Quote:
Originally Posted by estarzinger
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Andrew, Yes my boom section is massive, and I have a really long sleeve both inside and outside centered over the vang lug. I am a bit surprised you think mid-boom is better. Does not mid-boom have a 3m long (with Hawk's dimensions) lever arm acting on it (from the clew load) while end boom has zero lever arm?
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Evans
What you say is true from the narrow perspective of what's best for the boom, considered as a "beam in bending" (forgive the engineer-speak).
My concerns, in the general case, are the wider implications for the gooseneck and
mast, and for aspects of operation.
However you've been doing it with conspicuous success, in your specific case, for long enough that I'm sure you can safely treat my concerns as mildly interesting, at best.
So anyway, here they are:
(reminder to others reading this: these comments are to do with TOERAIL preventers, not bow preventers.
In a few cases, you might be able to extend or adapt the observations to apply to aspects of a bow preventer
(further note: they're based on beam at the chainplates falling within the 'normal'
monohull range relative to boom length... and to me Hawk's powerful form does fall within that, under
current norms.
My concerns become more strident as beam reduces):
When connecting the preventer to the toerail rather than the bow, I personally think mid boom is better, provided the boom can handle it.
My reasons:
1) The tension in the mid boom preventer, and hence the bending moment on the boom, reduces as the boom is eased across with the main backwinded *. If the preventer runs to the boom end, the tension
increases in the same situation.
While it is unlikely to bend the boom, because it's applied ever more in line with it, this increasingly steep tension rise could create a "toggle press" intensification effect, applying thrust loads into the
mast via the gooseneck, for which the geometry alone does not define an upper limit. (Engineer-speak for potentially crippling). Operation, however, can judiciously apply limits, and in Evans' and Beth's case, clearly that has worked well.
2) If end-boom preventers are both set up to stabilise the boom in extreme
weather when the main is down, or when reefing (performing the function of a gallows) the acute angles resulting from the boom being middled make this a suboptimal strategy (I'm picking my words carefully because it's nevertheless a great strategy, and 'circumstances alter cases').
The preventers have to resist the inertia of a (hopefully heavy) boom, when the boat may be taking serious sideways hits. In extreme cases, a wavecrest might land in the foot of the sail while it's being reefed. I would personally prefer the boom to attempt to take this on the chin, rather than kicking the stress into the mast, and with the deeply reefed main implied by such conditions, the mid boom preventer is actually better placed, even for boom bending considered in isolation.
In more normal conditions, there is still a case to be made that the higher tensions mean such preventers will stretch more (even with low-stretch line, which I consider desirable) than a mid-boom setup, allowing increased boom motion in a big swell – with the possibility of extra chafe to the blocks and the line. A mid boom preventer will need a multi-part tackle to achieve the same line loads as a single-part end-boom preventer, and this further reduces stretch (although admittedly it increases the localities for chafe)
3) Mid boom preventers remain permanently rigged. End-boom preventers need re-leading every time they transect the
lifelines, as Evans mentioned. This means the connection must be easily disconnectable, and even at grand-prix levels of
hardware, failures are inseperably linked with complexity of form and function.
This also implies leaving the
cockpit, and could expose the boat or crew to risk during the time they're disconnected. For solo sailors, especially when in confined waters with frequent course changes, this is particularly undesirable.
4) Permanently rigged preventers are useful in light variable winds, even hard on the wind. I often remove the mainsheet altogether under such circumstances, use the topping
lift to control leech tension, and use the windward mid-boom preventer to slew the boom.
The preventer hauls the boom delicately sideways, whereas a mainsheet, with traveller up to windward, hauls downwards, fighting with the topping
lift: "Lots of sound for very little action".
There is also gratifyingly little line to handle, given the rapid and radical changes in boom angle implied by shifting wind direction. It's like being back in a
dinghy....
5) For booms which are at
head height:
If it's sloppy, the leeward preventer can also be hardened in situation 4) without unduly loading things up. This can be a worthwhile precaution.
The hardest I've ever been hit by a boom was when it was sheeted amidships, in light winds in sheltered waters, and my
head was close by it on the windward side.
I was hanging onto the mainsheet falls as I looked up to windward, so I'd be warned of a shift which hadn't made it down to the surface yet. We hit a wake I hadn't noticed. The sail slatted once, the boom gave a twist (about the long axis) and caught me on a
tender cranial contour. Boy ! did that hurt!
An end boom preventer does little to prevent the boom twisting in this way, because the force is largely horizontal, rather than downwards.
I think I've got others, but I've run out of time.
Thanks for reading !
YMWV ...
* many sailors whose rigs rely on runner support never initiate a gybe without first middling the boom, in which case this might seem academic .... but even they might need to recover from an inadvertent gybe.
There may come a time when the opportunity to do so by turning the bow down has passed by. Generally they will be able to come up on the new tack, but in rare circumstances this will not be an option
Let's just say that I am not too highly worried about breaking either boom or gooseneck. So, I guess I am interested in other trade-offs?
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