boom partly broken at preventer

[foolishsailor]

Hey all,

Been lurking here forever and most of my questions seem to get answered before I need to ask them, this one however I cant seem to find in the archives...

A friend just made a crossing from Brazil to here (Cape Town) and his boom has parted partially at the point where preventer attaches.

His boat is 29' long and the boom is a hollow box construction of stika spruce from around the 30's. Very well designed with tapered ends.

The boom is parting PERPINDICULAR to the grain. i.e. 30% around the circumference at the connection to the preventer. Is this indicitive of any other unseen problems? There are no splits in grain or glue joints and the boom is otherwise in good shape...?

He wants to keep the boom, obviously...its well made, but we are concerned on placing a scarf joint on an area that has such high lateral stress.

Would a 12-1 nibbed scarf work here? WOuld a 20-1 or other type be better?

Which way should the scarf go for maximum strength, vertically or horizontally?

How large should the replacement piece extend fore and aft of the break?

Would adding a spreader wedge, either wooden or via several rope straps, under the boom where the preventer attaches spread the load or just dreate two new hard points for future breakage?

Is there anything we are not asking that we should???

This is a serious singlehander who has no engine and tends to sail the wrong way around (i.e. 8 week passage from brazil to cape town in april in horrible conditions) so I am hesitant to offer advice or help that may end up putting him in a terrible situation with a broken boom at sea...

[Ian McColgin]

Perpendicular to the grain? Like an essentially vertical crack across the boom?

The term preventer confuses me. Where I grew up, a preventer on a boom was rather like a fore guy really to keep he sail out and flat on a broad reach or run. It would be commonly lead boom to the rail either down or a bit forward. This preventer must be adjusted with every change of sail trim.

You could also mean what I call a boom vang, which leads from the boom to the mast and keeps the sail flat whatever the trim.

Either can cause tremendous point loaded bending stress caused on the boom. The rail led preventer can cause an especially strong loading if the boom hits the water. So the preventer is likely the cause of the problem.

First the fix. 12:1 should be adequate. Orient the grain the way the original grain is oriented. I take it this is a box construction. I’d duplicate that also, so you’re replacing wood on three sides. You don’t have much damage to replace so each of the pieces can be essentially a shallow V with each leg being the 12:1.

There is no need to make a fussy hooked scarf. With modern epoxy, the hook does not add strength. It is nice to nip off the feather edge and depress the V on the female side just a tad so that the last 1/32” or so is a vertical to the grain piece so you don’t have a hard glue line that sometimes leaves a minor bump when the piece is sanded out, but again, not at all necessary.

Now, preventing preventer damage. Spread the strain. I always strop such rigging anyway. Try two strops to spread the load a bit.

G’luck

[Tom Robb]

Given that there may be a language/name-of-the-thing question, pictures would be helpful.

[Jay Greer]

I gather that you are considering laying in scarfs all around the box section or is this just on the two side panels?

[foolishsailor]

Perpendicular to the grain? Like an essentially vertical crack across the boom?

Yes the crack is vertical and about 30% of the circumference.


...the term preventer confuses me. Where I grew up, a preventer on a boom was rather like a fore guy really to keep he sail out and flat on a broad reach or run...

Your use of the term preventer is the same as I inteneded, it is to prevent an accidental jibe while sailing downwind on open ocean conditions.


either can cause tremendous point loaded bending stress caused on the boom. The rail led preventer can cause an especially strong loading if the boom hits the water. So the preventer is likely the cause of the problem.

I am assuming it must be, there are no other attachment points near the crack and it is directly above and slightly towards the clew end of the boom (about 1 inch) from the preventer attachment.


First the fix. 12:1 should be adequate. Orient the grain the way the original grain is oriented. I take it this is a box construction. I’d duplicate that also, so you’re replacing wood on three sides. You don’t have much damage to replace so each of the pieces can be essentially a shallow V with each leg being the 12:1.

I am concerned that only replacing the wood in the immediate area of the damage is not adequate for this location. DO you feel this repair is enough for rough weather conditions? Epoxy and wood joints tend to have sudden catastrophic failure when exposed to long term repetitive flexing as this would be.



There is no need to make a fussy hooked scarf. With modern epoxy, the hook does not add strength. It is nice to nip off the feather edge and depress the V on the female side just a tad so that the last 1/32” or so is a vertical to the grain piece so you don’t have a hard glue line that sometimes leaves a minor bump when the piece is sanded out, but again, not at all necessary.

THanks I was hoping that was the case

[foolishsailor]

Given that there may be a language/name-of-the-thing question, pictures would be helpful.

No I am an american with my boat down here for the winter season in cape town. It was just my inability to articulate the problem very well, sorry...


