Replacing Base of Wood Mast

[Billy Buck]

Hey O! I've got rot in the base of my Sitka spruce 47' deck stepped mast (Cheoy Lee Perry 35). Any fantastic and informative links to show me what it means to scarf in a new one? I've found good stuff for further up the mast but nothing at the base. I've got good support around me (woodworker types) but I'm looking to do early stage research on my own.

[psk125]

Essentially a scarf is an angled join between pieces of wood. To scarf onto the bottom of a mast you would want to cut off the rotted section and then cut the bottom of the mast into a pointed "V" shape. For a spar that size you might want the arms of the "V" to be pretty long. Perhaps 2 or 3 feet. Your woodworker friends could then build up and cut blocks of wood into a female "V" shape that would extend the mast back to the original length. The joint needs to be quite tight. The two parts would be glued & screwed together (epoxy is our friend) and the bottom section shaped and smoothed to match the upper. The whole thing can then be varnished.

[fourlyons]

Originally Posted by psk125

Essentially a scarf is an angled join between pieces of wood. To scarf onto the bottom of a mast you would want to cut off the rotted section and then cut the bottom of the mast into a pointed "V" shape. For a spar that size you might want the arms of the "V" to be pretty long. Perhaps 2 or 3 feet. Your woodworker friends could then build up and cut blocks of wood into a female "V" shape that would extend the mast back to the original length. The joint needs to be quite tight. The two parts would be glued & screwed together (epoxy is our friend) and the bottom section shaped and smoothed to match the upper. The whole thing can then be varnished.

I would argue that this is correct for a solid mast, not a hollow mast like this one, even though the base will have solid blocking in the core.
I would suggest maintaining the original construction by cutting off the bad and scarfing (12 to 1 ratio) each of the four faces back to the correct length, offsetting each scarf (in theory not really needed), and installing solid core blocking before the fourth face goes on. Doing it this way also allows you to have vertical grain on every face.
And no screws, not needed.

[wingssail]

Originally Posted by fourlyons

I would argue that this is correct for a solid mast, not a hollow mast like this one, even though the base will have solid blocking in the core.
I would suggest maintaining the original construction by cutting off the bad and scarfing (12 to 1 ratio) each of the four faces back to the correct length, offsetting each scarf (in theory not really needed), and installing solid core blocking before the fourth face goes on. Doing it this way also allows you to have vertical grain on every face.
And no screws, not needed.

Me, I'd do it the solid block way. Easier, faster, just as strong, and vertical grain all the way around.

Sometimes people just want to make the job harder than it needs to be, for what reason, I don't know.

[fourlyons]

Originally Posted by wingssail

Me, I'd do it the solid block way. Easier, faster, just as strong, and vertical grain all the way around.



Sometimes people just want to make the job harder than it needs to be, for what reason, I don't know.

No, not necessarily. It'll be just as strong only as long as all of the mast is solid in that area. But if he has to cut back into the hollow part, there'll be correct length scarfs on two faces but the side faces will be too short.

Furthermore the only way to get vertical grain all the way around is to have each of the four faces cut separately, cut in a vertical grain orientation. So you still end up gluing four pieces plus the core together.

Is the mast likely to collapse, if he doesn't do it correctly or as it was originally engineered, no probably not. But doing it right doesn't actually take that much more time.

I'm not trying to make the job harder, but engineered structures are usually engineered a certain way for a reason.

[Spot]

It would be awesome to see a picture of the mast in question as-is where-is and the original construction details for said mast at the base. If it were mine to do, I would be inclined to replace the bottom 1/4 to 1/2 inch (6.4-12.7mm) with G10/FR4 'board' so that the wood base of the mast is not sitting in a puddle on the deck.

[bailsout]

I would not use epoxy for joints exposed to heat and sunlight. I recommend a water-based wood glue. Your mast probably sits in a boot with drain holes, but I did saturate my base with penetrating epoxy primer and finishing coats.

[phlygh]

Originally Posted by fourlyons

No, not necessarily. It'll be just as strong only as long as all of the mast is solid in that area. But if he has to cut back into the hollow part, there'll be correct length scarfs on two faces but the side faces will be too short.

Furthermore the only way to get vertical grain all the way around is to have each of the four faces cut separately, cut in a vertical grain orientation. So you still end up gluing four pieces plus the core together.

Is the mast likely to collapse, if he doesn't do it correctly or as it was originally engineered, no probably not. But doing it right doesn't actually take that much more time.

I'm not trying to make the job harder, but engineered structures are usually engineered a certain way for a reason.

Are laminated masts always vertical grain, also called quartersawn? Why?

[Cheechako]

I would probably cut the rot off and make a slip fit base for the mast out of solid Fir, Spruce or better Teak if you have it. Something like this, Resorcinol glued or 5200 together. Bronze threaded rod.
(Grain direction opposite of what is shown here, my software wont let me rotate it!)

