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boom-sweeper.jpg

The IRC rating offices have seen a growing number of instances and inquiries about additional cloth set under the boom, known by various names including lazy sweeper, deck sweeper, mainsail skirt, water sail.

In response to this, the IRC Technical Committee has issued IRC Notice 2021/02 which explains that this cloth is defined as a sail and explains how to measure the additional area so it can be accounted for in the rig factor.

Read it.

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8 minutes ago, cill123 said:

And how's the rail meat meant to get across the coach roof with that yoke in place

think mostly offshore use, so less rail meat and tacking. how efficient is it? did anyone do a CFD analysis yet? 

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weird.. it looks like they're just changing the size of the sail area polygons and basically counting *all* of the area under the boom as sail area.

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Quite simply, this bit of rag constitutes an end plate to the airfoil, damming slippage between the deck and under the boom, hopefully cleaning up airflow for a greater section of the main.  That's what all those cute little wingtips on virtually every airliner are doing nowadays: limiting wasted end-slippage of attached flow on the foil. Amazing it took so long to appear on a boom.

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8 minutes ago, Glenoid said:

Quite simply, this bit of rag constitutes an end plate to the airfoil, damming slippage between the deck and under the boom, hopefully cleaning up airflow for a greater section of the main.  That's what all those cute little wingtips on virtually every airliner are doing nowadays: limiting wasted end-slippage of attached flow on the foil. Amazing it took so long to appear on a boom.

This (decksweeper) makes a noticeable difference when we changed to this on the F18 cats. Much more power, even in the lighter stuff.

When I asked Pete Melvin what he would do to make my boat more efficient and faster (Lumbo 32 co-designed by his office) the first thing he said without hesitation was to add a decksweeper to the boom. Interesting to see it's starting to happen on monohull keelboats. I predict we'll see more in the near future.

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15 minutes ago, Glenoid said:

Quite simply, this bit of rag constitutes an end plate to the airfoil, damming slippage between the deck and under the boom, hopefully cleaning up airflow for a greater section of the main.  That's what all those cute little wingtips on virtually every airliner are doing nowadays: limiting wasted end-slippage of attached flow on the foil. Amazing it took so long to appear on a boom.

Agreed.

As you say, I believe the main advantage is end plate effect.

Any attempts to include it in the sail area calcs - even as inefficient sail area effectively bans it. 

Or at least anyone would be silly to have one if it was included.

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1 hour ago, Glenoid said:

Quite simply, this bit of rag constitutes an end plate to the airfoil, damming slippage between the deck and under the boom, hopefully cleaning up airflow for a greater section of the main.  That's what all those cute little wingtips on virtually every airliner are doing nowadays: limiting wasted end-slippage of attached flow on the foil. Amazing it took so long to appear on a boom.

If they're going to ding you for it and you still want to see the instruments, not block the crew and get the end-plate effect, why not go for a Park Avenue Boom?

image.jpeg

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It was amazing how much the decksweeper main helped on the small catamarans.  I've been wanting to try this on my keelboat but would make the foot shorter and not go over the companionway.  Keep it more comfortable to use.  I would like to try a tacking one that had some shape in it but maybe that is silly given the straight boom that messes these up compared to boomless or curved boom options.

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The park avenue boom is probably too narrow to substantially corral the end foil vortices, or more importantly, to create a flow to counter the spanwise flows created by the pressure differential on each side of the sail.  The deck is already there, so no price is paid in profile drag or increased wetted surface as the spanwise flow is stopped completely- at the bottom of the sail.  The top of the sail is still throwing vortices- a square top less violently than a pinhead.  Its not a mechanically practical arrangement and does not work well once off the wind but still in attached flow, but on the wind and strapped down, an effective endplate reduced induced drag by a noticeable amount.   Modern aircraft shapes actually cancel spanwise flow, which is fast.  Also note that at typical sailboat flow velocity, these effects will have less relative benefit.

http://www.mandhsoaring.com/why winglets/wl-it.pdf   

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4 minutes ago, bluelaser2 said:

The park avenue boom is probably too narrow to substantially corral the end foil vortices, or more importantly, to create a flow to counter the spanwise flows created by the pressure differential on each side of the sail.  The deck is already there, so no price is paid in profile drag or increased wetted surface as the spanwise flow is stopped completely- at the bottom of the sail.  The top of the sail is still throwing vortices- a square top less violently than a pinhead.  Its not a mechanically practical arrangement and does not work well once off the wind but still in attached flow, but on the wind and strapped down, an effective endplate reduced induced drag by a noticeable amount.   Modern aircraft shapes actually cancel spanwise flow, which is fast.  Also note that at typical sailboat flow velocity, these effects will have less relative benefit.

http://www.mandhsoaring.com/why winglets/wl-it.pdf   

Yeah, but in the area backwinded by the jib? I don't think in that type of rig it will do much for to-weather performance. It adds a wee bit of sail area for downwind though. 

