Recommendations requested


More info needed…

“SNAP” = ???

Windward, leeward or reaching?
Loose or tight diamonds?
Where did the break occur?
How much sail area was on the mast?
Was mast rotated - over rotated?

Looking at the break itself…

Any tearing or glass delam from the foam?
Wood core?
c/f uni-directional set into mast sides?
Were you using back stay? Forestay? One or two side shrouds?
Did mast bend in half toward windward when it broke - or did the top part fall away and the lower section come toward windward? (if you recall)

Typically, in big boats I have seen wing mast failure in one of several ways. In heavy air diamonds were too tight giving too full of sail and too much power - or - diamonds too loose allowing the mast to “S” bend and go out of column while trying to get mast to depower and flatten main. Finally - is there any possibilty of air bubble between foam and covering in the area of break because of loss of structural integrity. Finally - Force 5 (if true) is a pretty stiff wind for r/c boats (scale wise) and if mast was built without any shearweb, and over tight shrouds, a big gust beyond the F5 wind really could have simply “blown up” the rig.

Look closely elswhere on mast for cracks of additional minor failure. If mast did go out of column, there may be other places where it might have started to crack also. My initial guess is that the sear web (cross foil) failed - or wasn’t there. If there was no additional longitudinal stiffening, the diamonds may have been attached too high/low for the mast loads. You have probably seen the bendy carbon masts on DN iceboats - so there is a lot of latitude in how much a mast can bend - provided it is engineered properly, and actually designed to bend. Usually, wing masts aren’t supposed to bend too much (if at all) and assuring you have very stiff sidewall construction is warranted.

A little close-up detective work migh shed light on what caused failures. Good luck.

Glad to hear you’re building wingmasts, but too bad about the failures[:-banghead]. Some more info about the exact nature of the failure would be useful to determine what might be the problem. Also, I should note that my masts are only about 50% as tall as yours, so these thoughts are not based on direct personal experience, but on a general knowledge of statics. Masts are essentially beams in bending with some additional axial compression loads induced by the resultant forces of the stays.

That being said, here are my suggestions:
-If you are still are using a masthead rig for the stays, consider going to a 65-75% fractional rig.
-Increase the thickness at the 1400 mm midpoint. This will increase your section modulus for added strength, as well as give a more uniform airfoil shape (in terms of the ratio of thickness to chord length)
-Use a thicker skin, and/or add a strip of unidirectional carbon in the region where the previous failures occured
-If the failure occured in conjunction with the diamond stays, look at the connection points for adequate reinforcement and/or consider increasing the spreader length

Let me know if any of this helps


Thank you for your comments.

3500mm GLIDER wing construction as follows :

The 3300mm replica wings were constructed more or less as one would expect an aero modeller to build - out of very thin balsa sheet and 2mm x 2mm x long balsa sticks stuck with SP resin and placed on a wing construction jig to prevent warping. The base of the jig was made to take a flat ‘underwing shape’

Each frame of the wings were pre-cut to be identical. Each frame had a 1mm cut away on the underside 30mm aft of the leading edge. Some frames had longer cut aways than others because the fore-aft measurements differed, but all the cut aways began at 30mm. If you see what I mean…?

A releasing material agent was stretched over the frames.

20gm weight c/f material was used with an SP mixture which set off within a preheated enviroment of 85 degrees. The ends of the wings were left open.

When set, the wing was turned over and a length of c/f material with SP mixture was applied from the leading edge to 20mm aft and a 1mm strip @ 30mm from leading edge and the remaining 1mm cut-out section to the trailing edge. When set at 85 degrees, the balsa frames were slid out which left a rigid c/f wing shape.

A 15mm groovie c/f mast was temporarily fitted to the wing from under at the point 20mm aft of the leading edge with the groove facing aft. This allow one to see where the spreader/diamond arms would pass thru the mast and therefore we were able to apply extra c/f spacers between the outside of the groovie mast and the inside surface of the wing. Whilst we did this, we dicided that as a precaution, we would add spacers every 50mm (similar to that for the spreaders but not so beefed up) as these would prevent any sideways bend of the groovie whilst under load inside the wingmast. Again, whilst at this stage, we discussed the possiblity of the like;ihood of the 2mm gap (from groovie to trailing edge) to be foced open by sail…in our wisdom, we added short pieces of hardened c/f at the 30mm mark, again at 50mm intervals to the inside surface of the wingmast.

We attached the 2 sections of the wingmast together win the groovie section inside, binding it with removeable tape. It looked the business.

Then we constructed the 100mm head and heel sections to make it 3500mm long. The luff if you recall is 3300mm.

