Wishbone Boom Versus Conventional Boom

In another thread (Multihulls>Maneuverable Trimaran Design), Paul Scott asked why I chose a wishbone rig for Broomstick, my 15' hydrofoil trimaran. I thought the question of wishbone boom vs. conventional boom was of more general interest than the original thread and I wanted to see what other people thought, so I'm starting a new thread. I hope I'm not rehashing stuff that's been covered earlier, but I couldn't find much on wishbone rigs.

When I was designing the boat, these were my main considerations:
1. Using a wishbone boom reduces the compression on the mast & the bending of the main hull, so possibly I could make things lighter &/or sail in stronger winds than if I used a conventional boom.
2. I already had the mast & sail from a previous boat (1974 vintage) & the mast had been cut off too short to allow enough clearance for me to get under the sail when tacking & jibing.
3. I also already had a collection of old Windsurfer booms (1982 vintage) .


The first attached photo shows the original configuration. It has a 85 ft**2 main with wishboom & a 22ft**2 jib. At first, I had no real traveller. The mainsheet blocks attached to the aft crossbeam with a simple lashing that could only move a few inches off center. The wishboom attached to a track on the front of the mast, which I could adjust while rigging, but not while sailing.

After encountering several long stretches of fairly light air on the local lakes where I sail, I got a larger main (118 ft**2) which is essentially a loose-footed Hobie 14 sail and an old Hobie 14 mast. There was now ample clearance under the sail & I no longer had a wishboom long enough to use with it. So I went with a conventional boom & added a proper traveller. This allows much better control of the twist of the main (in spite of the huge twist in the second attached photo, which was taken an instant after leaving the beach & before I could get anything trimmed properly).

In most conditions, the larger main + jib is too much sail area (143 ft**2) for me to handle by myself, so I still sail with the original rig most of the time, but now I have a traveller to control the twist & the wishboom just allows me to do without a vang or preventer while jibing.

As explained in the earlier thread, I'm now converting to a cat rig without the jib. I'll probably be using the 118 ft**2 main with conventional boom most of the time, but save the 85 ft**2 main for really windy days. I'll probably be adding an extension to the base of its mast in order to step it on the foredeck instead of the main crossbeam, and allow enough clearance to use a conventional boom.

So far, I've only mentioned the twist control as the advantage of the conventional boom, but there's also the matter of the greater aerodynamic drag due to the wishboom. I have some vague memories about wing efficiency being improved by the proximity of a wake under it, so maybe the drag of the windward 1/2 of the wishboom might not be so bad, but I have a hard time believing that it could ever make up for the extra drag due to the leeward 1/2 of the wishboom. Does anybody have any information on the drag of wishbone booms or any other thoughts on wishboom vs. conventional rigs?

 

I did some experimentation a while back with a unique wishbone. Technically, it wasn't a wishbone but a half of one.
The boom went along the jib luff perpendicular. It was a T section (my quick solution to a rigid spar)--- the top of the tee faced the sail.
The flat tee-top tapered at its ends, and the stem of the tee was horizontal (right angles and away from the sail). The sideways oriented stem part wasn't nearly as tall as the flat top of the tee that ran parallel to the sail.
There were a few fairleads along the outer edge of the stem section, through which a spectra line ran. This allowed instant adjustment of draft. Since all the pressure tended to compress the spar, tightening the tension line always gave the perfect heavy air draft, and easing it relaxed it to its maximum "C" shape. The sail never touched it, but came close in the eased position.
This mono-bone was pretty light. It did need one "topping lift" to keep it from sagging, but that seemed to work fine no matter what the draft was adjusted to. It was self-tending and so by centering it, the draft line could be adjusted from just ahead of the mast. The performance was very good, adding 9/10 of a knot to the top speed previously measured.
The advantages of a single mono-bone would be a reduction by half of any interference caused by its proximity to the sail. It would be lighter too. The flat section might even make up for the tubulance by assisting in laminar flow.
Mine was on 100 sq ft of jib, and was 4" x 1/2" thick (flat of tee at middle), and 1" x 3/4" (stem of tee at middle). A double purchase was used on the draft line.
On a jib, tension is against the jibstay at right angles, and so any LP spar will push it out a bit, requiring good tensioning of the stays.
On a mast, the angle is maybe 60-70 degrees, and the mast is very stiff of course. So you're right that there's less pressure against the mast. Also, one could locate a jumper strut at that level in any case. I say that because while the pressure is less than at the normal boom level, the wishbone level probably falls on a part of the mast furthest from either deck or shrouds, and so it will flex more at lower pressures, maybe cancelling the lower pressure advantage.

