Jay Greer
10-23-2007, 06:51 PM
Hi Group,
I reciently recieved a letter from a person that is planning to build a box
section mast. I thought that the information I sent to him might be of some
interest to others. So here it is.
Jay Greer
Hi -------,
Here are a few thoughts on mast building in relation to your questions. For my own work, I prefer a bench
that is rock solid. This allows for accurate planing and sanding of the staves.
Mine is built like a ladder with cross spalls made of 2x4 material spaced four
inches apart. My own bench has the stringers made of 2x10 stock and is set
against the shop wall.
Each cross spall is drilled to take a bolt made of all thread that passes on
either side of the mast with a matching cap on top of the spar. These are
nutted for the glue up and set with a torque wrench at 1.5 lbs. Then, only the sides need C clamps to hold the panels against the inner edge of the rabbet.
There is an accurate center line down the bench to facilitate alignment of the
staves.
The top of the tree should face the top of the mast unless weight dictates otherwise. This can be observed by
reading the grain. I there is any slash grain showing the annular rings will
form a point towards the top. I always weigh my wood in order to place the
lightest stock up high. I also find the balance point of the staves in order to place the heavy end down.
Scarfs are always at least 12:1. The layout is such that the long end of the scarf faces down in order to present the least surface of the scraf to the sun; thereby minimizing the possibility of weather checking. I cut them by stacking all of
the staves so that the scarfs will be dead nuts on in one plane. I first rough cut with an adz, followed by a power plane cut and finish with a jointer plane. The surfaces are block sanded with 80 grit to give them a bit of tooth for gluing. I try to place scarfs near areas of support such as the upper or lower
spreaders if the mast has two. Under no circumstances would I put more than two scarfs in close proximity to one and other preferring to keep at least fifteen
percent of the mast length for the others.
As for glue, I have always used the brown Weldwood water mixed glue.
Others like epoxy and that, I suppose is OK. All I know is that I have never
had a mast delaminate. But I have seen plenty of sticks glued with resorcinol
fail as well are the two part Aerolite glue come apart. I am thinking of using
industrial grade Poly Urethane for the next spar I will build. I do believe that it
may be superior to Weldwood. Prior to gluing, I mask off the faying surfaces
and coat the interior faces with two coats of orange shellac. This effectively
seals the interior wood preventing moisture intrusion
I do not use blocking in my masts in the way of the spreaders as do some
builders; preferring instead to separate the spreader attachments with
compression tubes. I believe that this allows the mast to take a more natural
bend when loaded. Additionally I always use free swinging spreaders that will self align when loaded and not strain their fastenings as do spreaders that are solidly attached to the mast.
I have seen plenty of masts fail because they
were literally sawn in half by the spreader mount screws that moved as the rig bucks.
Doubling the wood near the gooseneck and lower mast hardware is a good
idea as so many fastenings are needed at that point. There should always be
a small hole at the bottom of the mast. This will allow the spar to breath with
changes in temperature and also provide condensation drainage.
I run electrical wires in a tube. I use PVC that is fixed with screw on nylon pipe brackets on one of the staves, fixing it in such a manner that new wire can be easily, fed if needed. I have also used closed cell foam donuts drilled for alignment of the tube as well as serreated edges to allow for drainage. Packing crumpled aluminum foil in the mast will turn it into a radar reflector.
So far as the cost of varnishing is concerned, I find that I can prep sand, tack
and apply one coat to a forty foot spar in an hour and a half. This includes straining the varnish and cleaning my brushes. Behr Varnish is nearly bullet proof. So I don't find it necessary to paint upper surfaces white. Of course, a
mast should be varnished nearly as often as the rest of the boat is.
Jay
I reciently recieved a letter from a person that is planning to build a box
section mast. I thought that the information I sent to him might be of some
interest to others. So here it is.
Jay Greer
Hi -------,
Here are a few thoughts on mast building in relation to your questions. For my own work, I prefer a bench
that is rock solid. This allows for accurate planing and sanding of the staves.
Mine is built like a ladder with cross spalls made of 2x4 material spaced four
inches apart. My own bench has the stringers made of 2x10 stock and is set
against the shop wall.
Each cross spall is drilled to take a bolt made of all thread that passes on
either side of the mast with a matching cap on top of the spar. These are
nutted for the glue up and set with a torque wrench at 1.5 lbs. Then, only the sides need C clamps to hold the panels against the inner edge of the rabbet.
There is an accurate center line down the bench to facilitate alignment of the
staves.
The top of the tree should face the top of the mast unless weight dictates otherwise. This can be observed by
reading the grain. I there is any slash grain showing the annular rings will
form a point towards the top. I always weigh my wood in order to place the
lightest stock up high. I also find the balance point of the staves in order to place the heavy end down.
Scarfs are always at least 12:1. The layout is such that the long end of the scarf faces down in order to present the least surface of the scraf to the sun; thereby minimizing the possibility of weather checking. I cut them by stacking all of
the staves so that the scarfs will be dead nuts on in one plane. I first rough cut with an adz, followed by a power plane cut and finish with a jointer plane. The surfaces are block sanded with 80 grit to give them a bit of tooth for gluing. I try to place scarfs near areas of support such as the upper or lower
spreaders if the mast has two. Under no circumstances would I put more than two scarfs in close proximity to one and other preferring to keep at least fifteen
percent of the mast length for the others.
As for glue, I have always used the brown Weldwood water mixed glue.
Others like epoxy and that, I suppose is OK. All I know is that I have never
had a mast delaminate. But I have seen plenty of sticks glued with resorcinol
fail as well are the two part Aerolite glue come apart. I am thinking of using
industrial grade Poly Urethane for the next spar I will build. I do believe that it
may be superior to Weldwood. Prior to gluing, I mask off the faying surfaces
and coat the interior faces with two coats of orange shellac. This effectively
seals the interior wood preventing moisture intrusion
I do not use blocking in my masts in the way of the spreaders as do some
builders; preferring instead to separate the spreader attachments with
compression tubes. I believe that this allows the mast to take a more natural
bend when loaded. Additionally I always use free swinging spreaders that will self align when loaded and not strain their fastenings as do spreaders that are solidly attached to the mast.
I have seen plenty of masts fail because they
were literally sawn in half by the spreader mount screws that moved as the rig bucks.
Doubling the wood near the gooseneck and lower mast hardware is a good
idea as so many fastenings are needed at that point. There should always be
a small hole at the bottom of the mast. This will allow the spar to breath with
changes in temperature and also provide condensation drainage.
I run electrical wires in a tube. I use PVC that is fixed with screw on nylon pipe brackets on one of the staves, fixing it in such a manner that new wire can be easily, fed if needed. I have also used closed cell foam donuts drilled for alignment of the tube as well as serreated edges to allow for drainage. Packing crumpled aluminum foil in the mast will turn it into a radar reflector.
So far as the cost of varnishing is concerned, I find that I can prep sand, tack
and apply one coat to a forty foot spar in an hour and a half. This includes straining the varnish and cleaning my brushes. Behr Varnish is nearly bullet proof. So I don't find it necessary to paint upper surfaces white. Of course, a
mast should be varnished nearly as often as the rest of the boat is.
Jay