We fitted a s/s dinghy burgee spindle down the inside of the leading edge of the mast and a short s/s spindle at the heel taking a ball race.

The kicking strap/vang was attached to a gudgeon/pintail fixture under the hel and trailing edge of mast. The gooseneck as similar at the trailing edge 200mm up the mast.

We decided that the mast was too long to transport in a motor car, so divided it into 2 manageable lengths. (approx 7’ & 4’). A long discussion ensued culminating in the divide being at 1300mm from heel. [by dividing 1300mm from head means that extra weight is added towards the head and we dont want weight up, if any weight is added, it should be lower].

The spreader/diamond arms were fixed at 1200mm from head and 1200mm from heal.

The arms are attached to a thin s/s rod that passes thru the mast and groovie and a welded washer butts up to the mast. Another washer is then pushed onto the rod from the other side and is held in place, so the rod cannot slid out, by a small splitpin. The tubular arms are then pushed onto the rod until butted up against the washers.

small s/s hooks attached to the diamond wires fit into small very thin s/s plates [with small circular holes therein] were added to the outside walls of the mast. 2 at the head.

The wires then pass thru small appertures at the outer ends of the arms and attach to a single bottlescrew which hooks into a small hole on the leading edge of the mast. This hole is situated 50mm below the lower spreader/diamond arms. The lower diamond wires affix in the same way but into small holes 50mm below the upper arms and to a bottlescrew 50mm above the mast heel, at the leading edge.

Now imaging a religeous type cross on a horizontal plane with the arms facing the gunwales and the longer to towards the transmom…the backstay attaches to this, the side shrouds attach to the arms and the forestay attaches to the shorter arm facing forward. Where the arms cross, there is a hole which takes a bearing thru which passes the thin s/s burgee spindle with washer under and screw nut to retain.

This set up allows the spreader/diamond arms to rotate inside the side shrouds.

…next text

Next text…continues…

All times on fresh open water lake [1 mile x 1/2 mile]

Failire 1. Sunny, dry day, Breeze Force 4 Full Sail and going nicely…
Gybed from dead run from Starboard to Port beam reach…SNAP…mainsail became flappy.
Kicking strap/vang had parted at base of mast.
Replaced mast attachment with s/s gudgeon/pintail fitting.

Failure 2. Cloudy dull day, Breeze Force 4, 5 at times. Full Sail and going very nicely with a few onlookers/commentors…
Starboad quarterly reach, mast over-rotated doing about 6-8 knots in swell with white horses
Appeared to catch up with preceeding wave pattern and nose initally buried before beginning to ride over…BANG!
Mast fell overboard to starboard.
Port side shroud had parted with bottlescrew at chain plate.
Replaced all single stranded s/s shroud wire with 7 spranded wire and beefed up ‘eyes’

Failure 3. Drizzle dull day, Breeze Force 2 going 4 gusting 5 at times. Full Sails WET
Starboard close haul. approx 4 knots looking superb!
Tacked onto Port…Squeek…er…CRACK! Limped ashore, sails flapping.
Mast split 350mm down leading edge at divide. Also noted small crazing on c/f by upper spreader/diamond arms
c/f beefed up along entire leading edge of mast and over crazed area [both sides of both arms - upper/lower].

Failure 4. Sunny but cold. Breeze Force 3-4 gust 5. water choppy white horses.
Just for fun, reaching this way and that - on starboard, approx 7 knots,
Mast fell to leeward, Oh dear…!!!
Thin s/s burgee spindle had somehow pulled sideways out of mast head causing severe ripping damage.
Increased length of s/s spindle also using thicker spindle with stronger bearings inside mast and on cross piece.
Added 3 extra layers of c/f at mast head.

Failure 5. Sunny warm dry day. Breeze Force 4-5 gusting 6 reduced sail [approx 3/4] with genoa roller reefed to 1/2.
Tacking, gybing - general this way and that…waves, WAVES…
BANG!! Mast heel fell off deck into water. Damn!!
Mast heel pivot sheared. How? I don’t know!!
More c/f, beefed up bearing and thicker s/s pivot rod.

Fauilure 6 NONE


Failure 6 - None

Congratulations !

Well, I was convinced there was something wrong - but in reading over the setup, the only thing I see is the wing is “one big muther” ! [:D]

Not related to problems, but would love to see a photo of this baby.

Back to your problems.

I thought I had keyed in to the problem - several times - but no - guess not. First is was construction and insufficient c/f running up/down the length. Then I thought perhaps too low of diamonds. Then thought perhaps too light of a build. Sticking bows into the backs of waves, etc. etc. Tonight, I am going to try to sketch this out with US standard dimensions - still have a problem with metrics - but unless I am seeing some huge surface area dimensions, it looks like the problem keeps moving around and first thought is that you built it “just right” - for particular conditions - and component parts are failing independant of the others. Each time you strengthen one - a different one lets go.

