The new rig for Shoestring has turned into a discussion about CF spars versus wood.

Arne, I passed on those formulas to C-Tech showing that, with a loaded displacement of 6 tonnes on Shoestring, I'd probably need  a breaking moment of at least 10,500kpa. Tim Willits from C-Tech and his engineer friend from Southern Spars got back to me saying....

Hi Roger

A friend of mine is a Composite Engineer and  Spar designer at Southern Spars and he previously worked at Hall Spars as head of Engineering.

He offered the advice below. 

For your information you wouldn’t worry about a stress at the surface of the tube as low as 15.84MPa as Carbon/Composite Spars can routinely see over 6 to 10 times that figure with ease.

We can take a closer look at the Spruce Spars you are currently using, as in what is their size and average wall thickness and if  you could tell us the most you have ever seen them bend, we could then tell you how our composite tube compares to that.

Our feeling is that the 10500kgm maximum moment is probably higher than you are going to see.

Mike felt that if your maximum righting moment is 3600 kgm then the mast tip is likely to bend about 940mm, and if you are sailing along and the mast starts to bend that much the boats going to get a pretty big lean on and you’re bound to ease the sheet!

Hi Tim

Got the usual pre xmas time crunch and am out on site so quick calcs show:

If we take his 10500kgm max moment and we take a case of a cantilevered beam for instance with 9m sticking out of the hole we would have a distributed load of 4666kg along the mast to balance that moment (4666/2*4.5).

If we then have your 250 x 298 x 70GPa tube which does indeed have and EI of 13651kNm and we solve for those and vary deflection we get a stress at the surface of the tube of 15.84MPa which isn’t a lot. The deflection though at the tip would be 2.75m!

No allowance has been made for taper and the load in reality wouldn’t be evenly distributed of course.

If we repeat the logic above with his maximum RM of 3600kgm then we get a distributed load of 1600kg, deflection of 943mm and stress of only 5.4MPa.

So it seems it’s not about the breaking stress as per his logic but really about the deflection. As you would expect.

Just thought this information might be valuable for you.

Mike is certainly confident in the spar suggestion we have offered you.

No problem though if its still not a goer for you.

Best Regards

Tim Willetts

C-Tech Ltd

PO Box 71-131 Rosebank 1348

Auckland, NZ.

To me it looks like they have a different way of calculating mast strength. Are you able to see through what they are trying to say? They emphasise 'deflection'  and put less priority on breaking stresses.

I am really grateful for all the information they shared, but still a lot of guess work.It would be great if there was a standard formula to calculate what thickness of CF is needed for JR tapered masts without going overboard on cost. 

Still waiting for Harry-Proa to get back to me. Those guys are making 12 and 14 metre 'hinged at the boom' free-standing masts for wing sail catamarans!

From: Roger Scott [mailto:rscot462@gmail.com]
Sent: Wednesday, 12 November 2014 9:41 p.m.