Loose footed mains

Loads on boom from sail panels.

Folks, if you want eveidence of how much load was been taken on the centre of the boom as opposed to the clew then look at photos of reef points of boats, modern and old. If the reef points were ever very strongly reinforced in the way the clew is then you have evidence there was significant load. If there were always lightly loaded then the reinforcement will be significantly less, as it is on a modern sail.

[digs out Uffa Fox books from the 30s]

I think the light load has it... While doubtless its possible to cut a sail so that the boom take up load mid length, it would appear that it has not been thought desirable within the last 70 years or so...

 

gggGuest:

I would swear that I had seen a technical sailing manual showing the boom bent in order to flatten the mainsail.

But, I ain't going to argue with the "reef point" issue. That is a very brilliant observation.

 

There was a brief fashion for bendy booms in a few dinghy classes in I think the late 60s. As for how well it worked - I did say it was a *brief* fashion! Because crew speed is so much more important than boatspeed a sufficiently talented crew can have a really stupid idea and still win, which is a great source of blind alleys.

 

The idea of bendy booms is a foolish one. Lowering the clew in order to gain some small increase in sail area has been tried, but apart from the fact this arrangement means a loss of sail area because the perpendicular from the lowered clew to the mast will always be shorter then the length of the foot measured along the boom, it is not absolutely certain that induced drag will be improved at all; on the other hand, flow over the lower part of the sail will be affected by the boom and this area will increase as the boat heels further.

 

Spar bend

I'm not sure that thinking about any changes in sail area is the best way of understanding the issues for the average sailor these days. After all modern sail materials may distort under load, but they don't really stretch.

Lets start with mast bend, which is conventionally regarded as a good thing, see what it achieves, and then think about how this might apply to booms.

If you consider a modern dinghy rig, which is a) the most advanced amd b) the one I understand best(!) then bend in the mast has two functions. The first is to change the aerofoil shape of the sail, and the second is gust reponse.

There are two ways that a sailmaker puts shape into the sail. The first is by shaping the seems, and the second is by cutting a concave curve in the luff, known as luff round. Luff round is what we are interested in in this discussion. If there is more curvature in the sail than in the mast then the extra curve results in a "pocket" of shape by the mast. You can demonstrate this quite nicely by cutting a very gentle curve in one edge of a bit of paper and gluing it to a chop stick. You get a sort of pocket of shape when the chopstick is straight, and if you bend it until it matches the curve in the paper edge then it goes flat.
Because the paper distorts less (on this scale) than sailcloth the curve looks pretty ugly,but in practice this is the way sailmakers put shape in the front of the sail, and you can control this shape by bending the mast more or less.
The downside of this is that the effect doesn't work over the whole width of the sail, just the luff. How much depends on a whole load of factors. Shape in the rest of the sail is given by seam shaping, and bending the mast does'n't have nearly such a big effect on it.

The second major thing mast bend is about is gust reponse. Whereas luff round is pretty well understood, the whole business of gust response has only really come to sailors attention in the last ten to 15 years at the top level, and hasn't necessarilly come to everyone's attention. Bethwaite is probably the best resource for a detailed treatment, but what it comes down to is that research has shown that the wind is far more erratic and changeable than anyone ever appreciated, and in practice there is no way any crew, no matter how talented, can trim the sails as fast as the wind is changing. If you're cruising the East Coast in a heavy displacement cruiser this may not matter that much to you, but on a sailboard or a high performance raceboat its vital. So rthe most sophisticated rigs are now made so that the top flexes continuously with every tiny variation in the wind. If you see a video of a modern sailboard rig you can see the top of the sail twisting off as the mast bends and straightens on every wave as the board minutesly slows and accelerates with the resultant change in apparent wind. This change is concentrated at the top of the rig, where the exaggerated roach of all these sophisticated modern rigs opens and closes. The very best sails are cut so that the leech at the top of the sail is really quite loose. Instead the kicking strap (vang) load is ditributed right through the cloth of the sail and evened out by the full length battens. Another effect of this is that when the tip of the mast bends only the top of the sail gives to leeward. The main body of the sail is supported by the load through the battens, doesn't twist off and so the boat can still point even with the top of the sail twisting off as if the kicking strap (vang) had been released.

OK, lecture over, what does this have to do with booms...
Well, you could have a sail cut with "foot round" laced tight to the boom, and bend in the boom could flatten this out. But we've established that this effect only happens in the area adjacent to the spar. So in practice it really wouldn't affect very much of the sail. Worse still it would tend to interfere with the effects that you are getting from mast bend and panel shaping, making the job of controlling the sail shape much more complex, and thus more difficult to get right. That I think was the big advantage of the shelf footed main, it meant that anything the boom did to bend wasn't going to affect the shape you were trying to control in the sail with the mast etc.

