Saturday, September 10, 2016

Mast Vertical Checking Question?

I have not been working on the boat project for over a month now, the last I had done was to power plane the trees down to size. One mast was ready for sanding, and the other was 75% done when my power plane melted it's little drive belt. I have since replaced the plane, an extended story in itself.

The plane is a discontinued model, and the belt I needed is also discontinued. However, because the product is not repairable due to this, they said I could replace it with a similar model, just bring it in with a receipt. It took me a while to locate the receipt, then drive to a store with stock, but they did a straight across the board exchange, with another identical old stock plane.

Other projects and a trip interrupted further work, and I went out to look at the masts today, and I found a lot of vertical checking. The question I have is how much checking is too much? Buehler's Backyard Boatbuilding makes it sound like the checks are no big deal, but he was also talking about a mast that had been oiled. I also ask, if this mast is considered OK, do I just go ahead, finish sanding and paint it? Or would it be considered too weak, and I am better off to start over with a fresh tree?

Feedback from anyone with mast making experience is welcome, as this is the first tree I have attempted to make into a mast.

Note the spiral checking, really obvious on the one mast, I believe this is why the lumber guys left it for the firewood guys, hard to make a straight board from it. I think it is Tamarack, but am not 100% sure.
The tamarack or hackmatack has been an excellent timber much used for ships. It is practically indestructible under water and stands very well even where exposed. It is used to be the colonial substitute for the ‘compass timber’ of English oak used in the ships of the Royal Navy, it’s roots furnishing the natural knees and other curved pieces so precious to the early shipbuilders. Unfortunately the tamarack as a commercial timber is no more, for some years ago an insect pest swept the country and destroyed all the trees of any size. Their gaunt skeletons, bare, grey and dry as tinder, may still be seen standing in northern bogs and muskegs, a tribute to the species durability. Fortunately new growth is rapidly coming on. (Lower, A. 1938)


  1. I have seen many clothesline poles still standing after many years with such checking. I think that there may be some structural loss but likely not much. Caulk up the cracks with polyurethane (NP-1 maybe?) and paint them up is what I would do given your sailing grounds. But I would also get Dave and Anke to wade in on this. Lot's of experience there :-)

  2. Hi Dennis,

    Vertical checking doesn't present a strength problem. The big spars on tall ships had cracks you slide your hands in. AK trollers use smaller diameter poles with bigger cracks than you've got now. We've gone with Beuhler and put 'em up green when necessary, usually with no coating. When the wood is sound, good grain and sized well, no problems.

    To slow checking, coat the ends with a thick glop (paint, sludge oil, anhydrous lanolin, etc)... so long as it is compatible with your finish. This slows loss of intercellular (vascular) moisture and allows wood time to adjust.

    The checks allow water to enter the interior. Two strategies: Leave them open to drain and air, or stop them with a filler.

    The filler problem is that the checks work with humidity and stress, so the fillers themselves open and let moisture in. Then it sometimes can't drain as well as if open, and rot in the interior can start up (serious problem).

    Our choice has been to leave the checks open. If we finish the mast (with hard drying house oil), we dribble it in, but drain it out, coating the inner check but not filling it. Then we watch for signs of rot at the outer edges.

    You may do better with fillers in your dryer climate... consider beeswax + pinetar and adjust viscosity with turpentine (about sticky earwax consistency). Run into checks hot, and once cool, topcoat mast with hard drying, UV resistant house oil (sometimes called 'log oil).

    Hope this helps!

    Dave Z

  3. Dave, do you have a brand of House Oil that you recommend? I admit I do like the look of varnished wood, but want something low maintenance as well. The folding masts will help compared with climbing