How many layers of epoxy to reinforce a bowsprit?

[Greenheart]

My carbon fibre bowsprit will poke through a small stainless U-bolt on the gunwale, adjacent to the forestay.

6ft will be sticking out beyond the bow, so the tack of the kite will press the sprit hard against the U-bolt.

It seemed wise to me to thicken the area of the sprit close to the U-bolt, partly to resist the abrasion it will suffer, but also to spread the sudden bending stresses that the carbon pole will experience at that point.

I initially hoped to use tight-fitting plastic drainpipe, but I couldn't find any of the right diameter...(about 55mm would suit)...and it struck me that too much rigidity would just transfer the sudden bending point to the end of the drainpipe section...

...so this weekend I used glass-tape with epoxy to thicken the sprit for several inches either side of the point where it will pass through the U-bolt. I suppose I've added around 4mm to the overall diameter.

What I can't estimate, is how much rigidity that epoxy 'tube' will have added. Too little, and it won't have been worth doing...too much, and it won't spread the sharp bending stresses, it'll just move them a few inches.

Anybody care to estimate how many layers of epoxied tape would create a medium-rigidity 52mm diameter tube?

Thanks!

 

[lw395]

How long is a piece of string?
Does the reinforcement taper or end abruptly?
How much glass is in your 4mm of lay-up?
The glass will be less stiff than the carbon, so probably not too rigid in terms of stopping the pole bending, but good for stopping it crushing.
When you look at a B14 pole and how that is point loaded at the bow, I'd say you are in with a chance.
I presume you're not looking to use it in 20 knots?

 

[macd]

Anybody care to estimate how many layers of epoxied tape would create a medium-rigidity 52mm diameter tube?

That's a bit tricky, Dan, without knowing the weight of cloth in the tape. 2mm added to the gauge of the tube doesn't sound an awful lot, but if the aim is solely to allow for chafe, I'd have thought it probably enough. What's the gauge of the carbon tube? I'm sure it occurred to you to feather the tape (as you wound it) to reduce stress concentrations, which I suspect is the most critical factor after chafe.

 

[Quandary]

6' seems an awful lot of projection for a 50 mm.tube, or is it in a dinghy?
On our 33 footer, the Selden sprit (alloy about 65mm.?) extended less than half of that and I was surprised by how much it bent in the moderate winds that I was allowed to use the gennaker in. I can not offer advice on the load resistance of reinforcement other than to suggest that you go for as much as you can manage.
If you have not reinforced at the bearing point but just either side why would it not collapse there where the load is greatest and the strength is least?

 

[Max Campbell]

I've got a 6' bowsprit on a 17' LWL boat, and it's got catstays and a bobstay, so that it doesn't touch the side of the gammon iron (your U bolt).

 

[Greenheart]

Thank you gentlemen, yours are all good points. If I wasn't simultaneously afflicted with laptop failure and obliged to use an iPad which cannot lift links for my images on Photobucket (which by itself is the other affliction), I would add pictures here of my planned bowsprit and foredeck set-up, which might make my question clearer.

Your descriptions of the stresses that will affect the point where the sprit will emerge, lead me to doubt whether anything my cack-handed epoxy-work can add, will do anything except defend the carbon against superficial scratching and chafe.

Given that I'm unlikely ever to hoist the kite in more than a force 3, I think I'll stop worrying and rely on the carbon's intrinsic strength. The layup of epoxy will also help fill a space, because the carbon tube was about 9mm less than the internal diameter of the "U".

 

[Neeves]

You could reinforce by adding an insert, rather than beefing up the outside. Might you find something that fits internally - and if its too small wind tape round the outside until is does fit.

I'd make the fit to the 'U' bolt snug- you do not need a snatch load.

Jonathan

 

[Ian_Edwards]

The epoxy and glass tape on the upper side of the tube will do very little to strengthen the tube, glass epoxy tape is very weak in compression, but it will obviously help with abrasion and help spread the load, best done by tapering the reinforcement, so you don't introduce a stress raiser.

Epoxy resin.

Tensile strength 85 N/mm²
Tensile Modulus 10,500 N/mm²
Elongation at break 0.8%
Flexural strength 112 N/mm²
Flexural Modulus 10,000 N/mm²
Compressive Strength 190 N/mm²

Note that the compressive strength epoxy is low, and the glass will add very little to the strength.

