Mast Strut Bolt

[travorgeron]

So when raising my mast yesterday, one of the stays got caught on a cleat. The minute I felt pressure I corrected the problem. What I didn’t know was that apparently it loosened the bolt on the port strut. While out sailing the bolt popped off and flew in the water. I immediately put the sails down and just motored. I ordered a new bolt from hunterowners but I need to take the boat out the water. Can I just put a regular bolt in its place to lower the mast and then when I get the new bolt install it. I know it’s a shoulder bolt but can’t find the size and I know it has to be a little loose to allow movement of the strut when lowering the mast.

 

[BobSail]

FWIW, that's what I did. Worked fine until the replacement came (bought two and keep spare on the boat).

 

[travorgeron]

FWIW, that's what I did. Worked fine until the replacement came (bought two and keep spare on the boat).

I ordered two as well. Did you just use a normal bolt and not tighten it super tight to allow for movement?

 

[Tedd]

Are you talking about the bolts on the baby struts? I'm not familiar with the details of the H240 installation so I'll reserve comment on your temporary solution other than to say that if the "bolt" is actually acting as a "pin" (i.e., the joint doesn't depend on bolt tension), then any bolt of the same major diameter and material will probably be fine as a temporary solution, so long as a proper method is employed to ensure that the nut doesn't come loose. "Proper method" in this case normally means that the nut and bolt are drilled and pinned. But if you're just using it once, to lower the mast, then two nuts torqued together will do. Or a new nyloc nut.

It's unlikely that a single application of load like that would loosen a bolt. It was probably loose already, so I would check the others.

If it's a nut on a shoulder bolt, then it should be a fairly reliable joint so long as the nut is properly tightened. A common mistake with shoulder bolts in this kind of application is to under-tighten the nut. The threaded shank of the shoulder bolt has a necked-down area at the base who's function is to stretch slightly when the nut is tightened, to provide a spring function that keeps tension on the nut, so that it doesn't loosen. I haven't been able to find torque specs for shoulder bolts but you can feel it when you torque the nut.

A nyloc nut is also a very good idea, if that's not already what's used. If they are nylocs and one came loose then it's probably time for new nylocs. Opinion varies on re-use of nylocs. I lean toward replacing them each time they come off, but you can feel the prevailing torque and judge based on that, and on how critical the joint is.

It's also probably worth putting a Belleville washer between the nut and shoulder, to help ensure that the nut stays tight. You can also get 316 shoulder bolts from McMaster-Carr that are likely cheaper than from marine sources.

 

[travorgeron]

Are you talking about the bolts on the baby struts? I'm not familiar with the details of the H240 installation so I'll reserve comment on your temporary solution other than to say that if the "bolt" is actually acting as a "pin" (i.e., the joint doesn't depend on bolt tension), then any bolt of the same major diameter and material will probably be fine as a temporary solution, so long as a proper method is employed to ensure that the nut doesn't come loose. "Proper method" in this case normally means that the nut and bolt are drilled and pinned. But if you're just using it once, to lower the mast, then two nuts torqued together will do. Or a new nyloc nut.

It's unlikely that a single application of load like that would loosen a bolt. It was probably loose already, so I would check the others.

If it's a nut on a shoulder bolt, then it should be a fairly reliable joint so long as the nut is properly tightened. A common mistake with shoulder bolts in this kind of application is to under-tighten the nut. The threaded shank of the shoulder bolt has a necked-down area at the base who's function is to stretch slightly when the nut is tightened, to provide a spring function that keeps tension on the nut, so that it doesn't loosen. I haven't been able to find torque specs for shoulder bolts but you can feel it when you torque the nut.

A nyloc nut is also a very good idea, if that's not already what's used. If they are nylocs and one came loose then it's probably time for new nylocs. Opinion varies on re-use of nylocs. I lean toward replacing them each time they come off, but you can feel the prevailing torque and judge based on that, and on how critical the joint is.

It's also probably worth putting a Belleville washer between the nut and shoulder, to help ensure that the nut stays tight. You can also get 316 shoulder bolts from McMaster-Carr that are likely cheaper than from marine sources.

Yes it was a nylock nut that was on it, not sure how it popped off. The boat is new to me and only the second time I use it. Still learning everything. I did not have a shoulder bolt to replace it but did have a regular bolt I used to at least hold it together. I left it at the marina in a temp slip for the week and plan on going sailing today. I will get a pic of it as well.

 

[Tedd]

Hi @travorgeron ,

There's no consensus among engineers on re-using nylocs, but there is agreement that if the resistance when installing the nyloc is low, then it should be replaced. I think there's even an aviation spec on the minimum "prevailing torque," but I'm too lazy to look it up.

In this case, it's not necessary to remove the nylocs in normal use of the boat, right? If so, I would replace them every time they come off. If they come of regularly I would replace them every third time.

 

[Tedd]

@travorgeron ,

Just had a closer look at your drawing. That's a terrible way to use a shoulder bolt. The threaded part should never be loaded in that way. If it were my boat I'd change that, if only so that I didn't shudder every time I looked at it. But changing it to something proper would be a big deal so, as long as the shoulder bolt is big enough...

