The other set-up looks more robust to me. The holes look like the failure point in this case.

Is it really preferable to drill holes in that ultra thin aluminum pole?

I didn't say it was a catilever under spi loads - it may or may not be depending on how well the pre-bend matches the spin loads.

It is most definately a cantilever when there are no spin loads due to the pre-bend, which is just as bad from a fatigue viewpoint.

The high degree of constraint at the bridle and the much lesser degree of constraint at the end of the pole is a pretty gross violation the truss idealization. This means there will certainly be some bending in the pole. To reduce or eliminate the bending on would need to:

A - put a hinge in the pole at the bridle. This would decrease the compression force resistance of the pole due to a different buckling mode, however. You would also need something to keep it from drooping down too far when the spin is not up.

B - use extreamly stiff lines between the bows and the end of the pole, and eliminate the pre-bend. As Berny said, because of the acute angle of those lines, the loads in them are greatly magnified. Stretch in those lines causes the tip of the pole to move upward (I figure this is why you use pre-bend, so that the tack of the spin doesn't go to high). Because the pole is highly constrained at the bridle, this leads to bending stress in the pole. You would need some super-stiff line (perhaps made of unobtainium) to keep the end of the pole from deflecting an appreciable ammount.

C- Decrease the constraint at the bridle, by removing the compression strut. This would decrease the apparent stiffness of the pole by increasing the length over which it is bending. This will make the lines take more of the upward force of the spin by decreasing the resistance of the pole to that force via bending.

Anyway, this may be a systematic problem, or just a random event. My understanding is that the Blade is pretty new design. Maybe other Blade runners will encounter this problem in the future, in which case the design of the pole needs to be changed. Or maybe they won't.

If I owned a blade, I wouldn't futz with my pole at this point. However if it broke in a similar manner, I would certainly consider modifications to the design.

Quite. It's hardly a unique design in this regard - Hobie Tiger is the same, if I recall (give or take the compression strut), although the pole is probably a bit more over-specified.

Paul

What is the consensus for pole set up?

1. Slight bend when attaching spi pole bridles (I assume this is what is being referred to as pre-bend).
2. No downward pressure on tip of spi pole
3. Remove pressure on spi pole when on trailer, beach, etc.

What else?

I leave my pole setup permanently with pre-bend, but then it is carbon rather than ali. If it is really windy, I'd ideally want to increase the pre-bend to bring the tip down lower and therefore make the luff tighter. An adjustable pair of tip stays is something I'm currently working out.

Take the strut and the saddle out of the equation [not hard to do] and the problem, [if there is one] is solved. As I've already said, I don't see the need to have a strut at the bridal. It's just unnecessary baggage IMHO and detracts from the proficiency of the pole.

Another idea I had was to put a 'bottle screw' adjuster in the end of the pole at the beam. This would help to pre-load the pole by adding tension the bow lines to improve the fixedness of the truss, sorry pole.

No strut, no rivets.

Bern,
I agree, if sailing cat rigged there is no need to use the compression strut and saddle.

When running a jib the compression strut is needed.
The jib comes down past the primary load bearing bridle wires to the pole and if you don't use a compression strut you can't get enough luff tension on the jib. I have tried it without the compression strut and found it really is needed when running the jib.

What is a bottle screw adjuster?

Regards,
Phill

Berny,

I'm going to be a little confrontational here. Try actually understanding my posts, if you don't then there is no point in continuing this discussion. Your idea's won't work on the F16's and remarkably enough the pole setup in your picture will actually make the F16 poles alot weaker. This will requiring sailing with a much more beefed up pole and that goes against the idea of a lightweight F16.

Also Berny I've been into this field of spi pole design and experimentation since 1998. I have used your setup myself back then and I can tell you what its downsides are.