Fair enough, but what about gust reponse. Well, consider that we want this gust reponse at the toip of the rig, and keep the bottom of the rig stable. Having the boom bend has excatly the opposite effect. It makes the sail twist off from the bottom, destroying pointing ability.

And finally, the bending boom is in opposition to the kicking trap - it makes the control less precise because when you pull on the kicking strap the first thing it does is to bend the boom, fighting against the changes you want to make in the sail.

So those are the downsides of a bendy boom...
1) it makes it more difficult to cut the shape in the bottom of the sail correctly.
2) It puts gust reponse exactly where you don't want it and harms pointing
3) Its makes the sail harder to control.

And given all that, especially the control thing, it means that the stiffer the boom the better, and given that its as stiff as can reasonably be arranged, there's not any structural advantage of a laced or bolt rope attached sail over a loose foooted one. We can see from the above that a loose footed sail is easier to control in shape because there are less variables. On the other hand a shelf foot might have a better end plate effect.

My gut feeling is that the area around the foot of the sail is such a turbulent aerodynamic nightmare with all the rubbish off sails, rigging, deck, crew, everything else that the gains from the end plate effect aren't as big as theory would suggest, but I have absolutely zero evidence to back this up, and am not aware of any research that has been done in this area. Even those most sophisticated of craft, the C Class Cats, don't seem to have much end plate treatment at the bottom of the sail (from the photos Ive looked at there is an air gap between trampoline and rig) and I'm sure they must have done studies of what's going on...

 

So far as I can tell, lowering the clew will probably not reduce the induced drag, unless you lower the clew a great deal. I see three ways lowering the clew would affect the induced drag:
- it increases the span
- it decreases the gap between sail and boat/surface
- it changes the shape of the spanload distribution

The conventional sail shape starts off with at least a 4% increase in induced drag compared to an optimal sail shape of the same span. Increasing the span by, say, 1% will reduce the induced drag by 2% if the spanload distribution stays the same. Decreasing the gap between entire foot and deck from, say, 5% span to 4% span is good for about a 2% reduction in induced drag. Just dropping the clew while keeping the tack at the same location won't reduce the drag as much as these changes.

Perhaps as important as the induced drag is the fact that the conventional sail shape doesn't make good use of the area at the foot where the sail has its greatest chord. The very high induced angle of attack at the foot - a header of the sail's own making - means the lift per unit area is greatly reduced at the foot.

A better approach would be to make the sail planform more like the optimum shape, which happens to look pretty much like a boardsail rig if the objective is minimum drag for a given heeling moment. This suggests raising the clew is the way to go, not lowering it. The optimum sail shape has the maximum chord at around 30% of the span. This requires a wishbone boom to get it to set properly. It reduces the sail area at the foot where it's not effective and the more gradual reduction in lift in the spanwise direction toward the foot cuts the intensity of the shed vortices there and makes what area remains much more effective.

 

Tom,
If you lower the clew so that it is in the windshadow of the boom (which doesn't take much in the way of lowering) then it does not increase the span, just the parasitic drag area.
No?
Steve

 

Yes, I think that'd be the case. At least for a conventional boom.

I've come up with a concept I call the lambda rig, in which the gooseneck is raised up to around the 30% span point and the boom angles down to the clew. The mast is highly raked so that the leech is vertical. Both mast and boom rotate to form smooth leeside contours, like a conventional wingmast. The sail is shaped like a square-top at both head and clew, possibly using a clew board for attaching the sheet. The area distribution is close to the optimal planform. The lambda rig planform tapers toward head and clew, but all the taper is in the leading edge, rather than the trailing edge.

Both mast and boom are leading edges, and well faired into the sail so separation is minimized for both. The high sweep angle of the leading edges turns the windward side separation bubble into a spanwise vortex, helping to minimize its extent. The exposed part of the mast below the gooseneck would be pretty well rotated into the apparent wind, minimizing its drag. The vertical leech minimizes the spanwise pressure gradient in the region where the streamwise pressure gradient is adverse. This helps to minimize separation and keep separation from propagating spanwise. Because of the downward rake of the boom it becomes self-vanging, like a wishbone boom, to control twist.

When combined with a center cockpit, aft cabin hull, the mast step is located forward of the forward companionway, say on top of the forward beam of a trimaran. This concentrates all the structural loads and minimizes the load paths. The cockpit is located in the triangular area under the boom. The clew can be brought right down to the top of the aft cabin, blocking off flow through the gap between sail and hull. Provided the cabin does not extend back too far, there is still safe headroom under the boom.

