Bowsprit loadings

[oldbilbo]

I'm considering fabricating an alloy bowsprit using some tube I have, which is 78mm OD and 3mm wall thickness, and hang a gennaker from the end. Here's the idea, from Selden....

I'm wondering how to proportion the 'overhang' and gain some idea of the loads/loadpaths on A: the business end, B: the bow mounting ring, C: the inner end. Any thoughts.....?

 

[lw395]

When I bought my cruising chute, I quizzed a couple of sailmakers on this point, the most useful answer I got was 'a lot'.
I enquired whether we were talking about a lot of bags of sugar or a lot of bags of cement, and the mumbling seemed to veer toward the latter.
Certainly the tack of a dinghy asymmetric generates a lot of force, even with just SWMBO on the sheet.

 

[sarabande]

bed time reading

here's some data on pole loads

http://www.boatdesign.net/forums/at...315504631-gennaker-bowsprit-load-abschn01.pdf

Table 1.4 gives approximate relative loads for tack, head and clew.

Once you find a figure for the tack load, finding the load on the ring, and the compression load on the inner end of the pole, is beetle maths (lots of squiggly figures).

Please remember that carbon fibre poles are anisotropic.

HTH

 

[Chalker]

Are the calculations transferable to boats less than 24m?

As a Chartered Engineer, interpreting these equations terrifies me!!!

 

[Norman_E]

Since Selden sell them in various diameters it is worth looking here http://www.seldenmast.com/download.cfm?download=7837&webnode_id=3014&filename=595-261-e.pdf
If anyone has one and can tell us the wall thickness of it, the answer to the OP's question should become clear. I would be interested to know the thickness of the 87mm tube, as my boat would need that size.

 

[oldbilbo]

As a Chartered Engineer, interpreting these equations terrifies me!!!

I dimly recall another pair of Chartered Engineers, both of them RAF, discussing the unexplained fracture/failure of one rudder hinge on a Canberra in flight a couple of days previous. My whole squadron was there 'cos we had a vested interest. The WingCo Engineering Officer, himself a Senior, practical AND qualified eng type vouched the view that the metal used would be found to be sub-standard, probably in its 'treatments'. One of his staff, a Very Senior, practical and experienced Warrant Officer Eng mentioned he'd been back through the manufacturer's specification sheets and he had 'several reservations about certain of the fatigue-life and loading cycle assumptions' which seemed to be based on WWII expectations of 'G-loadings' and metals material science. He muttered darkly about crack propagation and mainspar crack failures. A number of the aircrew present expressed their ill-ease. "I think you'll just have to rely on faith in the designers," offered the WO Eng.

Our QFI, himself a grizzled WWII veteran of Mosquitos, wryly opined "I have faith in The Lord - and Martin Baker...."

And I note that aluminium T6 alloy is isotropic - at least until indecent things are done to the shapes, holes and local loadings. That's why I prefer aluminium aeroplanes and aluminium spars. I've seen what happens to carbon-fibre components when one drives an F1 car into them, even quite gently.

 

[bbg]

I don't know if this helps the OP but I was told for my boat that I should size the equipment for the end of the pole so that it should be able to take the weight of the boat.

In my case, a 21 foot boat with a spi of 75 sq m and a weight of about a tonne. So I suspect it would be safe to assume a similar loading (a tonne) for a similar sized spi.

 

[sarabande]

I don't know if this helps the OP but I was told for my boat that I should size the equipment for the end of the pole so that it should be able to take the weight of the boat.

that seems to be an empirical suggestion for, if you think about it, a displacement boat flying a gennaker , coming off a big wave, could have about half the boat's mass being lifted by the sail (the rest being supported by the water. So fagpacket analysis suggests that the OP's boat at 3.6 tonnes might need a spar which can 'lift' 1.8 tonnes at the outer end.

That to me seems a figure which is within the realms of reality.

