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Wing-Masts on Multihulls

QUESTION:  As an avid reader of your website articles and appreciating your commonsense approach to potentially complex issues, I’d like to know what is your take on the use of Wing Masts, as a number of designers seem to consider them hazardous?   Thank you Mike.                                                                                              Bob, FL

ANSWER:   Thanks for that Bob …. oh yes, Wing Masts.

Personally, I am definitely a fan, but within certain limits.   But let’s first define what IS a wingmast.   

I consider any mast where the fore-and-aft chord is at least double the mast width, to be a wingmast., even if a very moderate one.   And they of course must then be rotatable.     To further control their area, I also recommend staying very close to a Chord of 0.025 (1/40th) of the mast height.    Personally I would not recommend a wingmast with a C/W (chord/width) in excess of 3 (or a chord of over 3% the mast height), for the reason that once the minimum width (W) is established for transverse strength and rigidity, it then has too much area and also, that a C/W in excess of 3 creates a heavy (and expensive) mast and overall, there is generally not the extra performance gain to justify it over a more moderate option. 

So yes, even a wing-mast with a C/W of 2.6 to 2.8 (as are my own designs) costs more and will be a little heavier, so it’s somewhat of a trade off with the performance gain one can achieve.    This means that to really get the most benefit and value, one needs to match such a mast with a boat designed for fairly high performance, not for a monohull cruiser or even a heavy fixed-keel multihull.  That, to me, would be like attaching a race horse to your family cart for a shopping trip to the village.  (You'd not get your money's worth from the horse and I doubt you'd get there much faster with the load  ;-)

So if you have or are considering a fairly sporty, moderately lightweight multihull with a centerboard (or daggerboard if you insist), then I’d certainly recommend a moderate wing-mast within the limits noted above (see photo at left).

But many will then ask ….  what about the often noted concern of high side windage and its affect at anchor or at the dock ?     

Here, I have perhaps a somewhat different response to many, so please bear with me on this.

Wind drag on a mast, is commonly based on this formula:   Drag = Cd x A x V2 x (0.5 x density)    [A = frontal area]

So it varies as: the square of the Wind Speed and directly with the Frontal Area and Drag Coefficient Cd.

Based on published NASA data, the Cd (drag coeff) of a moderate wingmast as noted above, is about 0.05.   But such low figures come from controlled tests in a highly directional wind tunnel and would be highly unlikely in a practical environment with even a slightly varying wind direction   So let's triple it to 0.15.  But the Cd of a totally round mast is about 0.50, or as low as 0.20 for a good pearshaped one.   So when pointed into the air flow. that still puts the wingmast at roughly 1/3rd the resistance of a round pole and less resistant than any fixed mast of typical pear shape.     (But IF it were indeed at 90 degrees to the wind, its Cd would jump up to about 1.00 … approaching that of a flat plate (at 1.25) … and then be about 5 times that of a good fixed mast! ... (but before any extra rigging is factored in).

So the range of Cd’s and its effect on drag is pretty amazing.  Take this example.

This large but ideally shaped aerofoil has the same drag resistance as the very small cylinder shown above it !!      In fact, adding an aerofoil cover to the small cylinder, could lower its drag by 90%  !  (again, from wind tunnel results).

So, IF we can manage to point a wing mast directly INTO the high wind, we can lower its resistance well down below what a fixed mast will be, and this is the secret of living with a wing mast.   No argument though, it will require more attention in very high winds to avoid being set broadside-on, particularly with a very light boat.

Re: Mast Staying.   It’s true that rotating masts typically have just 3 stays but also commonly have an additional halyard going to a bowsprit which gives 'some' limited redundancy.   So it’s essential to service these lines regularly, size them somewhat oversize and give particular care to prevent abrasion when the mast rotates.     But the limit of 3 stays is not all bad.   If you consider the very high drag of a small round wire or dyneema shroud (from the above example), you will be saving a LOT of wind drag compared to a fixed mast with the redundant rigging that cruisers seems so happy to pile on.   You just need to make sure those 3 wires have oversized attachments and are checked regularly.

Now to get the wing mast into any side wind will require not only a control of the mast tiller, but also the ability to rotate the mast a full 90 degrees.      Masts under 10m typically have their stays attached to a fixed beak on the forward edge of the mast and will probably need one of the shrouds to be slightly slackened to obtain the last 20 degrees of rotation.  This is fairly easy to arrange by having a shroud tensioner of dyneema that is typically installed anyway, to pull the shroud slightly aft.

For masts 10m or longer, I always recommend to replace the fixed beak with a swiveling one, as this will automatically line up with the loaded stays and generally not require that the shrouds be slackened.  The swiveling beak will also have a stronger attachment to the mast, particularly if of carbon fiber, as the upper and lower gudgeons can then be well wrapped with CF tows around the mast wall, even if the beak itself is of stainless which I would recommend.   (In such a case, the CF gudgeons should be built with a fiberglass bushing inside the carbon fiber gudgeons, as stainless and CF do not get along so well in salt water.


So as long as attention is given to align a wing mast into any strong wind, the windage issue is just not a major one, so allowing the sport sailor to benefit from the clear 10%+ performance advantage of a wing mast.    But note that with a wing-mast, the sail is now the tail part of the aerofoil and therefore needs to be adjusted accordingly with a flatter cut … so be sure your sailmaker is made aware of this.   With the wing-mast indicated in this rough sketch, the required camber is cut in half !

And there are other benefits too.    The amazing Dutch adventurer Henk de Velde is one of the few who have circumnavigated the globe 6 times under sail, and 5 times alone.  While that is already a major achievement, the really unique thing is that he has done this in a variety of vessels, including monohulls, catamarans and trimarans!   So I often turn to Henk to ask how various things compared.     When I asked him recently about wing masts on his world cruiser, this is what he had to say: 

“My last trip around was in Juniper, the comfortable and capable 52ft trimaran designed by Chris White.  Although initially rigged with two round unstayed masts, these were replaced by two large and rigid rotating wingmasts when I did my circumnavigation.   The forward mainmast was 16m high with a chord of 0.40m and the mizzen was 12m high with the same chord.  (Ed, note that his main-mast matched the 0.025 ratio I also recommend).    I loved them, as during two big 45-50kt storms that I remember, I was able to sail UPWIND with just ‘bare poles’ with sufficient speed (at 1.5+ kts) to use the autopilot and stay inside, under full control.   And downwind under just these two masts, I was able to make 7 kts.  I personally had no problem with docking and neither with anchoring which I did more often.  But I agree with you that one should be able to rotate such masts 90 degrees if ever needed.”   Henk de Velde Oct. 2018

This experience might at least counterbalance some of the skeptics ;-)

I have also found a micro high-aspect-ratio storm mainsail can be remarkably efficient working behind a rotating wing-mast, as shown here on a W17.    No doubt due to the wing mast then being a higher percentage of the total area.  (more elsewhere on this sail under ‘StormSail’ on this website).

                              …..  Mike 2018

 

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