Reefing would probably require the first reef to be a triangular section running from the gooseneck to the leech. The gooseneck would be lowered as the clew is raised, changing the planform to be more like that of a conventional rig. Slab reefing toward the boom without changing the gooseneck would probably result in unacceptable windage as the bunt would be parallel to the boom and angled to the wind. But in either case the center of area would move forward as the sail is reefed, due to the rake of the mast. The highest headroom is at the forward end of the cockpit, which is a good place to keep the crew weight.

If a jib is used, it could be a Yankee jib with a high clew that corresponds to the location of the gooseneck. The rake of the mast means forestay tension will be high. Visibility from the center cockpit of the lambda rig would be exceptional - only the mast and the tack of the Yankee would block the helm's vision. The jib sheets would lead farther aft than a conventional rig, possibly locating the jib tracks along the cockpit coaming. Main winches may well be mounted on the aft cabin top instead of the forward cabin.

I've not yet done any detailed flow analysis of the lambda rig, but I think it makes aerodynamic sense and it opens up some new possibilities for integration with the rest of the boat.

 

Tom,
It sounds like you may have an exciting, id expensive, summer ahead of you.
Do you have a model to test it on, or is it going on a Laser first?
Steve

 

Neither. I'm not pursuing the lambda rig development right now. Although if I get back to working on my trimaran design I'll be considering it as a possible trade study.

If I were working on it, I'd be using a panel code to compare it with a conventional rig and a wishbone boom rig. That won't answer questions like separation and maximum lift, but it would be a first step to understand where the problems might be.

 

In a multihull, with its large beam, is it necessary to have a boom? I don't see the difference between the movement of the aft end of the boom and a curved traveller. A traveller also avoids the need for a boom vang.

 

It looks like Ian Farrier may be thinking about a boomless full batten main for the F22 that is still in development. See <http://www.f-boat.com/pages/trimarans/F-22info.html>.

Note the bottom batten appears to be heavier than the others in the sailplan and that the main sheet pulls down and aft. In the early '90's either Hobie or Nacra - can't remember which - had a similar thing going on with their 18' cat. Had an 18" piece of traveller track running from the clew forward along the foot and a small car with an outhaul line to adjust the direction of pull from the mainsheet.

The dealer selling the boats said it was pretty futzy getting the proper balance of of downhaul vs outhaul, particularly since the straight, beam-mounted track made constant tweaking necessary. He also said getting whacked by that chunk of hardware was as bad or worse than getting hit by a boom. Don't recall seeing it on any recent production beach cats.

 

No, there are very competitive multihulls that don't have booms. An F-31 champion sails boomless, for example. One can use a traveler or dual sheets led outboard. Although there's a limit to how far you can let the sail out before the shape deteriorates if you don't have a boom

The issues tend to be more practical than performance related. Safety is a big motivation for going boomless. But reefing is not as easy. And you've nowhere to stow the main when you drop it.

 

Chris,
The traveller track you talk of is on the Nacra 14 sq (I think) The Nacra 5.8 has an aluminium plate about the same length with five holes in it. You select which hole you want for the day to give some control over the depth in the sail if you want to given the conditions of the day.

I currently sail an old 5.8 and this boomless setup works well. Just dive through in the tacks and gybes - just miss the great big block at the rear of the sail - it hurts.
Brett

F18's are probably the best place to look for these setups.

 

Wylie's wishboned mains

Not exactly 'boomless', rather wishboned boomed mainsail:

Kind of interesting comments on the shape of this alternately boomed mainsail....
* From Chuck Hawley (Regarding Fred Roswold's comments on Randy Repass's new boat, Convergence): I think there may be some misunderstanding on the need to reef this powerful cat ketch. I sailed on Convergence several times with Randy and Sally-Christine before they departed for French Polynesia, and we reefed the main on both occasions, although at higher windspeeds than might have been the case with a conventionally-rigged boat. The winds in Santa Cruz in the early summer are generally around 20-25 knots a few miles offshore, and we saw over 30 knots apparent. I think the point is that the unstayed rigs depower with each puff, bending off to leeward without actually flogging the sail, so that the rig appears to select the correct "gear" for the conditions. Owners of Wylie cats have reported this for years; now Wylie ketch owners are experiencing it!

What's really exciting to me is to see how perfectly both sails set without the intrusion of a boom along their feet (foots?) The wishboom booms create a sail which looks efficient from luff to clew, much more like a genoa than a conventional mains'l. We were able to hit 13.6 knots on a broad reach with excellent control, and with a very broad range of downwind angles that could be sailed without risking an accidental gibe. Truly an innovative and powerful boat.