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Thread: making a solid spar ( mast) hollow?

  1. #1
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    Default making a solid spar ( mast) hollow?

    I finished making the mast for my shellback. Came out looking quite good, but heavy! ( ( douglas fir) In a boat that is already known to be a bit tender, I am concerned that the weight would worsen that tendency. After reading about hollow spars I lamented that I hadn't made it hollow. Then I wondered, couldn't I cut the sides off with a band saw, very carefully constructing a sort of dolly for the spar to sit on as I cut it, and then reconstruct it so that I had a hollow spar. I could of course buy some spruce, but for a person who has just made a few boats, this process is not that easy to do, and the present spar is the end result of a final successful try at it after a few not quite successful attempts. Too bad masts with curves are not considered stylish. Yes, there would likely be a good deal of epoxy with filler used in the process unless I had the skills of a violin maker to do what I am proposing. I assume that this idea is folly, or it would have been done before ( perhaps it has). Tell me what is wrong this idea, I'm all ears.

    thanks

    David

  2. #2
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    Default Re: making a solid spar ( mast) hollow?

    I'd be tempted to keep the solid mast as backup and build a birdsmouth mast from a lighter wood. But if you're willing to deal with possible breakage (and I've broken two on my dory skiff), you could try cutting it in half lengthwise and using a router to remove material from the center. You probably want to leave it solid for a ways above the mast partner, but could remove some wood down there if you really want to.
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  3. #3
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    Default Re: making a solid spar ( mast) hollow?

    I can't imagine in a boat that's so small and ballasted by shifting your weight that a tender boat would be a problem. Your weight would so greatly overcome the tenderness that the point would be rendered moot. Unless you're unnaturally small of stature but even a child of 9 should be able to sail that boat without any stability issues brought about by her tenderness. Am I wrong?
    If one advances confidently in the direction of his dreams, and endeavors to live the life which he has imagined, he will meet with a success unexpected in common hours.
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    Default Re: making a solid spar ( mast) hollow?

    I've seen it done.
    But I wonder, if you have thought about, or even calculated, how thick your mast needs to be, will not hollowing it out make it too weak?
    If a thinner mast is OK, then perhaps it is easier to just make it thinner?
    You may earn a bit of weight-loss contra strength by hollowing out, but it will still weaken the mast, and the glue-line is a weakness by itself.

    /Mats
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  5. #5
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    Default Re: making a solid spar ( mast) hollow?

    you are brilliant!

  6. #6
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    Default Re: making a solid spar ( mast) hollow?

    Quote Originally Posted by mohsart View Post
    I've seen it done.
    But I wonder, if you have thought about, or even calculated, how thick your mast needs to be, will not hollowing it out make it too weak?
    If a thinner mast is OK, then perhaps it is easier to just make it thinner?
    You may earn a bit of weight-loss contra strength by hollowing out, but it will still weaken the mast, and the glue-line is a weakness by itself.

    /Mats
    Removing wood from the centre leaves the mast stronger than removing the same weight of wood from the OD.
    It really is quite difficult to build an ugly wooden boat.

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  7. #7
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    Default Re: making a solid spar ( mast) hollow?

    I have ripped solid masts in two in the band saw then hollowed them and glued them back together. Easy and quick with significant weight reduction. I left them solid from the partner down and from the sheave up.
    I did the hollowing with a skill saw set to rip to different depths and finished with a big gouge. A coat of epoxy painted on the interior while it was glued up sealed it.

  8. #8
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    Default Re: making a solid spar ( mast) hollow?

    Quote Originally Posted by Peerie Maa View Post
    Removing wood from the centre leaves the mast stronger than removing the same weight of wood from the OD.
    Perhaps I wasn't clear. That is true.

    /Mats
    Yes the avatar depicts me; yes I drew the comic boat pic, it's a joke on the pop song I'm Not a Robot by Marina and the Diamonds.
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  9. #9
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    Default Re: making a solid spar ( mast) hollow?

    I’d be interested in the volume of wood to be removed at 38 lbs/cubic foot. Is that weight worth the effort to remove it?

  10. #10
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    Default Re: making a solid spar ( mast) hollow?

    Do you know the weight is actually a problem? No point fixing something if it doesn't need fixing.

  11. #11
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    Default Re: making a solid spar ( mast) hollow?

    I will defer to Jim Michalak for a far better write up about strength and stiffness than I can manage. http://www.jimsboats.com/webarchives/2000/1feb00.htm

    I don't see much harm in cutting a solid mast in half, hollowing it out and gluing it back together. The effort required to hollow it might be close to or even more than making a new one though.
    Management is the art of counting beans. Leadership is the art of making every being count. --Joe Finch

  12. #12
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    Default Re: making a solid spar ( mast) hollow?

    A core box pattern plane is a great aid when hollowing a spar blank!
    The sole should be a 45 deg vee, with a vee shaped blade. Check on Utube for making a wood bodied plane and make it as described here. A pair of battens attached to the flats that will be glued will provide a track for the plane to follow. Battens can be attached with two sided extra strength tape as sailmakers use. The plane will smooth out gouge work and will ride in a half circle arc down the track. In truth making a new bird's mouth spar will be simpler.
    Jay

  13. #13
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    Default Re: making a solid spar ( mast) hollow?

    I just did a quick bit of math FWIW. For a spruce mast I’m making, if I hollow it with a 2” diameter x 16’ void rather than solid as designed, I’d be saving 10 pounds (spruce @27 lbs/cubic foot. Not worth it for me.

  14. #14
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    Default Re: making a solid spar ( mast) hollow?

    10 pounds/10 feet is 100 ft lbs
    You might be surprised, I certainly was.
    Especially in the ease of handling, it was totally worth it.

  15. #15
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    Default Re: making a solid spar ( mast) hollow?

    Just a thought for a hollowing jig using a router and or circular saw to hollow the mast.

    A long box can be made from say 2x4s and a 2x8 with shims, wedges screws or whatever to center the mast half and hold it even with the top of the rails. A sled guided on the outside of the rails could be used to center the router or saw to cut the hollow. A router would make quick work of a stepped cavity. A circular saw set at an angle can cut an elliptical groove. The saw can only remove a millimeter or two at a time, but it can leave a smooth oval.

    The depth of cut with a tapered mast could be continuously varied by mounting the mast with the bottom surface on the bottom of the guide for a constant wall thickness. if the thin end of the mast is raised, the wall thickness will decrease as it tapers. It will probably need to be hollowed in short stages as the set up for the fat end won't be right for the thin end.

    I would set up with a pine 2x4 to see how or if it works before committing to the mast. Top view left, end view right.
    Jig.JPG
    If this is a stupid idea, it will be more polite to say that it won't work well and give a reason that dances around the stupidity without being too direct. If it looks bad or dangerous enough that beating over the head is better than beating around the bush, fire away.
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  16. #16
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    Default Re: making a solid spar ( mast) hollow?

    "The effort required to hollow it might be close to or even more than making a new one though."

    I think this is probably true. I might just go sailing with the mast as it is, and then determine if the weight seems too much. If you still want a lighter mast after the season, you could make a new birdsmouth one, provided you have access to a decent table-saw. I started with a short mock-up, maybe 18" or so out of scrap. It's easier than it seemed to me before I tried it.

    Good luck, and have fun sailing your new boat!

    Mike
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  17. #17
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    Default Re: making a solid spar ( mast) hollow?

    Before firing up the saw, I would fire up my calculator. Calculate the negative righting moment contribution of the mast, with the mast as it is, for say 45 and 90 degrees heal. Then recalculate with the mast hollowed out.

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