Taper in a birdsmouth mast

Discussion in 'Wooden Boat Building and Restoration' started by dwalker, Nov 9, 2009.

  1. dwalker
    Joined: Nov 2009
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    dwalker Junior Member

    Hi All-

    I acquired my first boat this summer. While I've been sailing to some extent for the better part of my life, just over the last 3-4 years has my interest in sailing really jumped! The boat that I got is a 32 ft homemade glass-over coldmold plywood. I haven't yet tracked down the original owner/designer/builder, but I was told from the previous owner that this boat was designed for a school project or something like that. It has a cutter rig... that has seen better days.

    So my first project on the table is a new mast. The old one is solid, looks like a spruce/pir. 35' from the tabernacle which is 1' above the deck. 6" at the base and 3" at the top. There are a couple of horizontal cracks in the mast, and a fairly sharp aft bend centered around the point where the staysail stay connects. there is also a clothspin scarf at that point. This bend is there with the mast on the ground. While this mast is probably repairable... I'm going to build a new one... that's just my style. I did a temporary fix over the cracks and put her in the water to see how she sailed. I pushed her to about 20 knts(wind), and didn't break the mast!

    My plan is to make a hollow birdsmouth mast from doug fir. My question that I'd like to put out is regarding the taper in the mast. To me the taper from 6"-3" looks a little skinny at the top.

    Is there a traditional ratio of taper that is generally adhered to?

    While I could just match the original (I actually think that the mast I'm replacing is a replacement). I'm not convinced that whoever did that mast replacement really new what the original design was...I'm also considering running a halyard or 2 inside the mast, and 3" doesn't leave a lot of room in center of the top of the mast.

    The other question I have regarding the taper is about the rate of taper. Is it typical for a wood mast to have a linear taper? I.E. straight line from base to top. Or should the taper start some ways up the mast? Or should there be a increasing rate of taper the farther up the mast?

    Any thoughts would be much appreciated.

    I've been forcing as much into my head as I can in the last several weeks, triyng to figure out the best path for my scenario.(which involves as little money as possible).


    I should also mention that I'm a woodworker by trade, and am not afraid to try what some may think is a daunting project.

    -Dale
     

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  2. alan white
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    alan white Senior Member

    I don't know where the rule of thumb would be found for taper on a bermudan mast but it seems to me from what I've seen you'd see a straight taper from the base to the spreaders (single spreaders) and then some curvature to the masthead. However, the aft face of the mast should be straight all the way, so the curvature would all be on the forward face.
    What percentage of the base diameter the spreader diameter is, I don't know.
    However, a look through a Chapelle book would likely present some good examples of masts of the type you are building (though they would be solid).
     
  3. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Yes, there are starting and ending points for your tapers. Typically, you'll have a foot and masthead taper and the beginnings of these can be all over the place, depending on the needs of the rig. The whole point to these tapers is weight and windage aloft savings.

    If you provide the weight of the boat and general dimensions of your rig, I can get you very close with a fine "cruising" stick. Knowing the actual design would be much more handy from a specifications point of view.

    So the bulk of the answers you need are "it depends". Judging by the photo, you had a "bendy" top, which doesn't look right to me in the apparent conditions (wind) of the photo, suggesting it wasn't right.
     
  4. dwalker
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    dwalker Junior Member

    Any ideas on finding the weight of a boat that you can't just go look up the specs for?

    As for the rig dimensions, I'll have to go measure things. What are the most critical dimensions that would be helpful? I would assume boom length, and forestay lenghts and heights on the mast? and maybe spreader location?

    -Dale
     
  5. dwalker
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    dwalker Junior Member

    Alan-
    Is the Chapele book that you're refering to "Boatbuilding: A Complete Handbook of Wooden Boat Construction"? I would guess that it is

    -Dale
     
  6. PAR
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    PAR Yacht Designer/Builder

    Run over to the local marina and talk the travel lift operator into giving you a quick weigh.
     
  7. dwalker
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    dwalker Junior Member

    Boat weight will probably have to wait until spring... the boats up on stands buried in a full boat yard... Unless... I could probably get a rough idea of the volume below the waterline... that would get me displacement, right? That might get me within 500lbs.

    Maybe i'll give that a try this weekend.


    -Dale
     
  8. alan white
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    alan white Senior Member

    I'm thinking it is, without finding the book right now. Must be up in the attic.
     
  9. gonzo
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    gonzo Senior Member

    The taper can be straight but looks better with a curve. Displacement is easy to calculate with a boat out of the water. You need to take a few measurements to figure the volume of the submerged hull including all appendages. Multiply that by the density of water.
     
  10. PAR
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    PAR Yacht Designer/Builder

    If you can measure the length of the waterline, the depth of the hull not counting appendages and the maximum beam at the waterline, I can estimate the displacement. It would be helpful to describe (pictures) the hull a little better in case appendages do need to be included.
     
