Making a bird's mouth mast

Discussion in 'Boat Design' started by tsugak, Nov 23, 2010.

  1. tsugak
    Joined: Nov 2010
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    Location: Brunswick, Maine

    tsugak Darryl

    I have a 1968 Wittholz designed Herman built catboat that I would like to build a new mast for. Although the existing mast works OK, I plan on trailering this boat some and wish to have a hinged mast with tabernacle. I can buy this arrangement in aluminum but would prefer wood as the boom and gaff are douglas fir. The problem with the existing mast is it is oversized at 6 1/2" and weighs too much to covert to a tabernacle system. My question is this, what diameter and thickness, I was planning to use birds mouth construction, can I make the new mast and have it be strong enough? Some information. I have some old beautiful douglas fir I was planning to use and the existing mast is 23'5" long and keel stepped. I was thinking about 5" in diameter and the staves at 7/8" thick. One other piece of information, unlike most catboats that have only a forward stay, this boat has chainplates for port and starboard shrouds which I assume will allow the mast to be made lighter. The current mast does not use them. My primary desire for a lighter mast is to make it easier to raise and lower on the tabernacle. All advise would be appreciated
     
  2. alan white
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    alan white Senior Member

    The supplied shrouds will allow a lighter mast if they aren't too far forward to allow a reasonable staying base. One rule of thumb is that the angle between mast and shroud is at least 10%. When too narrowly stayed, mast compression can be extreme.
    There are formulas for mast diameter and wall thickness.
    Supply the following:

    Boat size and Displacement
    Sail area
    Mast height from tabernacle
    Type of wood you will be using (spruce is about 20% lighter than fir)
    Staying base (from chainplate to chainplate)
    Intended use of boat
    Gaff or Marconi sail

    Someone here will probably have some simple formulas (My books are all in storage).
     
  3. gonzo
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    gonzo Senior Member

    If the mast is in good condition, cut it in half, hollow it and glue it back. Is the mast oversized compared to the original design?
     
  4. Chuck Losness
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    Chuck Losness Senior Member

    Just google "Birdsmouth Mast" and you will get hits on how to size the mast. I think that Duckworths has an online calculator to determine mast size for wooden birdsmouth masts. You will need to know the righting moment for your boat. Can't do the calc's without it. The mast has to be strong enough to resist the stress caused by the righting moment plus a safety factor. Look at the thread that I started a few weeks ago on rotating free standing masts. That will give you an understanding of the design issues. Good luck with your project.
    Chuck
     
  5. alan white
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    alan white Senior Member

    That's really clever, Gonzo. The mast is probably tapered the whole way and then even more tapered just above the throat. What is the preferred method of creating a trough?
     
  6. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Why are you suggesting the mast is over size and how do you know it's too heavy?

    The DuckWorks calculator will provide you with birdsmouth stave widths and notch angles, but it will not properly size the staves or mast diameter. There is a solid mast calculator, but it's not something I'd trust on any thing more then a skiff like build.

    Cat boat masts, particularly free standing ones, are much larger (understandably) then conventionally stayed sticks. You can take a few good guesses at this and watch your rig come tumbling down in a gust or you can actually preform the math necessary to insure you have the safety margins necessary, to prevent getting clobbered in the head.

    There are some rules of thumb, but mostly it's just simple math.
     
  7. tsugak
    Joined: Nov 2010
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    tsugak Darryl

    Alan, here are the specs you asked for. Boat LOA 17', beam 7'11", draft (board up) 20", displacement 2800 lbs., sail area 240 sq. ft., mast length (from tabernacle) 19', port to starboard chain plate 54", gaff rigged, douglas fir for bird's mouth mast. Main use for pleasure sailing around the coast of Maine.

    I want to stick with douglas fir for a few reasons. The boom and gaff are fir and I plan to put a clear finish on them and the mast. I have some very nice 5/4 fir, 16' long and completely clear, that I've had in storage for years. I also figured that although the fir is heavier than spruce its also stronger so could be a little thinner.

    Gonzo, the original mast was aluminum. Somewhere along the way it got damaged and one of the previous owners made the fir mast.

    Chuck, I did find the Duckwoths article on mast construction with the calculator. It did give me stave length and general width but doesn't say anything about diameter size depending on boat size, mast length, or other pertinent info, hence my posting on this site. How can I find determine the "righting moment" of my boat?
     
