Mast Support Post

[richardbrennan]

The mast came off again today and the cabin top immediately sprang upwards; I will leave it a few days and see when the movement stops. The fabricator who made up the post has suggested a chock from high density nylon. This is not a material I have come across before and Googling it seems to bring up information on fabric. Apparently it does not compress and can be drilled or sawn like timber; has anybody familiar with it and would you think it suitable for the job?

 

[TimBennet]

I would certainly want to know if it is likely to 'creep' under load.

Breadboard type plastic certainly does, but I'm not sure about nylon.

My first choice would be wedge made from a piece of GRP 'plate'. You can sometimes find some derelict GRP lying around the boatyard and cut a piece from that and then grind it down to a wedge shape with an anglegrinder+sanding disk and belt sander.

If not, GRP plate is actually so useful around the boat that making up a couple of squarefeet of 10mm thick on a piece of flat window glass is a worthwhile investment.

Nylon doesn't look too bad. Certainly not as bad as breadboards! (click to enlarge)

 

[thalassa]

The mast came off again today and the cabin top immediately sprang upwards; I will leave it a few days and see when the movement stops. The fabricator who made up the post has suggested a chock from high density nylon. This is not a material I have come across before and Googling it seems to bring up information on fabric. Apparently it does not compress and can be drilled or sawn like timber; has anybody familiar with it and would you think it suitable for the job?

I've used it in the form of 'Ertalon', very easy to saw and file, even by hand. Gives a smooth finish easily. There are different grades, PA6 is the most common. Very useful on the boat making slides, clamps, holders.
Incredibly strong. Although basically a nylon , it absorbs very little water, and has uses even under water. U.V. resistance varies with the type.

 

[G12]

Have you asked this on the Westerly Owners Association forum at all? Huge knowledge base there.

 

[JumbleDuck]

I would certainly want to know if it is likely to 'creep' under load.

Plastics aren't normally my field, but I know the basics ...

I'll make a wild guess at the forces, and assume 1 ton (10kN) static load, spread over a wedge which is 10cm x 10cm (9cm diameter tube). That's 0.01m², so the static stress would be 1MPa. I've found a duPont spec sheet (http://plastics.dupont.com/plastics/pdflit/europe/zytel/ZYTDGe.pdf) which 2 - 4% and 3 - 6% for 10 - 20MPa loads after 10⁴ hours. In both cases the creep rate looks proportional to stress, so at 1Mpa I'd expect 0.2 - 0.3% strain after 10⁴ days, which is roughly a year.

Since the yield strain for steel bolts is typically 0.2%, and since they won't be done up that tight, this means that the creep in the nylon over a year will be enough to make all the bolts slack. I'd look for something at least an order of magnitude less creepy, and maybe two.

 

[richardbrennan]

I am totally ignorant on these matters, and "creep" is not something I would have considered so thanks for bringing it to my attention; perhaps teak is a better bet?

 

[30boat]

The post is made of stainless steel is it not? So why not weld up a stainless steel wedge and tack it into place?Zero creep and dead easy .That's what I'd do.

 

[richardbrennan]

Another question if I may. The post still has a slight bow, it's probably 2-3 mm over the 2 metre length. The fabricator says they may be able to straighten this, but will it always be a weakness?

I am still trying to sort out the best solution for the wedge, I think I'd like to go with steel if I can. The coachroof has not gone back to exactly where it was before, I intend to apply a little gentle pressure to see if it goes up a little more.

 

[JumbleDuck]

Another question if I may. The post still has a slight bow, it's probably 2-3 mm over the 2 metre length. The fabricator says they may be able to straighten this, but will it always be a weakness?

An unloaded bow doesn't change the buckling strength, although it may deflect more under loading. It won't fail sooner, but it may slecken the shrouds a bit in heavy conditions. Probably best if the fabricator can straighten it, but not a disaster if s/he can't.

 

[30boat]

I'm not an engineer, obviously, but I'd be very nervous with a mast support that flexible.Were it my boat I'd get a stiffer post and correct the gap under the coachroof.

 

[doug748]

The mast came off again today and the cabin top immediately sprang upwards; I will leave it a few days and see when the movement stops. The fabricator who made up the post has suggested a chock from high density nylon. This is not a material I have come across before and Googling it seems to bring up information on fabric. Apparently it does not compress and can be drilled or sawn like timber; has anybody familiar with it and would you think it suitable for the job?

Hello Richard, I have followed this whole thing with some interest and certainly feel you are due for a bit of good luck.

If the coachroof could be seen to move upwards it seems that whoever took up the bolts should have seen what was happening in the first place, and stopped. There may have been other issues I am not aware of - but it does sound like very poor practice.

Your man's plastic is probably perfectly adequate as would a hardwood wedge. I would wack in a full width wedge, mark out the holes from the deck, take it out , drill clearance holes and then assemble the whole thing, having ensured the post is vertical and reversing the bend 180deg (or having it straightened).

