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Wing-Mast Weights

QUESTION:  With your carbon fiber WingMasts, what sort of comparative weight can be expected vs. an alloy mast?   

Chris-G, San Francisco, USA

ANSWER:   Thanks for that Chris

I am reluctant to give a precise weight as it can vary so much with the builder and also, weights may appear to look heavy compared to Pro CF masts offered, due to many 'comparing a golf ball with a ping-pong ball'.   To my knowledge, there are no companies making CF masts much over 2.1 chord/width ratio and that only barely qualifies as a wingmast.

Most fail to acknowledge that a 2.8/1 rotating wingmast can NOT be fairly compared to a typical 1.9/1 regular fixed CF Pro-built mast.   There is at least 25% more material involved with the wider chord, so, it's a trade off of aerodynamic efficiency vs weight.

Another thing is that many mast manufacturers, in their desire to promote the lightest weight, only quote the weight of the actual mast spar .... or the weight of their section 'per meter'.  The final weight could be 20-25% more than that with the essential fittings.  On a Waters Wingmast, many of these are built in during the build, so there is never a stripped pole to weigh, though it can be estimated fairly well from the short test section that I insist be built as the full mast is made.

But it’s also important to put weight in perspective and recognize that extra weight on a multihull mast is not that critical, as these boats seldom heel over 20 degrees ... a very different case from a monohull sometimes heeling at 50o !, when that weight is now far outboard so causing even more heel.    For that reason, i would never promote a wingmast for a monohull, unless it were stabilized nearly upright with foils.      (Some may ask, well what about pitch reduction?    In my opinion, with two or 3 hulls involved, this is far more a hull shape, rocker, weight distribution issue, than it is a few pounds more up a mast).

Some might also cite the advantage of a lighter mast for on-shore handling and raising, but even there, much can be done with a winch or better tackles in the right places.

But having communicated that important info first, I would expect the 10m wingmast to be in the 70-80lb range complete with fittings., though the weight of a foot or meter of the section would be far less …. and could be well below 2lb/ft (3 kg/m).

Typically, a Waters CF Wingmast weight will be in the range of -15% to 0% difference with an alum-alloy mast of just 2/1 chord/width ratio, as much of its theoretical weight saving (with CF) will be offset due to the 30-35% extra material to create the aerodynamic wing and add the central web.

A 2.8/1 alloy wing would be significantly heavier and would also not be a very stable section unless a center web could be built into the extrusion.  I personally think a central web is a must for a deep chord wingmast as it breaks up what would otherwise be a wide and fairly flat area that would be ripe for buckling.  The almost flat tail panels of a wingmast must be limited in width to survive and even then, must still have a minimal convex curvature to help resist buckling.

I also consider that side mast bend MUST be more tightly limited, so that the center of compressive thrust does not go outside the geometric radius of gyration with such a slim, non-round section.  (See this article for a sketch).    Round tubes or rectangular sections close to this, with a Chord/Width of say 1.5 can accept far more bend without failure than would a wing mast shape without diamonds but fortunately, diamonds are easy to incorporate and are virtually standard now for all wingmasts.  In passing, diamond wires should not angle less than 10o from the mast without very good reason.

Your final weight result will depend a lot on your ability to follow and execute the manual instructions but either way, it’s not worth getting ulcers over a few kgs for a multihull mast IMHO.  Strength and resilience is what you will be praying for when caught out in bad weather, not having a mast a few kilos lighter.   And a stable multihull needs a stronger mast as its FAR more loaded for the same sail area than on a monohull or a beach cat, that both relieve load by heeling.    The added central web adds insurance here.  After all, have you seen many (if any!) airplane wings that are not built just like the WatersWing, starting with a central spar and then adding upper and lower skin surfaces to achieve both strength and aerodynamic efficiency ?   Such air-wings can accept their section without the equivalent of diamond wires, as, unlike a sailboat mast, they enjoy virtually zero compression.

While on the subject of mast weight comparison of CF with Al-Alloy, it should be noted that while theoretical strength values of CF indicate a useful margin over that of Al-Alloy (6061-T6), this is not often experienced outside the laboratory.     There is probably a variation of +/- 30% in strength of a composite layup** based on materials used and the process of construction, which can go a long way to explaining why values 'posted online' also vary considerably.   And then, there is the case of carrying high compression on the open edge of a laminated section.   Instead of acting as an isotropic material like Al-Alloy (which is excellent in compression), a layered composite skin can progressively be delaminated if the load is high and continuous, with the result that the remaining effective wall just peels away.    This never happens with alum-alloy and can be prevented with an CF lamination by wrapping the outer fibers with tows or bonding-in the outer surface to prevent it from spreading externally.  See sketch.

**While some may raise doubts that their prized composite designs can fail in seemingly unpredictable ways, I have long believed that due to the large variation in the way they are actually made, that we need to be prudent when designing with such theoretically strong fabrics.    This compression test video will show you what I am talking about in my above example.  In theory this tube was predicted to be much stronger. 

This is just one example of why it’s very high risk to use the theoretical strength values of CF composite panels without also considering its non-isotropic structure that for one thing, can result in a vulnerability to disintegrate by layers, something that a metal will not do (unless it was also made in layers). 

So as an example for a weight comparison, if we take Al-alloy to be 65-70% heavier per unit volume than a CF composite and that CF Wingmast needs 35% more material and finally that due to less production control, another 10% of CF wall thickness may be justified, then the different in weight will 1.65/1.35 x 1.10 =  approx. an 11%  heavier alloy mast for this example.   Either way, the gain in aerodynamic efficiency is worth the build effort IMHO.   

Here is a link to the WingMast Info page

I hope this helps to clear up the weight issue.    As I said at the beginning, it’s a trade off of weight against aerodynamics, but even then, using good technique and care, a weight saving from 0 to 15% is still possible, with vacuum-bagging assisting to achieve the best weight-saving results  ... all explained in the Manuals available,

February  2022     Mike


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