I can't remember the exact distance but there are a few factors to bare in mind. The distance should be measured from the forestay. The further aft the shrouds the more forestay tension you will achieve for the same rig load. The downside is that the boom won't go out as far and your crew also has to exit the boat on the trapeze further aft. That said, I rarely need the boom to go right out and the difference in reduced projected sail are is pretty small. Also we tend to trapeze further aft these days anyway so the crew issue has largely gone away.
We use a 11/2" by 2" high ubolt, we also have a 6" 3/42 dia length of Alloy rod under the gunnel which the u bolt is bolted through to spread the load.
The original location of the shroud points is in line with the front bulkhead of the side tanks. My guess without measuring my boat is my shroud points are 50 or 60mm aft of this position.
The original location of the shroud points is in line with the front bulkhead of the side tanks. My guess without measuring my boat is my shroud points are 50 or 60mm aft of this position.
Also bear in mind, that in most older Javelins, the ends of the side tanks are not exactly in the same position. Measuring from the forestay is the only way to get left and right in the same place. It's still a boat...... Even moulded GRP depends on how well the original model was made, so don't expect CNC-precision. Not even from the CNC
It's fairly common for the centre of the u-bolt to be 50mm aft of the bulkhead.Arent's advice is worth following,as is Mike's mention of a length of tube in the recess.The tube needs to be fairly thick walled to resist the compression load of the nuts.The tensile strength should not be an issue as the shrouds are much thinner than the diameter of the u-bolt.Am I correct in understanding that the boat in question is number 511?
This may be a little technical but I did quite a lot of analysis on rig loads on my previous Javelin GBR543 and is aimed towards those with a technical intrest rather than fora renovation. The issue was that I seemed to bend the boat when trying to apply the same sort of rig tension the top guys were using as 543 was very lightly built when new, unlike most of the earlier boats.
The aim in general is to get between 90 to 120 kg of tension on the forestay. The range varies due to conditions and also to some extent your style of sailing and your sail cut. I wanted to work out what the loads were and where they went.
For a starting point I've used the following settings. Shroud & forestay height, (deck to hounds) = 4.26m Distance from Forestay/deck point to the mast = 1.5m Shroud U-bolt position = 400mm aft of mast (bolted through the gunnel).
Given the above, to get 100kg on the forestay you would need to apply 178kg to each shroud.
For comparison if you moved the u-bolts aft by 50mm (2") and keep the other settings as they are, you would then need 158kg on each shroud to get 100kg on the forestay, some 20kg less.
If however your u-bolts are only 350mm aft of the mast, the shrouds would need to be tensioned to 203kg to get 100kg on the forestay.
The other important factor to keep in mind is the associated loads on other parts of the boat. For example the 400mm position with 178kg on the shrouds, you are compressing the mast into the mast foot at 449kg. At 350mm and 203kg the compression increases to 499kg. At 450mm and 158kg the compression drops to 408kg.
I've created an image below to show some a simple rundown of the loads but for clarity I've left out many of the vertical and diagonal loads.
However keep in mind that these are all static loads. Bouncing off a wave in a F4 will multiply some of these loads by over 4 times.
Many thanks for this. I think I have downloaded your excel spreadsheet that details the various tensions/
compressions that a space frame would be required to cope with. It has proved invaluable as I was able to send it to Dave Chisholm at Carbonology who was able to let me know exactly the diameters of the carbon that I would need and his thoughts around tensile strips. Also very helpful in terms of how to put the components together. By the way he reckons that with adhesives etc the whole thing can be done for under a kilo. Also given the dimensions of the tube he reckons that the frame should cope with loads that are many multiples of those that you calculated.
Given that I will be putting in your space frame (I hope there isn't a patent anywhere), the shroud position really needs to be just forward of the side tanks - I don't fancy cutting holes and fixing the shroud struts blind. If I successfully manage to get it all in place then god (and epoxy) willing the space frame should be able to cope with the various loads even if the shrouds are not taken aft of the side tank line.
The only thing that wasn't clear from your chart was B6 the bar at the bow, which I assume should also carry the forestay fitting or furler. I have yet to disappear up to the bow of the javelin to see what's there - it hits the garage on Sunday for the start of the refurb - but I think that to make the space frame work I will be looking at bonding a piece of 3.5 cm diameter carbon tube at the appropriate position. The other issue I still need to address is to make sure that the mast strut on the foredeck is also bonded to the space frame as well without damaging the frame's integrity.
I will let you know how I get on and give you a breakdown of what it all costs and how/if I manage to get it all done.
Andrew
PS Is it against the class rules to support the centreboard case with a couple of carbon tubes to give it added lateral stiffness. These should be relatively easy to fit but if they are illegal I don't want to have to pull them all out.
It's fairly common for the centre of the u-bolt to be 50mm aft of the bulkhead.Arent's advice is worth following,as is Mike's mention of a length of tube in the recess.The tube needs to be fairly thick walled to resist the compression load of the nuts.The tensile strength should not be an issue as the shrouds are much thinner than the diameter of the u-bolt.Am I correct in understanding that the boat in question is number 511?
Hi
Thanks for this. The boat is 511. Pretty sound but in need of a whole heap of TLC.
