Gull / Dolphin Striker Tension

[tamicatana]

Another way to try to guess at your gull striker tension would be: boat in water, set your rig tension, go sailing and see if you can tell if your fwd xbeam bends/deflects upward under normal upwind sailing conditions 10-12 kts. The beam should lie flat in that case

[meirriba]

Originally Posted by ausnp84

Sure thing - unfortunately I can’t edit the original post. To avoid any confusion, the stay tension we’re talking about is:

1.) The stay that connects at either side on top of the crossbeam, and has the A-frame in the middle.

2.) The stay that runs between the the forward, lower bows and connects to the forward lower of the sprit. I understand these are called martingales / bob stays / water stays.

N

So:
As to the crossbar, it should have a slight upward pre-bend.
I cannot check on my boat being far from the vessel (although not same model but measure should be similar) - I would guess in the vicinity of 1".
As to the bowsprit - see my previous post.

[ausnp84]

Originally Posted by tamicatana

Another way to try to guess at your gull striker tension would be: boat in water, set your rig tension, go sailing and see if you can tell if your fwd xbeam bends/deflects upward under normal upwind sailing conditions 10-12 kts. The beam should lie flat in that case

Thanks. Just waiting on new outboard remote controls and we’ll get a test sail in.

One thing this whole experience has taught me is asking three riggers gave three different answers, and that maybe it’d just be easier to buy a production cat next time😂

N

[boatman61]

My experience is limited to Wharram cats where the dolphin striker was under the main mast beam and tensioned to the max possible.. the foresail was much the same set up as yours with eye bolts in the bows and a wire each side running to the centre where a shackle linked them and the forestay was added using high strength cord to tension everything up.
Nice easy rig with just a port and starboard masthead stay set aft of the mast and the forestay making the triangle..

[tamicatana]

Again, confusion:

The gull striker, which is what was in the OP's picture, is upside down from a dolphin striker.

Quite often the forward x-beam on bigger cats will have a gull striker. The gull striker will deflect the beam downward. This deflection will oppose the upward forces generated by the forestay.

The central beam or the main beam on cats so designed will have a dolphin striker. The dolphin striker will deflect the beam upward. This deflection will oppose the downward forces generated by the mast and rigging.

Two of the three last posts have referred to the dolphin striker, whereas the OP was asking about his gull striker, as shown in the picture.

I have made no comment regarding the sprit other than to say it carries a dolphin striker.

[boatman61]

Originally Posted by tamicatana

Again, confusion:

The gull striker, which is what was in the OP's picture, is upside down from a dolphin striker.

Quite often the forward x-beam on bigger cats will have a gull striker. The gull striker will deflect the beam downward. This deflection will oppose the upward forces generated by the forestay.

The central beam or the main beam on cats so designed will have a dolphin striker. The dolphin striker will deflect the beam upward. This deflection will oppose the downward forces generated by the mast and rigging.

Two of the three last posts have referred to the dolphin striker, whereas the OP was asking about his gull striker, as shown in the picture.

From what I could see the gull striker only has to maintain down pressure on the fore beam to counter stresses from sails attached to the bowsprit like a gennaker/spinnaker etc so while important is not as critical as if the fore stay was connected to the beam.
Tension by hand till tight as possible.
But hey.. I'm not a rigger (since the 1970's)

[tamicatana]

Originally Posted by boatman61

From what I could see the gull striker only has to maintain down pressure on the fore beam to counter stresses from sails attached to the bowsprit like a gennaker/spinnaker etc so while important is not as critical as if the fore stay was connected to the beam.
Tension by hand till tight as possible.
But hey.. I'm not a rigger (since the 1970's)

https://www.cruisersforum.com/forums...1&d=1640713976

[boatman61]

Missed this pic which shows the dolphin striker is a on the bowsprit.. not the main beam as I assumed.
I will step back from the discussion as without being at/on the boat, any more from me would be theoretic assumption never having played with this type of set-up..
However practical experimentation looks fairly straightforward, but I'd wait till I was afloat to tension it up properly.

[catsketcher]

Originally Posted by ausnp84

One thing this whole experience has taught me is asking three riggers gave three different answers, and that maybe it’d just be easier to buy a production cat next time😂

N

I think that all the repondents have said the same thing about the forebeam - increase tension in the seagull striker wire when the beam bends upwards with rig tension on, ease it if it bends downwards with rig tension on. The seagull striker wire tension will depend on your rig tension. You would get the same advice for any catamaran, no matter what its pedigree.

The short vertical strut (dolphin striker) for the prodder means that the loads in the stay will be higher than with a longer strut. This is where your setup is different from normal - usually boats with integrated bowsprits - prodders - have deeper dolphin strikers. The race boat I sailed that used one like this anchored it way back on the beam, not on the walkway.

This is the part of the design I would be careful about. The upwards outer forestay tension load will pull the walkway down where the inner end of the strut attaches. It seems about the same length underneath as the amount it extends out front, so the load downwards will be equal to the stay tension - unless the waterstays do their job (which I recommend they do). Walkways are not usually designed to take much loading and so I am not sure why the prodder underwire is really there at all. If the dolphin striker was to take much load, then there would also be a lot of load on the inner part of the prodder and this would pull the walkway downwards. I have not seen any cat walkways designed to cater for this.

Most of the time prodders are designed with universal type fittings where they attach to the forebeam. Yours is quite different in this respect.

The racing cat Top Gun - has a fixed prodder with large dolphin striker that works well.

https://www.boatsonline.com.au/boats...pod-cat/263512

cheers

Phil

[Tupaia]

Originally Posted by meirriba

Dolphin striker should be tensioned to give slight upward curve to the crossbar.
Martingale wires - just hand-tight. The sail gives it upward tension, and the stays will keep the bowsprit from rising up.