  8. tsugak
    Joined: Nov 2010
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    tsugak Darryl

    Par,

    Its not exactly that the mast is too big, although the owner that built the wooden mast had to put additional lead under the cockpit to balance the additional weight, its that I want to convert it to a hinged tabernacle mast and the weight of this mast will make raising on the trailer particularly unfriendly to perform. Obviously a new aluminum hinged mast would be the lightest solution but if I can stick with a wooden mast and not have it be too heavy I would prefer this.
     
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  9. Chuck Losness
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    Chuck Losness Senior Member

    I would first try to see if the designer is still around and ask him what the righting moment is. You are looking for the righting moment at 30' angle of heel. Or if you know the size and wall thickness of the original aluminum mast, it might be possible to work backwards and get the strength that the aluminum mast was designed to. This of course assumes the original mast was designed properly. If that is not an option then you will have to conduct an inclining test. You can search "inclining test" or "inclining experiment" on this forum and you will get some posts that talk about how to do it accurately. I recall that Tad posted a method to do this. But I may be wrong. It could have been someone else.

    The basic calculation is that righting moment at 30' = (w*d/heel)*30. You basically hang a known weight off the side of your boat at a known distance and measure how far over the boat heels. The tricky part is getting an accurate measure of the angle of heel. For example. You get a couple of your friends to come down to the boat and you weigh them. Then you measure the angle of heel to get a base line. Next you have your friends stand on the gunnel at a know distance from the centerline of your boat and measure the angle of heel again. Finally plug the numbers into the formula and you will get the righting moment at 30'.
    Dave Gerr in his book the nature of boats also talked about timing the roll of the boat to get the righting moment. This seemed to me to be pretty difficult to do accurately.
    After you get the righting moment, then you have to calculate the bending stress. Go to the thread that I mentioned previously, rotating free standing mast design, and you will get the formula to calculate the bending stress. the thread is under sailboats. I had some difficulty in understanding this. But I got it finally. I believe the max bending stress for fir is 5,000psi. You want the answer from the bending stress formula to be 1/3 of 5000 or less. This gives you a safety factor of 3.
    Maybe Petros or Eric Sponberg will jump in here and help you out. I also recall seeing somewhere online where a guy had posted a calculator that would do the math on the bending stress for a round wood mast. I'll see if I can find it for you.
    Good Luck
    Chuck
     
  10. PAR
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    PAR Yacht Designer/Builder

    Given the general parameters you've listed, it appears the mast isn't overly big as some quick calculations suggest 5 15/16" at the base, just shy of 4"" at the head for a free standing Douglas fir stick, which will weigh in the 105 pound range. I didn't punch in all the variables, but a hollow, birdsmouth would be 3.9" at the base while 2.6 at the head, weighing about 30 pounds (20% wall thickness and Douglas fir - eastern white spruce will be 23 lbs.) and well stayed will carry the same sail area.
     
  11. tsugak
    Joined: Nov 2010
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    tsugak Darryl

    Par, I actually weighed the mast today and it came out at 190 lbs. 6 1/2" diameter at the base, 4 1/2" at the head and 23'5" long. It is bird's mouth construction at about 1 1/4" thick. It has all the rigging and hardware still on it so I figure about 20 lbs less in actual weight. 3.9" at the base? You really think I can go that small? I was figuring 5" at the base and 3 1/2" at the head. 30 lbs sure sounds great but I would be happy with anything under 70 lbs
     
  12. gonzo
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    gonzo Senior Member

    @ Alan White: there are several methods. A Gauge is one good way. Any carpenter can keep an even thickness. Go to any gift shop and check out the wooden salad bowls.
     
  13. TeddyDiver
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    TeddyDiver Gollywobbler

    As a carpenter I would build a new one instead (ask any carpenter.. ) :)
     
  14. gonzo
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    gonzo Senior Member

    I am a carpenter. If the wood is sound, the round and taper already made and all the attachment for rigging made, it is much faster and cheaper to hollow the existing mast. Price some good spruce or fir for spar making.
     

  15. tsugak
    Joined: Nov 2010
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    Location: Brunswick, Maine

    tsugak Darryl

    This mast is already hollow as it is bird's mouth construction. If I was going to reduce this existing mast I would be inclined to do it from the outside instead. Because it is 1 1/4" thick I could at least reduce the diameter by 1/2" to 3/4". I'm not sure if this will reduce the weight enough for my purposes, hence the interest in making a new one.
     
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