Basic Spar Building Questions

[BobBill]

Hello, this Yank has been hovering around this site and the spar building links for some time and have learned much, but have some ?s of a tech nature and have found little discussion on process.   

I am experimenting with older boat that is no longer a "class.   "

I am making carbon mast and boom, and decided to base the spars on used windsurf spars, adding carbon sock layer(s) to stiffen a bit.   

My concern is with openings in the spar and areas that anchor say a vang, a bail, or outhaul items.   .   .   so they do not compromise spar integrity etc.   

Basic stuff, for you sailors, I know, but no one on this side seems to mess with fashioning carbon spars.  I had inquired on SA and boatdesign. net and did received a few responses and even links to Cherub masts building, etc, but need a bit more. 

I did not wish to bother tech advisers with basic stuff, and the link material was not detailed enough for my applications, which are, essentially, quite simple.  

Appreciate the help.   

[Will_Lee]

Hello! (and excellent avatar you have there),

So you are making carbon spars for a boat you are restoring, and your plan is to put sock over windsurf tubes.

This is a good idea because you'll have something to start with so no problems removing the mandrel, but the big problem you have is with the diameter.

Booms tend to be 750-1000mm (3-4") across, masts about 45mm (a bit less than 2") for something cherub sized. The thickness of carbon you will add is less than 2mm. This means you would need  very unusual windsurfer masts to start with. Also the (glass?)  windsurfer masts you start with will be doing nothing for you when you are are sailing beyond being heavy, which is bad.

You may think that if you make the wall thick enough then you can make up the stiffness you need that way even though the overall diameter is a bit low. I would counsel against this because you are trying to make a light spar out of carbon and this is a one way ticket to one heavy mast.

My advice would be to start with a boom, which is a lot easier and when you have done that you'll be in a better place to do a mast. You can use a drain pipe as a mandrel, but if you do this is is advisable to take special care over removing the mandrel at the end.

High load areas are best coped with by first reducing them (vang looped over boom rather than a fitting. Mainsail clew is also just tied to the boom.) This spreads the load a lot. An extra layer or two of weave can also help. It depends on the loads (especially the shock loads). If the boat is never likely to take off off a wave (or at least never land....) then you don't need to worry so much. Big gybes in breeze are the other spar killer you perhaps won't need to worry about.

Good luck and keep in touch!

[BobBill]

Will.   Thanks for response and advice. 


Here is what I have in my head.   

The carbon spars are going to replace the top two sections of a Force Five mast.  .  .  and the -surfer spars are near the same diameters.  .  .  no fittings, as the orig wood mast is un-stayed.   

I thought I could use a carbon sock to stiffen those two sections, even epoxying into one-piece spar section and use with former base, which is metal.   The idea could be off base, I understand.  .  . 

Moreover, I can make the spars (spar) from scratch, on a mandrel, and use conduit or PVC etc for the mandrel, but figured the surf stuff might be a better beginning.   I have a bit of glass/epoxy experience, but no carbon work. 

The boom section was planned to be the base section of a surfer spar set.  .  .  beefed up a bit.  .  . 

This boom, at the gooseneck is a female fitting and, I think, easy to do.  .  .  the vang at the boom might be a problem, due to angle at boom and concerns me. 

The boat can be seen at this link: hxxp: forums.  sailinganarchy.  com/index.  php?showtopic=128233&st=0

(Could not upload pics here)

It has a mid boom traveler and that can be strapped to boom, I know.  .  . 

I thought to run a line from the clew around some sort of block or sheave in or at end of the boom and run line inside to another sheave and then to a block on the metal mast and back to driver.  .  .  but can live with limitations.  as in simple end-boom tether.

But wonder how to attach say a Clam Cleat to carbon boom.

Boat is 11'6" Kite Dinghy, Sitka mast and boom (original and now better than new), mahogany rudder.  .  .  1978.   Sails fine.   Wanted to lighten top and use with Force 5 sail etc and have the right spars, but guessing I could cut topside weight by half.  .  .  and then think about foiling dagger board and even rudder.  .  .  I like to fool around with stuff.  .  .  and why I decided to refurbish this dinghy.  .  .  it is a blast to sail, incidentally. 

