Mast extension

I currently have a 23' 6" mast (3"x4.25" teardrop section). It is deck stepped in an aluminum channel approximately 18"x4"x4" which is mounted on a glassed-in plywood reinforcement pad over an interior bulkhead mounted header. There is no compression post.

I would like to extend the mast by adding an additional 3' (three ft.) to its foot.

Can this be done without sacrificing the integrity of the mast? If not, what is the danger or difficulty? If it can be done, what are the options? Could a rectangular tube (3.5"x5") be welded or attached mechanically to the foot. I do not need to extend the sail slot. I would attach the boom gooseneck to the new section.

I'll appreciate any and all responses.:idea:

Thanks,

 

Let me guess that there is a plate or channel on the bottom that you could weld a square section tubing to.
The main problem this will create is a narrower angle for the shrouds to work at for keeping the mast up putting more strain on things. If you can move the chinplates out 5 or 6 inches each side this would be mitigated a lot. But probably you won't be able to.
I won't predict disaster but I won't say there will be no problem either.
If you think the benefits are going to outweigh the risks you can go ahead and try it.
I am assuming the mast would not be pinned to the channel three feet above the deck when I say this.

 

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hi i dont see why that would be a problem- but i would think that u would have to use the same section of aluminium for the extension, u could then just put a sleeve in it and pop rivet it together, with perhaps a thinnner sleve on the out side- this would rely on u finding the same section tear and the right bit for the inner sleeve

 

I would consider having it sleeved by a professional rigger. Then there would be no loss in integrity.

 

I did the exact same thing when my cat mast was too short for the new mainsail I found. I found a free tip off a H17 mast (fiberglass), cut it down, and this is where some will disagree, but took a treated 4x4 section, shaped each end of it to fit the very dissimilar mast sections (H17tip and a section such as yours), epoxied it in place, then put a fiberglass tape seam around the joint on the exterior to seal it shut. A cheap and easy alternative to fabricating aluminum. Yes the wood is heavier and eventually will get wet, and may break, but I doubt there is huge load near the tip of the mast (nothing to support that theory except I don't see many masts fail 3-4' from the tip).

 

Much depends on the location of the stays and upper shrouds. Masthead rig or fractional?
There is certainly a lot of stress at and below where tangs attach. When the jib is pulling, especially if it is a high aspect mainsail (and probably a high aspect jib), compression loads are very high.
The area of the boom is also highly stressed, especially if a vang is used.
Bottom line is you need a sleeve and a section of mast exactly matching the original. You can jury-rig something, but mast failures can also hurt or strand people, not to mention damage sails and rigging. You are going to have to replace your halyards as well, as they'll be 6 ft too short.
Often it's better to change the sail to a fully battened big roach style to gain area. Whether or not you can do this is dependant on whether you have backswept shrouds or a backstay, but if your shrouds are backswept (and I bet they are based on the length of your mast, which must be on a 16-20 ft boat), you might consider the idea, and even adding runners to handle the additional loading from the bigger sail.

Alan

 

with thanks

Thanks for the comments. I would need to add a plate to the inside of the mast foot and the extension tube would have a solid top, so that the original spar could be welded to the new section and attached mechanically as well.

The previous owner purchased this mast as a replacement for the original spar which had been broken. It is a mast head rig for an old Grampian 23. It is almost exactly three feet shorter than the specs for the original. Its cross-section is a bit larger than the original in its outside dimensions. The walls are equivalent in thickness or, if anything, slightly thicker.I contacted the original spar maker who said the original was a 3"x4" teardrop. He couldn't provide the wall specs.
If I extend the mast 3', relocate the shroud tangs on the mast according to the boat's original specs, and adjust the length of the spreaders, I should end up very close to the original stress parameters. I know its more complicated than that, but it seems like it might be acceptable. The new foot would be pinned at the mast step and both the boom and vang would be attached to the new section.

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If you were lengthening the mast, you likely have to re-calculate the panel loading, but since you're just bring it up to the original height, you'll be fine.

Since you have to scab a piece on the end of the stick, you might as well put some shape into it. The aluminum extrusions are straight sided because they're easier to produce. You could incorporate taper with pipe sections and some flat stock, pretty easy, save some small amount of weight and windage too, without worry of strength.

 

I had lost track of the aesthetics. Thanks for the suggestion.

 

I know this might be a bit late, but you might consider making a tall mast step that looks like a pedestal that fits on top of the old step location.

When you lengthen a mast you not only change the angle of the shrouds but also the bending loads in the mast and at the attach points. If your short mast is strong enough to take the loads you will be okay with lengthening it, but if it is not designed for the extra length, putting 3 feet at either end will introduce bigger bending loads over the whole length of the mast.

You use a tall pedestal the there no new loads introduced into the mast. It must be anchored to the deck and wide enough not to introduce bending loads to the base of the mast (so it does not become a "hinge" point) the stress in the mast will be the same.. By the sounds of it your rigging should handle the extra hight just fine. a pedestal, while not elegant, also would be easy enough to build yourself and would not require specialized skill of a welder, machinist or rigger. At the very least you can use this to sort out the restored rigging configuration until you find a longer mast, or can afford a proper mast extension.

 

Neillaun (thread starter) is doing little more then restoring the rig to it's original height, which was cut down for some reason. The only concern is the mast section, being sufficient for the taller sail plan.

 

I understand that he is restoring the original mast height, but he is doing it with a different mast that was not as long as the original. Making the current mast longer might increase the bending loads on the mast itself. With a braced pedestal the mast itself will have similar loads since it would not be lengthened.