Thread: Weight GIS box mast

Originally Posted by Joost

Hello All,

Today I had as great day's sailing with Watermaat. The wind wasn't much, but it was still nice to potter around on our Dutch waterways sailing GISwerk and discussing boats and other matters.

More of interest to you is that we (well Ralph) weighed GISwerk's box mast:

9250 grams for the completed mast with all fittings and the halyard.

This is the box mast as drawn by MIK for the GIS according to the exact specs provided in the plans:
- mast staves - 12 mm spar quality oregon pine
- core filler for the bottom part - softwood
- 2 layers of light (160 grams) glass on the bottom 3 ft of the mast, mast fully coated in 3 layers of epoxy
- mastfittings: 1 saddle in the top of the mast attached with 2 bolts and 1 small block, 1 small saddle with screws, 1 cleat with screws
- halyard of 12 meters 6 mm dyneema
- 6 layers of varnish

Earlier I weighed the standard GIS boom: 2400 grams for the completed boom (3 layers of epoxy, 6 layers of varnish, leather at mast area).

Hope this helps.

Best regards,

Joost

Just when I was about to ask what would be reasonable weights for the spars, along comes the information I wanted. Thanks, Joost. Just starting to produce them using a lightweight Brazilian wood that I am wondering if it is too light. I`ll finish off the first (boom) and then see how it feels. Is there any easy way to test bendiness and is there a benchmark? Apply a specific weight or force in the middle and measure deflection or something of the sort? It would be good to know how bendy a sitka spruce boom is.

Steve

That's pretty close to what my alloy pole weighs--19-20 pounds with fittings but no blocks or halyards. I used my bathroom scale, though, so it could be off a pound or two.

About the flexibility--MIK says the mast can be pretty stiff but the yard and boom need to be more flexible. My sailmaker sent detailed instructions about measuring the flexibilty of yard and boom and, based on my report, he cut the sail for my spars. If you plan to have a pro make your sail maybe he will do the same. MIK says to allow about two inches of flex in both. I assume that is what he expects in use, in strongish winds.

The mast is probably a bit over size and overstrong if anything.

I can double check the stiffenss for the yard and boom if you like. Do you have the name of the timber or its density?

Best wishes
Michael

Originally Posted by Boatmik

The mast is probably a bit over size and overstrong if anything.

I can double check the stiffenss for the yard and boom if you like. Do you have the name of the timber or its density?

Best wishes
Michael

The wood is almost certainly Tabebuia cassinoides (Caixota or Marupá) though it could be a closely related species, there being so many here. Density of about 0.380g/cm3. It is very stable and clear of grain problems, no knots, so am planning to use for the framing as well to keep the weight down.

For the mast, I will be using a more expensive wood Cordia goeldiana Huber, Boraginaceae (Freijó) which is what people here say is the nearest to Sitka in performance. Density seems to be 0.48 or thereabouts. Long lengths available so no splicing needed.

If the other spars dont work out, then no big problem making them with another wood but I would like to get the mast right first time, of course.

Steve

Hello

For comparison only - my piece of lumber for mainmast for Beth is from ordinary European pine (Pinus silvestris) 60mm/60mm/3.7m (glued from 3 timbers 60/20 mm) and weight circa 7.5 kg. Tapered mast (not empty box!!!) will be circa 4.5 kg I think so. Probably ordinary European spruce (Picea abies) would be a bit lighter, but I did'nt found it in good quality. Better species as a Douglas fir (Oregon) are more expensive in my area.

Hi Robert,

I had the same problem with the spruce I got in Australia for the original BETH. You almost never see spruce at all here and I was really excited.

When I started cutting it, it was so lively that it kept jamming the saw. I had to cut 50mm at a time and put a wedge in to prevent the blade from jammng. After thicknessing and gluing up into the mast it spent the next few years getting a strange spiral bend. You can see it looking at the mast by itself but not very noticeable on the boat.

Loved the smell of the spruce though.

MIK

Joost, I'd love to hear more about the new boom and yard you are making, the larger diameter ones.



Cheers,
Clint

Howdy,

For using the Tabebuia cassinoides the yard and boom will need to have their diameters increased by 5 percent.

I can send you a drawing with the alteration pretty easily if you want to email me.

MIK

MIK,

It would be nice to have some kind of "standard" flex test for these spars. I normally do a "test" by putting them horizontal, supported at both ends where te sail should be fixed ( or in case of the mast; between top and decklevel ) and put some weight in the middle. I use it to get this "feeling" if it's okay ( hardly any science here ) or compare this with the "feeling" of another spar.

It would be nice to get some real figures by standarizing and defining things.
For the mast you could sit on it and meassure distances to the floor ( don't sit on the yard ) Take f.e. an avarage 90kg person and ad some weight if you are lighter.......or if you are heavier, get your son, neighbour or anybody else less than 90kg. For yard and boom you probably need to think in in 20-40kg range ( use cement-bags ? )

Do you agree and have some suggestions?

Best Regards
Ralph

Originally Posted by Watermaat

MIK,

It would be nice to have some kind of "standard" flex test for these spars. I normally do a "test" by putting them horizontal, supported at both ends where te sail should be fixed ( or in case of the mast; between top and decklevel ) and put some weight in the middle. I use it to get this "feeling" if it's okay ( hardly any science here ) or compare this with the "feeling" of another spar.

It would be nice to get some real figures by standarizing and defining things.
For the mast you could sit on it and meassure distances to the floor ( don't sit on the yard ) Take f.e. an avarage 90kg person and ad some weight if you are lighter.......or if you are heavier, get your son, neighbour or anybody else less than 90kg. For yard and boom you probably need to think in in 20-40kg range ( use cement-bags ? )

Do you agree and have some suggestions?

Best Regards
Ralph

Hmm, keep adding weights until it breaks and then take one off

Richard

Howdy Ralph,

Gradually this sort of data will accumulate. Basically the plans make sure the character of the bend is right so the sail will set, but down the line I can see a more refined view appearing, which would be great for those who want to optimise their boats.

It will depend on boats getting on the same stretch of water and trying different things of course, otherwise it is just a set of figures not relating to any outcome.

One way of standardising it too might be to measure the deflection of the square blank before the yard or boom are tapered and rounded to get some idea of a material comparison as well.

MIK