Carbon fibre bowsprit. How strong? How thick the wall?

tudorsailor

tudorsailor

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I would like to have a bowsprit onto which I can then use a furler for the cruising chute. As I have an Rocna anchor, the standard solution of the bowsprit pass over the anchor pin does not work. Ideally I would like to pass the tube up the port side of the stem head, but space is limited. Selden make carbon sprits, but for a 20ton displacement, their sprit would be too fat to go up the channel.

I wonder why one cannot get a tube with the same out side diameter but with a thicker wall to have enough strength.

So how do I work out the "moment" for a 400mm unsupported length flying a cruising chute 1.5oz material and approx 152m2, on a 20 ton yacht

Does anyone know a good place to get a carbon fibre tube? I hope that by not mentioning a "marine" application, it might be affordable.

Thanks

TudorSailor​
 
tudorsailor said:
OK! I know the distance. How do I work out the force exerted by the sail through its attachment to the end of the pole??

TS
Click to expand...

Thats more difficult but there must be some stats somewhere. Perhaps a sailmaker would be able to tell you what they use as a rule of thumb, or a rigger. I have a book at home about yacht design and scantlings so I will have a look in it when I am back home and see if they give any figures.
Another thing you could try is to work out the strength in tension of the weakest part of the rig and build to match that.
 
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Marlow ropes have a calculator for sheet load

Genoa Sheet Load Formula - Metric
SL = SA x V² x 0.02104

Where:
SL = Sheet Load in kilograms
SA = Sail Area in square meters
V = wind speed in knots

Of course a cruising chute is different in that it is only attached at three points, rather than along the whole of the luff. I wonder how relevant this theoretical load would be

152sq metre sail on a max wind of 20knots s gives a load on the sheet of 1279Kg

I think I need to find a naval architect!

TS
 
I asked a well known sailmaker what the force was on a tackline for an asymmetric.
This was in the context of a lightish 40ft ish boat.
His answer was 'A lot'.
I asked was that a lot of bags of sugar or a lot of bags of cement.

I think we are in the many bags of cement arena.
I suppose you could design for the tack line to break first.

It's a difficult question, you can work out the steady state force if the boat is knocked down, but then you need to factor in dynamic loads up to surfing into the back of a big lumpy wave.

Or you could just use an ali scaffold pole and carry a spare.
 
What is it about the Selden one that you don't like? Fittings incompatable, design, capacity, price......?
 
What are the dimensions of the one that wont fir and whats the max diameter you can use? There must be a calculator to work out the wall thickness ratio given o/d ratio.
 
bluemoongaffer said:
I used these people for a one-off carbon gaff: http://www.aardvarkracing.co.uk/about/
V helpful. No connection except as satisfied customer.

I made my own CF bowsprit using a section of windsurfer mast and some flexiply round the outside to stiffen it. Mine's a small boat though so I doubt it would do for you but you could try
Click to expand...

i used a windsurfer mast, cut it in half and slid one section inside the other
supported with stays, not busted it yet
 
If you know the dimensions of the seldon sprit ( diameter, wall thickness )they will tell you the load it will take. Then get a carbon mast maker to make a lower diameter spar with thicker walls to achieve the same.
You could make an assumption about the stress carbon will take & so long as you use the same factor in the smaller diam spar you can just work out the extra thickness of the alternative spar. The material is the same so it is a matter of ratios

I googled "bending stresses in tubes" & came up with "Are hollow pipes less likely to bend under weight" & came up with a forum showing the formulae for a cantilevered tube
I think the link is:-
Reference https://www.physicsforums.com/threads/are-hollow-pipes-less-likely-to-bend-under-weight.37702/ (But i am too tired to sit & study it tonight)

As i said you can assume any stress for the carbon as you will be using the same material & get a spar of the same strength in cantilever as the seldon
 
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Daydream believer said:
If you know the dimensions of the seldon sprit ( diameter, wall thickness )they will tell you the load it will take. Then get a carbon mast maker to make a lower diameter spar with thicker walls to achieve the same.
You could make an assumption about the stress carbon will take & so long as you use the same factor in the smaller diam spar you can just work out the extra thickness of the alternative spar. The material is the same so it is a matter of ratios

I googled "bending stresses in tubes" & came up with "Are hollow pipes less likely to bend under weight" & came up with a forum showing the formulae for a cantilevered tube
I think the link is:-
Reference https://www.physicsforums.com/threads/are-hollow-pipes-less-likely-to-bend-under-weight.37702/ (But i am too tired to sit & study it tonight)

As i said you can assume any stress for the carbon as you will be using the same material & get a spar of the same strength in cantilever as the seldon
Click to expand...

I am hoping to get a quote from Selden for a carbon fibrepole of adequate strength. Then yes, I hope to reverse engineer to work out the dimensions of a pole with a thicker wall

TS
 
Why not go for two or three times the breaking strain (not the SWL) of the tack line? You presumably want to be sure that that would break first so at least you still had the sprit if it all went mammaries inverted.
 
I have no idea of actual load figures. However you seem to have decided that the pole must go through a channel of limited size. Dimensions of the pole as in diameter are what give strength. Typically 10cms in diameter for your sized boat. So a pole will have to have a lot more material in thickness to make up for the limited diameter. An alternative might be to have an oval cross or square section so that the height is a lot more to manage the primary upward load. However don't discount sideways loads.
Another alternative might be to fit a stay down to the lower bow to take some of the loads. olewill
 
snooks said:
You could always reinforce your sprit with a UHMPE bobstay to take the vertical load and then a couple of lines either side to help with the horizontal loading.
Click to expand...

Thanks for the suggestion. However the sprit would need to be removable as it will go through the hoop of the Rocna (no thread drift please). So stays and guys are not really practical.

Here is a pic of a postal tube in the channel to represent the sprit

b78f7996-81f8-423b-a6e3-fad7d46578e2.jpg


TS
 
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