If you want to strengthen the tube add some unidirectional carbon, axially along the underside of the tube (assuming it will bend upwards), where the tensile strength of the carbon will work well.

The tensile strength of unidirectional carbon in epoxy is very high, at least 1000 N/mm2, and could be more than twice that.

When adding additional laminations to manufacture carbon fibre tube, be very sure to remove all the release agent from the tube (best done with detergent and hot water) and thoroughly abrade the surface, or the bond will be poor. But you probably know all that.

 

[Greenheart]

You could reinforce by adding an insert, rather than beefing up the outside.

Good plan, thanks. It occurred to me for a while that my old alloy spinnaker-pole would fit inside the carbon tube; although I understood that conventional poles aren't much subject to bending forces, and aren't especially resistant to them.

The carbon pole had been a windsurfer mast in two parts, totalling 16ft assembled. That was much too long so I cut off the tapered top five foot, and stuffed it up the lower mast, where it wedged neatly, protruding about 30 inches out of the top...

...just far enough to create a substantial internal reinforcement of the sleeved join between the original upper and lower sections.

Once again, I may be worrying more than the bowsprit's use will justify...even assuming it is strong enough to hold the kite in 10+ knots of apparent wind, there'll be the question of whether I can keep the boat upright in that breeze!

Thanks also to Ian for the carbon-knowledge...

...I'll certainly come back to you when the whole thing separates in the first moderate gust.

Quandary, I must have explained poorly - I didn't only reinforce either side of the U-bolt position, but the whole length between points about five inches either side. I wish I'd paid closer attention to tapering, but p'raps I can take care of that with the sander - it's going to need some smoothing over.

 

[Neeves]

Funny, I was going to suggest a windsurfer mast but if you had a straight tube it would not work - but 2 parts of a mast might work well as they would fit quite snuggly.

 

[pagoda]

Good plan, thanks. It occurred to me for a while that my old alloy spinnaker-pole would fit inside the carbon tube; although I understood that conventional poles aren't much subject to bending forces, and aren't especially resistant to them.

The carbon pole had been a windsurfer mast in two parts, totalling 16ft assembled. That was much too long so I cut off the tapered top five foot, and stuffed it up the lower mast, where it wedged neatly, protruding about 30 inches out of the top...

...just far enough to create a substantial internal reinforcement of the sleeved join between the original upper and lower sections.

Once again, I may be worrying more than the bowsprit's use will justify...even assuming it is strong enough to hold the kite in 10+ knots of apparent wind, there'll be the question of whether I can keep the boat upright in that breeze!

Thanks also to Ian for the carbon-knowledge...

...I'll certainly come back to you when the whole thing separates in the first moderate gust.

Quandary, I must have explained poorly - I didn't only reinforce either side of the U-bolt position, but the whole length between points about five inches either side. I wish I'd paid closer attention to tapering, but p'raps I can take care of that with the sander - it's going to need some smoothing over.

What was the limitation in picking the size of U bolt? I did something similar on our 35ft boat(sold), using an old 75mm spinnaker tube (3mm wall thickness) , somewhat modified. The U bolt was very snug and chosen to have minimal slack. The beyond bow extension I left at 500mm after looking at Selden set ups. How much have you inboard of the U-bolt? If you have 6ft outside, the loads at the inboard end could be impressive, so make sure that is strongly attached. The down/side loads will depend a bit on how you plan to fly the sail - Deep reaching or just reaching?
Our new boat came with a Selden 'sprit from new, and the forward extension is not >500mm , but enough for flying an asymmetric. Inboard attaches to a D-ring with a backing plate below deck. If the wind were steady , up to 10-12Kts is "comfortable". Gusts are not a lot of fun if you are light on crew...

 

[Greenheart]

I chose the 316 stainless U-bolt from a UK manufacturer which offered internal diameters of 40mm, 50mm, 60mm etc...so with a 50mm pole, I went for the 60mm U-bolt.

It won't be hard to wrap something round the 'U' itself, to ensure there's a snug fit. I had tried those plastic rope-stoppers, like parrell-balls, for reduced friction - but there really won't be much movement, because the pole will only travel back and forth a few times in each day its used. Plus the parrell-balls took up more space than I'd foreseen.

