Straightening a bent mast

Moderator: GreenLake

Straightening a bent mast

Postby GreenLake » Sat Sep 05, 2015 3:41 am

At a recent sailboat race, while milling around waiting for the start, the windward stay parted in rather moderate winds.

After the initial shock, we immediately deployed the motor to motor home. With the situation stable, we checked for damage: the keel-stepped mast was bent, but the shroud seemed intact. Closer inspection revealed that the pin had failed, because the fork terminal had been installed next to the chain plate instead of straddling it. (The overlong clevis pin that was used had made that mistake possible).

The fork was bent, but not cracked, so I pried it open and with a spare pin that I keep on my keychain, we decided to see if we could reattach the shroud.

My DS1 has a mast jack, so we lowered the mast, and despite the bend in the mast, were able to attach the shroud and raise the mast. With the shrouds defining the distance between chainplates and hounds, the mast was forced back into position, although the bend must have set up a slight curve and most of all asymmetric tension.

Unfortunately the maneuver took some time, and we had motored away for a fair distance, so we didn't make it back to the starting line in time, but this being a fun regatta, we simply followed the fleet around the marks in generally light winds for a bit before retiring. We did not really notice any effects of the bent mast while sailing, but when trying to take it down, the bend made it difficult to handle. It didn't help that the ground was inclined sideways a bit in the same direction. In short, the mast was leaning over so far that we needed one person to counteract the sideways lean while the other person took it down.

Repairs were needed.

First, I used the occasion to upgrade to 1/8" stays from D&R Marine. Because I have the mast jack, the stays are normally fixed length. So I got the ones with the simple adjusters, which should be fine for a one-time rig trim, but can be locked with the locknut after that.

Second, I needed to figure out how to go about straightening a mast. By sighting along the sail slot I determined that there was a bend roughly at the partners (that is where the mast exits the deck), but that a second bend had occurred near the gooseneck. I confirmed the location of the bend with a straightedge. I measured the bend, crudely, and the deflection seemed to be a bit less than a full mast diameter at the mast foot. (Meaning, if the mast foot was held true, the mast tip was deflected several mast diameters because of the longer lever arm given where the bend was).

I took all shrouds, halyards, spreaders and jumper struts off the mast and laid it across an old tire, put a 2cu ft bag of bark across the tip and stood on the other end. While the mast would give, the deflection would all be elastic and it sprang back.

After a bit of this, I discovered that if I stood closer and closer to the end, there was a point where had just enough leverage given my weight to decrease the bend in the mast. This worked best at the upper bend, which, being further from the foot, gave me a better lever arm.

I repeated the exercise moving the tire along the mast to try to straighten different locations.

At the end of the day, it looked like progress, but a significant bend remained. Perhaps 1/2 mast diameter sideways deflection at the short end.

The next day, I repeated the effort with a helper acting as a spotter and adding a bit of weight on the long end, to keep it from flexing as much. Mostly because that flexing would limit the range of deflection for the short end (it would hit the ground, before it was fully deflected).

With that optimization we quickly took out almost all the bend near gooseneck, but for the bend near the partners the lever arm proved too short. Turns out, cat litter is an essential ingredient in this kind of operation. I had two 40# boxes of it sitting around, so I did some weight lifting while balancing on the very bottom tip of the mast. (Glad there aren't any pictures of this :) ).

That did the trick, and now we are down to a very slight deflection of between 1/2 and 1 mast diameter sideways deflection at the *long* end, or between 1/8 and 1/4" at the short end.

My inclination at this point, and after conferring with some local sailors, is to leave well-enough alone and try to raise the mast and trim the rig. Having been straightened, the mast isn't quite as strong as it used to be, however, most of the bend isn't in a location where failure due to buckling is a primary concern - where the mast goes through the deck, it is held in position, after all. Whether the slight asymmetry will show itself sailing the boat, I don't know. I doubt it, in fact, both from our initial experience and because the boats aren't fully symmetric in the first place. Just my luck if the two effects add up, though.

The first key to the success of the operation so far appears to be the use of soft yet firm support at the location of the bend. An old tire was the ticket, but by the second day, it started to get crushed, so I stuffed a partial bag of bark on the inside. The other key was to find a way to just apply enough force to start the bend. By slowly shifting my weight I had good control over the bending force, and I could actually feel when the mast "gave". Being able to work on the short end made everything quite simple, because a simple bag of bark could serve as a counter weight. In the reverse case, I would have had to wedge the short end under something really heavy, like a car and to find ways to cushion it from any hard spots.

I had to take the diamond stays and jumper struts off, as otherwise I couldn't lay flat the mast on its side. I'm planning to keep them off, because too many people recommend just that. Which, of course, means I won't have a perfect before and after comparison. Too bad :).

Finally, why did I have overlong pins in the first place? Turns out that the fork terminals on my stays were drilled for a size pin that's smaller than the pins used on the DS chainplates, and a slightly odd size it is, as it's less than 3/16". (The standard pin size is 1/4"). In fact, to get spares, I had to locate the same rigger that a previous owner had used to order replacement stays and he rummaged on his work bench and measured each pin, and finally located just enough so I would have some spares, but not enough so I could replace all the overlong pins that the PO had used.
~ green ~ lake ~ ~
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Re: Straightening a bent mast

Postby Interim » Tue Sep 15, 2015 5:18 pm

This is an interesting read, GL.

For one, I'm surprised that the two bags of bark were enough of a counterweight, even at the long end of the lever. Second, how was the tire positioned? That is, was it vertical so there was one point of contact (albeit a wide one)? or did the mast lie across a horizontal tire, and touch at two points? The reason I ask is that I don't understand how the vertical tire would stay in place. Perhaps I read to quickly and missed the details.

