A conversation with Bob Struble - by Bob Dill - December 1988
|Bob Struble and his son Matt from Bay City, Michigan have more experience building and
sailing wooden strip masts than anyone in the class. They have made many masts over the
past several years. I had a conversation with him a couple of weeks ago. This article
reviews Bob's opinions of the current state of the art.
If you are unfamiliar with the general principals of strip mast construction you will find reviewing the Gougeon strip mast plans and the articles that appeared in the last newsletter helpful .
Bob uses spruce for his masts. He has used birch strips in the sidewalls in some past masts. Now he feels he can get the same effect with spruce and a tow or two of carbon fiber.
He picks wood that is as good as he can find. Usually it isn't perfect. Grain run out can cause serious weakness in thin strips. Quarter sawn wood tends to have less run out than plane sawn boards. Run out should be very low or nonexistent in the critical section of the mast from 4 to 8 feet from the base. Near the ends. especially the top a little run out is OK.
Bob does a few things to distribute minor imperfections in the wood over the mast. He marks one end of the board with a magic marker. He flips the board end for end with each strip! then he mixes up the strips. The strips are all thickness planed to make them a uniform thickness. This eliminates the thin spots that are typical of saw cut strips. All of these things go a long way to eliminating mast failures.
Bob suggests testing the strips before using them. The following is Bob Dill's idea of how to test them. Bend a couple of good looking 3 or 4 foot pieces of scrap strip to see how much it takes to break them. Then bend the rest of the strips a bit less hard. Test them at about 3 foot intervals along their length to see it you have any weak strips. Better to break a strip now than a mast later.
Pieces that have serious imperfections often have a long enough piece to be used in a strip boom. Short strips can also be scarfed together. Chip Cartwright made a mast that is ash to the hound and spruce to the top. use a 12:1 scarf and scarf the strips before you thickness plane them.
Making the Mast
Bob glues the masts up on a male mold He wrote an article about it for the Nov/Dec '84 issue of the Boat Builder (the Gougeon Brothers ins newsletter) with a follow up article in the Jan/Feb '85 issue. The method is essentially the same as is described in the current Gougeon Plans.
Bob does not feel the glue joints between the strips are critical. He has not attributed any mast failures to the glue joints. You do not have to spend a lot of time making sure the gaps are perfectly filled. He uses microballoon thickened epoxy as glue. The consistency that works best is about like mayonnaise.
Most of the masts Bob has made recently have had 1/4 inch walls. These weigh about 13 lb. He is not happy about having to add two pounds to conform to the new mast spec. He suggests uses 3/8 walls to make the mast come out at the spec weight. Bob has not made any masts with walls this thick yet.
Note: the term "thickness refers to the side to side thickness (Specs 2-C and 2-D). Wall thickness is just that. With 1/4 inch walls he made the bottom thickness about 2 3/8" tapering to about 2" at the hound. The top is about 1 3/4". This uses the geometry of the mast to give the proper stiffness rather than depending on a lot of carbon fiber. With a 3/8" wall thickness Bob thinks the thickness should be closer to 2" at the bottom.
Note from B Dill: By my calculations you shouldn't be too close to 2". Comparing the moment of inertia (stiffness of the cross section) I calculated a 2.21" thickness mast with 3/8" walls is as stiff as a 2 3/8" mast with 1/4" walls. The overall thickness has a much stronger effect on stiffness than the wall thickness .
The width (tore and aft) of Bob's masts are about 3 3/4" to avoid crowding the 4" maximum spew
The web structure is similar to the one Joe Norton uses (see last newsletter). it starts with two pieces of 3/8" by 3/4" spruce. The 3/8" edge is glued to the to the back of each half to form the front of the bolt rope tunnel. Bob then planes the whole mast flat (with the exception of the last little bit of the trailing edge).
To make the sail slot in the back of the mast he glues a thin strip (about 3/16") to one of the web pieces to separate the back of the mast for the sail slot.
With the new weight spec he thinks half inch thick web pieces might be better. He has not had web failures on any of his masts.
Reinforcement: Bob puts a layer of 4 oz fiberglass over the inside of the halves before the carbon goes on. Bob uses West System, one inch wide carbon tows for his reinforcement. He has experimented with various levels of carbon in the mast. He has found that how much you use in the sidewalls is not important He uses the geometry of the mast (thickness in this case) to determine the basic stiffness.
The carbon is valuable for three things. It allows you to reinforce areas of maximum stress particularly the sidewall about 6 feet above the base. It s used in the nose and tail to increase the fore and aft stiffness or close to that of aluminum masts. Its most important use is in tuning of the mast to the correct stiffness. Bob is using less and less carbon as he gains more experience with masts.
For the tail carbon he uses about 3 tows in each half of the jolt rope tunnel against the web with another tow on the wall. One tow goes on the foreword side of the web but it is not as effective there. The nose gets a similar number of tows on the inside. He suggests the Gougeon plan nose and tail Carbon schedule is fine to. Note from B Dill: An aluminum bolt rope tube as allowed by the current specs will add a lot of stiffness to the tail. A 5/8 OD 049 wall tube with a 3/16 sail slot adds the same stiffness as about 16 tows of one inch 30ugeon carbon. Finding 16 foot pieces of 6061-T6 tubing may be difficult. It usually comes in 12 lengths. It can be Melded easily however. Sailing specialists sells long lengths for Skeeter masts. See Paul Goodwin's article in this newsletter for more information fin aluminum bolt rope tubes.
