Any Engineers who can do the calculations?

[Norman_E]

I am thinking of adding a short bowsprit to my boat, just to hold the tack of the asymetric clear of the pulpit. It will be 1450 mm long with 650 mm stuck out ahead of the forward attachment point with a welded eye about 25mm in to take the load, and the back end held in place as well. The asymetric will put an upward pull on the forward end, so the risk is that the pole could bend at the forward attachment point where the bowsprit will pass through a ring fitted to the heavy stainless bow fitting. The boat is a Jeanneau 45.2 with a 16.5 metre mast and an asymetric of about 80 square metres. I have a choice of materials to make the bowsprit, but favourites are 316 stainless tubes. A local company suggested 50mm diameter by 3mm wall thickness, but I thought that rather small, and can source 60mm x 3mm or 60mm x 4mm wall.

Are there any engineers here who can calculate the safe load of those tube sizes to resist being permanently bent, assuming the bowsprit being firmly held at the back end and at the bow fitting which is 625mm from the point where load is applied?

 

[awol]

Yes, I can, but rather than hand feed you I prefer to encourage you to fish for your answer. May I suggest Roark and Young as a suitable source?

 

[catmandoo]

Yes, I can, but rather than hand feed you I prefer to encourage you to fish for your answer. May I suggest Roark and Young as a suitable source?

Ah! R J Roarke , Happy memories . The stress engineers bible - that and Peary's Aircraft Structures

 

[Norman_E]

Yes, I can, but rather than hand feed you I prefer to encourage you to fish for your answer. May I suggest Roark and Young as a suitable source?

I might find a copy in the library, but if I did I still doubt that I would understand the mathematics. I think that what is being quoted to me in Marmaris as 60mm X 4mm wall is actually 2" schedule 40 pipe, like this.

As the tack line is 10mm braid on braid with a breaking strain of 2000 Kg and a safe working load of half that there is no point in having a bowsprit that would take more to bend it.

 

[ghostlymoron]

You're talking about buckling rather than bending. Any tube will bend under load but recover when the load is released. Euler's buckling theory will enable you to calculate the load at which the tube buckles but there are so many shock loads imposed on rigging that an empirical approach would be best. i.e. see what's been adequate in the past.

 

[alahol2]

This might give you some food for thought

 

[Norman_E]

This might give you some food for thought

I have seen those Selden aluminium ones, but my boat needs a fatter diameter than will pass under the lid of the anchor locker when retracted and it would be very awkward if it retracted over the locker lid. This is why I am thinking of a stainless one in smaller diameter thick walled tube, which will retract beneath the lid with only a minor modification to the forward edge of the lid.

 

[Quandary]

I have a much smaller boat with a Selden sprit (alloy tube about 75mm.) and with a much shorter extension limit, I have been impressed by how much upward bend there is in the thing in just moderate winds. The bending loads are certainly a lot greater than I expected but I am more concerned about the forward restraint point than tube failure.

 

[sarabande]

Norman - lots of pre-determined calculators here.

Any use ?

http://www.engineersedge.com/calculators.htm

 

[paulieraw]

Should not be a problem as long as you hold it down with a chain or cable to the bow at about 45 degrees, then the stress in shared within the structure of the boat. Stainless is fal more resistive to bending than alloy.

Just make sure your anchor point on the bow had the load spresd with a substantial backing.

 

[Norman_E]

Norman - lots of pre-determined calculators here.

Any use ?

http://www.engineersedge.com/calculators.htm

I had found that site, but could not get an answer as I could not work out or find some of the variables in terms of material strength. I do know that Southerly Yachts fit stainless bowsprits, but diameters and wall thicknesses are unknown to me.

 

[catmandoo]

I am thinking of adding a short bowsprit to my boat, just to hold the tack of the asymetric clear of the pulpit. It will be 1450 mm long with 650 mm stuck out ahead of the forward attachment point with a welded eye about 25mm in to take the load, and the back end held in place as well. The asymetric will put an upward pull on the forward end, so the risk is that the pole could bend at the forward attachment point where the bowsprit will pass through a ring fitted to the heavy stainless bow fitting. The boat is a Jeanneau 45.2 with a 16.5 metre mast and an asymetric of about 80 square metres. I have a choice of materials to make the bowsprit, but favourites are 316 stainless tubes. A local company suggested 50mm diameter by 3mm wall thickness, but I thought that rather small, and can source 60mm x 3mm or 60mm x 4mm wall.

Are there any engineers here who can calculate the safe load of those tube sizes to resist being permanently bent, assuming the bowsprit being firmly held at the back end and at the bow fitting which is 625mm from the point where load is applied?

From your description the load on the tube is is one of bending and compression . At 650 mm I doubt that buckling under compressive load component only will be the ruling factor .

In the first instance you will have to work out the force on the asymmetric presume the wind is horizontal but the sail will take a curved shape such that there will be horizontal and vertical components at each attachment point to transfer the load to the mast , the bow sprit and the sheets . . First you will have to determine the centre of pressure on the sail by looking at the geometry and then work out how much of that load is transferred to top and bottom . I suspect the centre of pressure is about 2 /3 down so proportion the loads appropriately so if X is the total load on the sail then you could assume 2/3 X is distributed between the sheets and the tack and 1/3 at the mast and then divide by 3 to get the load on the tack . The sail can take any shape from fully curved to a flatter format so I suppose the big problem is working out the angles of the sheets and halyards . Try sketching it out .
To simplify you could assume its vertical to get the worst case and give your self a factor of safety of 2 by doubling it to get force F at the eye. Having the vertical load at the eye then the bow bracket on the sprit must be strong enough to withstand an up ward force of 1450F/650. The moment on the tube is 650F but you can then calculate the moment of inertia of the tube to get the bending stress add to that the compressive stress from the horizontal component and compare the sum with the acceptable stress for the grade of material which you can get from any piping manufacturers catalogue .