Finally, you talk about the deflection. When I bent my mast--described somewhere in one of these threads--my local advisor talked about how far it was "out of column." That is, if a perfectly straight mast were encased in an imaginary column, how far out of that column would the mast be at the bend. I'm inferring from your mention of a full mast width--later a half mast width--that it was 4-5 inches at one point. Is this close?

I was told that an inch out of column was not worth trying to correct, so I didn't.

Regardless, I liked reading about your success and might try this method. Just to see if I can straighten it.

--john
1979 DSII
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Re: Straightening a bent mast

Postby GreenLake » Tue Sep 15, 2015 9:46 pm

Actually, a single bag was enough to counteract my weight pushing down on the other end. The mast is 24' 6" long, so 4' on one side and 20' on the other side of the bend, if the fulcrum is about 4' in, but 21' to 3' if it is about 3' in from the end. That makes for a ratio of between 5 and 7, the latter is clearly large enough to offset my weight on the short end. The second day, I had a helper push down in the middle of the mast as well.

The tire was laid flat, but with one side raised a bit, so that there was only one contact point. After a while, I noticed that I was crushing the tire, and I "stuffed" it wit the end of a second bag of bark to give it more resistance. The support at the fulcrum was effectively between 2-6" wide.

I don't know for sure. Your "out of column", if I understand it, is drawing a line from bottom to top and measuring the distance perpendicular at the bend. I didn't measure it that way, but estimated the deflection of the bottom of the mast, if the rest of the mast was held vertical. Given that the bend is close to the bottom end, both methods will yield similar numbers (I believe for a bend in the middle, my method would yield a number that is twice as large as your method).

So, the deflection was about 2-3" (a bit less than a mast diameter).

The problem with out of column as a measurement is that it doesn't describe the angle of the bend; the same amount of out of column corresponds to a sharper bend the closer the bend is to the end of the mast. It may be useful in terms of estimating how much more easily the mast will then buckle under compression. In may case, that's actually not the prime consideration, because most of the bend is at the partners. That means, at a place where the mast is supported from the side, and therefore a spot where I assume that it wouldn't buckle anyway.

The main issue for me was that with the mast bent, it was leaning over so far that I could no longer safely raise it while standing in the cockpit (too much sideways lean for me to control). This is made worse by the fact that our local ramp is not level (and leans in the same direction...).

I think my straightening succeeded to the point where, had the bend been at the middle, the mast would be 1/2" to 1" out of column, so just about the target that you report. As the bend is near the end, the actual out of column is less, and in fact the overall mast shape is dominated by a very shallow curve that seems to be distributed over the entire mast and goes the opposite direction.

As it is, I've rigged and used it (albeit in light winds) and can't tell any change in trim or helm balance. At the next occasion, I'll try to measure the mast rake, but as balanced as the boat is, I don't think it's changed (despite having new stays). I did verify that the distance from the mast tip to the chain plates is the same, by taking the main halyard to both.

In the process, I've removed the jumper struts and diamond stays, and not put them back. The wind was too light to notice any difference, but I'm also not sure what the difference would be. The new 1/8" stays are adding weight, the removed diamond stays are saving weight, so that cancels.
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Re: Straightening a bent mast

Postby Interim » Wed Sep 16, 2015 8:06 am

Being out of column near the step or base does create greater deflection at the top of the mast, and I suppose affects sail shape differently. I think my conversation about being out of column had more to do with the likelihood of the mast buckling, rather than affecting sail trim.

I find that the way I ask questions shapes the nature of the answers, so I may have been asking about safety rather than performance, but then generalized the answer.

Up until this summer, concern about performance was about learning. I held the local lake sailing championship for three years in a row, having been the only sailboat on the water. But two other boats appeared this summer, so I can no longer claim that. Performance now matters in a different way. :D

You also raised the issue of weakening the metal by bending it. My understanding is that bending and straightening once will slightly weaken the mast, but not to a point of concern. Of course, every time it happens it degrades more.

--john
1979 DSII
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Re: Straightening a bent mast

Postby GreenLake » Wed Sep 16, 2015 3:42 pm

Several people have reported that they broke a mast in an attempt to straighten it. So, I am lucky to have avoided that.

I am not sure what the full effect is of having an uncorrected mast bend at the partners. The shrouds will force the hounds to be in pretty much the old position, but the tension is now more uneven (I'm talking of total bend that's slight enough that the elastic range allows the mast to be temporarily bent back that far).

As I wrote, we sailed that way for a full lap of the course. It was mainly during rigging, with the shrouds loose, that the mast leaned over.

Absolutely valid point on framing the question.

And, good luck on eventually reclaiming the top of the fleet!
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Re: Straightening a bent mast

Postby GreenLake » Wed Feb 28, 2018 11:19 am

Follow up: after several seasons with the straightened mast I can report that in daily use I've completely "forgotten" that the mast was ever bent. Whatever residual bend is still there is so little that it's not noticeable unless you deliberately sight along the mast and I've not observed any changes in sailing behavior.

I did leave off the jumper struts and diamond stays, but also have not observed any noticeable change - in principle, leaving these off would allow the upper mast to deflect, say in a gust, but while this may be the case, it's not something that I can 'feel' by sailing the boat. Recently, I "tangled" with another boat's rigging on the race course and was fully prepared for the mast to come down -- after a loud "twang" the boats separated and other than the loss of a Windex vane and a few scratches there was no damage (the vane had previously been broken and glued with super glue - snapped at the glue joint).

Since the repair, I've had the boat out in conditions that had stronger winds than I normally enjoy sailing in, and perhaps strongest I ever sailed this boat in, but not had any issues related tho the mast.

One tack seems slightly favored, but that was the case before, so also not a real change.
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