Yes, I guessed that I would err on the safe side. At which heeling angle does the boat have max righting moment? Those 160degress must have been the capsize point , for sure?

Arne 

PS: Anyway, if we plan the mast with a strength factor of 2.5, with Mr=2600kpm, the breaking moment of the CF mast could be reduced to 6500kpm (from my pessimistic 9000kpm).

Thank you Roger.
With those data in place, it should be possible to guess the righting moment of the Underwood 33 within about 20% accuracy.

With a decent ballast ratio of 2000kg/4500kg = 44%, but with a quite shallow draught of 1.0m, I would guess that the righting lever is hardly longer than 1/5 of the beam of the boat (probably a bit shorter).

That is, Lr = Beam/5 = 3.4m/5 = 0.67m

Then the righting moment,
Mr = Displacement x Lr = 4500kg x 0.67m = 3015kpm, or rather, the righting moment,
Mr will sit somewhere between 2400kpm and 3600kpm (+/-20%).

Then it is the question of strength factor, depending on the kind of sailing one has in mind. As I did in an earlier posting, I suggest that the actual breaking moment of the mast should be between 3.0 x Mr for ocean voyaging, down to 2.0 x Mr for lighter coastal work.

Those numbers were with a spruce mast in mind. I think that one can cut these factors a bit lower when using aluminium poles or carbon masts (provided that the mast builder knows his/her stuff). The reason for me reasoning like this is that the lighter CF and AL masts put much less peak stress on themselves (whipping) as the boat is rocking and rolling in a seaway. Maybe the strength factors 3.0 and 2.0 could be reduced to 2.5 and 1.5 on CF masts.

The strongest CF mast should then  -  at strongest version  -  have a breaking moment
Mb = 3600kpm x 2.5 = 9000kpm

I realise that I am on thin ice on this matter of strength factor. Anyone would be. The only way to collect some real data on this, would be to fit pressure sensors with logging capacity on either the mast partner(s) or the mast step(s).

Cheers, Arne

PS: Roger, could it be that Gary Underwood already has worked out the curve of the righting lever (righting arm)?

Hi Peter,

No, I've done a brief search in Aus but too many outlets and didn't have the time to study all of them. That's a good help. Shipping would be a lot cheaper than from the US or Europe.

Thank you

Roger Scott wrote:

Hi Arne,

.. I am also thinking of a CF mast in the current position without a tabernacle. In that case the LAP would be 10.5 meters.

Roger

as you are considering CF masts, have you ever tried the following website?

http://www.harryproa.com/index.php/carbon-masts

They are situated in AUS and claim to manufacture CF masts at very reasonable prices.

Hope this is helpful.

Cheers
Peter

Hi David,

I am always open for ideas. Yes, I was looking at Poppy today in issue 65. It looks good. I'll need to study up though.

I had a chat to local boat builder today as well. He thinks CF is the way to go and recommended looking up Nigel Iren's luggers with CF masts. A 70kg mast sounds attractive. Easy to rake, shift position or even lift out, but still considering the loss of weight aloft and how it might affect moment. No decisions yet!