On my boat the pole would bend out of colom under spinnaker in anything over 15 knots. Luckily I had a bridle strop (which you don't) and that saved my pole otherwise it would have broken will sailing due to buckling failure mode. The solutions was adding the stabilizing midpole wires with a small upward prebend in the pole. Now the pole that was too weak for the boat and spi handle very well all I could throw at during 3 years. It was 35x2 mm alu pole length 3.50 mtr. Funny enough mr Rhino writes of this modified setup : "The high degree of constraint at the bridle and the much lesser degree of constraint at the end of the pole is a pretty gross violation the truss idealization." And he is correct in a theoretical sense, but in a practical sense this setup is much stronger and stiffer one then the former one. And accepting a small loss in accuracy it can still be modelled by a perfect truss model. That is when the upward prebend is not too excessive, in that situation the small bending can be approximated by a perfect hinge in the theoretical model. The results will still be accurate to 80-90 %

Since then a few new setups have passed on my boats and the final design is one extremely comparable to that on the Blade. I'm using a 40x2 mm pole now I don't know what the US sailors are using. 40x 1.6 mm will be most perfect. 40x1 mm will be to weak and 35x1.6 mm and less will be so too. Maybe the US sailors have a pole that is a little to much on the threshold. I know what the basic Blade design specifies and that is well tested and sufficient. I don't know to what extend VWM follows these building guide lines.


In your post you wrote :




If you just take it away then the pole will fall down. If you replace it with a line (and somehow undo the hole that is already there) then you can't fit the jib for you have no attachment point for the selftacking jib sheet and the jib luff will be badly supported leading to a large grease in the jib sail. We have been there and tried it all when prototyping. I'm sorry to say but we are a few iterations ahead of you in pole design.

No further disrespect intended.


Wouter

Rhino,

I'm sorry to say that you a make a few noteably errors in your modelling. Allow me to clearfy this point.





Fatigue crack growth propagation is a function of :

-1- the presence of changing stresses(loads)
-2- magnitude of average stress level
-3- the magnitude between extremes of constantly changing loads
-4- the direction of the stresses
-5- The number of stress changes
-6- presence of corrosive surroundings
-7- material properties, especially its relative resistance to shear stresses.
-8- geometrical properties of the element subject to changing stresses.


Without point -1- you don't even have fatique cracks.

point -8- can make strongly influence its growth rate. Small round holes are much less bad then large square like ones. The rivet holes do really qualify as small round holes in this respect.

Time to failure is most dependent on points -2-, -3-, -4-, -5-, -6-

It must be noted that when point -2- and -3- are below a certain threshold level that fatique cracks growth can be halted altogether and time to failure will be infinite.

Naturally salt water can be regarded as a corrosive environment stimulating the growth of fatique cracks.

Note how bending stresses of any other type of stresses don't really facture in this propagation of fatique cracks growth. As a matter of fact the very weak relationship that is between these two is such that under bending stresses the elment subject to fatique is actually MORE resilliant then one under tensile (non bending) stresses. This is because of the supporting function of less stressed material nearby.

Of course when there are no spi loads that the both the average stress level and the magnitude between extremes is MUCH Lower then under spi load. Also the frequency of stress changes is much much much lower if significant changes even occur at all. I would say that a spi pole without a spi hoisted is probably well in realm of indifinate extention of "time to failure" because of these points. Crack growth is halted and as such it is not "...Just as bad..."





You are right in a theoretical modelling sense, but this doesn't mean that it is a problem. The problem in this sense is on the side of the specific model that may not be entirely accurate in describing what happens in real life. But it does not mean that the real life situation has a problem simply because an overly idealized model (one of many) fails to describe it sufficiently accurately.

If you are technically schooled then the next comments will be helpful. The current spinnaker setup can still be accurately described by lineairized models as such it can be modelled by a truss model expanded with a simple cantilevel bending model and idealized buckling model where all results can be superpositioned on one another. This new model will be accurate in describing what happens in real life. When so desired when can look at each model component individually and then compare them to eachother. This is should still be within the capabilities of medium schooled engineers.





What if they bending stresses due to the prebend you are trying to elimate are not a significant part of the whole situation, then you may be trying to solve something that really isn't a problem. Additionally if it is significant then why not simply reduce the amount of prebend from say 2 inches to 1 and cut the importance of bending by halve ? Reduce it even more if that is not enough. Only a small amount of prebend is required to take the slack out of the system and to prevent unintended buckling bending to the sides. Why go for complex solutions when much simplier ones are available ? That is if the problem is present at all which I don't believe it is.





So what is it ? Either the much magnified compression loading is dominant or the bending due to prebend is. They can't be both dominant at the same time. I say that compression is dominant when there is no excessive prebend in the pole.