The B6 bar is a little contentious. This bar replaces the normal spinnaker chute mouth. Basically the chute mouth under the deck was removed and the B6 tube was bonded into the hull sides and foredeck to provide a good: 1, anchorage for the spaceframe 2, radiused entry for the spinnaker 3, solid take off for the base of the forestay
The rules don't explicitly disallow the removal of the Chute and a small number of boats have done this but some do view this as pushing the rules. However I don't think anybody really minds too much.
If I was to fit another spaceframe I would replace the tension wires with solid tubes. Whilst the wires work to some extent they were difficult to tension and keep tensioned and in the case of S2 & S3, solid tubes would also help stiffen sideways loads from great hulking brutes and brutesess who dangle on trapeze wires. Also the tension wire "S1" was difficult to achieve as it has to enter the forward bow tank at an angle and then split as inside the front tank there is a fore/aft bulkhead running from the rear face of the bow tank to the bow. S2 and S3 have to enter the Rear vertical face of the front tank as well and these three holes through the tank are a real pain to seal as the hull flexes at a slightly different rate to the frame.
I've not really got a better solution but I think a solid tube would be easier to seal than the wires and stainless brackets I used.
The front face of the side tank bulkheads can become porous over time and these might need a layer of resin and can be easily accessed from under the foredeck, especially if the c/board is removed. This would also allow you to glass in the B2 and B3 tubes if you did decide to go through the tank. The position of the side tank hatch holes on my boats also allows me to reach the bulkhead/hull joint so it is possible to bring the frame aft.
The Class rules stipulate that the Hull/deck should be mainly made from GRP. Essentially the rule is there to keep costs down and stop an arms race and end up with cheque book sailing that afflicts so many other Classes. So in short yes you can add your carbon tubes for the frame and case support. Image: http://www.javelinuk.com/e107_files/public/1245237900_1_FT527_spaceframe_.gif
If you wish to add carbon tubes,or other parts to the permitted structure,no problem.The rule limits the builder to GRP for the components defined as constituting a Javelin.What you choose to add to this structure is your choice,but you must not remove any structural elements from the original mouldings.Obviously holes for fastenings are OK but grinding away a few kilos of structure is not.The simplified spaceframe may not accomplish all that you might wish for as there are some unconstrained triangles.
Interesting comments on the wire. Carbonology were against wire as they sadi it would be very difficult to keep under tension. Dave Chisholm's solution was either vectran or he would make ups some unidirectional carbon tensile strips. I will revert to him with the idea of tube and see what he thinks. Probably better as it will be able to take the various knocks etc. that are likely to come its way.
In terms of sealing the various holes that the structure requires in the various tanks, what did you use that worked the best. A rubber seal allied with sikaflex would on first blush provide a good flexible and hopefully watertight seal. Any thoughts?
Thanks for the warning re the front strut B6. As I say I have yet to investigate the bow area but it would seem that you have already come up with a solution.
This is the first time that I have taken on a project of this nature. The intention at the end of the day is to have some fun completing it and then hopefully getting a reasonably competitive boat out of the exercise. Only time will tell if I am successful with either goal.
Many thanks for all the very useful comments. They really are much appreciated.
Hi I have now stripped the boat of all the fittings. A couple of quick questions - (1) the thwart and top of the cetreboard casing looks like it is held together with numerous pop rivets. On 511 they look old and tired. Should i replace and if so is Monel preferable to a good old fashioned ally rivet - I assume a washer behind would make sense. Failing a rivet any other suggestions? (2) In order to sort out the furler is it OK to cut a larger opening so that I can sort out all the gubbins?
If the rivets have not actually failed,why change them?If ,or when, they let go then replacing them with new ally rivets and washers would make sense;monel will be more durable and cost more.It is also likely to impose higher loads when you snap the stem.The other significant thing is to use a bonding paste as this causes less stress raising than rivets alone.You can use thickened epoxy or a polyester paste or an acrylic bonding paste,according to your access to suitable material.
Andy, all you need to sort out the furler gubbins is a small volunteer. My nine year old daughter can remove the furler and re-install it, by crawling inside the bow, in a couple of minutes. It's just a pin and a ring. You would probably need to cut quite a large hole. I'd bribe someone.
Carbon space frame is in as per your plan. I did have to cut some holes in the front of the 2 side tanks and also in the bow tank to accomodate the S1, 2 and 3 tubes. WIll now have to think about sealing everything.
I have one question for you though. The SI tube is nearly 2m long and although very strong under compression/ tension it will obviously flex across its length. In order to prevent this I thought of linking it to the B1 tube with a small vertical piece of carbon tube to give it lateral rigidity. Howver i am no expert when it comes to the various tensions/strains the various tubes will be under. Thus thought I would ask you to see what you thought.
I also have a couple of other questions. Do all the boats carry genoa furling gear and do they use it. It would be easier to just attach a strop to B6. Also I have put a plate on top of the tank that links the mast to the B5 strut - thought it might spread some of the loads. Would it be ok to glue the mast step to this as opposed to bolting it as there isn't much room in the tank as the B4 struts fill the area where you would put the rear most bolts.