The overall sprit length is around 10ft 6". I'm reckoning a little over six foot will be outside the boat, and the remainder (call it 52 inches?) on the foredeck...

...so I've devised a round, flat-topped plywood platform with a hole through the top, which will be bolted to the foredeck in an exact position yet to be decided...a rope will be drilled through the rear end of the sprit, and will lead through the hole in the ply platform...

...so pulling the rope (led aft to a fairlead and a cleat on the thwart, ten feet behind the bow) will pull the inside end of the sprit forward to the fixed platform on the foredeck. With that end of the sprit effectively cleated (non-stretch 6mm halyard, I'm thinking), and the padded U-bolt keeping the sprit's mid-point still, I'm hopeful that the outer end won't flit around much. Even if it isn't rock-steady, my purpose of keeping the kite's tack well out ahead of the bow will have been achieved. If it's too long, it won't be hard to trim the sprit.

Regarding how deep I plan to use the kite, I'm thinking it's a matter of waiting and seeing what's possible. I decided it was too complicated to make the sprit pivot to windward to allow deeper reaching...

...but if anyone knows why I can't still use my conventional spinnaker pole to get the clew out to windward opposite the mainsail, deep downwind, I'd like to hear their reasons. I know the asymmetric spinnaker is mainly meant to allow benefiting from apparent wind, but it'd be nice to use it on a run too, as my course may require.

 

[Ian_Edwards]

QUOTE "but if anyone knows why I can't still use my conventional spinnaker pole to get the clew out to windward opposite the mainsail, deep downwind, I'd like to hear their reasons. I know the asymmetric spinnaker is mainly meant to allow benefiting from apparent wind, but it'd be nice to use it on a run too, as my course may require."

A slight drift, but I find that the smaller of my two Asymmetrics will set goose winged to windward without pole. It's quite stable, sheeted well aft like spinnaker, and I find it easier to use like that when sailing dead downwind in moderate condition, rather than reaching and jibing, especially when short handed and the boat is traveling at close to hull speed (the boat is too heavy to plane).

 

[lw395]

RS200's used to pole their kites out, but the practice was banned in class racing.
I have tried goosewinging an asy in light airs in one of my boats. There is a bit of an issue with seeing where you are going.....

 

[Daydream believer]

Dan
Personally i reckon that a carbon pole is the wrong thing for this application. I know carbon is light but it can be brittle if loaded at a single point. So first thing is to find a stainless steel mast protector that sailboarders put round the carbon masts where the boom sits. These are about 225mm long & help to distribute the point load of the boom ( in your case "U" bolt). The protector is sprung loaded & is like a split tube about 1mm th walled.
However, in spite of the slight weight disadvantage i think that you need to find a glass wave mast. Mine are carbon reinforced along the bend line & are designed to take the shock of hitting the bottom if I get chucked off in a wave. My strongest one is a "Rotho Wave" & is far more rigid than any of my carbon ones. It would not need reinforcement at the u bolt. You may find that as you sail single handed you are not actually going to load the boom up that much so a normal glass mast would be better as once again it would resist the cracking at the "U" bolt better. I have cracked carbon masts just by dropping them on a stone on the beach whereas a glass one would only chip the outer gel coat.
They would accept holes for the pulley system to pull the foot of the sail out better as well. I know that you have the carbon mast but let's face it there are hundreds of un used sail boards in garages all over the country 7 if you cannot get what you want for less than a tenner then you are not trying. might even get a board thrown in & blow the Osprey out instead!!!!!!!!!!

 

[Greenheart]

I find this very interesting, thank you for the input. Daydream, you have replied while I was writing my reply...I'll post again, below. :encouragement:

I've also got a little, short-footed, flattish kite which I suspect will be very versatile and probably won't even need the sprit. Effectively it's like a taller, fuller genoa, nearly twice the area of my regular genoa. I'm hoping it will pull well on a very close reach, yet remain controllable right round to a run, when I'll goose-wing.

I'm bound to ask, why was goosewinging disallowed from RS200 racing? Not that it matters - I can ignore all class rules and use whatever techniques or additional equipment I find beneficial, so the prospect of either kite being used to bustle across the wind while reaching, then to goose-wing dead downwind with up to double my white sail-area, is very exciting.

