need for a fast tack, may be occupied with sail handling, conditions do not require full righting moment etc.

Keeping the DSS foil in a middle position supports the "keep heads out of the boat principle" quite well. Only the crew in place at the time can provide answers. Logic states there must be some reason why they've consciously chosen to center the board.

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CutOnce

 

It must have been. Otherwise why would <removed> post this?

Of course I don't always believe what I read on the internets. Those charts looked familiar. A quick check at the DSS site revealed a powerpoint sales pitch to Wally from 1988. Guess what the charts look like?

The versions <removed> presented have had the date removed. I'm sure there was a reason for that.

 

I’ll try to clarify:

In my post I suggested the crew had attempted to exceed the limits imposed by the available DSS lift, that is not quite the same thing as using excessive sail area for the conditions. The distinction is subtle but may be important. When the crew commented on pitch angle perhaps they meant that DSS lift can be increased with increased attack angle to permit more sail to be carried.

I imagine optimizing foil angle will be part of the learning process for DSS users; also any new boat goes through a tuning period.

My suggestion raises the possibility that the life will be excessive causing surface penetration and additional drag, but that can - I imagine - be improved by moving the foil inboard.

 

Dss

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The board is tacked side to side on most DSS boats done so far except for test models. Much data was gained on the experimental 27 footer by leaving the board out on one side only.
Both BBB and the Quant 28 use sliding boards; the Quant 30 uses dual pivoting(like a centerboard) DSS foils.
Welbourn supplies the boat owner with data showing at what speed the boats L/D ratio is improved using the foil-it may be that in the picture showing the board to windward the speed was too low for the board to be effective. Look at the info for the big boat-the board shows and improvement at fairly low speeds.

The picture below showing BBB going upwind shows the board retracted-which is why it sticks out on the windward side:

Pix-Quant 28 at rest-click on image:

 

Dss

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Terry, so far none of Welbourns DSS boats has a foil whose angle of incidence can be adjusted.The foil has a fairly wide range of + lift-well over two degrees-even with a slightly negative angle of attack-say -1.5 degrees. Also, retracting the foil partially will increase drag because it decreases aspect ratio.
If the SA poster is right then the crew did overpower the boat which would have a negative effect on any boat. The comments related by the SA poster and attributed to the crew regarding the foil don't make sense to me.

 

Doug: adjusting the foil during a race might not be practical or wise. My thoughts were to adjust it before the race to suit the conditions. That’s assuming there is some gain to be had from that, but I don’t have the data to analyze it properly - it would be complex I think, not something to be done ad hoc.

All foils have some lift left at negative incidence, that’s normal.

Retracting the foil partially would increase drag-lift ratio but should reduce total lift IMHO . . .

 

Saw this today on SA


steveromagnino


Group: Members
Posts: 2,355
Joined: 13-March 06
Location:Bangkok, Thailand

Posted Today, 12:41 AM
We've raced brace brace brace in the shaws.

I don't doubt the dss system can work but it requires very specific conditions. They don't seem to use it upwind much and downwind in a sea it seems very hard to keep the pitch maintained.

In big breeze I would say a shaw 650 is sailing lower and faster because the foil cycles through fast airbourne submarining start again. No doubt if u figured out how to sail it but tough when there are not millions of them whizzing around. Whereas a std boat just keeps trucking along.

In 10-12 knots the boat is very quick to be expected for a 7.5m boat around 10% quicker than us but no noticeable 40% speed increase on any angle that I have seen yet.

I don't doubt it can work but from what I can see there is a pretty big learning curve which no one seems to have mastered to get this mythical 40% speed increase.

Maybe a bonus 3-5% would be more realistic. In certain directions and strengths.