So bowsprit at 2m length, retaining clip at 1m (middle of spar) and upload of 1.8 tonnes at the outer end, means that the retaining clip should be capable of 'lifting' 3.6 tonnes.

At this point my solar-powered fagpacket computer runs out of zeroes, and I start looking at the additional fittings such as bob-stay to share this load - say halving it down to 1.8 again - before considering the resolving of the triangulation forces from the sail which include compression towards the inner end of the bowsprit..


Some standard fittings and their loads are listed here :-
http://www.classicmarine.co.uk/Articles/Bowsprit Article.htm


And soemthing more, with some practical figures, on the same question:-
http://www.boatdesign.net/forums/sailboats/bowsprit-design-resolving-forces-34108.html

 

[sarabande]

ah, some progress.

Messrs Selden have some details.

http://www.google.co.uk/url?sa=t&rc...=b9ZLHd81YcUZetxBkofXOw&bvm=bv.45645796,d.d2k


View attachment 31189

That suggests for a 75mm OD carbon pole, with their fittings, you could have 1.117m sticking out the front. And quite a lot less (0.7m) for a small reduction in OD to 72mm.

 

[PeterR]

I'm considering fabricating an alloy bowsprit using some tube I have, which is 78mm OD and 3mm wall thickness, and hang a gennaker from the end. Here's the idea, from Selden....

I'm wondering how to proportion the 'overhang' and gain some idea of the loads/loadpaths on A: the business end, B: the bow mounting ring, C: the inner end. Any thoughts.....?

I have a Seldon 75mm sprit which I believe has a wall thickness of 4mm. Calculating the loads is hugely problematical. If you look up the references you will find numerous academic papers and MSc theses but no clear cut simple formulae. The closest I got to that was the Harken formula for calculating genoa sheet loads which gives load in Kg as being equal to the sail area in metres x square of apparent wind speed in knots x 0.02104. Tack and clew loads are generally accepted as being similar so the formula should be in the right ball park. However, that is for static loading and a kite can subject the pole to much greater loads e.g. if it flogs, the boat runs into a wave or the kite goes overboard.

One problem with these poles is that using Seldon's guide, unless you use a bobstay, you don't get much projection beyond the front ring. If you have to mount that ring behind the forestay you lose a fair amount of the allowable projection just reaching the bow, unless you can mount the ring on the bowroller. However, if all you are trying to achieve is a mounting point in front of the pulpit that keeps the downhaul clear of the pulpit they do the job and allow you to fly the kite a little further off the wind. However, if you want a really long extension that allows you to carry a bigger sail there is not much option but to use a bob stay and that still leaves you vulnerable to lateral forces.

If your gennaker is very flat cut and you fly it closer than about 80 degrees apparent the allowable extension will be even less than Seldon's table suggests. I have been toying with the idea of getting a flat cut close reaching sail but apart from the eye watering expense of the rollers to go with it the allowable extension falls to about 350mm. Although that is largely because of the necessary luff tension it is also due to the increasing lateral forces as you get closer to the wind.

 

[CAPTAIN FANTASTIC]

My thinking is that the tube should be restrained at one end on the deck, then at two thirds along, by means of a ring at the edge of the bow; thus leaving one third of free tube hanging over to attach the spinnaker at the end of the tube. The tube thickness is proportional to the diameter and naturally the material (eg bigger diameter, thinner the tube)

 

[H4B]

Fitting bow sprit kit to Westerly Fulmar

My thinking is that the tube should be restrained at one end on the deck, then at two thirds along, by means of a ring at the edge of the bow; thus leaving one third of free tube hanging over to attach the spinnaker at the end of the tube. The tube thickness is proportional to the diameter and naturally the material (eg bigger diameter, thinner the tube)

I am in the process of fitting a Selden 87mm kit to a westerly Fulmar . I am cutting one side off the existing double bow roller and bolting the bracket to the bow roller fitting which is also the forestay anchor point. Using the the information supplied I can achieve a maximum forward projection of 1200mm beyond the ring without a bob stay, agreed with the Dealer . This size of pole must also have a minimum of 700mm inboard of the ring which means even if I fitted a bob stay and considering the overall length of the supplied tube I could only achieve 1400 mm projection , so hardly seems worth all the extra hassle. I have also got an extra pad eye and will fit that further back to allow a projection of about 600mm for my flatter cut reaching Assymetric. I have a new A2 down wind Sail to get the most of the bow sprit.