Additionally even infinately stiff lines won't cancel at all the need to spi pole prebend. The prebend is there to pull these lines taught, which is not a function of stiffness of the wires but of the stiffness of the pole ! Additionally the prebend is there to force the pole to always try to bend upwards under compression loads. This is much preferred over it bending sideways or even downwards. It is locked in place much better that way. Additionally it is near impossible to get the length of the midpole support wires such that both these and the tip wires are perfectly taught with a prefectly straight pole. Ergo you try to have the midpole support wires a little more loose then the end pole ones and prebend the pole to get all of them taught.

The reason for the prebend is much of practical nature and the attractiveness of the fact that we know its failure direction in advantage. The last point allows a designer to optimize the design further. It will always fail in one direction before failing in another. But we are getting into details here. There is a more important error point to be discussed here.





I'm sorry to say that this is wrong on multiple points and as such is totally wrong in its end conclusion.

First of all if the wires stretch and the pole tip moves upwards then that is actually a good thing and not a bad one. Afterall by moving th tip up you are REDUCING the net prebent of the pole and thus lowering the bending stresses ! So this would mean that when a spi is hoisted that the pole experiences smaller bending loads/stresses then when the spi is snuffed. This is actually a good reason to have prebend in the pole and not start out with a perfectly straight pole. However the dyneema lines used or so low stretch that this is really not a significant issue.

So the basic error here it that deflection under spi load is actually a bad thing on a prebend pole when it really isn't it is actually a good thing. If the lines would stretch more then you would increase prebend to compensate for it.

The assumption that the fact of the bridle point being highly constrained will lead to increased stresses is an overall unsupported statement and false at that. First you can not ever say it like that as there are more then a few exceptions and secondly it is patently false in this situation where buckling failure mode is a component. Buckling failure mode is actually enhanced by any deflections (in its halveway point). By fixing the middle of the pole very well (constraining) the pole will become much more resiliant to buckling failure. So my other recent post to Berny.

Or do this home experience. Grap a small diameter by long rod. Stand it up straight and press down on its top. At a certainly level of force the rod will deflect outwards and bending it will become easier (requiring less downward force). Now have a family member hold the middle of the rod so that that point stays on the vertical between both ends. No repeat the experiment, what do you notice about the required force to buckle the rod ?

Now you will understand why the next point you wrote down is actually a very bad move.




I also am a bit confused by the fact that at first you try to lower the bending stresses but adding hinges etc and now you are actually increasing bending stresses by removing the constrained at the midpoint. These two actions are in direct conflict with eachother. You are, I'm afraid, not consistent in your solution.





The general spi pole design is now widely accepted in beach catamarans so that is not it. The only difference in F16's is that we use an eyestrap with 2 rivets to secure the strut too, although we are not unique in this aspect as several other builders use the same eyestrap to fit the retaining line to as well. I do feel that we F16 sailors are looking for the most lightweight setup while other builders tend to overdimensionalize components like these. A Tiger spi pole feels MUCH heavier then our F16 pole while being almost the same length.

During prototyping neither Phill not I encountered any problems with the saddle over a time span several years. And phill have a large gaping hole there. So either Robi's pole was at one time seriously mishandled or the VWM pole are using a much smaller wallthickness or small diameter pole.

Can anybody measure this ? Because now I want to know.





I would first try to establish what the reason for the failure was and attack that issue directly. I dare wager a good amount that it is not the saddle or the rivet holes but something else.

Wouter

Slight prebend is good ! No more then needed to get the support lines taught and force upward deflection against the bridle strut under spi loads. I think 1 inch is sufficient in any respect.

However I do run more prebend to get some more luff length on my spi, this hasn't lead to any failures yet. But the amount I'm running is not needed for any construction/design reasons.





Learn this trick when beach handling the boat. Untie the top and tack halyard from the spi and tie them of to one another then pull the spi halyard tight and cleat it. Now the pole tip is also fully supported in the vertical direction and you can now fully handle your boat from the spi pole tip. You can now even excert large downward forces on the tip. As a matter of fact it will then behave just as if a spi is hoisted and the whole system is now fully locking up and very strong. The spi luff is actually a crucial part of the whole design. You don't see any more poles with pelican strikers etc anymore, do you ? The current spi pole has evolved to this sophistication that only the bear essentials are needed and the spi pole setup only weights 2.5 kg (incl snuffer).