The visibility point is a valid one, I'm already accustomed to slewing about in order to scan the area ahead of the boat. It's a particular issue when singlehanding, as the job of keeping the boat upright often restricts my movement! Hopefully not downwind, though.

I was surprised in recent years to read symmetric spinnaker fans seeming derisive about poling out the clew of an asymmetric opposite the main, on the grounds that an asymmetric was meant to do away with the hassle of the pole. I think they'd missed the point. I hope I get a chance next year to find out how ably my bigger, curvy spritted kite can switch from deep-reaching to goosewinging dead-downwind...

...but I'm guessing that even with a pole to allow goose-winging, there must be a 'dead zone' when the boat is pointing too far off, for the asymm kite to benefit from apparent wind, yet not deep enough to allow it to fill, opposite the main. Is this a recognised problem, or something I'm imagining?

 

[Greenheart]

Daydream, I was dissuaded from using my old alloy symm spinnaker pole as a bowsprit, on the grounds that they're inclined to bend more easily than the go-to choice for dinghy bowsprits - carbon fibre. If anyone had suggested a fibreglass sprit, I would have welcomed the idea, because I'm more familiar with GRP than carbon.

A GRP windsurfer mast does sound good - very robust. And I'm reminded that there is the warning 'Not for use in surf' on my carbon pole, which I expect was to prevent windsurfers' crash damage such as you describe.

But still - most dinghy bowsprits do seem mainly to be carbon - so I doubt I've gone far wrong using what I've got.

The stainless gooseneck reinforcement sounds smart - but in my application, I wonder whether it wouldn't just transfer the loading to the next free spot, where the carbon emerges from the stainless?

 

[thinwater]

I'm baffled. We don't know...

* How big the boat is.
* Style of chute.
* How high on the wind you plan to carry it.
* Anything about the construction of the existing carbon tube. Not one thing.

And that fact that it is carbon is NOT magic. So is my fishing pole. It depends on the layup and exact dimentions.

Comparing a sprit to a spinnaker pole is comparing a baseball to a handgrenade; they are both round, but that is it. One is designed for compression only, the other one a cantilevered load. In fact, if the sprit were finely designed, the layup tapers and the u-bolt point would have been considered.

Either go for it, post full engineering information, or hire a rigger. There isn't enough information here to do any math. The equations are all full of blanks.

 

[Greenheart]

Good answer. Evidently you already know more than I do.

All I know about the sprit, is what's written on it.

It was a windsurfer mast. The name is "Technofibre." It was sold to me as 100% carbon. It's 490cm long (two parts, sleeved join). For most of its length, it is 50mm outside diameter.

I quote the following, as I can't use Photobucket at the moment:
Bend curve cc
MCS# 26
IMCS# 29

I'd say the tube wall looks a little over 3mm thick.

The boat is an elderly, fairly sound GRP Mk2 Osprey, probably 150kg, 17ft 6 overall, 5ft 9 beam.

The sail is an RS 400 kite, very full cut, made by Ullmans. I understand it was ordered but never sold. I expect the customer realised too late that a sail issued by anyone other than RS Sailboats, couldn't be used in class racing. None of which bothers me in the least; it was a true bargain.

I'd like to believe the sail will allow fast reaching in windspeeds up to 10 knots. Singlehanding in more than ten knots, the main and genoa are plenty for me.

Regarding close reaching, I'm reminded that the flat, black kite I bought a year ago (which I think had been made as a 'screecher' for a catamaran, then once again, wasn't paid for by the original customer) will offer lots more power than the Osprey's genoa. I made a height-adjustable spinnaker halyard point to enable use of longer luffs...

...but let's not go down the road of danger to the unstayed upper mast...I'll be very wary (chicken) about using either non-standard kite, and the hoisting height will be limited to about 18" above the standard spinn halyard bolt, where the mast hasn't begun to taper, so should retain most of its strength.

The black kite is not very relevant here because its foot isn't much longer than the genoa's, so it may be most easily handled without any bowsprit. I only mention it here because you asked how high I hope to point.

I'd like to know how good or hopeless you think the windsurfer mast will be as a bowsprit. As soon as I get my laptop fixed, I'll be able to illustrate my posts with Photobucket.

 

[Quandary]

Looking forward to some pics. of the boat under sail, I insist that you post some eventually, good luck with your project and stay upright.