Tappered Birdsmouth Mast

[D Gobby]

I'm building a 15' daysailor designed by A.Mason.

It's time for me to start building the mast. The plans call for all spars to be solid select grade Sitka Spruce.

The mast is 22'11" long,3" in dia. , with a tapper from 3" dia to 1 3/8" dia over the last 9'. The plans call for the aft side of the mast to be straight.

How do I taper the mast only from the sides and front and not the aft side.

I was thinking of using Douglas fir instead of Sitka Spruce. If I have read correctly I should increase the dia by 10% to make for the mast not being solid. Should I also increase a little to compensate for the Doug. fir.

Any and all Help welcomed!!!!

Darrel Gobby

[NormMessinger]

Make yourself a long flat table. A couple of 2x8's end to end on several saw horses will do iif the boards are a streight plane. Mash your spar down on the flat surface and that side of the spar will not be tapered. There is plenty of slip in the staves after glue has been applied so you will have no trouble doing this. Stretch a chalk line above or on each side of the mast so you don't get a side bend or end up with a mild dog leg in other directions.

[George Roberts]

D Gobby ---

Your hollow mast should be designed based on the design loads of the original mast not on the size of the original mast. But there are several mast programs on-line that will give you reasonable numbers for a hollow mast based on a solid mast size.

As for the aft side being straight. When you glue up the staves to make the mast, lay the aft side flat on a surface as you clamp it together. I would not taper the aft stave, but that is optional.

[Paul Scheuer]

I'm with Norm. You can do all the intricate geometry, but its all going to come out in the clamping.

George, why, and how can you not taper one stave without not tapering the opposite piece, and have all the birds bite right ?

[NormMessinger]

"When you glue up the staves to make the mast, lay the aft side flat on a surface as you clamp it together."

Yup, that would work too. tongue.gif

Not tapering a stave would yeald an oval section, rather, sorta more "D" shaped. Oval would form is staves of opposite sides were wider than the others.

[George Roberts]

I got to thinking about not tapering the aft stave and ...

(lots of excuses to cover my butt.)

It seemed like a good idea at the time, but times change and it is not a good idea now. It was always a bad idea. (that was tough.)

[ 01-25-2004, 07:51 PM: Message edited by: George Roberts ]

[NormMessinger]

Ya know, George, I don't care what your wife says, you're okay. smile.gif

[Keith Wilson]

To take a stab at the other question - Douglas fir is a bit stronger and heavier than Sitka Spruce. You certainly don't have to make the spar bigger if it's Fir, in fact, a slightly thinner wall would probably be OK. An equally strong mast of similar construction will be heavier out of fir than spruce, but a hollow mast of equal strength will be considerably lighter than a solid one.

[ 01-26-2004, 12:21 AM: Message edited by: Keith Wilson ]

[SailBoatDude]

I ran some numbers quickly for a hollow bird's mouth w/ 20% wall thickness in Doug fir and typical taper on a mast head stayed rig. Your taper seems very dramatic, but the stick should weigh between 20 and 25 pounds, depending on intended use. 20 for a racer and 25 for a light cruiser. 1 3/8" at the top seems very light. Typical taper would put it close to 2" at least.

These numbers would change if it's a cat, or unstayed, or bendy rig or fractional, etc.

[Nicholas Carey]

Originally posted by SailBoatDude:
I ran some numbers quickly for a hollow bird's mouth w/ 20% wall thickness in Doug fir and typical taper on a mast head stayed rig.Nevin's and Herreshoff's scantling rules both call for wall thickness of 20% of maximum diameter of the spar when built with sitka spruce.

douglas fir is both strong and stiffer: you can probably safely reduce the wall thickness to 15% of the max. diameter (or even 10% if you're brave :D ) and wind up with a suffiently stiff and strong spar.

[NormMessinger]

Carlson Design (http://www.carlsondesign.com/) has a couple of dandy little programs that will help. Free, too. Scroll down the page. One will calculate the stave thickness and width for any given diameter. Get a vernier to measure the widths 'cause a hundredth here and a hundredth there and pretty soon you are talking about really different sizes.

The other will help understand the relationship between various species, Sitka and Doug Fir among them as well as the scantlings for a given use, stayed or not, etc.

[D Gobby]

Thank You all for your reply's

I think the best way would be to lay it out flat when I do the glue up making sure to keep the aft stave straight and flat on some 2x or whatever I have handy. Thanks for that link to the carlson site Norm.

Due to my computer illiteracy I can't seem to open the mast program. It trys to open it as a pdf file and I can't seem to change that.

Checking the taper on the plans again it does taper from 3" to 1 3/8" over 9 feet. Checking the taper with a scale the 1 3/8" does measure out at 2" could be slighty less. do you all think that it could be 1 7/8" instead of the 1 3/8" as call out on the plans.

This boat I'm building has never been built so I really don't have anything to compare it too.

As all ways thanks again for the help.

Darrel

[SailBoatDude]

I've built quite a few sticks over the years and 20% is the standard. 15% is used for the brave and racing, though in this case he's using solid stick dimensions for a hollow mast and Doug fur instead of Sitka. I'd not change the wall thickness less then 20% in light of these facts.