Nothing seems to be problematic with the actual construction of the wing itself.

I’m going to have to take some time to digest all of this information. Will also share with a few guys who are into wings that might have a viewpoint I’m not seeing or considering.


All in good time - perhaps Video as well.

I need to get this right, before the website viewer see the vessel fail (er, yet again)…

As a matter of interest, I have a 762mm (30") keel that fits 23mm (8") into the keelbox so the keel is level with the decking and 559mm (22") is showing under hull with 38lbs under.

Originally, the keel was hollow to allow additional weights to be added…This failed as the keel was not strong enough and fractured under load [particularly when the craft was heeling more than 40 degrees] Because of the hollow, the keel would bend under the weight and the c/f would craze and eventually splinter and then snap.

The keels were bench tested with the keel being inserted into a vice so that the keel was horizontal and the weight fitted. The early keels just snapped at the point where the keel was just clear of the vice, so the keels were stiffened with additional layers of c/f.

When the keels tood up to 50lbs being applied, we tried them on the BIG E (that’s the name I have given to the rig).

What I havent been able to work out as yet, is with the total sail area (see 1st screed) how long a keel do I require with a 38lb ballast to keep the vessel from heeling more than 45 degrees or sinking with too much ballast??

Any takers??


Hmmm - what you describe of the keel seems almost consistent with the mast failure - except for the shrouds/attachment ones.

For photos - I was asking about the mast itself - not the entire boat. Description and actual visualization can be completely different between what one reads and what one sees. You know …" a picture is worth a thousand words" ?

[:-chef]Man this is some model yacht you have cooked up.[:-chef]
I am really hanging out to see a picture and get to grips with the sheer size of this project.
What a lot of fun[:-jump2]
IMHO any equipment failures are simply down to the size of the beast and the loads accordingly.
Like the Americas Cup craft[/b]<font size="2"><font color="blue"> "If it hasent broken once then it is built too heavy"</font id=“blue”></font id=“size2”>[b]
Can you tell us where you are from please. Best of luck with your project, does it have a name?[:-?help]

Do it NOW before it`s too late.

Herewith a JPG drawing to give readers just an idea…


Download Attachment: [ wing mast.jpg]( mast.jpg)

With your shrouds attaching to the side holes of your “cross” - unless you have extremely loose shooruds - how does the mast rotate?

Looking at the top view of the cross - as the mast rotates, it tightens (even more) the shrouds. It (looks) impossible to over rotate, since the leeward shroud would be so tight as to keep the mast from rotating. Seems shrouds need to connect at some point that allows rotation, or else shrouds need to be on small traveller cars to allow them to remain in one physical location, yet still allow the mast to turn. In your drawing, as the mast begins to turn, and shrouds don’t stretch, you are basically “screwing” the mast base directly downwards and into the hull (or deck). The more the mast turns, the tighter the shrouds get and the more pressure is put upon the mast trying to push it further down onto it’s base.

Again - strictly speculation from my part, not seeing exactly how far it will rotate/turn before any shroud slack is taken up. Also - it is rare to use any kind of backstay, as that too restricts mast rotation. As the mast rotates, it is the diamonds that should take up the job of stiffening the mast and keep it from bending out of column.

Yep, I knew someone would not see what I mean…

(and apologies for the poor quality drawing)

If you look closely at the drawing, you will see that the diamond wires at the end of spreader arms are no where near the side shrouds!!

The side shrouds start at the chain plates (gunwales) and attach directly onto the outstretched arms at the cross with bottlescrew at deck level - therefore, with forestay and backstay attached to forward and aft facing cross tangs, the mast is free to rotate between the centre cross bearing and the at the mast heel. (as the mast and spreaders/diamonds are a completely separate and indepenant unit, the mast etc can pivot on it’s bearings [top n bottom] without restriction. By attaching a cord to both sides of the boom, running the cords forward to attach at the leading edge of the mast, you can control the rotation - as the mast rotates, the leeward cord tightens!

By tightening the backstay, in effect you are also tightening the forestay (genoa luff).

Inspiration or what?



As from now on…known as the <font color=“red”><font size=“6”>BIG E</font id=“size6”></font id=“red”>

If I’m looking at your drawing right it looks as though your shrouds are parallel to each other(the short arms on the cross are the same length as the beam on the boat). If I am right then you will be getting very little support from the shrouds as they form a rectangle instead of a triangle. This would put tremendous loads on the top and bottom bearings and would try and force the mast into an ‘S’ shape. This could explain the mast splitting. If I am missing something please ignore this post.

