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Rcsailing.net Technology Discussions
21

<blockquote id=“quote”><font size=“1” face=“Verdana, Arial, Helvetica” id=“quote”>quote:<hr height=“1” noshade id=“quote”>Originally posted by NIMSS

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.

Regards,

NIMSS
<hr height=“1” noshade id=“quote”></blockquote id=“quote”></font id=“quote”>

You are correct that the windward shroud will be in tension. However, since that force is now acting at the end of a long lever arm attached to the top of the mast, it will induce a bending load in addition to compression.

You can proove this to yourself with a simple exercise. Get yourself a wooden dowel (or one of your carbon tube masts if you prefer). clamp the end of the dowel or tube into a vice so that it is held fairly solid. Tie a wire or string to the tip of the mast and pull on it from a point that would simulate the relative position of where the shroud attaches to the deck relative to the base of the mast (if you have a way of measuring the force, that is great) and see how much the dowle or tube bends. Now repeat that exercise with your cross bar in place. You will see that the bending is a lot higher for the same amount of force.

I’m pretty sure this is the cause of your failure (although you never really described the failure well enough in terms of location and failure mode to be sure).

If you want to keep the diamonds and the mast head shrouds, then you can do that. Simply have a single diamond stay with the spreader short enough to pass inside the shroud as the mast rotates. Take a look at rigs on some of the Vendee boats as they have a rig very much like this (rotating mast with diamonds and shrouds).

  • Will

Will Gorgen

22

I believe, that the side shrouds need to be connected to the front of the wing mast. While it is a solid wing - just consider the front as our mast. (On the attached top photo)… Note how all standing rigging runs to a single location on the leading edge? [just above the apparent wind indicators] Not visible, is the stainless curved rod that acts as a traveller, and the blocks at the end of the rigging which roll on the stainless rod. This allows the mast to turn - without any tightening of the shrouds.

I am still lost on how your mast can turn if the side shrouds are tight but fastened to the sides of the mast? If you have a forestay to front of mast, and side shrouds to each side arm that sticks out from your “cross” - it is impossible for that side cross arm to move forward in rotation arc if shroud length is kept constant! If side shrouds all fasten to the front as the foestay does, - only then will the mast be able to turn. (Unless I am missing something) All wings and wingmasts that I am familiar with have hounds on leading edge and all standing rigging attaches there.

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23

Hi Will,

Yes, but I had already thought of that concept.

If the CROSS arms are built strong enough (titanium core inside an aerofoil shaped carbon CROSS) the tension on windward shroud will have the effect of increasing the tension on the leeward shroud and therefore all is equal.

The effect of all this shroud tension will in effect have a pressure effect on the mast, trying to compress the length of the mast into itself, [ah, but] as the mast is made suitably strong enough to withstand compression stresses, because of : 1. the build quality 2. the for=aft measurements 3. the width measurements 4. the spreader/diamond arm positions together with diamond wire anchor points 5. ball bearings at head and heel, the only weak point will be at these bearings. Because they are metal, they will take the forces exerted upon them.

The only problem I can see is how do I obtain the 15 degree twist in the top third on the mast/sail?

Regards,

NIMSS

24

Will,

Both side shrouds, forestay & backstay atach to their individual points on the CROSS.

A vertical rod from the centre point of the CROSS inserted into the top of the mast [in line with the heel pivot] also on bearings…because of the tremendous forces exerted into the top of the mast, er, trying to force the rod sideways out of the mast, the top section of the mast must be strong enough [ie the use of titanium bands linked by very thin web like mesh].

The CROSS remains static.

The wingmast pivots on the bearings.

The mast including spreader/diamond arms pass inside the side shrouds.

To demonstrate this, take a sheet of paper and wrap one side of the paper around the length of the pencil and stick paper to paper, thus creating a sleeve. Hold the pencil vertically, one end on desktop and the other with finger. Turn leech of paper around the pencil. It turns freely doesnt it? Thats the basis of my concept. Your finger holding the pencil top is acting as the CROSS will do!!

However, having looked at your top photo [nice], my main concern is…when the mast is turned one way and then the other [on different tacks] the leeward shroud obvious becomes increasingly tensioned - what happens to the windward shroud? It gets slacker, therefore the mast falls to leeward, which is opposite to what is required. It should really fall to windward because the vessel will heel to leeward with the wind-on-sail pressure anyway…

Regards,

NIMSS

25

Will,

Apologies, having re-read your article with pic, I see what you mean.

I have taken your roller bearing and attached it to the top of the mast instead, mainly because I have a large mainsail roach and needed a backstay!!

NIMSS

26

Don,

I do believe you have a valid point.

I did wonder why I had to tighten the shrouds to a point where the chain-plates were about to be ripped out of hull side…!

If I were to make the arms of the CROSS shorter and the upper spreader arms shorter as well, it would be possible to lean the shrouds inwards to the mast head and get sufficient leverage to keep the mast upright…mmmmm, why didn’t I think of that? (fool that I am!)[:(!][:-[:-[:-paperbag]grumpy][:-paperbag]censored][:-grumpy][:-grumpy]

27

NIMSS,

I can’t tell from your replies as to whether or not you have seen the light that all of us are pointing you toward.

The effect that Don is pointing to is the “matchbox” effect (or the “house of cards” effect). Basically, there is nothing to keep your whole masts system from faling to the side.

Take your pencil (the one with the piece of paper wrapped around it) and push your finger (the cross holding the top of the mast) sideways. What happens? There is nothing to prevent the system from falling over! Since your finger is staying parallel to the desk while you do that, there is no shortening or lengthening of the sidestays - i.e. no change in sidestay tension. Thus the sidestays are NOT supporting the mast laterally.

Imagine that instead of your bearings, you attached the cross to the top of the mast with a ball in socket joint (and the same at the deck) what would happen? The whole thing would be free to fall to the side. The cross would remain horizontal (when viewed from the front) the whole time, but there is no support. Now, take the same system and remove the cross and attach the sidestays to the top of the mast instead. Now it will stand up.

This is your whole problem!

Making the cross arms shorter is not going to completely solve the problem. You need to eliminate the mechanism mode. Reacting it through the bearings is not wise. Eliminate it altogether!!!

Why do you feel you need the diamond stays anyway? If the carbon tube in the middle of the mast does not rotate, then the diamond stays are only supporting the foam and carbon wing shape that rotates around the center mast tube, right? Get rid of them! While your mast is pretty that tall compared to typical RC masts, it is also very large diameter and should easily be able to handle the bending loads without the need for diamond stays. There are a ton of small boat masts that are similar in size to yours. They do just fine with little or no support other than the sidestays. Heck, look at windsurfer masts. Fully 5 meters long and no sidestays at all (much less diamond stays). Try it our way. If the bend is unacceptable, a partial span, single spreader diamond below the shrouds will be more than sufficient to keep it straight.

In summary, here is how to fix your problem:

  1. Get rid of the diamond stays
  2. Attach your sidestays and forestay directly to the mast as shown in Dicks pictures (you can either attach them to the top of the mast of part of the way up)
  3. Keep the bearing supported cross only for the backstay

Trust us (all of us have come to the same conclusions).

  • Will

Will Gorgen

28

The bright idea bulb has dimmed and has been snuffed out…but I have seen the light at the end of a very long tunnel…Thanks all !!