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21 minutes ago, quod umbra said:

Is it about sail area or using the deck as an endplate?

To weather it's mainly about end-plate, however the front of the main is an aerodynamic mess, what with the mast as a spoiler and being in the backwind of the headsail. Don't think a half-assed end-plate is going to do much for that. 

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22 hours ago, Glenoid said:

Quite simply, this bit of rag constitutes an end plate to the airfoil, damming slippage between the deck and under the boom, hopefully cleaning up airflow for a greater section of the main.  That's what all those cute little wingtips on virtually every airliner are doing nowadays: limiting wasted end-slippage of attached flow on the foil. Amazing it took so long to appear on a boom.

So when we see boom with winglets if this gets rule hit.

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Well if a certain hack R/P 50 can sail around SD with a fathead main and a longer prod for a year with no penalty, then I should be able to add one of these, right? Wrong - They will immediately hit me.

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22 hours ago, Irrational 14 said:

This (decksweeper) makes a noticeable difference when we changed to this on the F18 cats. Much more power, even in the lighter stuff.

When I asked Pete Melvin what he would do to make my boat more efficient and faster (Lumbo 32 co-designed by his office) the first thing he said without hesitation was to add a decksweeper to the boom. Interesting to see it's starting to happen on monohull keelboats. I predict we'll see more in the near future.

I tried to fit one on a 14 during a long lockdown winter. Tried several configurations but it would be either illegal or an exploit (besides beeing impossible to get around a course :lol:). Looks cool though.

side.thumb.PNG.7ed170c1237f598d00ec763fa9ca920e.PNGfront.thumb.PNG.c174af43dd78cdb77fd040437fe81842.PNG

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19 hours ago, Chris in Santa Cruz, CA said:

just get a few really thin dwarfs to stand on the cabin top in formation in squirrel suits

Years ago Lee Helm (Latitude  38) mentioned this idea.  In lighter air where weather hiking isn't necessary have the crew sit shoulder to shoulder directly under the boom to decrease airflow under the boom and increase the endplate effect of the deck.

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1 hour ago, quod umbra said:

Way back when Love Machine first appeared with her B&R rig she had these strips of aluminum foil with bumps (think the opposite of golf ball surface). The idea was it broke the initial laminar flow across the  the front part of the extrusion which induced laminar flow to re-attach earlier on the main...... 
After she dropped her rig at BIRW in whatever that was 1977, 1979 or there abouts, the new rig did not have the strips. So I guess they decided it was of little value.

Once flow has tripped from laminar to turbulent, there is no going back....it stays turbulent.

You are right about the re-attaching part, just that it would be turbulent not laminar flow.

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7 hours ago, quod umbra said:

Way back when Love Machine first appeared with her B&R rig she had these strips of aluminum foil with bumps (think the opposite of golf ball surface). The idea was it broke the initial laminar flow across the  the front part of the extrusion which induced laminar flow to re-attach earlier on the main...... 
After she dropped her rig at BIRW in whatever that was 1977, 1979 or there abouts, the new rig did not have the strips. So I guess they decided it was of little value.
Maybe Fastyacht can chime in on this stuff......

Artemis (Holland 35 (1975) too). Seemed like a good idea but the B&R rig didn’t have the bendability to power/depower and really needed the luff curve matched up with the bend more than most so at the end of the day I think getting the basics right was more important than the esoteric stuff.

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Steve Dashew experimented with "end-plating" in the 1960s, in racing, and in the 2000s in Beowulf, in a cruising context.

He wrote something about it, with some real-world data:

“End-Plating” the Main and Mizzen
https://setsail.com/end-plating-main-and-mizzen/

"... Do they work? You bet! Upwind, in 13 to 14 knots of breeze, BEOWULF now averages between 9.7 and 10.00 knots, making 95 degrees between tacks including leeway ... This is five to seven degrees closer than before, at a speed improvement of around two percent... And reaching, we seem to be going five to seven percent faster than was previously the case..."

Older entries like this from 2003 are missing the pictures, I managed to find one from mizzen.

miz_seal.jpg.86d3bd0e05e7bb51ea33ea414a7dc0b4.jpg

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11 hours ago, Editor said:

Well if a certain hack R/P 50 can sail around SD with a fathead main and a longer prod for a year with no penalty, then I should be able to add one of these, right? Wrong - They will immediately hit me.

That would be the RP50 of the gray persuasion?

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14 hours ago, Virgulino Ferreira said:

Steve Dashew experimented with "end-plating" in the 1960s, in racing, and in the 2000s in Beowulf, in a cruising context.

He wrote something about it, with some real-world data:

“End-Plating” the Main and Mizzen
https://setsail.com/end-plating-main-and-mizzen/

I am a bit wary of Dashew's claims.

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23 hours ago, 12 metre said:

^^This

Decksweepers have been around for ages - or at least since 1970.

Here we have Ficker driving Intrepid during the '70 AC.

intrepid.jpg

I'd forgotten about that.

Talk about stupid dangerous.

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