The quick numbers I got gave a ball park taper (at 9') of 2". The odd eighth shouldn't hurt, but I do think a mast that tapers from 3" to a healthy broom stick dimension (1 3/8") should be strongly reconsidered.

Lets look at this stick. It's near 24' long, meaning it'll have a 7' to 9' boom if proportioned normally. Just guessing on the bury or tabernacle to gooseneck height she'd carry something like 60 to 90 sq. on that track, he's trying to figure out how to do in bird's mouth, this is assuming no jib.

I know all about bendy sticks , but 1 3/8", whoaaa . . .

[D Gobby]

Some more info. The boom is 8' 7" and the main sail is 74 sq ft. This is a stayed mast approx 2' of bury. There is a jib stay as well and the jib size is 36 sq ft. The taper starts 4' below the shroud tang.

I hope that info helps

Darrel

[Keith Wilson]

1.375 dia at the top for a 74 sq ft mainsail doesn't seem that small to me. I have a boat with a 60 sq ft sail and an unstayed hollow mast that tapers to about 1" dia at the top. Remember that with a normal birdsmouth spar the wall thickness is constant, so it's almost solid at the top. If the the shroud attachments aren't located at the masthead, stresses at the end are small. Al Mason knew what he was doing.

Consider, also, what will happen if it's a little heavier: nothing really. The mast is already lighter than the plans show, whatever the wall thickness an the size at the top. It'll be a little harder to step, and less bendy. You're not racing,and I dont think you'll be using mast bend to control sail shape, right? Just be careful gluing it up; it'll be fine. Birdsmouth spars are surprisingly easy to build, and very light and strong.

[ 01-27-2004, 10:55 AM: Message edited by: Keith Wilson ]

[Bill Perkins]

Darrel you read right .Increaseing the hollow spars' diameter 10% over a given solid one while retaining a 20% wall thickness is a sound rule of thumb right out of Skene's Elements of Yacht Design .Mr. Mason is mentioned in the Acknowlegments of my copy for his assistance to the author.

[ 01-27-2004, 03:05 PM: Message edited by: Bill Perkins ]

[SailBoatDude]

Okay, that sounds better. That hanky has to come in well below the top, a 3/4 rig, I like the 1 3/8" now, though the newest information produces figures requiring over 1 3/8" at the top, but not by much. With the use of Doug fir and 20% wall thickness I'd think it would be safe in these dimensions and it would look as the designer had it scaled.

[D Gobby]

Thanks agian for the reply's

Let me see if I've got this right. Building in Doug Fir keep the Dia of the mast at 3" and keep the mast as designed tappering to 1 3/8". The wall thickness should be 20% of the largest Dia. 20% of 3" inches is .6" and the wall thickness remains constant for the length of the mast. The staves should be 1.2" for the 3" dia and .55" for the 1 3/8". Does that sound right.

Just playing around after work tonight. the taper is not correct but I was just seeing if I could build the darn thing. http://im1.shutterfly.com/procserv/47b4df08b3127cce87c1338465300000001610 http://im1.shutterfly.com/procserv/47b4df08b3127cce87c13385e4010000001610 http://im1.shutterfly.com/procserv/47b4df08b3127cce87c137ffe4790000001610

[brian.cunningham]

Nice!

[scepticus]

Originally posted by D Gobby:
Thanks agian for the reply's

Let me see if I've got this right. Building in Doug Fir keep the Dia of the mast at 3" and keep the mast as designed tappering to 1 3/8". The wall thickness should be 20% of the largest Dia. 20% of 3" inches is .6" and the wall thickness remains constant for the length of the mast. The staves should be 1.2" for the 3" dia and .55" for the 1 3/8". Does that sound right.

by my quick calculations, after you cut the bird's mouth in the .55" end of the .6" thick stave, you will only have .25" of material left at the center of each stave. Probably workable, but small enough to cause me to make sure my experiments actually reproduce the small end of the mast.

[SailBoatDude]

Your thickness seems heavy, but not by much, my numbers show 1.162" stave at 3" with a .669" wall using the asymmetrical layout shown in the photos. A symmetrical layout would require a 1.243" stave with the same wall thickness. You get a larger gluing surface with the asymmetric layout, so I'd use that. Symmetric layout produces a nice 8 sided box where you can blend or melt the roundness into the crisp edged box. It looks nice in some applications, there is a bit more figuring in the cutting stage, but I go for the max gluing surface of asymmetric.

The little end of this stick still seems small to me, but I'm a bit weird anyway.

[ 02-02-2004, 10:11 PM: Message edited by: SailBoatDude ]

[D Gobby]

I will use the asymetric layout when building this mast. I will also due a mock up of the taper to see if its doable at 1 3/8".

I due have another question re: the grain of the staves. Should the vert grain run out from the center of the mast or across the wide part of the stave? Seems like running across the wide part would be more like the growth rings of a tree.

In the picture of the large end of my mock up I have both. Vert grain running from the center out to the left side of the mock up and more flat grain at the top of the picture. Which way is best.

Thanks again for all the help and advice.

Darrel