Bowsprit design - How strong is strong enough?

Norman_E

Norman_E

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I want to fit a short retractable bowsprit to attach the cruising chute. I do not want to buy the Selden one because its design would obstruct opening the anchor chain locker. My idea is to attach the bowsprit to the very substantial Jeanneau bow roller fitting via a new part through which it can slide, with the back of the bowsprit capable of being slid back inside the chain locker, The whole bowsprit would be just under 1500mm long with 600mm projecting ahead of the fixing point on the bow fitting, just enough to get the tack line forward of the pulpit. The Selden aluminium bowsprit recommended for my boat is 87mm diameter, too big to pass back into the locker.

In Turkey I was recommended to use 51mm OD x 3mm wall stainless tube, which seems to me to be a bit small. My own idea was to look for something bigger, but the idea of a wooden bowsprit appeals as I can easily make it myself. Would oak nominally 3 inches square, but finished to a little over 2 1/2 inched be strong enough? The boat is a Jeanneau 45.2 and the cruising chute is about 1000 square feet.
 
Very difficult to work out from the description, but from what I can make out you're talking about some serious loads; don't think wood will do it, but as I say difficult to make out.
 
What is the internal diameter of tube or wall thickness used in the Selden 'sprit, and what is the aluminium grade and temper?

If you know that, it is not difficult to work out what size stainless tube would be equivalent along with what grade would be required.
 
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If Selden are saying an 87mm dia tube is their recommendation, then a 2 1/2 inch wooden section is going to need a lot of bracing with dolphin strikers, stays, etc, if it going to take the compression and torsion loads from your big spi.

Perhaps some serious through-bolted steel work along the sides of the wood to give it the stiffness to resist deflection and help prevent deformation ?
 
I do not know the gauge of tube used by Selden, but their bowsprits can extend further forward than mine needs to. In my case the bowsprit will be very short, it is only intended to move the tack attachment point clear of the pulpit. My present arrangement is to attach the tack line block directly to the bow fitting with a "Tacker" on the genoa to keep the line central. All I need to do is extend it forward between 500 and 600 mm with the inner end of the bowsprit braced against the deck just ahead of the anchor locker where the downward force of about 2/3rds of the upward pull on the tack will be taken by a steel reinforcement.
 
My timber bowsprit, on a 24 footer, varies between 3" and 4" square. It's also rigged properly, with shrouds and a bobstay, so that (in theory at least) the only force on it is compression between the fittings at each end.

It looks like you're planning to set yours up with a downwards force in the middle and upwards at each end - essentially snapping it over your (upside-down) knee. Intuitively, 2 1/2" seems far too small to me, especially for such a big boat (although admittedly I'm used to builder's softwoods rather than oak).

Pete
 
One thing this is convincing me, is that the suggested use of 51mm x 3mm wall 316 stainless tube is not strong enough, though I think that if I do go for stainless, standard 2 inch heavy pipe, which is actually 60mm OD x 3.9mm wall, might be worth trying.
 
Norman_E said:
One thing this is convincing me, is that the suggested use of 51mm x 3mm wall 316 stainless tube is not strong enough, though I think that if I do go for stainless, standard 2 inch heavy pipe, which is actually 60mm OD x 3.9mm wall, might be worth trying.
Click to expand...

Here's another suggestion.

On my boat, the spinnaker pole is designed to also be used as a bowsprit for a cruising chute or gennaker.

The pole passes through a s/steel, rubber-padded 'eye' welded on the pulpit leg, and it protrudes about a meter from the stem and about 10cm above it. The aft end of the pole clips to a bracket on deck alongside the coachroof.

Yes, it does limit the amount the anchor locker lid can open when rigged, but it doesn't when stowed away.

Pole diameter is 100mm with a 4mm wall. Cruising Chute is 13.6m x 7.65m.

This is a standard (Dehler) design, not an owner modification.

Hope this helps
 
Norman_E said:
attach the tack line block directly to the bow fitting with a "Tacker" on the genoa to keep the line central.
Click to expand...

Norman,

I'm also struggling with exactly this issue and afraid I can't add to your answers. I would be interested to know what a "tacker" is though; never heard that one before.

Rob
 
Norman_E said:
One thing this is convincing me, is that the suggested use of 51mm x 3mm wall 316 stainless tube is not strong enough, though I think that if I do go for stainless, standard 2 inch heavy pipe, which is actually 60mm OD x 3.9mm wall, might be worth trying.
Click to expand...
It seems you have no idea - hence the queries above regarding the one yardstick you have, the Selden. Phone them up and enquire re the wall thickness and alloy grade - then you know what you need to work to in order to get the equivalent. With a shorter sprit it is a simple lever equation to know what kind of reduction that will allow.

I wouldn't even bother looking at 316, I'm guessing you will find it will need to be heavier than desirable to be strong enough. It's yield strength is typically weaker than a hardened/tempered aluminium. Find options for seamless tube in stainless 2205 duplex. Then you are starting to compensate for the reduced section size with some proper strength. Fewer corrosion worries too and almost no surprises possible courtesy of stress corrosion cracking.

For instance, your suggested 60 mm OD x 3.9 mm tube in 316L has a bending moment approx 20% (yes one fifth) that of 87 mm OD x 6 mm tube in 5083 tempered alloy. (The 6 mm is just a guess for the sake of the argument.)
Your 'sprit being shorter will make up some of this, but is it 1/5th of the Selden extension?

If you can come up with numbers you can work things out properly, otherwise you're just guessing in the dark.
 
