Anonymous wrote:

Thanks for the replies.

Arne. 

Are you using modulus of rupture in your calculations? For a 310/62 mm hollow spruce that would be 72 Mpa with a breaking strength of approx. 16300 kpm...

There were three numbers to pick from, tearing force, crushing force and breaking force. Not knowing what to choose, I picked the 'weakest' one, the crushing force at 45MPa. If I had chosen 71MPa instead, then the breaking moment would rise from 11600kpm to around 18500. I have later looked up data about wood, and my 45MPa are most probably on the low side. However, I have just used local trees for masts, not knowing how good quality they were, so I my thinking was 'Better safe than sorry'...

Arne

PS:
As early as in 2006, I had a go on finding the strength of wooden masts. I have later 'refined' the way to guess the righting moment of the boat, but apart from that, the write-up still makes some sense, hopefully. 
The uncertain factor is the strength of the wood material.

Hi Annie. I haven't decided yet whether to go ahead with the build or not. There are a couple of things that need to fall in place. I want to be able to finish in four years and my budget would be around 35 k euros.. So before I jump in I want to be sure that it can be done within my limits.

So far she's staked out in the garden..

It's great to see another new boat about to be built!  I shall be following this with interest, Frederik.  I dare say your progress will be more rapid than mine!

Tystie's 220mm dia x 5mm wall mainmast actually had a doubler inside, 3m of the same section as the mast, with a sliver cut out of it so that it could be squeezed in and persuaded inside the mast. Because of the longitudinal cut, it may have doubled the wall thickness but I don't think it doubled the strength of the mast, though it must have added something.

The original LAP was 10.5m, for a low AR rig, so somewhat shorter than this proposed design. When I lengthened it for a high AR rig, it was with a 10in dia x 0.25in wall tube.

Hi Frederik

When using the formulas in Chapter 6 of TCPJR, I too came up with a mast diameter of 31cm (hollow spruce). That should give a bending strength of at least 11600kpm

The other method is to look for an aluminium mast, starting with finding the ultimate righting moment of the vessel: As shown in chapter 6b  -  and guessing the max righting arm to be 0.18 times the beam that, I find that:

0.18 x Beam x Displacement = 0.18 x 3.05m x 8000kg = 4392kpm

Depending on sailing ambitions, the yield (bending) strength of the aluminium mast should then be between 8700 and 13000kpm, that is 2-3 times the vessel’s righting moment. I would guess that 10000kpm should do well. The challenge is to find a suitable tube dimension with the strongest temper ( 6082-T6 alloy, at 250(N/sqmm)).

It appears that a 220 x 5mm tube will only take 4524kpm bending. I guess a 220 tube with 10mm wall (8447kpm) would be better.

Whatever material you make the mast of, it is a good idea to check its weight against the displacement of the boat. If it weighs 3% or less of the boat, you should be fine.

Arne