Why do we all use circular section tubes or dowels for booms?

A more logical solution might be a trapezoidal section boom (trapezoidal carbon sections are available for model aircraft wing trailing edges.

Note that at 1:1 booms of this type (which do exist, although they are not common) have the thick end of the trapezium upwards. On a model yacht with cntre sheeting and a loose-footed sail, the thick end should be downwards.

Just a thought

Interesting, but what does this buy us? Would a trapezoid be better than a flat boom? Why?

Not arguing, just want to learn.


Two schools of thought. A wide flat top boom MAY provide an endplate for the main. This increases the effective aspect ratio. I say ‘may’ because any endplate produces its own parasitic drag, so the effect may be positive or negative.

A vertically deep boom gives added stiffness. This means that sheet/vang loads are transmitted to the leech of the sail more precisely. Again, this may or not be an advantage, but the case is more clear cut.

If the mast bends, we have to adjust the luff round to the shape of the bent mast (to flatten the sail) and adjust the stiffness of the topmast, which eases the leech as it bends. There is little, if any, theoretical difference between a boom that bends upwards at the end and a topmast that bends backwards at the tip - but if we keep the boom rigid we have eliminated one unknowmn and only have to concentrate on the mast.


why not something like the south african AC team’s boom? it is just framed, [i think it is triangular in shape] might save some weight up top… sigh, gonna have to figure out how to try it now…

I see your point, but doesn’t that suggest that the bending force on a carbon arrow shaft boom over a span of maybe 100 mm (from vang to end) is going to be significant enough to deflect said shaft appreciably? Arrow shafts’ “spine deflection” is measured with a 2 lb (1kg) weight suspended on the shaft which is supported 26 inches (660 mm) apart and produce deflections on the order of 10 mm. I would think that the rig could generate <100mg of force at most in that direction.


Angus or anyone else who knows this stuff…

Is there any evidence that these kinds of things have any effect at Footy speeds, and how much?

I’m guessing it would be awfully small compared to plowing into a wave, or sailing just a bit too closely hauled, or hesitating a second too long to take advantage of a wind shift or…

Bill H

I think this is one of those theoretical worries which just does not apply to us. A 3mmOD 2mmID carbon pultruded boom is very light and still seriously over strong for our needs. On the main boom the leech tension required to bend the boom between the kicking strap attachment and the rear sail attachment would be quite ridiculous… you would rip the sail apart first. Likewise the jib boom… imagine how much forestay tension that really is.

Conversely a vertical rectangular or trapezoidal boom in carbon may make certain attachment points easier to configure… and that makes it an idea worth looking at for a completely different reason.

Thankfully what Bill says about decision making is right IMHO… miss a lay line when your competitor hits it and you’re done… which is what makes the designer/builder classes so much fun.


  1. Graham. You need surprisingly little defelection in the boom to make a difference to the shape of the leech - and the higher the span:chord ratio of the sail, the smaller it is.

  2. Doug. Apart from the fact that 100 mg HAS to be a typo :slight_smile: , what you’re actally saying (as is Graham) is that your arrow shaft boom is over strength and hence probably over weight.

  3. No Bill, the differences are not readily demonstratable, but small incremental benefits add up - and tenths of seconds turn into minutes. The fact that the benefit of each feature is not individually not clear is that why we get fashions for this, that or the other gewgaw. What prompted me to think about it is that most classes restrict boom diameter (presumably for ease of measurement) but the Footy has no restriction whatsoever. I quite certainly wouldn’t throw away an existing boom, but I might think of a less conventional sectionif I were making a new one.

There is, of course, another reason. Let me tell you a little story - never before revealed to the public.

Once upon a time I fitted a tip spike to te fin of the mini-tonner Black Dog just before the last and crucial race in a series. If we won, we won the series. If our Dreaded Rival won, he did.

We fitted the spike late in the evening with the boat in the slings of the Travel-lift after we’d scrubbed her bottom. We put a 'skirt a la Australia II round the fin and retired to the bar, looking suitably conspiratorial. Dreaded Rival was there drinking a pint.

After a bit (and dark) he slipped out and disappeared in the direction of te hoist, coming back 10 minutes later, looking worried.

In the morning we slaughtered him. He was spedning all his time looking at us and not concentrating. He seemed to expect us to go into Warp Factor 7 at any moment.

What he did not know was that we’d stuck the spike on with water soluble glue!

Flash fittings frighten the opposition!:devil3:

Erm I have a big problem with being told what I am saying…

What I am saying… again, is that a boom has other functions and as such needs to be of a certain size to have fittings attached to it. At that more or less minimum practical size (diameter in this case) a carbon tube boom will not bend under the small stress of a filling sail by any significant amount. Yes that is gut rather than numbers but I have a very reliable gut… So I don’t believe I can go much lighter due to practical size, and at this size (3x2) I guess I am happy.

I still may try a flat boom though ‘if’ it is lighter.


Great story, Angus, and your point about incremental benefits is well taken. I was just trying to say that IMHO race-day decisions and educated thumbs make the biggest difference.

I’ve been a competitive (more or less) archer for a long time, and I love seeing opponents who are constantly trying to get the perfect tune on their gear. They spend their time in the workshop. I spend mine on the practice range.

Bill H