Alan

 

wishbone

Hey, Doug. The attached sketch shows a wishbone bone used similarly to a regular boom. The only point of it is to allow the forward part of the main to come right down to the deck. I hope to use this on my new boat where fully 40% of the foot of the main will come down to the deck. At the point of the mainsheet the two sides of the wishbone are attached with a "V" carbon tube glased on to each carbon half of the wishbone. Untested but holds promise...

 

Doug H.: You show the front of the boom attaching to the mast well above the clew. If the front were lower (boom more horizontal), then there would be less aerodynamic penalty.
Perhaps more interestingly, the angle of the boom should affect the relative amounts of tension in the foot and the leech, which should alter the degree of twist in the sail. Has anyone got more/clearer thoughts on this? Doug L. it looks like your sketched boat has a leech-tensioning device under the middle of the boom , or is that just the mainsheet? Alan: Did you control tension/twist somehow?

Matt

 

Hi Matt,

My only control (remember this was a jib) was the line used to straighten the spar, which flattened the sail. Untensioned, it was set up for full draft.
So the spar itself was "active". The control line ran along the outside of the curve on stand-offs, so the limit of curve depended on the line not stretching.
So I used spectra, and might have used wire, but for the chafe. A really nice setup would have roller fairleads for low friction, and a wire instead of rope.
The fore end of the spar was lashed to the sail grommets. Adjustment under sail required the boom to be over the deck. If this were used on a mast as a mailsail, the line could run down and back to the cockpit from the front of the spar. Then you could "shift on the fly".

A.

 

wishbone

=======================
Matt, on the square topped rigs I've experimented with the most important twist control is the vang and/or a mid-boom mainsheet. The nature of the square top seems to be automatic gust response.
The vang/sheet just sets the range of response.
On the aeroSKIFF sketch what is shown is the main sheet which is on a traveler. It also would act like a vang which may not be necessary with the mainsheet so far forward. It was important to me to have the sheet lead from the front with no lines aft off the boom to get caught in the dual hiking sticks.

 

Matth:
We are getting excellent draft and twist adjustability in unstayed uni-rigs by linking the boom-end to the mast base with an adjustable in-haul running through the foot of the sail and controlling mast bend with 'running' topping lifts from the mast head to the boom end. This solves the typical wishbone mixing and coupling of outhaul with leech tension allowing total control of draft and twist in square-headed 'chop top' sails. At the same time we are lowering the wishbone on the mast to reduce weight and windage aloft while preserving an end-plated boom-less foot planform. The uni-rig really comes into it's own with 'chop tops' and finally, under this arrangement, ultimate shape/twist control is attainable with a minimum of rigging. In some cases the running tops are not necessary but they are easy to rig and there is no penalty for failing to switch them in a timely fashion during close maneuvering or accidental jibes on larger rigs.
Bill Hansen
Hansen Design
Recent Projects:
Wyliecat 30
Wyliecat 17
Wylie 'Taxi Dancer'
Hoot Dinghy
Laser Turbo
Antrim Wing Dinghy
Cal-20+ Unirig

<<Matth wrote:
the angle of the boom should affect the relative amounts of tension in the foot and the leech, which should alter the degree of twist in the sail. Has anyone got more/clearer thoughts on this?>>

 

Back in the eighties, Rotating Asymmetric Foils (RAF sails) were just coming into wind surfing. http://wind.surf.vl.ru/equipment/sails/

Looking at the images you posted, I see the sloping wishbone presents a larger cross section to wind flow, which the RAF rig would not. Neither would a camber induced rig, but the RAF is intended to mimic an lifting surface shape more effectively. The windsurfer rig also controls twist more efficiently and boom bangs on the head won't happen.

A look at more recent wind surfing rigs might be inspirational.

Good luck

Pericles

 

Hansen: I've looked at the Hoot website; do you have links to see the rest, or is there a Hansen Design website?

