Removing the bridle foil in order to make a F16/Tornado style bridle set up on my slightly widened 5.5(9.7ft). Will use a self tacking jib with its tack at the spin pole/bridle strut junction and a F18 spinnaker.
Trying the wire angle from the 5.5 foil, the bridle/forestay crossing point is rather high at 3.5ft.
Wonder if the 5.5 hulls can take the beating from a lower bridle crossing point and if so, how low can you go?
See pics from some trial-and-error in my workshop, where the simulated forestay should lean a little more to the rear.
If we look at the plane formed by the bridles and a= the angle between one bridle and the horizontal, here is what happens.
Bridle tension may be decomposed in two forces
a) one in line with the forestay, = T/2 = half the tension of the forestay
b) one horizontal, from hull to hull = F
F = T / (2 • tan a)
If a=45, F= 0.5•T (half the forestay tension)
If a=30, F= 0.87•T
If a=15, F= 1.87•T
With the bridle foil on you apply the same formula (the a angle is bigger compared to no foil and same jib base height). So you can measure the angle with foil and without foil at the desired position and calculate the factor by which the tension will be increased.
Nice to hear from you again. All you say is correct, I also made calculations, but looked at tension in line with the bridle wire BT, where forestay tension is FT.
This is the ballpark figures I get with 2500mm hull distance at the chainplates:
a=41 BT=FT*.75 for each wire H=1071mm, equivalent to 5.5 foil
a=34 BT=FT*.9 for each wire, H=838mm, Tornado
a=30 BT=FT*1 for each wire, H=722mm
If we look at the Tornado rules, triangle height in line with the forestay is min. 838mm and I guess width between hulls is about the same as my widened 5.5, 2500mm.
Think I´ll go for the 838mm triangle height of Tornado. This will give a BT increase of ca 1.2x. Sailing only inlake with flat water and never over 18-20knots, makes me think this will work.
Still the question is how strong the hulls are in the bows and how well the chainplates are anchored inside the hulls? I rely on the Nacra boys built them with some kind of safety factor.
Static tension is easy to measure, wonder how much forestay tension is with full sheeting upwind?
Could you please double check my calculated 1.2x load increase when going down to Tornado bridle height?
It seems right:
From a= 41 to a=34 BT increase = 17%
I would look more at the horizontal component where the hulls are weaker. Mine have some scary wrinkles near the front beam showing that lateral effect..
Increase on the horizontal component = 29%
But more importantly, the biggest impact might not come from the bridle modification but from adding a spi. My gut feeling (I'm just an electronic engineer...) is that the combination of both things mean a real risk of breaking those hulls.
As I see here, F20s have the bridles/forestay connection higher than the spin pole but the jib start from the jib pole. That might be safer. https://www.youtube.com/watch?v=rZqPSBJcJvc
The F20 system is actually what I am aiming for, this is also how Tornado and Nacra 17 did it. My concern was, that the bridle height when going by the 5.5 angles where very high and I wanted to have a shorter compression strut for the jib to be Velcroed around.
Tornado bridle heigth is ca 800mm, but jib is at ca 300mm.
Will anyway wait with shortening the bridle until the rest of the rig is up and the Infusion jib is hoisted. Either way I will get rid of the bulky bridle foil.
Hey Lars,
It was merely a tongue in cheek comment on the beauty of your shop. Most, including myselt, would love to have such a clean organized workspace. Perhaps the intended meaning gets lost?
Sheesh, lighten up a bit...we’re all on your side.
-- Hobie 18 Magnum
Dart 15
Mystere 6.0XL Sold Was a handful solo
Nacra 5.7
Nacra 5.0
Bombardier Invitation (Now officially DEAD)
Various other Dock cluttering WaterCrap --
Hi Edchris177,
Really sorry for my "Grumpy Old Man" over reacting comment. Must admit I actually thought you where bullying me, as I consider the workshop being a total mess at the moment, when I´m trying to force things, to get this cat out on the water. Outside Frankencat2 is waiting to come in and inside Frankenboat4 is waiting for rebuild.
Apology accepted. I enjoy following your projects. In much the same vein as Carolina Catamarans idea a while ago of trying to re-invent Beachwheels, one has to admire a guy who gets n idea, & tries to make it work.
I'm jealous, most times a cow couldn't find her calf in my workshop.
-- Hobie 18 Magnum
Dart 15
Mystere 6.0XL Sold Was a handful solo
Nacra 5.7
Nacra 5.0
Bombardier Invitation (Now officially DEAD)
Various other Dock cluttering WaterCrap --
Did some more thinking about how to lower the bridle. By adding a third bridle part to the rear, with a pelican striker passing the striker rod going to the rear beam this should be possible. With a 460mm high bridle angle=20 and BT goes from 1,5 to 2,9, ie if we still want each bridle wire take FT/2=0.75 there will a substantial tension to the pelican striker that has a=22. As the spi pole acts as a compression rod, more tension can be added to the pelican striker than the bridle wires.
Hi Lars, that’s very creative. And your boat looks very nice, congratulations!
I think it would be hard to adjust load sharing between the central wire and the bridles. If the central wire is lose it just doesn’t get any load. Maybe with a turnbuckle.
