Thread: Birdsmouth for GIS Main Mast

I think Bruce is the current titleholder!

MIK

Hello MIK,

I think so as well.

The yard and boom are probably the same (my 2400 grams is with a large piece of leather in place on the yard and boom to protect the mast). Clean weight epoxy coated and varnished cannot be more than 2200 grams.

All the mast fittings and halyard and such might weigh 750 grams which would mean a clean weight of 8500 grams for my mast.

So Bruce's mast is clearly a lot lighter with 7300 grams. The weight difference is probably mostly in the bottom part of the mast since I have 2 layers of glass on it for the first meter. This probably also accounts for most of the difference in weight (I believe Bruce has not glassed the bottom part).

Joost

What are the general dimensions of the GIS main mast (height, diameter, tapers, sail area, etc.) and I'll work out a cruising and racing birdsmouth scantlings. I prefer feet and inches, but I'll supply both. If my quick math is right, you have a 16 - 18 pound mast? Seems heavy, but I'm not working with all the details.

Hello Par,

Bruce's mast weighs 16 pounds, my mast 18,75 pounds. Both coated in epoxy and varnished, otherwise clean of fittings.

Difference in weight is probably caused by difference in wall thickness of the staves, different types of wood and 2 layers of glass on the bottom part of my mast.

Both masts are hollow boxes tapering from 82 mm at the bottom to 47 mm at the top. Mast height is 4.7 meters, sail area 9.7 m2.

Best regards,

Joost

A birdsmouth mast 3.25" (82 mm) at the heel and tapering to 2.25 (57 mm) at the head would weigh about 14 pounds (6.3 kilo) is Douglas fir and would easily handle the 104 sq. ft. of sail. If sitka spruce was used it would be about 11 pounds (4.9 kilo) and if white spruce about 10 pounds (4.5 kilo). All assuming a 20% wall thickness.

This is on a 15' 5" (4.7 m) mast carrying 104 square feet (9.7 m2) of sail area on a free standing stick. This offers a reasonable set of dimensions, though at 15% wall thickness you can lose a few pounds off the mast (Douglas fir drops to 11 pounds, sitka to 9), but it will be delicate. I made several guesses about the GIS, but they shouldn't be very far off.

The heaviest mast mentioned here (14 pound Douglas fir) is 26% lighter then the 18.75 pound mast mentioned above. The lightest mast here (10 pound white spruce) is nearly 40% lighter then the 16 pound mast listed above. A few pounds (kilos) do make a difference.

Thanks for that Paul,

Particularly the weights of the different species.

The problem with boats that can be built world wide is that the wood itself is so variable as well.

I would not be too worried about using the white spruce for building either because the mast is quite conservative ... too strong. It assumes three people hiking hard which could happen some time.

I don't recommend the 15% wall thickness for the same reasons you add the caveat. This is a general use boat, not a raceboat which have rescue not far away and is almost expected to break things occasionally!

The thinner wall might be fine but it adds some little risk.

MIK

Originally Posted by Boatmik

Thanks for that Paul,

Particularly the weights of the different species.

The problem with boats that can be built world wide is that the wood itself is so variable as well.
MIK

That is quite the difficult thing here in the Netherlands: it is hard to get any decent timber nowadays, especially for spars. And if available, it does have its price.

I have an outstanding order for some Douglas fir since the beginning of October. The specialist just has not been able to put his hands on some decent quality wood yet.

Ordinary European pine is the only readily available (and cheap) stuff over here, but will always have some knots and pinholes in it. Ralph has made quite a nice mast out of it though, so it might be worth some more experimenting.

I have a client in Finland building one of my schooner designs (a glued lap build) who's having the same difficulty with lumber. The nice thing about birdsmouth masts is you can piece together the staves from fairly crappy lumber, scarf around the knots and the stick is no worse off.

What would be the thoughts with going with a hybrid aluminum-birdsmouth spar? The Core Sound's that B & B designed have mostly aluminum sticks. It would cost them more to go with wood if they were to have someone build it for them. What about an aluminum base and wood top since they are often two-piece anyway? What would be the advantage over all aluminum or all wood?

The Core Sound series has three different mast building options on the plans, an all wood version and two hybrid aluminum/birdsmouth versions. The all wood version is likely the least costly, but requires the most labor (a non-issue on home built), but is the most flexible and not something that Graham really likes. The two hybrids are compromises over more costly versions (aluminum/carbon fiber).

Maintenance and ease of construction would be two primary reasons for these hybrid sticks. I'm not so much of a traditionalist to not want aluminum masts, gaffs, booms, etc. It's an easy material to work, has consistent physical properties and much of the hardware on the market is geared for this material.

On these hybrids, are the bottoms or tops aluminum? What does an all aluminum set of spars go for on a Core Sound? I'd be curious to know.

Costs are a funny thing, especially with aluminum. The last time I purchased T-6 was this summer and I drove about 5 mile to my buddy the welder, who'd ordered two 2.75", 6061 with a .125" wall 24' long tubes. I cut what I needed and paid about $3.50 a foot. I don't know anyone that can get 6061 that cheap. I'm lucky, living in an area where doing marine stuff is common, so this alloy isn't unusual, plus having old Bob right down the road, so shipping is as difficult as whatever I can toss on the trailer. I know folks that are paying 3 times this for the same tubing, in different parts of the country.

The aluminum tubing is the lower section(s) on the Core Sound series. Again one of the methods shows a two piece aluminum and wooden top. While the other shows an aluminum lower and a wooden upper section.

I am doing some demos on making birdsmouth things...feel free to check out my page I created for this

Birdsmouth Masts & Spars - Maine Boatbuilder + Repairer Wood + Plywood Boats Oarmakers Birdsmouth Mast + Sparmakers Foils Boat Plans + Kits



I always take comments, questions or bad jokes.

Cheers,
Clint

Those bird's mouths are looking really sharp!