Mast Update

[ScottWidmier]

Thought I would share some pictures.  I finally got around to taking off the solid mast and prepping the boat for the new birdsmouth mast.  The birdsmouth is 3/8" larger in diameter and a lot stiffer but is the same weight (20 lbs) as the solid mast.  It is stiff enough that I can go without the forstay removing a problem I had with clearance for gaff jaws.  I did experiment with a gaff saddle but can't get that to work quite right and I prefer the simplicity of jaws.

Here are two pictures of the old and new mast.  The solid mast is on the right and shows the warp it developed just after being laminated.

Here is my boat with the new mast in place.  It is a thick mast for the length of the boat but this seems to be the tradition with catboats.  Probably because shrouds and forestay really don't work well given how far forward the mast is postitioned.

[dale]

Looks great, Scott.  I'm sure you'll be happy with it.  I sure wouldn't worry about the thickness of it. 

I wonder if there is a way to prevent the laminated mast from warping? 

[ScottWidmier]

I have made four mast out of 2X material and only had problems with one warping.  Of the four, one was out of douglas fir but the other three were out of spruce bought from Home Depot.  Now, if I were to make some guesses...

1) this was the only mast for which I used 2X4.  The others were at least out of a 2X6 which may mean better wood.

2) this wood seemed to have a much higher moisture content (more green) than the other wood I used.

I do have to mention I always look at the endgrain and try to pick pieces with verticle rather than concave grain pattern.

[PAR]

The thicker and wider the stock you build your mast from, the more subject it will be from "internal stresses". You'll usually find these stresses as you rip the stock on a table saw. Ever rip a piece and have it curl into or away from the blade as it passes through? You relieved some of the internal stresses in the lumber and the wood took a natural "set", governed by the remaining internal stresses. Straight grain run and quarter or rift cut stock will be less prone, but all lumber will have these internal stresses.

As a tree grows, it twists, bends and turns in relation to the available light, branch weight, terrain, soil condition, climate and other things. This causes grain patterns to move from growth ring to growth ring, some years having more density, others less, maybe a bad nutrient year, followed by a very wet year, etc. This places the tree under strain for that growing period, but the previous and subsequent years will have different rates of environmental strain on the life of the tree. These changes in the growth pattern generate dense and less dense areas within the tree. The denser areas pull on the weaker areas, bending it toward them, but the bulk of the tree prevents a major event (like it toppling itself over for example) from changing the basic shape of the trunk (in the short term).

Once the tree is cut, you begin to open up these areas, which can release some of this stored energy. I've rough cut logs on a portable mill and had some pretty violent changes take place, as I've passed through a particularly stressed area. I've had the logs jump nearly off the mill, lifts (rough cut planks) hop right off the mill bed or worse, bind so hard as to smoke the 20 HP engine's clutch.

Kiln dried lumber is the worst. It is forced to dry and the natural tendency of the lumber is to slowly release some of the internal stresses as it seasons, but it gets robbed of this and locks in the stresses, making them tighter (just waiting for a saw to come along and free them). Also some species are more prone to this then others.

[Hirilonde]

When cutting taffrails, bulwark caprails and other curved pieces from wide stock, these hidden stresses Par speaks of play a major role in lay-out and cutting.  There isn't much one can do to anticipate stress relief bends and twists when fabricating masts except looking for the truest grain pieces.

When I lay out a curved piece on a wide board I make a pattern and locate the shape on the board with a margin of wood all around it.  Next I cut out the shape on the band saw still leaving a margin around the shape.  This will relieve some stresses and thus the board will distort to the shape it wants to be.  But my piece of wood  is still large enough to make my piece.  I then sand/erase the marked shape and lay it out again with my pattern.  Usually the one stress relief cutting step is enough, but some situations warrant 2 such steps before making the final accurate cuts. 

Sorry if this is a little off-topic, but it seemed related. 

[Norman Colter]

No apologies needed. One of the things I love about this forum is that threads often go on a tangent course. All good information. Thanks for sharing.

Norm