Subject: Re: [harryproa] Re:: Carbon fiber spares at home?
From: "Rob Denney harryproa@gmail.com [harryproa]" <harryproa@yahoogroups.com.au>
Date: 10/26/2015, 6:39 AM
To: harryproa@yahoogroups.com.au
Reply-to:
harryproa@yahoogroups.com.au

 



On Mon, Oct 26, 2015 at 4:59 AM, robriley@rocketmail.com [harryproa] <harryproa@yahoogroups.com.au> wrote:
 

Im interested in this too, just a few observations

These are free standing rigs and depend on a bearing within the foot of the mast to be able to rotate. Ive no idea of the loads imposed or the safety factors employed. I realise that carbon is the ultimate material choice, but perhaps for a home built something out of E Glass or Kevlar 49 reinforcing is better. 

The loads imposed are large, but highly localised in the hull.  Unlike a stayed mast, all the loads are sideways, so they are lower and easier to resist than the vertical loads from a stayed mast.    Ring frames around the mast and fore and aft stringers at the keel and the deck are sufficient.     Alternatively, carbon tow can be wrapped around the bearings to spread the load. 
A big advantage of carbon unstayed masts is that if they withstand their ultimate load (flying a hull in a harry) once, they will probably never fail.  This load can be applied as a bench test or on the boat.  Stayed masts almost always fail because the stays are not tightened correctly, or one of the many small components breaks.  Neither of these are issues for unstayed masts.

Unstayed masts are stiffness dependant.  If they are stiff enough, they will be strong enough.  This means the breaking safety factors are very large.  If they are not stiff enough, they will bend a lot, but won't break.  
Fibreglass is about 1/6th the stiffness of carbon.  Kevlar behaves very poorly in compression, so neither of these are suitable for masts. 
The cost of carbon is quite low, and still falling.    For example, a mast for the Cruiser 50  would weigh about 110 kgs.  20 kgs of this would be E glass double bias or uni, applied around the mast at +/- 45 and 90 degrees, 50 kgs would be lengthwise carbon and 40 kgs would be resin.  Carbon uni currently sells for about $US50/kg, so there is about $2,500 worth of carbon per mast.  


Because they are a cantilever design with no standing rigging, they face quite high loads just above the deck. This necessarily brings large diameters to masts in the order of 10".

Correct.  This is a large amount of drag at rest, but  probably less than your typical stayed mast with spreaders and diamonds).  It also screws up the flow onto the sails.  This can be avoided by rotating the mast relative to the sail so the sail is tangential to the lee side of the mast.   This is a more powerful shape than a fixed smaller mast provides and is quite easy to do with an unstayed rig.   Max loads are at the deck bearing.


There are several ways to go about fabrication, you could have a strip plank mast that is glassed over in a system recommended by junk rig advocates, and it appears in the Gougeon Brothers on Boat Construction book, but its very brief and simply descriptive.

A tube mast built from wood will not be much better than a fibreglass one.  Both will need to be much bigger diameter to get the same stiffness and/or   benefit from having carbon applied.    But because carbon is so much stiffer, the wood or glass does not get anywhere near loaded by the time the carbon does.  Hence the wood and glass are just dead weight.  The exception to this is wing masts, where wall thickness stops the unsupported sides buckling.  A better solution is foam (lighter) and better again is a sheer web, although these are quite hard to incorporate.

Alternately a buck or mandrel could be used and removed after construction.

This is what we use for the filament wound masts.  As long as they are perfectly smooth, evenly tapered and you have a 20 tonne press to extract them, they are hard work.  Or you use a mandrel which can be dissolved out.  The main drawback is that it is very difficult to wrap the fibres tightly round the mandrel, unless they are filament wound.


Layouts differ too, in that the strip plank makes a hollow shape that is a facsimile of the finished product, or a box spar could be made and faired on the outside with non structural components much like how aircraft wings are made

Ultimately as more money can be saved on this part of the boats costs schedule than any other component sans the hulls, I think it is important to examine ways of fabricating masts that are safe and wont collapse.

Could not agree more.  The best way to make them is in one piece,from carbon. Either using filament winding for tubes or one piece moulds for wings.  These are the techniques Etamax (harryproa engineers and spar suppliers) use.   Joins on wing or round sections are  difficult, both lengthwise and to join 2 half sections.  Unfortunately, 2 half sections is the easiest way for amateurs to build. 

if any of the above is not clear, please let me know.

rob

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Posted by: Rob Denney <harryproa@gmail.com>
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