  11. dwalker
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    dwalker Junior Member

    Hull and rig Dimensions

    Thank you everyone for your help with this.

    Hull Dimensions:
    LOA - 28'10
    Length of Water Line - 24'4"
    depth of hull (from waterline to lowest point of hull with out appendage) - 20"
    Widest Beam at waterline - 8'6

    Average length of keel - 8'3
    Maximum width of keel - 1'7
    average depth of keel( I forgot to measure this one...) - it's about 22"


    Rig Dimensions:

    Mast Length from Desk - 36'
    Forestay attachment point from deck - 35'
    Staysil Stay attachment from deck - 25'2"
    Aftstay chainplate to mast - 15'2
    Forestay chain plate (is this called a chainplate if it is on the bowsprit?) to mast - 14'11
    Staysil stay chainplate to mast - 10'9"
    Spreader height from deck - 15'7"
    Spreader Length - 3'10"
    Cap Shroud chainplate 4'2 atwartship from mast, inline with mast.
    Lower Shroud chainplates 2' fore and aft of cap shroud chainplate.
    Mainsail track length - 33'10"
    Boom Length - 12'6

    Would anything else be helpful?

    -Dale
     

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  12. dwalker
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    dwalker Junior Member

    Any Thoughts?
     
  13. PAR
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    PAR Yacht Designer/Builder

    Your approximate displacement is about 4 tons. Your sail area will be in the 400 to 500 square foot range, but this seems on the heavy side, though your dimensions suggest this. I'll assume 450 sq. ft.

    At some point you're going to have to hire a professional or make some very accurate drawings and work up the math yourself, as the measurements don't include the for triangle or main sail dimensions, so I took a guess based on what was offered. Seeing this, suggests you aren't familiar with the lettering system standards used in rig dimensions, further suggesting you'll need professional help to sort this out.

    In spite of this, a stayed mast with 5" diameter at the deck, a foot taper of 1" (4") if stepped on the keel and a masthead diameter of 3.75", with the taper beginning at the spreader (single spreader rig), with a 20% stave thickness, hollow birdsmouth, made of sitka spruce will weigh about 75 pounds, Douglas fir about 95 pounds.

    I can't tell you the crap load of assumptions I've made, so these mast dimensions are just a ball park and would be typical of a 4 ton coastal cruiser, carrying a cutter rig. A heavy duty version would have a 5.5 to 5.75" mast diameter at the deck. You could also get lighter with an oval section birdsmouth.
     
  14. dwalker
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    dwalker Junior Member

    Thanks for the advice PAR. I'll learn correct standards along the way on this project.

    I measured the sails, and as you thought... your estimates were a bit high. The main is about 162 sqft, and the largest jib is about 213 sqft, totaling to 375 sqft.

    I don't think it's in my budget currently to hire a naval architect ( I assume this is who would be the most help?) to help me out with this... so I'm just going to fumble through it.

    I know that it is certainly more complex than a simple relation, but is there a general rule about the ratio of displacement to sail area?

    I think at this point, I'm going to plan on over building slightly. I'm not too concerned about the weight of the new mast... it will certain be less that the old solid one. And this certainly isn't any kind of race boat... I'll probably stick with 6" at the deck and maybe taper to 4". I think I can probably take a foot or so off the length of the mast looking at the photos of the rig.

    Thanks for your help

    -Dale
     

  15. Tuva
    Joined: Dec 2009
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    Tuva New Member

    Mast size

    The size of all the elements of a rig is really dependent on stability, not displacement. A boat can be heavy with little stability, or light with good stability, depending largely on beam and the depth and percentage of ballast. With low stability, the boat will just heel over in a breeze and put little stress on her rig.

    Stability is rather tricky to figure, but you can do an inclined experiment with the boat in the water which will give good results. The design of the actual mast and wire sizes is a bit complicated, however.

    For a masthead rig, there should be very little taper, because the entire mast is in compression. Also, for a marconi rig, the mast should have an oval or rectangular section -- round masts should be reserved for gaff or very low aspect ratio leg-o-mutton rigs, i.e. rigs with no intermediate shrouds or stays. Deck stepped masts need to be bigger in cross section than keel stepped masts.

    See if you can find a sister to your boat, and look at her rig. Better yet, see if you can find the plans or designer. Barring that, go oversize if you will be sailing in places where a mast failure will put you in severe danger. If the current mast is solid, and has not failed in use, a new hollow box section mast with outside dimensions about 15% greater than the old mast will give about the same stiffness and be a good deal lighter. The wall thickness should be about 20% of the outside dimension. In other words, say the mast is 3" x 5" -- then the thickness of the fore & aft staves would be 1" and the side staves about 5/8". You can go a little thinner if you use good Douglas fir instead of spruce
     
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