Carbon Spinnaker Poles

Hi,

I am investigating the feasibility of building a carbon spinnaker pole (4000kg 40’ boat)

Can anyone give me information regarding how a spinnaker pole is loaded?
Obviously most of the load is compression, however there would also be some torsion, and sometimes bending and/or tension.
I am looking for how the total load maybe broken down, or put a different way for a given compression load what are reasonable ratio’s for torsion, hoop, bending etc?

Then the question: what is the compression component on a pole? One could use the calculation method using sheet tension etc, however trying to calculate the various loads and how they relate to each other is very complex and beyond my meager resource’s.

I have heard the following rule of thumb from experienced filament winding people:
When replacing an aluminium pole with a carbon one, make the carbon pole to about the same dimensions (diameter and wall thickness) as the aluminium pole.
The apparent differences between filament winding and hand laid include:
Fibre/resin ratio, fibre pre-tension, fibre orientation, and consistency of the above.

I figure that by using the current pole as a starting point, quantifying and accounting for the difference’s, I should be able to come up with a reasonable laminate schedule.

Actual construction method is a whole new problem (I have read with interest the various discussions on this)

Any thoughts?

Rob

 

I'd think the stress distribution would be different for each rig, but what you describe sounds about right to me. Mostly compression, some bending. Probably not much torsion since the sail's still on a sheet off the end. The actual calculation would indeed be tedious...

From what you describe it sounds like you already know a lot more than I do about this. My experience with carbon is mostly in solar cars and the like, which are under very different loads from a sail rig. From what I do know I'd suggest largely unidirectional fibre, a combination of linear and at 45-degrees each direction. But that's just a guess.

Good luck, Rob. And happy sailing.

 

Rob,

So, you wanna make yourself a carbon pole eh?

Splendid idea, but before I give you my thoughts on the subject, I want to pin down a few assumptions, stipulations and caveats.

Assumptions; I will assume you have some experience and working knowledge of building composite structures, since this is not really a good project for a beginner, nor is it a good project for a person that isn't pragmatic or requires hand-holding every step of the way. I will also assume that you want to do this project both to save a few dollars and for the satisfaction of having done it yourself.

Stipulations; my advice must NEVER, for any reason, be construed as gospel. I feel very strongly that one mustn't take ones own bullcrap too seriously, and besides, there are a lot of people cleverer than me here, so keep in mind that this is only one of many approaches, so use what you can of it, disregard what you don't like, or, simply laugh at it. I sincerely hope that you aren't a theoretical physicist (I noticed that your name isn't Feynman) or (shudder) an electrical engineer.

Caveats; While I haven't actually strung these techniques together to make an actual spinnaker pole, I have used most of them in different guises over the years, and I'm quite willing to bet your money that it will work.

Designing a spinnaker pole; Once you make the wall thickness heavy enough to handle the normal use and abuse on a smaller boat, I think you'll have more than plenty of material in it to deal with the loads it will see in service. However, 40 feet might be getting close to the crossover point where that isn't true anymore, so I'd suggest going down to the nearest well-stocked rigger and measuring the wall thickness on a carbon tube that would be suitable for your boat. But, do keep in mind, if you are looking at a tube that is prepreg, filament wound, or processed in an autoclave, it may have a higher fiber-content than you can acheive, so round the number to the next millimeter or sixteenth or whatever upwards. What I would do is to mimic the layup used by forespar, wich consists of a couple of wraps of 0/90 cloth, followed by zero degree unis that make up the bulk of the stack, then topped with another couple of wraps of cloth, the beauty of this seemingly simplistic method is that the inner and outer cloths use the longitudinal unis as a "core" to increase the stiffness of the wall to localised loads, quite an elegant solution in my estimation.