You might reconsider driving a timber pole up the middle of the post, which would be cheap and offer a bit more reassurance.

 

[richardbrennan]

Doug

Thank you for the kind words.

Unfortunately, I am getting rather varied advice at the moment. Obviously I would like to beef up the post somewhat, but whether this can be done to the existing post or whether I need another new one with a greater wall thickness does not seem clear. What I do know is that SS tubing is difficult to source and does not come cheap in relatively short lengths. Similarly some suggest a hard wood wedge and others, especially one of my neighbours, is dead against this.

I have been down to the boat again today and applied a little upwards pressure to the coachroof with a jack as it does not seem to me as though it has currently gone back up as far as was before we tightened the bolts. It's "mea culpa" on this, as there was nothing there previously, I assumed that pulling down the coachroof would be OK, which was clearly a great mistake on my part. I may also be imagining it, but the bowing seems to be lessening. The top plate measures 25 cm by 12.5 cm and the gap at the aft end is currently about 6 mm, which according to the old one in sixty rule, means the angle is just under 1.5 degrees. So whatever it's finally made from, the wedge is going to be pretty slim.

 

[30boat]

Doug

Thank you for the kind words.

Unfortunately, I am getting rather varied advice at the moment. Obviously I would like to beef up the post somewhat, but whether this can be done to the existing post or whether I need another new one with a greater wall thickness does not seem clear. What I do know is that SS tubing is difficult to source and does not come cheap in relatively short lengths. Similarly some suggest a hard wood wedge and others, especially one of my neighbours, is dead against this.

I have been down to the boat again today and applied a little upwards pressure to the coachroof with a jack as it does not seem to me as though it has currently gone back up as far as was before we tightened the bolts. It's "mea culpa" on this, as there was nothing there previously, I assumed that pulling down the coachroof would be OK, which was clearly a great mistake on my part. I may also be imagining it, but the bowing seems to be lessening. The top plate measures 25 cm by 12.5 cm and the gap at the aft end is currently about 6 mm, which according to the old one in sixty rule, means the angle is just under 1.5 degrees. So whatever it's finally made from, the wedge is going to be pretty slim.

I quite like the idea of filling the tube with a wooden insert .You can have one turned out of a piece of solid oak or mahogany for not a lot of money.It can be pressed in with some epoxy as a lubricant that will ,after hardening,make the whole thing very strong.
I don't really understand what you say about the size of the wedge.Isn't the cachroof slowly going back to its normal shape?Doesn't this mean that the space between the top of the post and the underside of the coachroof is widening?Or are you intending to fit the wedge under the mast step and thus compress the coachroof again?

 

[JumbleDuck]

I quite like the idea of filling the tube with a wooden insert .You can have one turned out of a piece of solid oak or mahogany for not a lot of money.It can be pressed in with some epoxy as a lubricant that will ,after hardening,make the whole thing very strong.

The bending stiffness depends on the flexural rigidity, EI. For a 9cm diameter tube with wall thickness 3.5 mm, made of stainless, E = 210GPa and I = 102cm⁴ (post #11) so EI = 214 MNm².

A solid wooden inset of diameter 9cm has E = 322cm⁴ and I = 11GPa, so the additional flexural rigidity is 35MNm², an additional 16%. Probably not work it.

In view of the tightness of the angle, I thing welding a new top plate on at an appropriate angle and then perhaps cutting small recesses for the washers is the way to go. I would expect this to be done by the people who have bodged the job so far, and I would be also expect them to be making good any damage they have done to the coachroof by bending it. I might well want a surveyor's opinion.

 

[William_H]

The high density nylon is easiest bought in the form of plastic cutting boards for the kitchen. Usually around 6mm thick. Easily cut and a variety of colours. olewill
Edit I see people think that this material will creep under a lot of pressure. Maybe but I would give it a go considering the area and pressure.

 

[JumbleDuck]

I see people think that this material will creep under a lot of pressure. Maybe but I would give it a go considering the area and pressure.

I wouldn't. I'd use a creep resistant material in the first place. Why go to all the bother of making a wedge out of unsuitable material when it would be the same amount of work to use a suitable one?

 

[richardbrennan]

Unfortunately I really have only myself to blame for what's gone on as I have pretty much done the project management myself. What I really don't understand, and probably never will, is why the original 6 cm post, in apparently the wrong position, did not give problems for 17 years? However, I really do need to get it right this time, apart from anything else, the costs are getting silly, for instance it's £138 each way just for stepping and unstepping the mast.

The fabricator, at one point, indicated that he could put a cross piece inside the tube so I will investigate that next week and also discuss the wedge with him. Assuming I get that all sorted out, then that only leaves me with getting the cabin table rebuilt around the new position of the post, which is a challenge in itself!

Thanks again to everybody for your views, they've been most helpful.