Boom Measurement

There are generally 2 types of rules within a class: rules for safety and rules to prevent an unfair speed advantage. Here is a rule from the VO70 class:

6.3 Boom Measurement

6.3.1 The top edge of the boom shall be straight within a tolerance of 20mm between points 500mm from the extreme ends of the boom. The cross section of the boom shall not exceed 400mm at any point, in any transverse orientation. Outriggers and/or diamond struts are not permitted on booms.



So, this is probably not a safety rule. What were the rule writers trying to avoid? What could the rest of the non-VO70 sailors do to their booms that could increase the speed of their craft?

If a "crooked" boom was allowed, what configurations might be tried to go faster? Is there such a thing as an articulated/gybing boom? Is a convex or a concave boom superior to a straight boom? Should boom bend be as easy to adjust as mast bend? Are there any classes that do not have boom limitations and have demonstrated innovation in boom design?

It is not beneath me to cheat when it comes to boat design. Here is a rule just begging to be broken.

 

boom

I can see making the boom deep like the ACC guys do for whatever "endplate" effect may be gained that way and the extra area dead downwind. Or a "park avenue" type boom for just the endplate effect but, so far I don't see any advantage in a bent or articulated boom.
I think my favorite boom that would be illegal in most classes and rules would be a modification of a wishbone boom(parallel to a "normal" boom) that allows the fwd third or so of the main to come right down to the deck and uses twin vangs-one on each side. Maybe with an athwharthip structural member that takes a midboom mainsheet...

 

The rulemakers wanted to define a maximum endplate effect, which means that the effective aspect ratio of the mainsail cannot be increased by ever wider booms.
Articulated booms (articulating on the deck that is) are seen on Mini-650, Open60 and other boats. The main object of this mounting point is to do away with the strain that the gooseneck otherwise would have put on the mast. Lighter masts can be built this way, which lowers CG, which increases stability, etc. Of course, it's also a way to sheet the mainsail a bit lower, so overall there's a perfomance angle on things as well.

bye
Hans

 

Doug's Boom Design

Wow, that's unique, Doug. A wishbone or even A-frame boom would be inherently stiff and structurally strong. You do think outside of the ordinary.

What would the gooseneck resemble? How about a cabin-top traveller/car system with an elliptical arc to automatically tension the outhaul for upwind/downwind sail trim. Put a brake on it to prevent accidental gybes . . .

 

modified wishbone

Thanks, SP. The boom will be used on the 14' foiler Eric and I are working on. On the model of the boat the "gooseneck" is on the front of the mast and thats probably a good place for it so as not to disrupt the sail more than necessary.It would have to be a more robust version of a windsurfer type gooseneck but essentially the same. I think it is an important detail to get as much of the main as low as possible; this boom allows 30 to 40% of the forward end of the sail to be as low as it can go...
Got to think about your traveler/car arrangement-off the top of my head sounds like it would work.

 

Its a mistake to think that every rule is precisely targeted and there's an advantage to be had by going outside it which the rule writers had clearly in mind. Rule making just isn't that precise an art. In most rules sets there are going to be things that are in there to keep everyone in the same sort of ballpark and discourage people from thinking about something weird without any great idea of what that weird might be.

On the wishbone type boom. this is something I've thought about too without coming to any great conclusions. Its clearly desirable on a singlehanded boat to have the mainsail as near as possible to the floor round the luff, and perhaps eveb more so on a two sail boat. Normally on the other hand you have to get the crew under the boom as well though.

The conventional boom and kicking strap is simple, light, and far too far off the deck, with all sorts of drag happening underneath it.

The inverted rigid vang without pocket is aerodynamically ghastly, doing nasty things to saiil shape on one tack and not the other.

The Bethwaite pocket arrangement with the same thing is consistent on both tacks, still doing things to windflow and complex to make and rig.

Wishbone type designs have more drag and all sorts of problems dealing with the loads.
If you take the sailboard arrangement as regards wishbone and gooseneck with the booms being curved and then bang on a klicking strap each side as Doug suggests then you have really odd things happening to the beams load wise, and all sorts of interesting challenges in the structure - the loads from the kicker will tend to rotate the booms around their end joins for instance, so the structure will have to be stronger in torsion than most standard carbon tubes and the joins very well engineered, so lots to think about. Wishbones have trouble with class rules in some classes too.

Then you could work rouind thiis by making the boom tunes straight and have muich wider end fittings to cope with the curve of the sail, but that's big and heavy and draggy and nasty.

You could have a single sided wishbone in the manner of singlesided swinging arms seen on some motorcycles. It feels horribly heavy to me, but probabaly a simpler frabrication exercise than the symettrical one. Then, with fully battened sails that have low outhaul tensions if huge vertical ones you could consider fabricating an S shaped or curved boom vertically to get the luff low and the leech higher for clearance, but again fabrication and weight, rules too often...

Then there is the problem of kicker loads - given a non-inverted vang then as the boom gets close to the floor then the kicker is acting with more and more of a forward vector and less vertical, so the compression loads on the goosenck become vicious and most of the effort tensioning the thing is wasted in putting insane loads into the structure. Maybe you could think about a radial track like some keelboats, but then there are consequant loads to transfer in the underlying structure...

Don't think there are any easy answers with this... when you consider all the problems of the most likely alternatives its hardly suprisng the traditional version, for all its faults, still dominates...But maybe there's some new concept I haven't thought of, but I don't see an obvious best compromise staring me in the face.

 

aeroSKIFF boom

The modified wishbone boom is being used on the 14 footer to get as much of the main right down to the deck as possible-no gap. It doesn't look like a "normal" wishbone but is at the same height and approximately parallel to a normal boom(which is approximately parallel to the waterline). By modifying the wishbone we seal the forward 30-40% of the main and we wind up with enough height aft of the sail for the crew to easily pass under the boom. This boat also has carbon tubes that support a sliding bench seat and it is important to me that the mainsheet leads from forward so the skippers forward hand takes the sheet and his/her aft hand takes the extension tiller. This works out so that, with an athwhartsip structural member between the two halves of the boom , the sheet is led down from the boom to the forward carbon tube and to a traveler mounted there or alternatively , after testing, we may just stick with a block and the twin vang concept.
At any rate, the three critical factors are: 1) getting as much of the main down to form as good a seal as possible with the deck,2) maintain enough headroom under the boom for the crew to cross without a headache, 3) Having the mainsheet lead from forward without any part of the sheet being capable of fouling the extension tillers.
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edit: a word on drag: I'm fairly confidant that a sorta wishbone + sail, as I've described, would have less drag than almost any rig I have yet seen; maybe we'll be able to test it accurately one day.

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This is for a wanabe production boat not just a oneoff( I hope).The rig(unstayed, 130 sq.ft.) is being looked at very carefuly as is everything to refine it as much as is practical.