I gather that you are considering laying in scarfs all around the box section or is this just on the two side panels?

I dont really know what to have him do. If we were just going out to sail on san francisco bay I would just lay in a scarf replacing the 3 broken planks in the boom. But I think it needs to be approached with the most conservative and strongest possible solution I just dont know what that would be. If you all think that scarfing 3 pieces in with a 12-1 slope is adequate then I will tell him and we can start forward with it. It just seems that with this repair now at the weak point you have lines of epoxy which are not as flexible as the spruce and they are both flexing at the same area. Would this not be a problem? Or would Ians recommendation of adding strops to spread the load to either side of the repaired break now be enough?

It is not just a building issue but a safety issue because of where he is going. He is leaving here to go up the east coast of africa and across to madagascar. That is some very heavy seas and weather and the boom would be under serious strain and breaking would not be a pretty thing which makes me so hesitant to offer help or knowledge that is beyond my scope...

...thanks again for the help

[Ian McColgin]

The longer the scarf, the less likely that repatitive flexing will increase the chance of failure. So, if you've doubts go 20:1.

But booms don't flex that much when correctly loaded. Maybe once it's fixed your pal should just not strap her down so hard. I don't know - to get where he's got and all it's not like the guy could possibly be inexperienced, but sometimes we all get into a way of doing that's open to radical reevaluation.

Booms are designed for compression loading and some well spread bending. If they get really hard point loading, like with mid-boom sheeting, they are made for it.

Once fixed, I think the preventer attachment needs some very serious thought. I like spreading the load more than reinforcing the point because the reinforcement just makes two somewhat unpredictable hard spots, but it's a guess either way.

Why not move the preventer attachment(s) way further out? This will increase the force on the preventer, translate more of that stress to a compression stress - gooseneck up for it? - and most importantly give a bit more flex to the system. The last is important as wood cracks more readily with shock bending than slow flexing to the same angle.

Whatever your pal does, he probably has the wherewithal to make a new boom or a new rudder at sea if he must. Or at least sister strap his spinnaker pole, boat hook, an oar or something to the boom to get to his next port.

One of the interesting differences between sailing around the coasts, even is several hundred mile stand-offs, and ocean cruising is repatitve stress. Like how the (falsly called as it's only half way) Trans-Pac racers have to regularly overhaul the spinnaker halyards. So, I'm wondering if your guy is overly tight in the preventer department and thus introducing stresses that could as easily be avoided. What about one of those hugely strong rubber strops - you know the 1" diameter units that take the shock off big mooring systems?

Whatever. Anyone who can make it from one stormy cape to another has my admiration.

G'luck

[P.I. Stazzer-Newt]

Scott Boomlock (http://www.boomlock.com/howitworks.htm) is one kind of "preventer" which is well capable of introducing huge point loads to a boom - never seen one fitted to a wooden stick.

The phrase in this which I find most troublesome is "boom has parted partially" - assesing the limits of the damaged area is no small job neither is it very sure.
Combining it with a desire to keep as much of the original structure as possibe could easily be a recipe for disaster.

[Jay Greer]

Splicing can be acomplished on three sides, using the third member for alignment. Here, he will need to use a filler block to back up the scarfs during clamping In the case of this situation where both ends of the scarf are essentualy closed, the blocking will need to be of a sliding multi part set up to allow both ends to fill the cavities at both ends. It is a tricky job and will require a means to stabilize the boom such as a temporary spar bench built on a pier or fence. I was faced with a similar problem when the mast of my H28 broke. You can see the methods I used by searching the pictures I posted under Jay's H28. If materials are available and time is critical, it may be more pratical to build a new boom from a stand point of being less hassel.
And, if push comes to shove, one can always be made in a shop that has the proper set up and shipped to him.
Jay

[py]

Just wondering if a permanent filler block, making the boom solid a foot or 18" fore and aft of the stress point might be a good idea?

[Paul Fitzgerald]

I always attach a preventer to the end of the boom in the open ocean.
You dont tack much out there so setting it isnt a chore.
The problem is broaching with a full main and the boom in the water, stresses are too much for a mid boom preventer.
The preventer is also designed to stretch and let go before the boom breaks, thin 6 to 8mm nylon line rigged 2:1 on a trapeze clamcleat clipped to the chainplates.
I usually use the chainplates as a base, but some take it as far as the foredeck.

[foolishsailor]

Thanks for the info guys.

I passed it on and he is going to decide whether he wants to take on the scarfing or not. But he definitely agrees with moving the attachment points farther out and spreading the load via strops.

Appreciate the help.

Jay, great pics. I like the mast shots where you can clearly see the jig setup. Nice work, wow!!!