bigwow
Well-Known Member
The A frame is under compression from the bridle to counteract the pull of the forestay, so I would think that as long as it is still in the correct alignment it should be OK. I'd forget using the gennaker though. IMHO
Mojomo is a bit bust... any ideas?
- Thread starter tcm
- Start date
ParaHandy
Well-Known Member
yes, it could well be dangerous. What is beginning to puzzle me is that the strop attaching the gennaker is only 14 or 12mm rope and I wonder why that did not break rather than the welding. Even allowing for the leverage applied to the A frame I would have bet that the cord would break long before the weld. I think I can see from Matt's pic that the whole of the weld seam has lifted. There doesn't appear any buckling or distortion of the component parts - just a clean break.
Ariadne
Well-Known Member
Sorry, I think you guys are missing the point of the post.
At this stage of the game, with the boat being 700 miles from a landfall, and possibly needing to use fore sail to get there, shouldn't this discussion be more about how to resolve the problem rather than how ill designed it is!
I am no navel architect, but if it was me I would be thinking about riviting it back into place. It is not a perfect solution but as form of jury rig/repair it may just me to shore quicker so it can be professionally repaired.
At this stage of the game, with the boat being 700 miles from a landfall, and possibly needing to use fore sail to get there, shouldn't this discussion be more about how to resolve the problem rather than how ill designed it is!
I am no navel architect, but if it was me I would be thinking about riviting it back into place. It is not a perfect solution but as form of jury rig/repair it may just me to shore quicker so it can be professionally repaired.
halcyon
Well-Known Member
Do not think there is any weld ? I think the load has splade out the A frame legs slightly. This has then caused the out ends of the A frame seats that sit on the cross beam to lift, giving the gap.
The danger, if any, is when the load comes on the forestay, this will then put the A frame under load, cause a chance of A frame legs spreading more. Also due the seating pads being bent up, it will give a much higher load on the cross beam, due to point loading on inner edge.
Brian
The danger, if any, is when the load comes on the forestay, this will then put the A frame under load, cause a chance of A frame legs spreading more. Also due the seating pads being bent up, it will give a much higher load on the cross beam, due to point loading on inner edge.
Brian
Ariadne
Well-Known Member
Could they use a 'Spanish windlass' to hold or stop the spread on the A frame? Then they have to find a means of re fixing it to the X members.
snowleopard
Well-Known Member
I am alarmed by comments that the heavy wire over the A-frame (aka seagull striker) is redundant! The purpose of the A frame and the wire are to counteract the upward pull of the forestay and prevent the crossbeam buckling. The A frame needs to be kept in line with the forestay and must not move in relation to the crossbeam.
I would avoid any rivetting without consulting the designer as it could weaken the beam.
My plan would be: (a) prevent the legs of the A from moving along the beam with lashings around the beam and a lashing between the legs, (b) prevent it bending forward by running some lines back from the top of the A to strong points aft (the shorter the distance the less chance of stretching, (c) minimize the load on the forestay by reducing the headsail.
I would avoid any rivetting without consulting the designer as it could weaken the beam.
My plan would be: (a) prevent the legs of the A from moving along the beam with lashings around the beam and a lashing between the legs, (b) prevent it bending forward by running some lines back from the top of the A to strong points aft (the shorter the distance the less chance of stretching, (c) minimize the load on the forestay by reducing the headsail.
halcyon
Well-Known Member
Think I would be tempted to run a few loops around the outside of the base of the A frame, then loosely lash the A frame and loops to crossbeam, put bar in loops and wind up. This will pull the A frame base in, and stop it spreading, the lashing to crossbeam will stop it sliding up A frame.
Then avoid any heavy loading till it can be removed and reset, then rebeded and retension.
Brian
Then avoid any heavy loading till it can be removed and reset, then rebeded and retension.
Brian
Blueboatman
Well-Known Member
Hi Matt,
Twisterken is correct-That A frame functions primarily as the spreaders do on a mast, its job is to prevent the crossbeam from bending under the upward load from the rollerjib.
As said, get a lashing around the A Frame's two base saddles/crossbeam and tighten with a bar or by driving wedges in..
Further, if it were me I would arrange a rope bridle from the top of the A-at those lugs on each side- and back to the anchor winch drum,this will triangulate and counter the forces that were exerted by the bowprit dipping down.
