6m Mast suggestions

[stevevw]

After the Nationals last weekend I have come to the conclusion that I need a stiffer mast than the rolled 12mm carbon tube from Easy Composites I have at the moment. Other than 2 one metre lengths of high modulus carbon for well over £100 are there any other options?

[John949]

Several:

1) Use pultruded tube.  This is stiffer in bending but more prone to splitting.  Cost is little more than roll-wrapped.

2) Use a bigger diameter.  Going from 12 to 14 will give about 60% increase in stiffness.

3) Use a thicker walled tube. Not as much improvement as you might first think and heavier of course.

4) Rigging. You don't say what you use at the moment but lower shrouds, diamonds and jumpers can make a mast remarkably stiff, at the expense of windage of course.

it also depends whether it is fore/aft or sideways stiffness you want to increase.  You can increase forestay tension by just letting the mast bend, BUT the mainsail has to be cut to accommodate the increased bend.  I find you have to match the mast bend to the sail.  Too much or too little and the sail shape is awful.

Mr B's very expensive 'high modulus' tubes would probably make a stiffer mast but the graph on his web-site doesn't tell the whole story because the (Young's) Modulus of the material given is almost certainly the figure for just the fibres in tension. The bending stiffness of a tube depends on how you form these fibres into a tube.  It would be very interesting to compare one of these tubes with a commercial pultruded tube.  

 

[stevevw]

Thanks for the reply john.

Yes I have thought of giving pultruded tube a try and probably will.

12 to a 14mm tube is not an option for this boat. The built in mast tube is 12.5mm wide plus the the gooseneck pivots round the mast on 12mm id ballraces. Although I could try sliding a 14mm tube over a 12mm stub mast down to the top of the gooseneck.

Easy Composites do a 10mm tube with a .5mm wall that I could possibly run down the top half of the mast. I already have a piece of 10mm x 1mm wall from the mast foot to 60mm above the gooseneck.

I have 1 pair of shrouds with a spreader. The shrouds attach to the mast at the same height as the forestay. I am reasonably happy with the mast bend below this point but above is a different story, far to much flex for not much more jib tension.

To be fair the boat goes very well in light winds with this soft set up.

As a last resort I could use the groovey mast from my Ravenna.

There was some pictures of 56 my Kalulu on the 6m Nationals report but they have disapeared for now.

[John949]

No easy solution to that one.  I've always felt that the way model yachts are rigged is fundamentally flawed.  3/4 rigged big boats use runners to tension the forestay.  Dinghies have the shrouds well aft of the mast to resist the forestay tension.  I did try moving the shrouds aft on my 10R but moving the hounds higher (you can have a masthead rig on a 10R if you want) is a better solution for that boat.  You could try jumpers but it would be messy and draggy (and make it look like a J class!).

A stiffer tube would be the best solution.  I originally made my M & 10R masts with step down sizes (14, 12, 10) but I now think that isn't so clever.  For the lower rigs I am moving to one piece 12mm tubes on the 10R (with shrouds, 7/8ths rig and a parrot perch) and I'm considering a 16mm tube for the shroudless M B rig (with a pocket luff mainsail).   I also have removable diamonds on my swing rigs - perhaps it's the way I make sails but I get horrible creases from clew to about 1/3 mast height if I don't use the diamonds when the wind gets towards the top end of the rig's range.

These people sell 1m tubes at reasonable cost:

https://www.bucks-composites.com/products/carbon-fibre-pultruded-hollow-round-tube-1-metre-long

And these have 3M long tubes 

https://carbonfibreprofiles.com/products/3m-carbon-fibre-tubes

 

 

[stevevw]

Thanks John very helpful. 

[Brad Gibson]

Hi Steve,

Also worth checking the length of your backstay crane. This is a common area where an over length crane will add unnecessary bend to your mast without effectively tightening the forestay through greater leverage. In simple terms, if you transfer an imaginary line of the mast upwards to meet an imaginary continuation of the backstay, it will be much higher on one with a long crane. Any backstay crane should only just be long enough to clear the mainsail leech in the lightest of winds.

Outside of that, John is right in looking at shroud positioning on the deck relative to the mast. If they are not aft of the mast they are doing nothing to help rig tension. A look at IOM rigging shows moving shrouds down the mast, combined with them being aft on the deck helps induce a small amount of reverse prebend that when taken out with the backstay applied, adds more tension to the forestay.

I would avoid an overly bent mast with matching luff curve as when the sail is eased for a run, that luff curve has nowhere to go and sets terribly with less sail area being projected.  I would also avoid weight and windage on a class that is tender as the wind gets up due to the short fin. 
Sadly that all adds up to the stiffest tube you can find (we all know the source) for best fore and aft stiffness and then set the shroud heights and spreader lengths to get it to bend and de-power sideways to match the boats displacement.

Cheers

Brad

Edited October 13, 2022 by Brad Gibson

[stevevw]

Brad thanks for the reply.

The original crane and backstay is at the bottom of the Bournville pond after a gust inducing summersault, but was tight to the sail originally. the bodged replacement using a piece of stainless from one of Damian's vane gears was rather further away, which as you say definitely made matters worse to the point that on the run the jib did not look like it had a boom attached. thankfully both the jib and main were spilling plenty of power so the boat went forward and not impersonating a submarine.

The shrouds are 12mm behind the mast but I have the option of moving them back to 25mm.

So I think for this weekend I will just repair the rig to pre nationals spec. (better the devil I know for the two comps this weekend) Then next week will give a pultruded tube a try with lowered and further back shrouds.

Thanks to all, Steve