Norman_E said:
The whole bowsprit would be just under 1500mm long with 600mm projecting ahead of the fixing point on the bow fitting, just enough to get the tack line forward of the pulpit. The Selden aluminium bowsprit recommended for my boat is 87mm diameter, too big to pass back into the locker.

In Turkey I was recommended to use 51mm OD x 3mm wall stainless tube, which seems to me to be a bit small. My own idea was to look for something bigger, but the idea of a wooden bowsprit appeals as I can easily make it myself. Would oak nominally 3 inches square, but finished to a little over 2 1/2 inched be strong enough? The boat is a Jeanneau 45.2 and the cruising chute is about 1000 square feet.
Click to expand...

There are basically three ways that a bowsprit might fail, plus combinations thereof:
  1. Compression. Failure load depends on area x yield strength (A x s)
  2. Buckling. Failure load depends on second moment of area x Young's modulus (I x E)
  3. Bending. Failure load depends on elastic section modulus x yield strength (Z x s)
So to compare the three criteria you need dimensions and material properties for each. I can't dig out an inner diameter for the Selden bowsprit online, but for the other choices...
  • Steel: A = 452 mm^2, I =131000 mm^2, Z = 5000 mm^3
  • Wood: A = 4000 mm^2, I = 1355000 mm^2, Z = 43000 mm^3
Taking typical values for Young's modulus (210 kN/mm^2 for steel, 10 kN/mm^2 for wood) and yield/failure stress (200 N/mm^2 for steel, 50 N/mm^2 for wood) gives
  1. Compression: 90 for steel vs 200 for wood (A x s / 1000)
  2. Buckling: 27.5 for steel vs 13.5 for wood (E x I / 1000000)
  3. Bending: 1 for steel vs 2.15 for wood (Z x s / 1000000)
In other words, I'd expect your wood to take roughly twice the pure compression or pure bending loads before failing, compared to the stainless tube, but to fail in buckling at about half the load. You'd need to do a bit more calculation to work out if this mattered, but I'd expect it to be critical. However, if you took the wood up to 3" square it would match the steel for buckling strength too.

Note: All done off the top of my head as a displacement activity. No guarantees given. Not to be used for navigation.
 
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Norman,

Thanks for the link to the "Tacker". Given that you're after a bowsprit would it be fair to say that the Tacker doesn't work too well?

rob
 
I had some similar questions a while back.
I was trying to get a definitive answer to the actual loads.
I failed on that.
My approach was then going to be make sure the tack fitting is the first thing to break.
You could perhaps work out how much force is required to tear the corner off the sail?
Don't forget the downward force on the deck is as big as the upward force from the tack, times the relevant leverage.
If you bend a bit of ali tube, you just buy another one. If you crush the deck or tear the bow fitting off, it's more of a problem.

Do you want it to get your light air chute a little more efficient, or are you going to wind on the stresses in some serious apparent wind sailing?
Something that is plenty strong enough for what you intend to do might be inadequate as soon as the racers get on board and hit a few waves at speed...

I sold the boat before I got anywhere with this, but since then I have been sailing dinghies with gennakers and I'm hooked, so I am interested for my next yacht.
BTW being able to swing the pole even 20degrees to windward can make a huge difference to 'projecting' the kite out of the main's windshadow when trying to run as deep as possible.
 
craigsmith said:
For instance, your suggested 60 mm OD x 3.9 mm tube in 316L has a bending moment approx 20% (yes one fifth) that of 87 mm OD x 6 mm tube in 5083 tempered alloy.
Click to expand...

That's the ratio of I values. Z, which is what actually matters for bending failure, is only three times larger for the bigger section. Still quite a difference, though!
 
No it's not, I used the section modulus and the relevant yields. Since I didn't reference what they were, I don't see how you could possibly critique it anyway. Your yield figure of 200 MPa for steel is a little optimistic for 316L stainless.
 
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craigsmith said:
No it's not, I used the section modulus and the relevant yields. Since I didn't reference what they were, I don't see how you could possibly critique it anyway. Your yield figure of 200 MPa for steel is a little optimistic for 316L stainless.
Click to expand...

I for 87mm x 6mm circular section = 1258410.74 mm^4

I for 60mm x 3.9mm circular section = 271573.05 mm^4

Ratio 4.6. Pretty close to your claimed 5. You'll note that I gave my assumed figures to allow intelligent criticism.
 
Actually the Tacker works OK. I think that the intention of it is to fit the tack of the sail and the tack line to it and allow it to move up and down the furled sail, however I use it by running the tack line through it with the Tacker fitted just above the furling drum and prevented from moving up by a tie. My objection to it is that it another thing to fit and later remove every time the cruising chute is used.
 
bowsprit

Would definately have thought that a stainless tube would be the way to go. It would be very difficult to make a wooden one that was small enough, be able to stand up to the loads and be retractable. as somebody else has stated it would have to be held with a bobstay etc. the selden and similar bowsprits are easily removed from deck and theren are a variety of different brackets that probably would be suitable. have fitten one to an oyster 46 before and they just got it out when it was required rather than leaving it permanently stowed on deck
Hope this is some help!?!
 
We have one on our Beneteau 323 . Its a Selden one extending out the bows by about 600mm . We can open the chain locker with the pole on deck .

DSCF0239.jpg


The pad eye by the right hand stantion is the position the pole goes in when its extended . The tip of the pole is then directly inline with the bows :D You can also use the anchor . I fitted a Selden fitting and made a mounting bracket for the side of the bowroller
 
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