For the inside of the sidewall 30b suggests a couple of alternatives. One is the Gougeon Plan schedule. The second is putting about three Layers (each layer about 3 tows wide) of carbon in the critical area of the mast: from about 3 feet to 9 feet. Most masts break about 6 feet (give or take a foot) above the base. Whenever carbon is used it is very important to stagger the ends of the tows by about 6 per tow.
The goal is to have the mast be a little too limber after the halves are glued together. It is much easier to add a couple of tows of carbon to the outside sidewalls than to make a mast less stiff.
There are three internal wood blocks in the mast: the base, hound and top. All the blocks have fore and aft Uv's" about 2 or 3 inches deep to allow them to taper into the sidewalls. This cuts down the stress concentration at the end of the block. Bob makes the base and top blocks about 6" long. The hound block is 9" with a UV" in both ends. The hound block is really a half block measuring about 3/4" by 3/4". it fits in the front part of the mast.
He Often uses lower density woods like redwood for the base and top blocks. He uses spruce for the hound block. With the new weight spec this may not be as important. A slot is cut in the top block to allow the internal halyard to get through.
For a base he has been using 1/2" aluminum plate (he would have preferred 314" if he had any). He drills 3/4" diameter holes for the mast sockets. These are just deep enough to peek through the top of the plate. He puts screws up through these little holes to be epoxied into the base block. He suggests pan head screws. The screw heads serve as a pivot for the mast step. The base plate is held in place by the screws and about 4 wraps of 2" wide 9 oz. glass tape around the base of the mast.
Note from B Dill: An alternative is the screw in mast socket available from Bill Sarns. They are just like the Kenyon socket. They screw into a 5/8 coarse threaded hole. The 5/8 stud on the cup is 3/4" long. I recess a two to four 1/4" by 2" anon head cap screws in the plate so they can be epoxied to the base block. The base is fiberglassed the same way. Bob has found that he only uses the two back positions in the base plate. The front one makes the mast too hard to rotate.
Bob is using a different hound construction method than is shown in the Gougeon Plans. He makes the hound out of 1/4- cold rolled steel. The dimensions are roughly 2" by 2". There are two 1/4" holes in the part of the hound that is inside the mast. Holes are drilled in the mast and hound block to line up with these holes. Two pieces of 1/4" threaded rod are epoxied into these holes. Bob also drills a bunch of smaller holes in the internal part of the hound to give the epoxy something else to hold onto. This hound construction has worked without failures so far.
Assembly of the mast is straight forward. The Gougeon Plans have some suggestions that may be helpful.
After assembly he wraps the base as described. The Boom jaw area can be protected with a couple of short tows of carbon or some fiberglass. The very top gets wrapped with 4 layers of carbon. The bolt rope grove is reopened by cutting the reinforcement out with a hacksaw blade. A layer of 4 oz glass is put over the outside of the entire mast to protect the wood. The mast is now ready to be tuned.
Tuning the mast
If you have done things right the mast will be a little too limber. Sail with it. Bob suggests setting the boom blocks so the boom exerts little pressure or even pulls away from the mast slightly when the sheet is tensioned. The masts sail better this way and the boom won't dig up the mast.
As you sail have a friend watch where it bends the most. He can use the sail battens as reference points. After this is done, remove the mast from the boat and sail with another one for the rest of the day.
That night put a tow of carbon to about 2 feet each side of the maximum bend point. If it is way too soft a couple or three tows might be applied. If applying multiple tows put them side by side. Remember to stagger the tow ends.
Repeat the tuning procedure until you are happy with the stiffness. The tows are only about 1/100th of an Inch thick. It shouldn't be necessary to sand them to fare them in. In any case, any sanding of the carbon should be done very carefully to avoid thin spots. If you want to fare it in doing it with a microballoon faring putty might be safer.
Sailing with the Mast
Bob says the main sailing advantage of these masts is their ease of sailing. He feels an aluminum mast is just as fast but it is much more work to sail in puffy conditions. The masts are not indestructible. It is tempting to just hold on to the sheet in a big puff or an overpowered windward mark rounding. The mast will break at some point. He suggests easing the sheet when it really gets out there. He points out that it may go out to 18" or so a few times but each time reduces the life of the mast. In any case the sail is usually very distorted with the mast bent this much. use the forestay as a reference line to see how far the mast is bent.
Note from Bob Dill: I have taped a marked stick onto the mast at the maximum bend point to actually measure the amount of bend. This is quite instructive.
Off the line the wood masts allow a fuller sail than is optimum for max overall speed with an aluminum mast. As the boat accelerates the sail is flattened by the mast bend. Finally, the leach is tightened by the mast derotating. This gives both speed and pointing ability.
Learning to sail these masts takes a little time. The most common mistake is to not get the boat wound up before sheeting in hard . it is very easy to over flatten the sail robbing the sail of the power it needs to accelerate the boat at lower speeds.
Bob and others have been using all condition sails with vest success. They have found that full, sticky ice sails do not rave the top end that all round sails do.
Bob sees several advantages of the strip mast. They are homebuildable and inexpensive n build. They can be sailed warder than aluminum masts and rave better flex fatigue Properties. And, for us old men, they are easier to sail in puffy conditions.
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