Only you can decide whether to then continue using the genniker or whether you have enough diesel with mainsail to broadreach off to the nearest island bar and competent surveyor,repair facility(Antigua ?).
Not sure I would want to use that front jib either, but I bet you could fly a spi attached to the two mooring cleats on the 2 foredecks, in light airs(!)..
Hey, breaking something proves that the boys are trying hard enough.
Good luck.
Twisterken is correct-That A frame functions primarily as the spreaders do on a mast, its job is to prevent the crossbeam from bending under the upward load from the rollerjib.
As said, get a lashing around the A Frame's two base saddles/crossbeam and tighten with a bar or by driving wedges in..
Further, if it were me I would arrange a rope bridle from the top of the A-at those lugs on each side- and back to the anchor winch drum,this will triangulate and counter the forces that were exerted by the bowprit dipping down.
Only you can decide whether to then continue using the genniker or whether you have enough diesel with mainsail to broadreach off to the nearest island bar and competent surveyor,repair facility(Antigua ?).
Not sure I would want to use that front jib either, but I bet you could fly a spi attached to the two mooring cleats on the 2 foredecks, in light airs(!)..
Hey, breaking something proves that the boys are trying hard enough.
Good luck.
Roger_D
Well-Known Member
The luff rope of the geneker was too elastic to prevent downward rotation of the sprit wich then loaded the much stiffer wire up to the A frame. The A frame then twisted the cross beam which is torsionally very stiff. The connectors to the hulls at the ends of the beam then were also twisted but had greater resistance than did the base flanges of the A frame which then parted. It would be worth while checking the cross beam connections with the hulls for damage but I suspect that they will be OK.
Yes, the tension in the forestay tries to bend the cross beam upwards and is resisted by tension in the very heavy cross wire (upside down dolphin striker?) The A frame legs are in compression from both the prestress in the cross wire (how tight is it?) and the loading from the forestay. The horizontal member of the A frame may or may not be in tension depending on how the legs are attached to the beam. It may be that even if they are unattached there is sufficient friction generated to prevent the legs from spreading along the beam.(not good) Indeed this would be a desirable design concept. The static coefficient of friction of steel on steel (if they are steel) is 0.7, the arctangent of which (34 degrees) looks greater than the angle of the legs from the vertical, so it should not slide under loading.......but it would be nice to make sure it would not and I concur that a good stiff lashing to the beam and to the other leg would give confidence. The point made about uneven loading of he foot of the leg on the beam being damaging to the beam depends on the loads, thicknesses etc, but epoxy filling the gap would help.
I don't think Mojo has too much to worry about but I would keep the genny luff as tight as possible and slacken off the wire from it to the A frame in case it happens again, in order to try to prevent further forward loads on the A frame.
I would be happy to fly foresails, but would sail a bit undersailed to keep the rigging loads down, until expert help is at hand.
Yes, the tension in the forestay tries to bend the cross beam upwards and is resisted by tension in the very heavy cross wire (upside down dolphin striker?) The A frame legs are in compression from both the prestress in the cross wire (how tight is it?) and the loading from the forestay. The horizontal member of the A frame may or may not be in tension depending on how the legs are attached to the beam. It may be that even if they are unattached there is sufficient friction generated to prevent the legs from spreading along the beam.(not good) Indeed this would be a desirable design concept. The static coefficient of friction of steel on steel (if they are steel) is 0.7, the arctangent of which (34 degrees) looks greater than the angle of the legs from the vertical, so it should not slide under loading.......but it would be nice to make sure it would not and I concur that a good stiff lashing to the beam and to the other leg would give confidence. The point made about uneven loading of he foot of the leg on the beam being damaging to the beam depends on the loads, thicknesses etc, but epoxy filling the gap would help.
I don't think Mojo has too much to worry about but I would keep the genny luff as tight as possible and slacken off the wire from it to the A frame in case it happens again, in order to try to prevent further forward loads on the A frame.
I would be happy to fly foresails, but would sail a bit undersailed to keep the rigging loads down, until expert help is at hand.
ParaHandy
Well-Known Member
Latest from Mailasail blog
"So the Aframe is held on the beam in compression. Welded to the bottom of
the A-frame are c-shaped feet that fit around the bottom, front and
underside of the beam. Think of these as just locating the A on the beam
prior to tightening up the cable over the top of the A. The A frame doesn't
need to be welded down on to the top of the beam - and as far as we can see,
it isn't. Quite right really,cos that would compromise the intergrity of
the beam I suppose."