No need, just use trick described at 2. All the spi boat sailors do it like that overhere as small children tend to hang on the spi pole tips when the boat owners are away. We are at a public beach overhere. Anybody not using the described trick risks a bend or broken pole not matter how the pole was build up, ring, saddle, strut or line. I've seen a good number of bend and broken poles this way.

Wouter

Good stuff, Wouter. Thanks.

Yep, agreed...good stuff. Probably a good idea to do this at regattas when kept overnight...as these are usually at public beaches where kids possibly can hang from the spi' pole.

Obviously there is tensile stress in the Blade pole setup. That stress is cyclic - higher tensile stress when the spin is not up, reduced or no tensile stress when the spin is loaded. Since the pole is Aluminum fatigue damage will accumulate regardless of the magnitude of that stress. The question is at what rate does it accumulate, which I can only guess at with the information available to me.

However, the failure does not look like it is due to a single overload to me, which leads me to belive that fatigue was a large factor.

The pre-bend or lack of thereof doesn't make a difference regarding the presence of bending moment in the pole. With pre-bend we expect tensile stress at the top of the pole when the spin is not up. Without pre-bend we expect tensile stress at the bottom of the pole when the spin is up.

That's really the entire point of what I've been saying. As I stated, the options I gave are ways to reduce that tensile stress. And yes, they will reduce the tensile stress despite your irrelevant attempts to score points. They may or may not be good solutions for other reasons, but that's a different matter.

If you want to claim that you can model the Blade pole setup as a truss with any degree of accuracy, you are wrong. There is no hinge at the bridle, which fails the idealization. In addition, we know there is a significant bending moment there - enough to cause 2" of deflection at the pole tip - which means you can't ignore the failure of the idealization.

The problem is not statically determinate either. That has nothing to do with whether it's a truss or not - trusses like any other type of structure can be either determinate or indeterminate.

Now, claiming that an actual engineer would model this as a truss plus a cantilever using superposition is bizarre. You can't cleanly split this problem that way, at best you could arrive at a solution iteratively. The obvious and easiest solution to this indeterminate problem is to replace the lines to the end of the pole with virtual forces. A virtual moment at the bridle, which is what I think you are getting at, is only something you would do to prove a silly point on some discussion forum.

Anyway, this has gone on long enough. I belive that fatigue is a large factor in that failure. You don't. Let's leave it at that.

Interesting stuff.

Out of interest, how much does 3.5m of that alu pole cost?

Assuming I know how do use a drill and a rivet gun, should I be more worried about this, or the fact that the 10 quid block at the top of the mast never lasts more than half a season?

Paul

PD; What's a quid? Seriously.

Our Tornado is eating spi halyard blocks <img src="http://www.catsailor.com/forums/images/graemlins/frown.gif" alt="" />
Next one will be without roller bearings!

quid = £
buck = $
Quid ~= Buck

Thanks Rolf.

<img src="http://www.catsailor.com/forums/images/graemlins/blush.gif" alt="" /> <img src="http://www.catsailor.com/forums/images/graemlins/shocked.gif" alt="" />

Not where I'm sitting; If I could buy all my kit in $ uquality to £ then I'd be very happy !

Right, hence the ~= instead of the = or ==.

<img src="http://www.catsailor.com/forums/images/graemlins/shocked.gif" alt="" /> <img src="http://www.catsailor.com/forums/images/graemlins/blush.gif" alt="" /> <img src="http://www.catsailor.com/forums/images/graemlins/shocked.gif" alt="" /> <img src="http://www.catsailor.com/forums/images/graemlins/blush.gif" alt="" /> <img src="http://www.catsailor.com/forums/images/graemlins/shocked.gif" alt="" /> <img src="http://www.catsailor.com/forums/images/graemlins/blush.gif" alt="" /> <img src="http://www.catsailor.com/forums/images/graemlins/shocked.gif" alt="" /> <img src="http://www.catsailor.com/forums/images/graemlins/blush.gif" alt="" /> <img src="http://www.catsailor.com/forums/images/graemlins/shocked.gif" alt="" />

the $ to £ is about 1.9 today - BBC as an example

I don't call that ~= in any way shape or form.

Can I send you some $ and you send me the same as £ !