[Lemsteraak]

Hi Billy,

You might want to check out the Woodenboat Forum to research. There is a wealth of knowledge there. I can say you are very wise to first determine how it is constructed to determine your plan of action.

I used to race in a class where we were required to use wood masts and would have a few dismastings every year and it wasn't a big deal. The group was fortunate to have an old Danish shipwright who was gifted and could build and repair masts. The repair that is being discussed above is what he called a clothespin scarf and it worked well with our solid masts. If you can find a woodworker like this, they make the repair look like childsplay. It is quick, takes longer to repair the varnish. Sadly, he has retired so the fleet now allows alloy masts and when a mast breaks it is usually replaced with a shiny new stick.

Your mast will be a different animal. Choy Lee was an excellent builder and no doubt built a very high quality mast but you have age against you. Hollow masts tend to degrade from the inside out. Determine what caused the failure. End grain will draw water right up if left exposed to standing water, just like if it still is part of the tree. This may be the cause, or it could be that water entered above and pooled in the base. The next wear area would be your glue lines, these can be easily splined if one is bad but they all may be suffering from old age. Last, check every screw and fastener, if the wood is deteriorated, it is easy to make what is called a dutchman, an inlay to repair. Replace all stainless fasteners, this is cheap insurance

I'm doing a 50 year refit on an old topsail gaff cutter I acquired. It has a spruce mast that I didn't bother to go up to inspect. I just assumed it would be toast because it had horrible maintenance ..... turns out, we were able to save it. The builder did such a nice job 50 years ago that the mast survived. I put in one spline and one small dutchman and it is good to go for another 50 years. Wood can be good.



Rich

[fourlyons]

Well, first I would draw a distinction between laminated and hollow. A laminated mast or structure is made up of multiple layers of wood, but is solid, plywood is a laminated structural panel. Hollow wood masts or spars are not really laminated structures as I understand the term.
Yes, traditionally vertical grain or quartersawn wood is spec'd for hollow wood masts or other spars. I believe the main reason is that this produces the stiffest structure. Perhaps it is only tradition. It is also the most dimensionally stable. Wood swells most with the growth rings, so a flat sawn board expands more across its face than a quarter sawn board does.
Having said that, a mast built of flat sawn lumber, that is well sealed, with no grain runout, is probably perfectly adequate, as long as it is built with a healthy safety margin.

[fourlyons]

"Hollow masts tend to degrade from the inside out."

Rich, I would love to know how it is you come to this conclusion?
How would that even happen, all of the environmental exposure is on the exterior. I know one common theory is water condensing on the surface of the wood inside, that has presumably not been sealed.

That's certainly not been my experience. When I find failures in wood masts, it is always either where fasteners have been attached, or the wood has sat in water, or something was attached to the wood that was loose enough that it held water against it (the bedding failed), or the glue seams failed.

[Cheechako]

Originally Posted by fourlyons

Well, first I would draw a distinction between laminated and hollow. A laminated mast or structure is made up of multiple layers of wood, but is solid, plywood is a laminated structural panel. Hollow wood masts or spars are not really laminated structures as I understand the term.
Yes, traditionally vertical grain or quartersawn wood is spec'd for hollow wood masts or other spars. I believe the main reason is that this produces the stiffest structure. Perhaps it is only tradition. It is also the most dimensionally stable. Wood swells most with the growth rings, so a flat sawn board expands more across its face than a quarter sawn board does.
Having said that, a mast built of flat sawn lumber, that is well sealed, with no grain runout, is probably perfectly adequate, as long as it is built with a healthy safety margin.

One would think that flat sawn would be better on the long sides of the rectangle as quarter sawn would be more prone to cracking.... the grain lines running only the thickness of the board. Albeit flat sawn more prone to warpage.

[fourlyons]

Originally Posted by Cheechako

One would think that flat sawn would be better on the long sides of the rectangle as quarter sawn would be more prone to cracking.... the grain lines running only the thickness of the board. Albeit flat sawn more prone to warpage.

As you note, flat sawn is more prone to cupping and twisting which will stress the glue lines and want to distort the box in general. The consensus is actually that quarter sawn checks less, not more, than flat sawn lumber.
A flat sawn board 6 in wide has a great deal more movement than than a board one and a half inches thick, that is quarter sawn. The comparison is 6 in to one and a half inches, that is going to encourage checking. If you look at the typical residential wood deck closely, you will see this is the case. For the same reason wood decks on boats are always, ideally, spec'd for quarter sawn lumber.

[Lemsteraak]

Fourlyons,

Let me rephrase that. If water gets into a hollow mast, it will rot from the inside out. Don't ask me how I know that. The mast I showed in the picture is a round hollow mast but because it has very few fasteners was able to survive 50 years with minimal maintenance. It literally had moss hanging from the spreaders. Blew me away.