Thanks,
Matt

 

Hi Bill,

Thoughts?
A vertical seperation (maybe by a few feet) of the forward ends of the wishbone (assymetrical wishbone) might be an excellent way to get total adjustment (by raising and lowering the mast end of the spars independantly to tension and also apprortion that tension).
No topping lifts, no inhaul. Track on forward edge of mast. Two looped control lines.

alan

 

Matth:
Most of these projects are presently in testing and remain proprietary to the client. Once they are approved and released, pics will be posted here:
http://www.hansenaerosports.com/

Alan:
Your suggestion is possible but seems complicated. The common wishbone 'choker' system provides a similar function but both mix outhaul with vang. The in-haul is a vang replacement and outhaul limiter which provides leech tension without flattening the sail's lower sections. Running 'tops' may sound complicated but they are actually very simple and the benefit is independent shaping of mast bend and upper sail shape and twist. It is equivalent to an adjustable backstay or adjustable uppers on stayed rigs but also allows a rotating rig. With these three adustments, one has complete control of sail shape on an unstayed uni-rig with complete freedom as to roach or extent of 'chop top' planform. IMHO, conventional rigs become cumbersome, highly loaded and expensive once the sail diverges from a simple triangle. My prediction is 'chop tops' will become the future in yachts just as they have in windsurfing and an unstayed uni-rig is the best choice being less expensive, less complicated and more efficient.
- Bill Hansen

 

Thanks, Bill. I guess the question of complication of an offset wishbone is at least still a question. The means to adjust the two independant parts may possibly take more time than the two minutes I used up to think up the control system. I was more thinking of the basis of the idea than the control lines, and I totally agree that your system is simple and reliable not to mention fine-tunable. I'll work on the idea some more and see how elegant it could actually be. As with most new ideas, it is very likely to be inferior to well tested existing setups. My own feeling is that the idea is sound enough to noodle on for a while.

Alan

 

Bill, I remember back in the the late 70's, being an ex Finn sailor, and being frustrated with the lack of sail controls on windsurfers, I was messing with modifying windsurfing sail controls, and one of the criticisms of what I think it is you're calling the inhaul (if I understand it correctly) was that when I increased it's tension, the foot of the sail increased in depth of camber (or as some called it (me & a couple of I14 sailors))- went baggy, because the end of the boom was moving independently of the mast, which, if the boom end moved down, tightened the leach but released the foot (which is why, I think, you've gone with the running backs???????????) so we thought it was because the inhaul was stressing the wishbooms apart, so I tried stabilizing the outward flex of the wishboom by a small rod going through the sail from one boom to the other (horizontally), which helped some (at least it helped stabilize the point of maximum camber (CE) a bit.). So then I tried turning the inhaul (which by the way, I was thinking of as a vang) also into an outhaul, which also messed with the leach, but not as much. So, remembering what the Soling class was doing with their self tacking jibs as far as leach control, I installed a 'card' at the outhaul point of the sail that had different hole positions for the inhaul/vang, which was better, but not that much better, until I combined the card, a typical wishboom outhaul, and the vang also as an outhaul (both lines on different positions on the card), routed around different geometric points (heights) on the end of the boom, so I could balance the two as far as controlling the foot bagginess and leach control, but it was all a lot to ask of a sailor on a Div II log, it was so difficult and time consuming to adjust and I got made fun of a lot, so I gave up, going on to dynamic outhaul control via the use of bicycle brake handles, but that is another story. So is foot bagginess a problem with your system? Or do you use stretchy stuff in the foot to take up the slack? I also tried solid rods as a vang using turnbuckles to adjust it, but that was worse. And hurt the board and me.

Paul

 

Rethinking the mono-bone I mentioned earlier (which as said, I used on a jib with good success, which naturally had a straight leech), I do like the simplicity of a spar on one side only that has an outboard tensioning line (on stand-offs which act as mini spreaders) which straighten the spar (as much as the sail will do so) when the outboard line is tensioned. The idea is that it is extremely rigid in any position, as all forces tend to be compressive.
Overall lengthening of boom length is a single line down the mast from a block at the forward end of the spar. What's left is clew height, which is necessarily by way of topping lifts at boom-end, and a vanging type control below the sail's foot from the same point to mast below tack.
Windage of spar is reduced by half by using a single (one side only) spar. Weight aloft also is reduced. Question remaining is whether vertical positioning of the forward end of the spar could be fine-tuned enough to negate the need for the lifting line, leaving only the vanging line to adjust leech tension relative to foot tension.

A.

 

Paul:
Experience has shown that significant draft low in the sail works best on uni-rigs without jibs. Hence, I always design a full batten fairly close to the foot such that in-haul compresses the batten against the mast and locks in the shape. If it becomes necessaary to flatten the sail it is eased and 'choker' (outhaul) applied. An added refinement is a sculpted 'shelf' foot with a pocket along the lower edge containing the in-haul. This also works well on sails without a foot batten.

<<So is foot bagginess a problem with your system? Or do you use stretchy stuff in the foot to take up the slack?>>