Seems that the c-shaped feet have been distorted by being twisted forward when the sail dropped into the sea.
Matt goes on to say that he thinks the A Frame does naff-all and he might be right as I remember seeing the paint finish where the cup sits on the frame and there was no visible crack or abrasion which you might expect given that it is fitted as Matt describes.
"So the Aframe is held on the beam in compression. Welded to the bottom of
the A-frame are c-shaped feet that fit around the bottom, front and
underside of the beam. Think of these as just locating the A on the beam
prior to tightening up the cable over the top of the A. The A frame doesn't
need to be welded down on to the top of the beam - and as far as we can see,
it isn't. Quite right really,cos that would compromise the intergrity of
the beam I suppose."
Seems that the c-shaped feet have been distorted by being twisted forward when the sail dropped into the sea.
Matt goes on to say that he thinks the A Frame does naff-all and he might be right as I remember seeing the paint finish where the cup sits on the frame and there was no visible crack or abrasion which you might expect given that it is fitted as Matt describes.
Twister_Ken
Well-Known Member
If it does nothing, firstly, why is it there and secondly, why is there a girt big cable in tension running over it?
bilbobaggins
N/A
Para,
The A-frame or 'seagull striker' in many cats is a VIP part of the structure - as several others above have indicated. Its purpose is to support the very substantial bending loads imparted by the forestay into the front crossbeam, when loaded highly by a large headsail - ESPECIALLY when working into a head sea.
Consider the weight/cost/strength of the quite massive A-frame 'shroud' - usually the heaviest bit of rigging wire on a cat - to get a clue re the loads that can and are cyclically imparted into the crossbeam by the boat's working upwind, then consider what happens to the structure of the fore part of the boat if the crossbeam gets pulled out-of-column i.e. as in mast failure.
It is advised several times above to make damn sure the A-frame is relocated and prevented from further movement or twist by supporting lashings, so that it can continue to do its primary job. The s/s cable from the A-frame to the prodder-end works only to stop that pole drooping. A doubled length of fat bungy rope will also do that job, and be sacrificial should the genneker again try to pull metal off the boat.
Inspection of the forebeam end fastenings is probably indicated, as is a regular sighting along the forebeam to spot any bending before it causes an 'out-of-column' beam failure, followed by a sideways flexing or panting of the fore hulls a la 'Team Philips', with resultant catastrophic structural failure. It's happened before...
That's the risk. And that's what the fore beam, in part, is there to prevent. So protect that beam.
/forums/images/graemlins/wink.gif
The A-frame or 'seagull striker' in many cats is a VIP part of the structure - as several others above have indicated. Its purpose is to support the very substantial bending loads imparted by the forestay into the front crossbeam, when loaded highly by a large headsail - ESPECIALLY when working into a head sea.
Consider the weight/cost/strength of the quite massive A-frame 'shroud' - usually the heaviest bit of rigging wire on a cat - to get a clue re the loads that can and are cyclically imparted into the crossbeam by the boat's working upwind, then consider what happens to the structure of the fore part of the boat if the crossbeam gets pulled out-of-column i.e. as in mast failure.
It is advised several times above to make damn sure the A-frame is relocated and prevented from further movement or twist by supporting lashings, so that it can continue to do its primary job. The s/s cable from the A-frame to the prodder-end works only to stop that pole drooping. A doubled length of fat bungy rope will also do that job, and be sacrificial should the genneker again try to pull metal off the boat.
Inspection of the forebeam end fastenings is probably indicated, as is a regular sighting along the forebeam to spot any bending before it causes an 'out-of-column' beam failure, followed by a sideways flexing or panting of the fore hulls a la 'Team Philips', with resultant catastrophic structural failure. It's happened before...
That's the risk. And that's what the fore beam, in part, is there to prevent. So protect that beam.
/forums/images/graemlins/wink.gif
KevB
Well-Known Member
There's a nice big anchor chain and windless there, I'd possibly lash the chain around the A frame and take up the tension on the chain. Not to pull the A frame back in to position but to prevent it going any further forward.
Twister_Ken
Well-Known Member
TCM's thoughts are here
As he can actually do the touchy feely thing with the A frame, I guess his opinion is the best.
