Deck sweeping sails

im not sure if you exactly understand what im attempting to state here..

 

decksweeper

I guess the thing to point out,if necessary, is that the flow over the windward edge of the hull when it is heeled going upwind does not blanket the lower portion of the sail; on the contrary it forms a vortex which interacts positively with the flow at the bottom of the sail.
Without spending a lot of time researching exact quotes the best reference that comes to mind is Marchaj 's "Sail Performance" at the bottom of page 184..

 

I thought about doing the same thing in one of my classes, but some leading skiff designers who are also involved in rigs said it's not worth it, compared to shoving the available sail area up high.

It's been tried many, many times before. Miss Nylex, the C Class in 1975 IIRC, and some Nacras I think. Plus some other C classers; Helios? Frank Bethwaite noted the movement of spray over a Tasar in Australian Sailing magazine about '81 and queried whether the deck sweeper was worth it; the spray indicated very little flow over the deck but of course that was ignoring the end plate effect. 18' skiffs have moved to higher booms and become faster. Windsurfers talk a lot about "closing the gap", but the rig and hull configuration means that many other effects occur when you close the gap and therefore it's hard to see how much effect the end plate has.

The endplate effect seems to come and go. The Dashews like it, but the speed increase they talk about is totally out of kilter with the proven impact (or lack of it) in other classes and if the Dashew boats are such great performers, they wouldn't be slow according to their PHRF ratings. Certainly in boards we can easily futz around with the height of the foot off the deck and while it's perhaps important in terms of gaining a few boatlengths, it doesn't seem all-important in terms of being a breakthrough.

 

sail power

249, seems to me that nobody in this thread has claimed a deck sweeping sail is any form of breakthru only as you yourself mentioned:that "it might be worth a few boat lengths" which on a race course, of course, could be a solid margin of victory.
Seems appropriate to consider a decksweeper or any other detailing of a rig in the same or similar manner to be a small but potentially important contribution to the overall aerodynamic effectiveness of a sailboat rig.
Whether or not it is beneficial in a specific class depends on what the tradeoffs are....

 

Re "249, seems to me that nobody in this thread has claimed a deck sweeping sail is any form of breakthru"

Well, the Dashew link you posted makes some pretty damn big claims; "This is five to seven degrees closer than before, at a speed improvement of around two percent. That is a huge difference."....."more increase in performance for less cost than anything else you can do.".... "a good seal was worth at least five degrees in tacking angle with no loss in boat speed."......"huge" increases in speed sound a bit like a breakthrough considering how a few boatlengths can, as you say, change a race. There were also other claims for 5% more efficiency which may well translate into a "breakthrough".

 

Hey hey hey Chris, you're getting you're knickers in a twist again, what kind of an Australian are you?

Yokebutt.

 

Aaaahhh, yeah mate, ya may well be only too right, I admit I get my knickers in a twist faster than a goanna going up a gum tree. Only on the web, though.

 

Well, Chris, if you and your buddy Lorsail doesn't soften up soon, I'll tow you both behind my ute on a rope down a gravel-road 'til you do.

That's not a threat, that's a promise.

Yokebutt.

 

logic betrayed

CT, what a load --whups- I better soften up a bit so I don't get to find out what a ute is...

 

Hi David,

How did the decksweeper work out for you?

We have seen Mischa Heemskerk and Glenn Ashby dominating the Worlds with a decksweeper (and their sailing skills). 2 Other dutch sailors, that Help Mischa tune the sail, did very well to.

Do you sail a foiling A or one with c-boards? When foiling, the lowewr center of effort is an important gain. When floating the gain is upwind and you seem to lose a bit of competitiveness downwind when light. What is your experience?

Best
GJ

 

I suspect the improvement would be better with a loose footed sail and even more so with a boomless one.

The reason I say this is that most main sails I've seen have quite a pouch where they attach to the boom. This pouch may act as an end plate.

Also, there's the problem of holding the boom end down. With a deck sweeping boom, there is no room for a boom vang.

The design and operational compromises need to make this so, such as needing a hold down strut, on top of the boom, and having to move behind the main boom with every tack, may totally negate the gains.

With a loose footed sail, held out with a sprit rather than a boom, the engineering would be simpler, but then you'd have the sprit bisecting the sail on every other tack.

A boomless sail could be treated like a jib, with a sheet line on each side. The clew could then be allowed to move forward, while changing tacks, so you wouldn't have go as far aft to get around it.

 

Wow, stone the crows, CT has an Aussie vernacular sense of humour, well done cobra.
Regarding Steve Dashew; he used main/trampoline end plate effect on his then, world record speed holding and class stomping and mixed fleet destroying D Class catamaran Beowulf IV - and that was way back in the 1960s/'70s. Repeat that is 40 plus, no 50, years ago - he was way ahead of the game.
No one here has mentioned the latest French C Class champion Groupama; C Class winner in a fleet of the very best and very innovative and very highly tuned cats. Yes, the G crew had a very sophisticated foil control but also had the only deck wing sealing set up too. And now that configuration has been carried over to the A Class winners as well. And these are flying boats, not displacement designs. So comments about air escape under high flying trampoline does not seem to be of importance; what happens above the tramp is what matters. Agreed, these are flat or near flat sailing multihulls; they are not well heeled over monos with windward deck screwing up wind flow over the lower sections of the sails.
Here's another very early deck sweeper, C Class Scimitar, also a top boat of the times.

 

For the following I have no personal experience, but as an A-Cat sailor I am keeping a very close eye on the development...

The deck sweater has entered the class before, but has never been shown to really help. In large part because sail area is restricted and deck sweapers count as sail area. They did help cut down on drag, and were fast upwind, but the lost sail area high on the sail killed downwind performance so the total time around the track was actually lower.

This all changed with the advent of foiling boats. Now downwind speed is restricted by RM and not sail area, so moving the area out of the top of the main to the bottom seems to gain you a few things...

1) lowered CE of the sail plan equates to effectively more RM
2) lowered drag, particularly upwind from the end-plate effect
3) lowered drag downwind seems to add more torque and allow a quicker transition to foiling (6kn wind speed is the new lower threshold)

So a deck sweaper alone is really a negative because of what it does to downwind performance, but when combined with lifting foils they come into their own.

Additionally new trampolines are being made that are double layered (top and bottom with a gap in between), sealed to the sides, as well as made from air tight materials (old 3dl and 3di mains seems a particular favorite). This also helps minimize drag and help get the boat up on the foils sooner.

For the first time it seems aero drag is going to play a larger and larger part in speed around the race course. Just because the marginal gains all effect how quickly the boat can getup on the foils. The new mantra then is 'first to foil wins.'

 

The America's cup multi hull uses deck sweeping jib and main. On a multi-hull I would think there is a big advanage. On a monohull that operates heeled over, I suspect not so much, and that is why you do not see them on large mono hulls.

 

As a newbie to this forum, just thought I would add from personal experience, that the 18 Square Meter class (in late 1970's) had a cat design called COYOTE which had a standard sail profile with a deck-sweeping cut. The sail was boomless, having a plate with holes fastened to the sail and at the rear, just behind the sweeper part. A Hook on the end of the mainsheet attached to the plate. The front hole helped flatten the sail, while using the back hole allowed maximum camber. The sail configuration did have a positive effect on speed, but I believe maximum was when sail was coupled with a curved traveler which handled leech tension regardless of sail position - inhauled or outhauled.

I find the claims of "New" technology to be humorous, since obviously the concept and design was used nearly 40 years ago.

Below are photos (probably circa 1979 or early 1980s - unidentified sailors with a COYOTE with deck sweeper main.