One thing you must make sure is that at the point of bending the tube remains circular other wise the wall could buckle

Alternatively you could add a tie downwards from the opposite side of the eye to the bow at the water line thus canceling out the bending and just dealing with straight compressive load

Bit of a ramble . hope it helps

 

[awol]

The question asked was - what is the safe point load on a 650mm long built in cantilever made of 316L tube (i.e. E=200GPa) with a) 50mm dia x 3mm wall, b) 60mm dia x 3mm wall and c) 60mm dia x 4mm wall? An interesting sideline is the trade-off between weight/diameter/wall thickness for an aluminium tube at 1/3 of the weight and 1/3 Young's Modulus cf steel.

 

[catmandoo]

The question asked was - what is the safe point load on a 650mm long built in cantilever made of 316L tube (i.e. E=200GPa) with a) 50mm dia x 3mm wall, b) 60mm dia x 3mm wall and c) 60mm dia x 4mm wall? An interesting sideline is the trade-off between weight/diameter/wall thickness for an aluminium tube at 1/3 of the weight and 1/3 Young's Modulus cf steel.

yes but all he wants to know is what size of tube will do the job . could cut out all the waffle and obfuscation by fitting a strop from the sea side of the sprit to just below the WL then moment is cancelled out just as our fore fathers did and then all we have to deal with is compressive load on the sprit which is easy particularly with a small outstand of 650mm .

SIMPLES SIMPLES SIMPLES


KISS principle

lots of love and kisses too

 

[B&M]

It is possible to calculate the maximum load that a tube will take. But the shock load applied by a flogging jib could be several times the steady load and therefore unless you know what factor of safety to apply you are wasting your time.

 

[prv]

cut out all the waffle and obfuscation by fitting a strop from the sea side of the sprit to just below the WL

Well, that's what I do with my five-foot timber bowsprit, but those with modern white lozenges tend to prefer a simple cantilever. Avoids the need to fit a bobstay chainplate, and is less obtrusive when the whole shebang is removed, as it often is.

Pete

 

[Norman_E]

Awol summed up the problem nicely. With a modern GRP boat fitting a strop to the bow is an added complication, and would require me to lift the boat out to do it. I did think of flying the asymetric directly from the bow fitting as I do now and estimating the pull on the tack line, but a safer course seems to be to find out the amount of force needed to bend the available tube sections, and compare that with the breaking load of the tack line, bearing in mind that the knot used to attach the shackle has probably halved its break strength.

 

[paulieraw]

An alternative is to have a long wire strop made that sits above the pulpit, its what we used to use on service boats like the victoria 34, this gives access. The strop can have a fiting that clips it to the pulpit when not in use.

Of course service boats do not use furling headsails, but that should not be a problem.

 

[misterg]

A local company suggested 50mm diameter by 3mm wall thickness, but I thought that rather small, and can source 60mm x 3mm or 60mm x 4mm wall.

Are there any engineers here who can calculate the safe load of those tube sizes to resist being permanently bent, assuming the bowsprit being firmly held at the back end and at the bow fitting which is 625mm from the point where load is applied?

Moment of inertia (I) of a tube is:

I = Pi x (OD^4 - ID^4)/64

1) 50 OD / 44 ID tube I = 122812 mm^4
2) 60 OD / 54 ID tube I = 218780 mm^4
3) 60 OD / 52 ID tube I = 277264 mm^4

Max stress (S) from a bending moment M:

S = (M x OD )/ (2 x I)

The above is all independent of material.

Yield strength (Y) of 316L is ~170MPa (Young's modulus is irrelevant to the calculation)

Max bending moment in each case to cause yield (assuming well engineered supports):

1)835121 Nmm
2)1239752 Nmm
3)1571165 Nmm

On a 625mm spacing between load and support, and assuming the load is acting at right angles to the tube, the corresponding forces are:

1) 1336N / 133 kg
2) 1984N / 202 kg
3) 2514N / 257 kg

Decide on safety factor: 2.5 for example (you may want to go higher) and the 'usable' loads are:

1) 55 kg
2) 81 kg
3) 103 kg

When the pedants have finished ripping this to pieces, you may or may not conclude that you're 'gonna need a bigger tube...'

Let it begin.....

Andy

 

[Norman_E]

Thank you for that. Whilst the figures look a bit lower than I hoped the largest tube should do the job, as sailing last year with the tack attached via a block fixed to the bow fitting never seemed to require much force to pull in the tack line, which was always pulling forward as well as up unless I used the tacker at the base of the Genoa foil. I gave the 625mm figure because that will put the line well forward of the pulpit, but in practice I hope to get away with pushing it out a bit less, perhaps only 500mm.

In a rising wind I suppose I could always let the tack line out so that the pull becomes more forward than up, and the bowsprit will have a bit of an upward tilt because the inner end will sit below deck level, whilst the forward attachment will be above the bow fitting by a few inches. I am hoping that the force on it will be at about 45 degrees rather than 90 degrees, thus cutting in half the component that is trying to bend it.

Am I right in thinking that the yield strength of 170Mpa is the minimum for 316L with ordinary 316 being higher, and typical values for both being about 300Mpa?

Paulieraw's suggestion of a wire strop won't work, as the current problem is that a tack line, or a strop, attached to the bow fitting passes inside the pulpit, and if the sail flies out to the side the line imposes an unwanted load on the pulpit. The idea of the bowsprit is to get the tack line just out in front. Routing the line outside the pulpit from the bow fitting is not an option as it is likely to snag on one or other of the navigation lights.

P.S. It would be very interesting if anyone with a Southerly fitted with one of their stainless steel bowsprits could measure it.