dgadee
Well-Known Member
Where are the horizontal stresses at the kicker take-off point? My boom brake pulls the boom down towards the deck. Can't visualise any sideways pull on the boom.
Boom Brake Question.
- Thread starter goeasy123
- Start date
thinwater
Well-Known Member
Think back to school physics.
In math you learned that 2(2+3)=2x2=2x3. In physics you learn that forces can be separated into their vertical an horizontal components (or any angle) and treated separately, using the appropriate maths. Same idea.
The sole purpose of a boom brake is to resist a side force that can be as great as several thousand pounds. A byproduct of resisting this force, because the rope is angled downwards, is a downwards force. In other words, a boom brake is equivalent to a vang plus a horizontal rope directed toward the beam.
duncan99210
Well-Known Member
I think that there’s potential for confusion here. I regard the function of a boom brake differently from the function of a preventer.
A boom brake is there to absorb some of the momentum of an unexpected gybe by means of friction. It isn’t there to hold the boom in place, just to slow the movement down. This means that you can attach the brake to the same point on the boom as the mainsheet as the loads you’re seeking to minimise would have fallen on those point if the brake wasn’t there.
A preventer’s function is to hold the boom in place against the wind, thus preventing the boom moving across the boat in an unplanned gybe. The loads are greater and therefore need to go to the end of the boom to avoid over stressing the mid point of the boom.
If a boom brake can be applied so hard as to hold the boom in place, to my mind it stops being a brake and becomes a preventer and should be rigged to the end of the boom.
A boom brake is there to absorb some of the momentum of an unexpected gybe by means of friction. It isn’t there to hold the boom in place, just to slow the movement down. This means that you can attach the brake to the same point on the boom as the mainsheet as the loads you’re seeking to minimise would have fallen on those point if the brake wasn’t there.
A preventer’s function is to hold the boom in place against the wind, thus preventing the boom moving across the boat in an unplanned gybe. The loads are greater and therefore need to go to the end of the boom to avoid over stressing the mid point of the boom.
If a boom brake can be applied so hard as to hold the boom in place, to my mind it stops being a brake and becomes a preventer and should be rigged to the end of the boom.
dgadee
Well-Known Member
I agree a preventer is a stop on movement, but when my boom accidentally goes over with a boom brake there is a lot of creaking (and probably heat produced on rope) but no stop on movement. Mine has a central wheel you can tighten or slacken to change resistance. There is obviously some horizontal pull but not as much as on a preventer. I would only use the latter in light winds.
Edit: the ropes from the boom brake go down to the base of the stays on each side, which is why I think there is much more vertical pull than horizontal pull. It's the 'Dutchman' model.
Edit: the ropes from the boom brake go down to the base of the stays on each side, which is why I think there is much more vertical pull than horizontal pull. It's the 'Dutchman' model.
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thinwater
Well-Known Member
Yes... and no.
By angling the rope downward there is the SAME amount of horizontal pull but increased vertical pull.
dgadee
Well-Known Member
Isn't that what I said? What I am not sure about is just how much horizontal force is on the boom. Seeems (much?) less to me than on a prevernter because a boom brake gives rather than stops.
RJJ
Well-Known Member
Yes, and if you have an aft mainsheet then the boom brake would be operating a most unfavourable angle.
thinwater
Well-Known Member
That's a really interesting question, and the answer surely depends on how the tight the brake is.
The scary parts are:
- What if the brake is over tensioned?
- What is the ratio of control line tension to friction? This is adjustable with all brakes and also depends on the rope. In other words, 200 pounds on the control line might be 2000 pounds of friction. For comparison, a climber's figure 8 is about 7:1, depending on the rope. Some boom brakes are capable of 20:1.
- Tension varies with the point in the swing. How much depends on the height difference between the boom and deck.
- The application point for side force is all wrong. Even though the force is 5 times less, the leverage is 5 times worse than boom-end, and the boom is NOT strong there.
- How does the force compare to vang tension? I suspect it can be similar (the friction multiplier and the vang tackle multipliers are similar). If the vang is on, how additive are they?
- I'd be VERY careful how much tackle or winch I put on the control line.
No single answer. An interesting discussion question. And a good cautionary statement.
goeasy123
Well-Known Member
This, I think, is all irrelivant and the answer is a simple mechanics problem. What really matters is the relative attachment points of the main sheet, clew and the preventer or brake. If they are not coincident them a large bending moment is applied to the boom. This is what causes the boom to break.
The only way a brake should be used is loose so that the boom always moves slowly in an accidental gybe thus relieving the bending moment. The only place a preventer (or locked brake) should be fixed is at the same point as the main sheet. Note: that this might not be the end of the boom as most experts seem to advocate.
thinwater
Well-Known Member
Of course, they never are coincident.
You may want to rethink your "only point" statement for conventional preventers. There are more factors than bending moment around the mainsheet attachment, such as leverage on the preventer line to the bow, the possibility of dropping a boom in the water 9mid-boom preventers don't do well in this case), and load on the gooseneck. You will loose that discussion; history and engineering say different.
You DO imply an interesting point. A boom designed for mid-boom sheeting is a lot beefier, both vertically and horizontally. Where that not true, the moment applied by the clew would snap the boom. Thus, they are inherently more suitable for heavy use of brakes.
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Personally, I have little use for a preventer. Multihulls barely have use for them because we have a traveler wide enough to control the boom and don't square the boom off downwind. But I like hearing out-side-the-box discussions. I'm guessing the boom brake resulted from an outside-the-box discussion. Just maybe, there is a next-evolution on the horizon.
solo 32
New Member
Hi just seen this post and I'm planning to buy a boom brake / preventer i have been lookin at the dutchman model... Is it worth the money and what wind speeds do they work in.
I have a jeanneau sun oddessey 32 so not sure what size to get etc?
I'm a newbie and feel safer when going on wing on wing etc
dgadee
Well-Known Member
I brought mine back from the US when the £ was higher than it is now. I probably wouldn't have bought it at UK prices. So I can't say whether it is worth the money to you.
Which model? Ask them: Boom Brake - Dutchmar
solo 32
New Member
Thank you for the info about the price etc
The next question is are they any good and what wind speed are the good upto?
Thanks Neil
dgadee
Well-Known Member
My experience is that it works when well reefed down in strong wind. Once, going downwind in short, steep seas we broached constantly. It was difficult to stop the odd accidental gybe. The boombrake took the force out of the gybe.