I would change those shoulder bolts every year, though.

(I know somebody is going to be thinking, "He's full of crap, I've been using mine like that for x years and never had a problem." To which I would reply, "Yes, and you won't have a problem, until you do." Past performance is not a predictor of future performance where fatigue is concerned.)

 

[BobSail]

I ordered two as well. Did you just use a normal bolt and not tighten it super tight to allow for movement?

Correct. Used a spare bolt with same thread diameter as shoulder, double locked nuts and washers. Was only in place for a week or two.

 

[rgranger]

@travorgeron , To which I would reply, "Yes, and you won't have a problem, until you do." Past performance is not a predictor of future performance where fatigue is concerned.)

You are not wrong but I think those baby stays are only under load during raising and lowering of the mast and keep the mast from swaying side-to-side during the process. I doubt much load is transfered to the bolt.

 

[Tedd]

I doubt much load is transfered to the bolt.

It's a low-cycle fatigue issue. Due to the leverage inherent in the installation, it might require only a very small lateral load on the mast (such as induced by it's own mass during swaying of the trailer as you walk around while raising the mast) to move the necked-down portion of the thread into the plastic range. Bear in mind that the angle of the baby stays is quite steep, thereby multiplying the load, and also that the baby stays attach to the mast well below its center, further multiplying the load. Each such loading weakens the thread root and then, just when you really need it to hold--say, a wind gust happens while you're raising the mast--another such event pushes it over the fatigue threshold.

It's a bad idea to incorrectly load a fastener, ever, and especially one that's structurally critical, which this bolt is during raising and lowering of the mast. The "it's only subject to small loads" theory has been the root cause of many failures. It would have been much better for the designer to simply use a plain bolt (with the diameter of the shoulder bolt's shoulder, not the thread diameter) with a nut on either side of the mounting flange. Still dodgy, but much better than what the designer did do.

I realize I'm being a curmudgeon. In my defense, in my career I've seen so many failures due to incorrect fastener use--some of which ended in injury--that I have a low tolerance for it. It's especially egregious in a case like this, when a better solution could have been employed at zero cost. (Possibly even cheaper, given the price of shoulder bolts relative to plain bolts.)

[Edited to add: Most likely, the failure mechanism I described above is exactly what happened to @travorgeron , as described in his original post. So it's not a far-fetched scenario.]

 

[rgranger]

It's a low-cycle fatigue issue. Due to the leverage inherent in the installation, it might require only a very small lateral load on the mast (such as induced by it's own mass during swaying of the trailer as you walk around while raising the mast) to move the necked-down portion of the thread into the plastic range. Bear in mind that the angle of the baby stays is quite steep, thereby multiplying the load, and also that the baby stays attach to the mast well below its center, further multiplying the load. Each such loading weakens the thread root and then, just when you really need it to hold--say, a wind gust happens while you're raising the mast--another such event pushes it over the fatigue threshold.

It's a bad idea to incorrectly load a fastener, ever, and especially one that's structurally critical, which this bolt is during raising and lowering of the mast. The "it's only subject to small loads" theory has been the root cause of many failures. It would have been much better for the designer to simply use a plain bolt (with the diameter of the shoulder bolt's shoulder, not the thread diameter) with a nut on either side of the mounting flange. Still dodgy, but much better than what the designer did do.

I realize I'm being a curmudgeon. In my defense, in my career I've seen so many failures due to incorrect fastener use--some of which ended in injury--that I have a low tolerance for it. It's especially egregious in a case like this, when a better solution could have been employed at zero cost. (Possibly even cheaper, given the price of shoulder bolts relative to plain bolts.)

[Edited to add: Most likely, the failure mechanism I described above is exactly what happened to @travorgeron , as described in his original post. So it's not a far-fetched scenario.]

Okay.... you have changed my perspective. I appreciate the thoughtful explanation.

 

[BobSail]

in my career I've seen so many failures due to incorrect fastener use--some of which ended in injury--that I have a low tolerance for it. It's especially egregious in a case like this, when a better solution could have been employed at zero cost.

So given the existing design and what you know, how would you go about modifying it to improve/mitigate the situation? I'm not sure I correctly visualize the double nut approach mentioned above.

Thanks

 

[Tedd]

@BobSail ,

I'm reluctant to propose a solution without being able to see the parts involved. The drawing from the manual doesn't provide enough detail. But I'll make a couple of suggestions.

I lean toward overkill so, if it were my boat, I would probably replace the deck bracket with one that supported a bolt in double shear--i.e., a clevis-shaped bracket that clamped the bolt from either side. But that's going pretty far.

A simpler approach that's probably good enough is to find or make a sleeve that fits in the hole on the baby stay and then attach it with a regular hex bolt and washers, as shown in this sketch.

 

[Crazy Dave Condon]

I sent a private message to poster if fixed and photo. The mast strut is the same for 260, 240 and later models. I am not sure but washers were used. Yes the bolt was slightly loose to allow the strut to rotate with the mast as it replaced putting on baby stays which were on the 19, 23.5 and 26 to keep the mast from falling sideways