Compression force on whisker pole

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Anchorite

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I can't find this quantified anywhere! There must be figures behind the choice of pole sections (somewhere): I'm trying to find the force transmitted onto the mast fitting. This concerns a 35m2 poled out genoa. An approximate percentage would be a good start... no decimal places required (thanks).
 
Anchorite said:
I can't find this quantified anywhere! There must be figures behind the choice of pole sections (somewhere): I'm trying to find the force transmitted onto the mast fitting. This concerns a 35m2 poled out genoa. An approximate percentage would be a good start... no decimal places required (thanks).
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The force due to wind pressures on the poled-out genoa will be countered by the forestay, the sheet and the whisker pole. These are all acting in different directions but my guess is that the compressive load on the pole will be about equal to the tension in the sheet. Look up the SWL for your sheet material and that will probably be the max. compressive load that the pole will ever see. Buckling is the main hazard - you'll need Euler's equation in order to determine pole dimensions. I didn't bother when I made a whisker pole - just judged it by eye or instinct [hasn't broken yet].
 
I've checked various books that I've got and they all give graphs from which you can read off the recomended sizes for spinnaker poles by boat length or sail size. None give any indication of the forces involved.

I'm sure similar graphs are available for whisker poles but I suspect that the choice is usally determined by the spar manufacturers experience rather than calculation.

What exactly are you trying to do?
 
Unless you're racing you should just get something substantial.

Trying to calculate the strength is a bit of a waste of time because the shock loads that occur when the poled-out sail gybes are off the scale compared to the steady state that gets used in the Maths. A mast designer would do the calcs and then use experience to multiply by a factor. Possibly as much as an order of five.
 
The loads ona whisker pole depend entirely on the length as related to the size of the jib. So a jib in quartering breeze will stay stable and the sheet goes from the jib sheet block on the deck to the jib clue without a pole. So if you had a pole no compressive force. If you had an adjustable length pole as you make it longer you force the jib clue outwards deviating from the straight line of no pole. Compressivee forces increase with pole length.
If you go to the extrreme of having a pole too long like using a spin pole for a whisker pole you force the jib clue way out so that the sheet ends up providing end force on the pole as well as simply pulling the clue backwards.
If you want to squeeze every ounce of drive from the jib when running and especially when the jib is flying to windward then you need the longer pole. which in being longer also can not handle the compressive forces so well.
But if you are satisfied to simply make the jib more stable by holding the clue out a little way then end forces are much less and of course a shorter pole is stronger.
So it is up to the skipper what he wants to do with the whisker pole and hence how heavy it must be. There might even be a case for a shorter pole and a longer heavy one depending on skippers needs or mood and crew 's ability to set it up.
Yes I use a carbon fibre section of wind surfer mast on the little boat for spin pole. It is about 3 metres long. It does bend a lot on occasions but apparenently happily. It wouyld suit I think a much bigger boat as a whisker pole.
I occasionally use it as a whisker pole but it is really too long yet does the job. I don't use a whisker pole enough to have a dedicated short pole. good luck olewill
good luck olewill
 
boguing said:
Unless you're racing you should just get something substantial.

Trying to calculate the strength is a bit of a waste of time because the shock loads that occur when the poled-out sail gybes are off the scale compared to the steady state that gets used in the Maths. A mast designer would do the calcs and then use experience to multiply by a factor. Possibly as much as an order of five.
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Agreed, a really light pole we had used a great deal for 8 years buckled when a broach when running with full sail slammed the pole against the shroud. However had it been heavy enough to withstand this we would probably seldom use it. As a light pole on 30 square metres of genoa I can just grab it with one hand and fit it without faffing with up or down hauls. The problem was a lack of seamanship, a club race and pushing with too much rag flying. A whisker pole can be a lot lighter than a spinnaker pole.
 
Just in case you need to decide on pole length, I found a website that said 1.1J which I adopted although it sounded rather long but it's just right for a jib ... not sure if a genoa would need something different.
 
Mast abeam!

reeac said:
Just in case you need to decide on pole length, I found a website that said 1.1J which I adopted although it sounded rather long but it's just right for a jib ... not sure if a genoa would need something different.
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Length is not the problem: it's the fitting on the mast. Some sources say that beyond a certain pole length (not a specified sail area) a track must (should) be fitted and not a simple ring. I feel this information is aimed at the racing fraternity (not my case...) but with a 4.5m pole I'd like to know what effort the rivets might have to handle.
 
A lot of boats used to have rings on the sides of the mast which are far better able to deal with whisker poles and reaching struts. Anything on the fwd face of the mast is going to suffer from twisting.
 
reeac said:
Just in case you need to decide on pole length, I found a website that said 1.1J which I adopted although it sounded rather long but it's just right for a jib ... not sure if a genoa would need something different.
Click to expand...

One of my books suggests the whisker pole should be the same length as the foot of the foresail.


Maybe I'm missing something, but I would have thought that, if the OP's fitting is also for his spinnaker pole, (I'm assuming it is) then he can safely assume that it will be fine for a whisker pole.
 
merlin3688 said:
A fixed eye either side of the mast is ideal for a Whisker pole, a track up the front is a bit over the top! As mentioned the Whisker pole can be a lot lighter if not used as a Spinnaker pole, let me know which boat its for and I can recommend a section size.
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It's the mast fitting that he is unsure about.
 
There is no mast fitting at present, this is a winter project (mast is down and out!). I haven't seen or heard of twin fixed eyes before: this seems a better engineered solution and one I will adopt.
Many thanks to all for your responses.
 
Anchorite said:
There is no mast fitting at present, this is a winter project (mast is down and out!). I haven't seen or heard of twin fixed eyes before: this seems a better engineered solution and one I will adopt.
Many thanks to all for your responses.
Click to expand...

When I bought my present boat, it came with a brand-new cruising chute, which I had no intention of using. She also had a massive, heavy, telescopic spinnaker pole, which I was incapable of using safely. I made and fitted eyes on each side of the mast, and also made the whisker-pole, mentioned in my previous text. It works very well for poling out the genoa on a run, and is light enough that it doesn't need any uphauls or guys.
 
Around here the loop fixed to the front of the mast for attachment of spin pole is called a "snotter". Is this name universal and if so what do we call similar rings on each side of the mast for whisker poles "ears".
As a matter of interest slightly related to this subject I fitted a similar ring at the bottom of the mast on one side. Well up the mast I fitted a plastic pole clamp. This becomes the storage for the spin pole. It has proved very successful for stowage of the wind surfer mast type pole about 3 metres long. The advantages are that topping lift remains attached so you can't lose the pole and it does not clutter foredeck or get fouled with sheets. good luck olewill
 
Rather than using a seperate whisker pole I use my spinnaker pole with a snap shackle lashed at the right spot (~1m) from the outer end. I can then use the up & down hauls to manage the pole (which occasionally I leave up even at anchor in the SE trade winds) including takeing weight off the jib in light airs. No sign of the snap shackle moving (if it did a simple line to the end would stop it) or of the jib sheet chafing (I keep an eye on it). Andrew
 
As far as the size of a whisker pole is concerned, the word is empirical. You could calculate or measure forces till the cows come home but as a previous posters have indicated an unfair load or a slam against rigging will break it. It's just a case of specifying something that can be used 95% of the time hopefully without an issue.
 
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