Building a spinnaker pole; Start by finding a section of aluminium pole that is at least a couple of feet longer than half the length of the pole you want to build, a section of somebodys broken pole will do just fine, but make sure it's nice and straight and free of surface irregularities, of course, you could spring for a full-length blank, but that would increase the budget substantially. Don't worry too much about having a tube the right diameter for the end-fittings, you can always have a machine-shop turn a couple of bushings from delrin to make the ends fit, this is especially important when using aluminium end-fittings.

OK, so you've got your mandrel, now wax the bugger up real good, green wax seems to work well, and don't bother polishing the excess off either, it'll be on the inside anyway.

Now you do the first layup, and since the mandrel is only slightly longer longer than half the pole, you have to make two parts, just a couple of wraps of 10oz carbon cloth will probably work, but do some experiments first to make sure you have sufficient stiffness for the rest of the process. In order to get the part off the mandrel, you have to heat it during the cure-cycle, what I've done in the past is to rig up a heat-gun blowing into one end through a tin-foil funnel, and having a shop-vac in the other end to distribute the heat, of course, you have to dilute the hot air going into the vacuum quite a bit, to keep it from overheating. (do invest in a few digital cooking thermometers) Once you have the two parts you have to join them up, to get a flange big enough to bond, first find a plastic cup that has a bottom smaller than than the inside diameter, and the top larger, push the cup into the tube and cut if off at the height where it will just slip in, then push it in with the big end first until the bottom of the cup is just sticking out the end, now you can carefully dribble in resin thickened with Q-cell into the wedge-shaped annulus to make a flange large enough to bond the two ends together, then all you need is a length of straight angle iron or U-channel to line them up properly.

This all leaves us with a tube the right length, but very fragile, now you can heap on all the unidirectionals, fair it out, and then do the final couple of turns of cloth.

If you find this missive difficult to comprehend, I apologize profusely, English is not my native tongue, but I thoroughly enjoy mangling it.

Yoke.

 

Rob,

Instead of bothering with wrapping cloth around the mandrel, you might want to investigate braided carbon socks instead, that'll probably do to handle off-axis/abuse loading of the pole, and you won't have to deal with the edge of the wrap if you're clear-finishing it.

Yoke.

 

If your time is worth anything, I think you will find it a much better approach to just order the plain carbon tube and assemble the thing yourself. IMHO

 

Doug,

Certainly agree with you there, that's why I've never built one myself, they're too cheap to buy, even repairing the smaller ones often come perilously close to replacement cost. But since this concerns sailboats, I'd argue we've already left what's strictly logical well behind us anyway.

Did you sail on Gruntled last year?

Yoke.

 

You rght of course, somtimes I loose sight of the logical, I'm so busy trying to fill up that hole in the water with every pay check. Yea, I sailed with Bart a couple of times last year, and we did the DH Farallones this year.

 

Cool, I've been sailing a wabbit with Simon for a couple of years.

Yoke.

 

You guys do the ditch run- I heard a Hobie 33 broke a spin pole or rig or somthing. must have been a good ride. Say hi to Simon.

 

Hey Guys,

Thanks for your time and input.
One of the primary reasons for building it instead of buying it is that we have the uni's already. The other reason is that it seemed like a good idea at the time...
And as far as sailing being logical, hmmm, well, nothing makes miles like a 747.

Anyway, back to the pole.
I was not thinking of a specific torsional load, however looking at the wear on pole beaks it is evident there is some torsional loading. Maybe during setting/retrieving, or gybing/crash gybing - I don't know.

Of all the ways of building it I keep returning the idea of somehow laying up a light shell in two sections, joining them and then using that as a former for the remaining laminate.
I had thought to split the pole on its longest axis. -make a semi circular plug from pvc pipe, foam, whatever. the two benefits to doing it that way is that the profile of the pole can be exactly as we want it (so what is the "ideal" profile?), and we can guarantee draw in the plug.
Aluminium poles are parallel sided. The pole being replaced is 5 meters long. so making the initial shell in two full circle halves (split the pole at the mid point between one end and the other) means over 2.5 meters with zero draw. I imagine pulling the mandrel out would be fun.
Yokebutt, you mention heating the mandrel during cure to help this. Why does that help?
Another way might be to lay up the whole initial shell over the complete pole and then cut it lengthwise to release it. Although that means (A) we need a suitable pole to use, and (B) we can't laminate the overlap join into the part.