A dolphin striker, downward pointing, would never be fitted to a cross bar as it would apply force in the same direction as the forestay that it is designed to resist.



A gull striker, upward pointing is fitted above the cross beam and if anything should be tensioned to give a slight downward curve to resist the tension of the forestay attached directly to it.


A martingdale is a y shaped piece of a horse bridle as a head control / restraint. Never heard it used in this context.



The two lower stays connecting the bows to the outer end of the sprit are called water stays and resist the upward tension of free flying headsails flown from the end of the sprit.



The arrangement being questioned here is complicated by the fact the sprit is not pivoted at the cross beam and there is a rigid furler attached to the end of the sprit. The sprit therefore applies additional upward force to the cross beam and it appears the water stays provide a downward force to counteract it. The sprit itself has a dolphin striker with a tensioning wire that will resist upward bending of the sprit around the cross beam.


So in essence there are multiple downward restraints on the sprit, water stays and crossbeam fixing. IMO these will need to be balanced unless they are all over engineered and that kinda defeats the object of having both of them. If you pivot the sprit at the cross beam the forces will be taken by the water stays and there will be no additional upward force on the crossbeam. If you remove the water stays the upward force will be taken by the cross beam and the gull striker and the dolphin striker will prevent the sprit bending. Fountain Pajot use this method albeit composite. The water stays could be there purely to prevent the sprit from pulling to port or starboard and not contribute anything to resisting the upward forces but it appears that the water stay wire passes through a ring on the sprit and would not be able to provide any sideways resistance.



Assuming a static situation and the sprit is rigid all the way to the apron then the downward component of the water stays should equal the upward component added to the cross beam by the sprit proportioned by the distance from the apron to the cross beam otherwise the difference in forces are simply trying to bend the sprit.


Given the dynamic conditions a boat must operate under the arrangement does not make sense from an engineering point of view.


Anyone want to shoot me down or explain why they have this arrangement.

[ausnp84]

Originally Posted by catsketcher

I think that all the repondents have said the same thing about the forebeam - increase tension in the seagull striker wire when the beam bends upwards with rig tension on, ease it if it bends downwards with rig tension on. The seagull striker wire tension will depend on your rig tension. You would get the same advice for any catamaran, no matter what its pedigree.

The short vertical strut (dolphin striker) for the prodder means that the loads in the stay will be higher than with a longer strut. This is where your setup is different from normal - usually boats with integrated bowsprits - prodders - have deeper dolphin strikers. The race boat I sailed that used one like this anchored it way back on the beam, not on the walkway.

This is the part of the design I would be careful about. The upwards outer forestay tension load will pull the walkway down where the inner end of the strut attaches. It seems about the same length underneath as the amount it extends out front, so the load downwards will be equal to the stay tension - unless the waterstays do their job (which I recommend they do). Walkways are not usually designed to take much loading and so I am not sure why the prodder underwire is really there at all. If the dolphin striker was to take much load, then there would also be a lot of load on the inner part of the prodder and this would pull the walkway downwards. I have not seen any cat walkways designed to cater for this.

Most of the time prodders are designed with universal type fittings where they attach to the forebeam. Yours is quite different in this respect.

The racing cat Top Gun - has a fixed prodder with large dolphin striker that works well.

cheers

Phil

Thanks Phil.

The comment about the riggers was based on one advising it to be “tight, but not too tight”, another advising “bar taut”, and another advising it to look tight but have some give. Hence the post on CF instead for more info.

Ta for the info as well - we’ve noticed the forward end of the walkway has been pushed slightly downwards at some point (the fore most bolt is missing / out of alignment) and I suspect it’s been caused by the water stays not having sufficient tension.

The wind has finally eased here so I’ll get the dink in the water today and re-tension / check.

N

[Tupaia]

Originally Posted by ausnp84

Thanks Phil.

Ta for the info as well - we’ve noticed the forward end of the walkway has been pushed slightly downwards at some point (the fore most bolt is missing / out of alignment) and I suspect it’s been caused by the water stays not having sufficient tension.

This bears out my previous comment.

"Assuming a static situation and the sprit is rigid all the way to the apron then the downward component of the water stays should equal the upward component added to the cross beam by the sprit proportioned by the distance from the apron to the cross beam otherwise the difference in forces are simply trying to bend the sprit."

The forces are bending the sprit and the weak point is the joint with the walkway.


I think if it was my boat I would cut and install a pin joint in the sprit below the cross beam and fit individual adjustable water stays. The current setup means that conflicting forces are applying an unnecessary bending moment to the sprit.

[ausnp84]

Originally Posted by Tupaia

This bears out my previous comment.

The forces are bending the sprit and the weak point is the joint with the walkway.


I think if it was my boat I would cut and install a pin joint in the sprit below the cross beam and fit individual adjustable water stays. The current setup means that conflicting forces are applying an unnecessary bending moment to the sprit.

Agreed - it would make more sense to have two seperate stays to ensure no lateral movement.

From looking at old photos of the boat it appears to have previously had a single dyneema water stay, however this was broken during a storm when the marina experienced 1-1.5m waves and a concrete pontoon float broke loose and sat between the hulls for 24 hours. The hulls only experienced abrasion damage but it appears even then that the bolts connecting the sprit base to the walkway had been pushed out slightly, and that the front of the walkway had been dented slightly.

I’ve got the dink in the water today so we’ll get some measuring done in order to organise new stays.

Thanks again.

N