If it really is still solid, I'd be wondering about the feasibility of flying the gennaker from the windward bow, when the winds drop away a bit.
As he can actually do the touchy feely thing with the A frame, I guess his opinion is the best.
If it really is still solid, I'd be wondering about the feasibility of flying the gennaker from the windward bow, when the winds drop away a bit.
ParaHandy
Well-Known Member
There are bits of Matt's text that are worth reading before leaping to smite the messenger! The Alliaura cat body comes forward all the way to this beam and is bolted thereto. In this respect it is unlike most other cats of similar size where the beam has no support - at its centre - from the boat's structure. There is, therefore, a considerable additional support provided by his cat at that point. The observation I made about there being no visible sign of movement around where the foot of the A Frame is attached - at the least I would expect some fracture of the paint finish - may be wrong, may be entirely a mis-interpretation of the forces but it suckered me into thinking that it was welded (until I began to question why a welded construction failed before either a 10mm, which it is, strop or any of the other fittings) .... but, I take your point that Alliaura wouldn't have fitted it without good reason and the resolution of the forces on this beam, as expounded by many on here, is correct.
halcyon
Well-Known Member
One thing I do not think I have read, check the tension in the wire over the A frame. If the A frame has distorted, then it could have had it's preload reduced, increasing stress in the cross tube.
Brian
Brian
ParaHandy
Well-Known Member
rather perversely in all of this, the gennaker structure and support is unaffected, if youse thinks aboot it .... so long as there's nae a repetition
matelot
Well-Known Member
[ QUOTE ]
For my part I dont think that wire from the A-frame to the bowsprit should be there. But I am not an Marine Architect.
[/ QUOTE ]
If the A frame wsnt there then the cross beam would bend up wards under the load of the forestay and the mast would come down. The A frame is vital to ensure rigidity of the cross beam.
Its not clear from the photos just how the base of the legs of the A frame sit on the cross beam, but I cant see how they can rotate forward without some metal somewhere giving way. Has the forward movement put dents in the cross beam? If so that would make me very nervous. Has the movement simply bent the flanges at the bottom of the A beam legs - if so I would be less worried.
We cant really advise from this distance except to say that the A beam is vital and must be secured in position on the cross beam until there is a pemanent repair
For my part I dont think that wire from the A-frame to the bowsprit should be there. But I am not an Marine Architect.
[/ QUOTE ]
If the A frame wsnt there then the cross beam would bend up wards under the load of the forestay and the mast would come down. The A frame is vital to ensure rigidity of the cross beam.
Its not clear from the photos just how the base of the legs of the A frame sit on the cross beam, but I cant see how they can rotate forward without some metal somewhere giving way. Has the forward movement put dents in the cross beam? If so that would make me very nervous. Has the movement simply bent the flanges at the bottom of the A beam legs - if so I would be less worried.
We cant really advise from this distance except to say that the A beam is vital and must be secured in position on the cross beam until there is a pemanent repair
bilbobaggins
N/A
Yep. Agreed. No smiting attempted or intended - just that your reply was then the last on the list, and had the benefit of a better knowledge of the boat, better fotties, and better comms with the miscreant.
While I hadn't picked up that a centre-line structure supported/was supported by the forward crossbeam on that boat, we are all agreed that the 'seagull striker' assembly and heavy rigging is there for good structural reasons, and it would make sound sense to secure it and protect it until pro help could be reached.
Awrabrest.
/forums/images/graemlins/wink.gif
While I hadn't picked up that a centre-line structure supported/was supported by the forward crossbeam on that boat, we are all agreed that the 'seagull striker' assembly and heavy rigging is there for good structural reasons, and it would make sound sense to secure it and protect it until pro help could be reached.
Awrabrest.
/forums/images/graemlins/wink.gif
bilbobaggins
N/A
[ QUOTE ]
"TCM's thoughts are here........."
[/ QUOTE ]
Och! Aye! Well, now! That's different.
And if he's not going to get exercised about it, sure as buggery I'm not. Let us know in the morning...... /forums/images/graemlins/wink.gif
"TCM's thoughts are here........."
[/ QUOTE ]
Och! Aye! Well, now! That's different.
And if he's not going to get exercised about it, sure as buggery I'm not. Let us know in the morning...... /forums/images/graemlins/wink.gif