Don Case
 Vancouver Island


The funny thing about your setup is that because of how the shrouds attach to that cross arms, when the windward shroud is tensioned, it will induce a large bending load into the mast. This could be a big part of the reason why it is breaking. This is especially true when the mast rotates as this bending load will be in a direction that is unsupported by the diamond stays (i.e. some component of fore-aft bending in addition to the lateral bending).

If I were you, I would try one of two things:

  1. Go with an unshrouded rig. Extend the mast base into the boat and go with a bearing on the keel and a bearing at the deck to react the bending loads and still alllow the mast to rotate.

  2. Eliminate the diamond stays and attach the shrouds to the mast 75% of the way up. This will induce the typical compression loads but will not induce the large bending loads of that top mast cross. The unsupported section of the mast above the shrouds will help counteract the bending loads induced between the deck and the shrouds. If you attach these to a single point “crane” extending off the front of the mast, you will still be free to rotate.

  • Will

Will Gorgen

NOT A PROBLEM as such…

Personally, I cannot see what difference whether the shrouds attach to the CROSS arms or at the normal hounds (70%). The windward shroud will always be under tension, wherever it is attached to therefore creating various forces…Okay, so the shroud attached to the CROSS arm(s) and will generate a force directed onto the bearing at the mast head - this bearing is man enough!

One thing that emerged from tests is that the s/s burgee rod was not long enough [spreading the load] or the c/f cloth/SP mixture was too weak - & therefore was pushed/pulled sideways out of the mast head.

I agree that by tightening the fore & back stay as well as the shrouds, forces are generated by this action into the mast head - depressing it, but the actual construction of the mast and the spreader/diamond wires will prevent a buckled mast.

Even though the mast is under compression, because the bearings are man enough, the mast will [as does] freely rotate.

I am constructing a more or less accuate drawing to show size.



Wow, that’s a very interesting rig.[:-hspin] It sounds like several of the failures were caused by underdesigned components that can simply be built stronger. This is the nature of empirically designed prototypes: test until failure occurs, redo the weak point, test again etc etc. It’s the old of “the chain breaks at the weakest point” concept.

That being said, I have to agree with Don & Will’s comments about the masthead crossbar possibly exerting some unusual loads on the rig. Plus it adds weight & windage up high where it’s least desired. Will’s #2 suggestion echoes previous comments I have made about the benefits of a fractional rig, & it would eliminate alot of complexity aloft. The upper diamonds may not be needed. The backstay could be eliminated, so the rig would need to be raked & the side stays swept aft. Just an idea, but keep up the enthusiasm & perhaps you’ll prove us wrong [:D][:D][:D]

ps to Will: I see that Erich S took 2nd place at the stern steerer regatta in Green Bay sailing “Taku” in Class A

Hi Bill,

Yes, I agree that there is windage aloft, but it’s a trade-off.

I wanted to be able to get the mast rotating without complexity and that the genoa leech will not snag against any fixture projecting forward from the leading edge of the mast.


I had seriously considered my previous input :

“Take a 110 degree set-square with equal lengths and place it in front of the mast on a horizontal plane with the outer corner facing forward. Each end will take a shroud. Attach a short bar to the inside corner of the set-square and attach the aft end to a fixture on the leading edge of the mast with a gudgeon and pintail set-up. The shrouds could be fractional or at mast height.”

The main problem with this rig set-up is that the mast requires to be much stiffer and therefore much heavier - which defeats the object of a wing mast.

I know this because I have tried it on an 18’ sailing dinghy. The mast has difficulty in rotating because when on one tack it has a bend in it for that mainsail shape and when the boat is tacked or gybed, one has to physically pull it round to flex to the new tack - not suitable for an RC yacht. And the more important aspect is that you can only use a tubular mast section!



B&W photo shows a tubular mast section with the forward facing [set-square] spreader set-up.

You will see that when the boat is tacked, the jib/genoa leech will obviously snag the spreader arm…


Download Attachment: [ B&W SH16.jpg]( SH16.jpg)

Although is not my “A” Class RC yacht, but mine is the same class.

If you were to print this pic and knowing that the total length of the mast above deck is 2200mm, all you have to do is divide 2200 by 2 and then multiply by 3 and perhaps you may start to see just how large the rig is.

Particularly if you draw a line from the masthead to about 50mm aft of the stem and the boom measure 950mm.

The sail area is about 3.5 times what you can see in this pic.


Download Attachment: passive_vane_1_lge.jpg