Wait a minute, I forgot you have the real bridles further up. Those are the ones taking the main part of the load. If you tension the central wire it may contribute but is it really worth it?
Yep, the bridle is at 107cm now, but it will be lowered to 46cm. A turnbuckle was my idea too, to get the tension right in the central wire. As I have a Spinlock Rig-Sense, keeping track of the wire tension can be done quite easily.
The reason for the lowered bridle is, that I want it below the jib tack. To much work to make the jib luff pocket pass the highly placed bridle as I first planned.
If my guesstimate of 1,3x is correct, this means there will be a compression of the spin pole of 0,3x, so no problem for the 40x2mm alu pole.
Have ordered 5mm Hampidjan Dynice Dux(spliced breaking load 4300kg!) for the new forestay. Will also use it for the pelican striker.
Good luck then!
Just in case: Years ago I had a Laser II, it’s jib had a wire inside the luff (permanently) and wasn’t rigged through the forestay. That would work I think, with a good halyard
The real tension is only applied when sailing though. I think there might
Be some flexibility on the central wire/pole that may make it hard to hold enough tension compared to the bridles. Just my impression, no maths..
At last I found some info about Nacra bridles, they use the same angle as Tornado. Calculated 5.0 and 5.8(not NA) and they both are 34 degrees.
My previous figures for Nacra 5.5 where way off. With Frankencat bridle height of should be min. 85cm, not 107cm.
If you go down to 46cm, load is 2.9 which means the rear going will have to take 1.1 of the forestay tension.
If you go down to 33cm, load is 3.9 which means the rear going will have to take 2.1 of the forestay tension.
A static tension of 100kg would be no problem.
Still wonder how many times higher the dynamic load upwind will be?
If we tie the rear going wire(5mm Dux) to the center of the rear beam we will get severe deflection with a dynamic load of maybe 4-500kg.
If we instead split the wire in two, to another bridle behind the striker rod, with each end tied to the inside beam ends(tube walls 5mm at the ends), things look a little better. It should be made of a single loop turning back at the rear end of the turnbuckle.
The striker rod is made up of M16 threaded stainless rod, with a 20x2mm alu tube covering it. It is used to prebend the main beam with a 40x3mm stainless striker band. As the rear wires only will go through an eye at the bottom no for/aft forces will be present.
Cleaned up the GC32 to show my idea. You will have to imagine the forestay bridle and bowsprit is lifted 34cm. The rear bridle tension is a guesstimate.
Using the same 5mm Hampidjan Dynice Dux as in the forestay, makes this a light construction, although the rope is $7.50/m in Sweden.
Made a spreadsheet for the COS/SIN part to find out the forces and tensions involved. Formulas below.
Have done something wrong and then gone blind as neither signs(T3 should be negative) nor values add upp. A little ashamed as this is basic math/physics. Do you mind taking a look?
Sorry, found some errors and measured some more on the boat. Looks as we will have ca 3x compression force to the rear at the front beam and ca 2.7x at the pelican striker. Looks reasonable to the earlier, no math, ballpark figure of 2.2.
1kN fore stay tension; Compression force and pelican striker tension are close to the same at 2.5kN each when keeping the intitial a=34 degree bridle tension. Used 35cm bridle height and 30cm from c/c beam to end striker rod.
Still, question is what happens to the main beam with the spin pole force of 2.5kN pressing against the front side.
I store my righting pole under the tramp, after a while the tramp started to show signs of wear, I suggest to add some protection, like a cordura or vynil patch. Maybe add some color above, around the area, to identify it easily, stepping on the cable or rod can be painful, particularly when stepping the mast.
Hi Andinista,
Thanks for the advice. My idea was a max 20cm strut for the DS. Will use 5.5mm(when stretched) Dyneema for the strap, thinner might hurt more. Hopefully the DS not in the way, but beware of traps on the trampoline ! Will publish an image when it is done, but will first test deflection without the rearward DS.
it appears that you are experts on Nacra 5.5 SL. Just purchased one and mast came down in a storm, cat moored in 5 feet of water so my initial reaction was to save mast as quickly as i could and dove in and disconnected all ropes etc without paying attention to detail, mast and stays ok But i have no idea where everything is supposed to go once mast is raised! all u tube and other instructions on catsailor & this site not specific as to 5.5 show 5.2 which i previously owned and know the details, only sailed this 1994 Nacra 5.5 once- for example there is a stainless square ish apparatus that attaches to mast and pulleys at base of mast dont remember correct configuration! are photos available? WOULD APPRECIATE ANY HELP, PS FOIL FOR JIB OK BUT PROPER REATTACHMENT ON THAT WOULD BE APPRECIATED AS WELL
On a boomless cat you want to induce rotation more often than to limit it, don’t you?
That’s my case at least, I even miss sometimes the wide rotator that allows over rotating. But for me the real deal is to be able to firmly lock the mast centered when not sailing, especially after a shroud failed at night on a buoy. The configuration pointing fowrward works perfectly, the other one doesn’t seem so. Still much better than not locking at all, though.
Posted: May 22, 2018 - 04:44 AM
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! Will publish an image when it is done, but will first test deflection without the rearward DS.