If I am off base, tell me, no problem. 

I have been perusing the Cherub material for some time and you guys seem to really have something going for small boats and have earned my interest, as well as respect.   Hi tech. 

I just have it in my bean that a light spar set will be easier etc and at least as swift.   Of course, I have been wrong many times before.  .  . 

And thanks for compliment, feel free.  .  . 

[phil_kirk]

Attaching fittings to Carbon.

For attaching a vang to the boom consider the following.

Take a stainless steel ring and bond it in place with an epoxy/filler mix.
Then lay strips of uni directional carbon through the ring and around the boom.  Set each strip at a different angle os that the strips of uni are laid onto different parts of the boom surface.  You can then shackle or lash the vang to the ring.

For small fittings away from the middle of the boom or away from the vang attachment you can thicken up the wall thicknes of the carbon tube locally and use pop rivets. If you are fitting aluminium fittings try to isolate them from the carbon with a layer of glass epoxy.  Aluminium fittings will waste away due to galvanic action from the carbon. 

Nearer the vang or middle of the boom. bolt the fitting on to a peice of carbon plate and bond this onto the boom with an epoxy filler mix.  Protect the fitting with tape.  The nuts become encased in the epoxy filler allowing the bolts and the fitting to be removed. Don't forget to use release wax on the bolt thread. For aded strength remove the fitting and put a layer of carbon over the plate, filler and local part of boom. re drill the bolt holes through this layer of carbon .

For high loaded areas eg. the tube of a mast it is not a good idea to put lots of holes in it. 
Either thicken up the wall thickness and through bolt eg. for shrouds        or laminate a bump onto the tube and use a lashing around the tube.  The bump stops the lashing from sliding up or down as required. the bump can be made by epoxy filler overlayed with a layer or two of carbon.

[BobBill]

Thank-you, Phil. . . by thickening, you mean adding layers of glass/carbon. . . the ring thing is excellent idea.


What about using the windsurf stuff as base? Still seems a good beginning, and they are not expensive, the used ones that is. . . a mini-recycling market. . . ?

[phil_kirk]

Bob, Yes by thickening I do mean adding additional layers of glass or carbon.  These help by giving the fitting more tube wall thickness to bear against and compensate for the higher stresses due to stress concentrations around holes. 

The windsurf masts i have seen taper along their whole length and are thinner than the UK Cherubs require to take the compreesion loads from shrouds and forestay. If the masts you have are relatively thin walled and the boat you are rigging is not too powerful then a mast based on a windsurf mast may be acceptable.

For info. current uK cherub masts are about 48-52mm diameter and abut 2mm wall thickness from the mast base to the upper spreaders. this is a length of about 4.6m. Above this point the mast tapers to a diameter of 20-30mm with the top of the mast about 1.9-2.0m above the upper spreaders. Our masts have 2 pairs of spreaders and are suported by cap shrouds, main shrouds, D2's and lowers.
Without shrouds our masts weight between 3.5 and 4kg. You can appreciate that there is a lot of compression down the mast tube.  If your mast is shorter or you do not use a trapeeze, or are carrying smaller sails you may well be able to get away with thinner mast diameters. 

As Will said adding more material to a small diameter mast will be a heavy approach.  if you play around with formulae for buckling of a tube you will appreciate that the second moment of inertia of the tube's cross section is much more significant that the wall thickness.

I understand you are working to a budget and have access to some windsurf kit at a reasonable price.  It is much easier to laminate some sock over a windsuf mast than make a new mast over a mandrel. 

[BobBill]

Phil, Good stuff.  Had no idea.  Much appreciated.

Will have to attend the measurements closely.  Am guessing that one or even two socks will put the spars in spec.

Orig wood mast is like old Finn.  Force 5 is metal and more like Laser, a bit stiffer.

I figure to make a lighter version of the original. . . and it is coming together well, thanks to your (and others) kind sharing of your experience and knowledge.

Who knows, may have people in all classes fashioning their own carbon rigs in the future.

Very good. . .