Birdmouth spars

[Greg Luckett]

I mentioned at boat building class that I would like to learn how to make a birdsmouth spar. The instructor showed me a section of one he had made and told me he would never use one again. He explained that the one he had made bent radically when under load. I think he also said it was an unstayed rig. Now I am confused. His opinion carries a lot of weight with me, but I have doubts about this. Anyone here had trouble with this sort of spar flexing too much?

Thanks,

Greg.

[Bud_Wilson]

Greg: I think the birdmouth works great and not to hard to build. I think if you loaded one up enough to bend on a boat with stayes you could also check out your PFD and see how it works. I think Frank or Charlie posted something on it in the last couple of mounths Y'all Come Cap'N Bud.

[Barry Pyeatt]

I think the challenge is if the new birdsmouth spar is replacing a solid one in an unstayed rig. Masts in particular, if solid wood, have a degree of flexibility that differes in different points of orientation, and grain orientation can make a difference on how well the mast will work without flexing too much. With a birdsmouth you are making an artificial grain pattern that is fairly even on all points so orientation isn't as big a factor but overall strength and resistance to flexation can be an issue.

Obviously a mast with stays will have additional support and resistance to flexing on some points of orientation. If properly fabricated, it should still have the strength it needs to do the job and lower weight than its solid counterpart. On booms or bowsprits, flexing is going to be a part of the issue. With a main lashed to the boom it will have fairly constant pressure along the length. With a loose footed main the boom will flex a lot more and a birdsmouth spar may not have the strength it needs to do the job without overbuilding it. Weight isn't as much of a factor on the boom on most cruising boats so a birdsmouth boom isn't gaining much, if anything. Same with a bowsprit. Too much tension and loading stress without adding stays to make it work on anything but a light weight/small boat. Hence the dolphin striker and bobstays on them. Otherwise they would just break on a lot of boats from the uneven loading.

Also cost being a consideration, finding solid wood that is appropriate for a good mast is more expensive than finding much less expensive wood that can be used to fabricate a birdsmouth. Shorter lengths can be scarfed together and defects can be eliminated. If built as Charlie builds them, then I don't know why they wouldn't work out on most smaller boats with stays.

I'm in the process of completing a 24' Sitka Spruce birdsmouth mast for a friend to replace a solid Doug Fir mast. It weights just over half as much and is stayed. We have run wiring and two halyards up thru the center, installed plugs where it has thru fasteners and it has a solid top and bottom. It is a constant diameter for 21 feet and then tapers to the top. It is being run thru a spar lathe to make it nice and round and is being glassed full length. Old mast lasted for nearly 25 years and if all works well this should last just as long. But if it weren't stayed, I doubt if it would work as well.

[Charlie Jones]

nope- in fact we experienced exactly the opposite. The spar came out too stiff. Sails will most likely have to be recut because they were cut for a bendy spar. You can control the amount of bend by the thickness of the walls of the spar. You can offset the birdsmouth slightly and get more beef inside or out, depending on how you use them. Also you can use thinner or thicker staves.

Free standing masts depend on that bend to let pressure out of the top of the sail also. Here's a pic of a 22 foot spar I built for a gaffer in Florida- weighed 42 pounds. 3 1/2 inches aroun d up to the 18 foot mark, thentapered to 2 inches. He's delighted with it.

I've done 6 spars so far using Birdsmouth - all worked very well

[mrthethird]

Just adding my opinion and limited experience

Sometimes, if not most times, opinion has to be seperated from facts to get the truth. Sometimes it can't be done. I have worked, in the past, with at least three extremely skilled woodworkers. Almost everything I know about woodworking, I learned from these three individuals. If I asked them the best way to build a particular wood item, I would get three very different, but seemingly correct answers. If, being the pot-stirring kind of guy I am, I confronted each of them with each others viewpoint, opinion then takes over, possibly mixed with some ego :wink: , and each theory of building is summarily taken apart, ridiculed, and proven inferior. The "buffet" approach to learning, then, is best.

I have been a bird's mouth spar making fool the last few weeks, and have some reservations with the application of a hollow wood spar to reduce weight, but lack of stiffness has not been a problem. I am a little concerned that the spars may not flex enough....

Questions arise, what kind of wood was used, the diameter and length of the mast, the thickness of the sections compared to the final diameter, what adhesive was used, what kind of sail load there was, etc. In my experiences, these are not necessarily politically correct questions when addressed to an instructor you wish to learn more from. Take his comments for what they are worth, do some more research. I know very little about sailing or masts, but the engineered strength of a vertical grain fir hollow spar, following the formulas given by Frank and others, is gigantic. I can't see how one could overflex the spar without severely overloading it. I would bet I could pry my son's car off jacks with the mast for my weekender (wouldn't try it on my own vehicle)

Like speed limits, its just an opinion. Then again, I'm a land-locked, land lubbin' budget-restricted pleasure boater, not an old salt :wink:

[Greg Luckett]

nope- in fact we experienced exactly the opposite. The spar came out too stiff. Sails will most likely have to be recut because they were cut for a bendy spar. You can control the amount of bend by the thickness of the walls of the spar. You can offset the birdsmouth slightly and get more beef inside or out' date=' depending on how you use them. Also you can use thinner or thicker staves.

Free standing masts depend on that bend to let pressure out of the top of the sail also. Here's a pic of a 22 foot spar I built for a gaffer in Florida- weighed 42 pounds. 3 1/2 inches aroun d up to the 18 foot mark, thentapered to 2 inches. He's delighted with it.

I've done 6 spars so far using Birdsmouth - all worked very well[/quote']

That speaks well to me, Charlie. Did you do some T&E to arrive at how to build them or is there somewhere I might look to learn how to? I think I saw a post earlier that mentioned varying the amount of wood between the interior rabbit and the exterior and the width of the board, or something to that effect. I will keep looking for it. I expect to try my hand at this process later this year. My Spindrift will need spars as will the PS26 later. Thanks.

[Charlie Jones]

Greg- there's an article in one of the back issues of WoodenBoat. I'll look it up this evening. There's also a small article on the Duckworks site. Plus Frank has formula's on his site for calcing the stave size.

Plus there's a learning curve, but the basic's are pretty straight forward. I'm continually amazed at the range of possiblities the technique opens up. I saw some gorgeous double paddles made using Birdsmouth- oval 1 1/4 shafts with several different kinds of wood alternating in them. Staves were only 1/8 thick.

Tapers, double tapers, ovals, egg shapes, etc- all are possible just by varying how you cut the staves.

I'm seriously thinking of a double tapered spinnaker pole for Tehani using Birdsmouth.