My trained up foredeck crew bought his own boat !!!

 

[H4B]

Since Selden sell them in various diameters it is worth looking here http://www.seldenmast.com/download.cfm?download=7837&webnode_id=3014&filename=595-261-e.pdf
If anyone has one and can tell us the wall thickness of it, the answer to the OP's question should become clear. I would be interested to know the thickness of the 87mm tube, as my boat would need that size.

The tube is not a uniform circular section but an octagonal section with two beams and varying wall thickness . It would appear to be at most 4mm top and bottom and slightly less on the sides.

Hope that helps.

 

[simonfraser]

I have one of these :

http://www.harken.com/productdetail.aspx?id=5722&taxid=537

On a stayed bowsprit, carrying 25.5 m sq of flat asymetric, max boat speed 15.5 knots.

Has not busted, yet.

 

[saltylegs]

ah, some progress.

Messrs Selden have some details.

http://www.google.co.uk/url?sa=t&rc...=b9ZLHd81YcUZetxBkofXOw&bvm=bv.45645796,d.d2k


View attachment 31189

That suggests for a 75mm OD carbon pole, with their fittings, you could have 1.117m sticking out the front. And quite a lot less (0.7m) for a small reduction in OD to 72mm.

I think that detail is for the kit with an alloy pole which is handy as the OP has some 78mm od alloy.

 

[rudolph_hart]

I have a system that was factory-fitted by Dehler on my 35CWS, which uses the standard spinnaker pole as a bowsprit.

Pole is 77mm OD with 3mm wall and is 3.75m long. It passes through a rubber-lined s/steel ring on pulpit and protrudes 1m forward of stemhead when the inboard end is clipped to a stowage bracket on sidedeck.

I set a masthead cruising chute of 13.6 luff x 7.65 foot via a tack line led through a block clipped to the outboard pole end.

Admittedly I only use it in light (ish) airs, but nothing has bent or broken - yet

Edit: pole is aluminium alloy.

 

[Inselaffe]

Hi,
I think estimating the loads is always going to be pretty difficult (or even to know when it will break - gust? slam? sail in water? etc), and then estimating a relevant safety factor will then also be difficult.

What about?:
1. Think how long you want the thing to stick out the front.
2. Work out how much force this length of your diameter pole will need to break (as ever, working out strength should be straightforward compared to trying to estimate the loads, simple cantilever in this case?)
3. Use a tack line that will break with less force than this pole breaking force as a 'fuse'
4. Use it, and:
(a) If the line keeps breaking, shorten the pole and go back to 2,
(b) if it works, then job done

or,
1. Guess and if pole breaks make it a bit shorter next time.

 

[oldbilbo]

Hi,
I think estimating the loads is always going to be pretty difficult (or even to know when it will break - gust? slam? sail in water? etc), and then estimating a relevant safety factor will then also be difficult.

What about?:
1. Think how long you want the thing to stick out the front.
2. Work out how much force this length of your diameter pole will need to break (as ever, working out strength should be straightforward compared to trying to estimate the loads, simple cantilever in this case?)
3. Use a tack line that will break with less force than this pole breaking force as a 'fuse'
4. Use it, and:
(a) If the line keeps breaking, shorten the pole and go back to 2,
(b) if it works, then job done

or,
1. Guess and if pole breaks make it a bit shorter next time.

This appeals to my 'empirical' mindset.... and I have three such tubes to hand!