THICKNESS OF RIGGING
30/01/2019 at 12:26 pm #17324
I just finished with the construction of the wooden mast for my GP. I want to order the rigging.
I am wondering if there is a rule for the thickness of the rigging according the class.
Also if it is better regarding the forestay to use a turnbuckle, or to be stable or with rope.
30/01/2019 at 2:27 pm #17325
I think the most useful thing would be to start from the “Rope and Wire Lengths & Thicknesses” quoted in the Members’ area, page “All about your GP14“. I can’t comment on the forestay for a vintage boat, but the purpose (as I understand it) of the forestay is a) to support the mast when the sails are not rigged, and b) to keep the mast up if the sail are rigged but the jib/genoa tension were to suddenly release/break. On my boat (MkII, approx 20 years old) I have a wire forestay but secured to the bowplate via 6” rope (and also a separate bungee to take out the slack, but allow mast to move back in the slot – helps when rigging the shrouds). I am sure Oliver can advise more accurately and with relevance to much older boats.
30/01/2019 at 8:17 pm #17326
On a wooden mast you will never get anywhere near break loads but 3mm 1×19 stainless wire would be usual. Roll swaged terminals are unnecessary on a wooden spar, standard copper ferrules/thimbles (Hard eye) will be perfectly adequate.
I’d use the same on your forestay as it will be supporting the jib. Again, you’ll be nowhere near the breakloads.
31/01/2019 at 11:30 am #17327
A handful of other points.
The lengths data document that Chris Hearn mentions is appropriate for the usual situation, i.e. the modern boat. However the height of the chainplates was not standard on the early boats, and as originally designed they extended up through the deck, and the shrouds were attached above deck (like most dinghies of the time). Although the more modern system (chainplates below decks and the shrouds passing through the deck) was approved in (I think) 1952, boats continued to be built to the original design for many years afterwards. And even for the modernised version I doubt whether the detail was specified; certainly I have known – in both cases on professionally built boats by top builders of the day – shrouds pass through the centre of the deck bushes (outside a pin placed above the chainplates and rigging screws) and shrouds bearing on the sides of the deck bushes and going direct to the adjusters (on the chainplates).
Also, on wooden masts it is not unknown for there to be some slight variation in the design of the tangs of the hounds band – and even slight variation in the height at which it is screwed onto the mast. And where shackles are used to attach shrouds (at either end) the length of shackles varies.
All these factors of course affect the length of the shrouds, so measure up your old ones; alternatively, if more convenient, step the mast on your boat and then measure directly.
Forestay. The original arrangement, almost universally used on early boats, was for both shrouds and forestay to be secured (and adjusted) by rigging screws. Halliard tensions were very much lower, and the forestay was responsible for supporting both the mast and the jib luff, which was hanked on to the forestay. With greater halliard tensions the situation changed, gradually, to the point where in modern boats the rig tension is determined by the genoa halliard and the forestay is often much smaller diameter than the shrouds, and is secured by only a lanyard (albeit with several turns). The forestay then serves only to support the mast when the genoa is not set, and it normally goes slack (unless tensioned by a bungee cord) once the genoa is set and full rig tension applied.
A half-way house in terms of rig tension arises in boats with Highfield levers to tension the halliard.
So the question of whether to use a rigging screw or a lanyard for your forestay, and what diameter the wire needs to be, depends very much on how much rig tension you intend to apply, and that in turn interfaces with what hardware you have for tensioning the halliard.
Finally, don’t be tempted to apply full modern rig tensions on an old boat unless she has had the approved mast step conversion done; the original design never envisaged the tensions that would be developed many decades later, and there is a real risk of structural damage.
01/02/2019 at 12:35 pm #17336
As always Oliver you are a fantastic mine of useful information on early boats!
01/02/2019 at 12:53 pm #17339
Thank you all for your suggestions .
I agree totaly with you, Oliver , that i have to step the mast on the boat to take measurements.
Regarding tension I will discuss it with my sailmaker to design the sails according the “true” bent rake & forestay tension.
Regarding thickness I believe that 3mm 19×1 stainless wire is strong enough !
These are my thoughts after your comments.I hope you agree.
Thank you all again!
01/02/2019 at 1:21 pm #17341
If you can hang fire for another few days I will aim to measure the thickness of some of my shrouds as soon as convenient.
01/02/2019 at 1:34 pm #17342
I planed to make the measurements next weekend ( 09-10 / 02) .
I you are ok , it will be great. !
Thank you again.
By the way, I also just finished all the restoration of the boat (hull no 4288) !
She is like new !!!!!
02/02/2019 at 10:35 am #17351
I have recently been reading about the rescue of the erstwhile America’s Cup challenger Sceptre from a stripped down and gutted state (a conversion interrupted by the death of her owner, resulting in an executor sale “as is”) into a prestigious cruising and racing yacht. Bear with me; this is relevant!
One detail which I picked up was Uffa Fox’s dictum for the strength of shrouds; the total breaking strength on each side of the boat should equal the displacement. For a dinghy, of course, displacement is in sailing trim, so it includes crew weight.
Uffa knew a thing or two about boat design; he and Jack Holt were immediate contemporaries, and were the two leading dinghy designers of their period, joined a little later in a claim to that accolade by Ian Proctor.
This standard seems intuitively sensible, but as a physicist I wanted to validate it. And I suspect that Uffa may have been wrongly quoted; I personally would prefer a safe working load of that much, rather than just the breaking strength. However it seemed to be backed up by a mental “back of the envelope” very rough calculation in a semi-drowsy state while I was in bed but not sleeping in the wee small hours of this morning. I estimated the maximum forseeable wind loading in the windward shroud as perhaps 265 kgf, for the situation where two 80 kg sailors are driving the boat to windward and sitting out seriously hard. Given the mass for the bare boat of perhaps 150 kg, the total displacement would be 310 kg, so Uffa’s dictum seems sensible, with just a modest safety margin.
That estimate does rely on some fairly approximate estimates of the lateral distances involved in the calculation, so it is no more than a very rough guide.
Allowing a factor of 3 to convert from breaking strength to safe working load, and using my mental “back of the envelope” figure, we are looking for a safe working load of 265 kgf, so a breaking strength of 790 kgf. Note that this is for the wind loading when driving the boat seriously hard.
Looking up strengths of rigging wire online http://www.riggingandsails.com/rigging-breaking-strengths.shtml#top I find that 3 mm of 316-type 1 x 19 wire has a breaking strength of 720 kgf, which would indeed seem to be almost sufficient.
However this takes no account of static loading; and in Uffa’s day the static load was far less extreme than in the modern GP14 rig. But since you are dealing with an early boat and a wooden mast, you won’t be using those extreme static loads either.
So it does look as though you are probably right in thinking that 3 mm is JUST strong enough – provided you don’t push her too hard in the heaviest weather.
Hope this is helpful.
15/02/2019 at 4:12 pm #17410
With apologies for the delay, I have just measured up two sets of shrouds. One set is 3 mm diameter, and the other is 2.5 mm. I have to say that – now that I have measured it, and done the above calculations – I think the 2.5 mm pair are a bit light, but I am reasonably confident that 3 mm should be fine in everything except the most strenuous conditions.
Hope this helps.
You must be logged in to reply to this topic.