Braided sock is definitely on the list.

I'm also thinking up way's to tension the uni's a little to make them straight while they cure.
The problem is how to do it with out interfering with the vac bag. I don't want to completely encapsulate the initial shell with the vac bag as it may collapse. I will make the initial shell longer than I need and then stick the vac bag to it with the ends of the shell open (atmospheric pressure on both sides of the job)

Given the explosive nature of failing carbon, is it feasible to put kevlar into the laminate to retard it's bad manners? Would a 400gm 0/90 cloth be enough to help or is it a waste of weight.

Thanks again,
Rob

 

Rob,

I'm quite relieved too see that you appear to have the right attitude and way of approaching this project to make it work, if you enjoy doing this kind of ****, by all means, go ahead.

The reason for heating the mandrel during the cure-cycle is to make it expand, that makes it easier to remove the part when it cools down and the aluminium shrinks away from the laminate, it also improves the final properties of the epoxy, provided you use one that will tolerate elevated temperature curing, but wait, there's more, the expanding mandrel will also serve to stretch the fibers.......pretty clever eh?

I've had pretty good luck so far by gently squeezing the mandrel/laminate combo along its length, first along one line, then rotating it 90 degrees and doing it again, but do use a smooth-jawed vise. You'll hear popping noises when the part releases, but if you hear cracking noises or make dents in the mandrel, ask somebody to kick you in the bum, hard.

Almost every broken pole I've ever seen was the result of rounding down, ie, wrapping it around the shrouds with the outboard end dragging in the water, making a super-sturdy pole may very well result in bringing the rig down instead, so I think there is some merit to considering it as a fusible link under certain conditions.

Making a thin shell and then removing it from the tooling before adding the bulk of the laminate is an idea I find very appealing, if everything goes totally to hell, at least you haven't wasted a huge amount of materials. Stiffness is to a great extent a matter of geometry rather than amount of material, so a thin shell will have most of its global stiffness already, (ie, sag when supported between two saw-horses) but much less locally. (ie, squeezing it out-of-round)

Joining up two halves lengthwise, a friend of mine makes curved wishbone booms that way, so the method is proven. Let's say that you do it in a female mold, male molding would certainly work, but I don't think it has as many advantages going for it. First you'll need to make a little lip that jogs inward and bonds to the inside of its matching part, I think that could be rather easily accomplished by making the mold slightly larger than half, say 200 degrees of arc instead of 180 perhaps, and then building up a lengthwise "bump" with masking tape to the thickness of the shell along one edge. That would facilitate a nice lap-joint. I would also make the first layer of the shell peelply, that way you wouldn't have to sand off the mold-release.

You could try tensioning the fibers while curing, I've done it before, but that was for a composite chainplate, 12 layers of uni at five slightly different angles laminated all in one go, fricking nightmare it was. I don't have any simple solutions for it in this case, although perhaps a truck lugging away in drive might work.......(oh, wait, just a loose thought here, how about bending the mold into a shallow U- shape while you laminate, and then straightening it out before the resin cures?)

Using a kevlar sock as the outermost layer wouldn't increase damage tolerance all that much other than for abrasion, but it would keep the shards contained if it were to fail, but then again, we're not exactly talking fragmenting ordnance here.

Alright mate, that'll have to do for now, run with it!

P.S. Doug, the ditch didn't blow at all this year, mid-teens at the most perhaps, marginal planing conditions for us. Man, I was in pain from pumping the kite when the morning came, and the stack of drink-tickets somebody gave me as they were leaving didn't help either.

P.P.S Devise a few experiments on a smaller scale first, I've often been to self-assured and arrogant to do that myself, and it's bitten me in the buttocks more times than I care to admit.

Joakim.