Replace centerboard control line

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Re: Replace centerboard control line

Postby Castaway » Mon Apr 09, 2018 5:44 pm

Andy,

Thanks very much for the description and links; I was quite mistaken about the fisherman's bend, of course; I tend to tie whichever of my restricted repertoire seems suitable, but don't really keep a visual memory of their construction. The links are very helpful, though the author is rather dismissive about the holding power of the Halyard Knot in Dyneema or the like, and, as you say, twice through the becket would be impractical. Perhaps the end could be made off to another shackle or the board itself, though that would probably intersect with the body of the block in use?

Your revised drawing look pretty much as I remember mine, though it is now two years since last I looked. The picture I posted was of a previous pennant, from 2010, when I had to replace it in Helsinki after a block failed – not one I had chosen, and nominally well within its load capacity; but it also had had the pennant rove in a sub-optimal manner, and the twisting contributed to the failure. The new, substantial block was eye-wateringly expensive – €75 – and the Swedish yard had lost the Tufnol blocks I put in the previous year.

Erik – that's why I think it is so important to get the route of the pennant just right, and why it's worth taking a little time to check that nothing is twisted or chafes.

Regards,

Gerald
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Re: Replace centerboard control line

Postby andygc » Tue Apr 10, 2018 6:22 am

€75? That's a bargain! The Selden PBB80 double with becket is around £125 now. And others are much more. Fortunately this sort of gear does not need or want ball-bearing blocks.

The halyard knot works well for Dyneema - unless the end is cut very short - it's not easy to tie the knot tight, so there is always some slip initially - even with conventional braid on braid. That's the reason for the comment - if the end is very short it can slip through as the knot beds in. There are plenty of people who use it with Dyneema halyards, and they are loaded much more than the purchase on a half ton keel which is partly supported on a pin.

I'm interested that you are thinking of going to thinner line. Do you think it will make moving the plate easier? The original bronze sheaves on the fixed blocks are certainly sized for 12mm line, and that size of line works well on a self-tailing winch. (I don't think the 28/30 ever had a wire pennant for the keel - there's only one winch)

Cheers,
Andy
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Re: Replace centerboard control line

Postby Castaway » Tue Apr 10, 2018 10:52 am

Andy,

The smaller line seemed to require less effort, and still held in both the clutch and the self tailer, without slipping. However, the thicker rope has only had about 6 weeks under tension so far, and it may get easier once it has properly bedded in. Of course, it may be that I am just getting weaker! The value of Dyneema would just be the extra margin of working strength, though I am not sure if it is more susceptible to chafe than braid-on-braid polyester.

I upgraded the diameter from that used by the original owner when I replaced the line for the fourth time, as the first signs of wear appeared. I lie awake at night (at sea) listening to the board moving as we crest the waves, and waiting for it to fall off; just a pessimist at heart. It has done around 90,000 nm so far, much of that across the Atlantic, so it probably won't come off on a Baltic cruise.

The magnum opus of my declining years will be a short treatise on the F35 centreboard, and the several problems that arise should you choose not to leave well alone.

Gerald
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Re: Replace centerboard control line

Postby Erik » Wed Apr 11, 2018 6:25 pm

Thanks to you guys for the info. I just got a look at my centerboard through the top of the trunk. The arrangement I have is one single block attached to the board with two single sheaves located on the front wall of the trunk.

To increase the mechanical advantage I could attache a fiddle block to the board .
Then, attaching the line to the upper trunk support run it through the fiddle block and two stationary sheaves.

What do you think.

Again thanks
Erik
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Re: Replace centerboard control line

Postby Castaway » Thu Apr 12, 2018 1:35 pm

Hi Erik,

Anything that increases the mechanical advantage will be good news, so long as it can run freely. I'm surprised it can be lifted with the current arrangement, unless you have a very powerful winch. A double block (sheaves side by side) might be better than a fiddle block, if there is little room when the board is raised. My long bowline actually prevented the complete raising of the board because it touched the fixed sheaves on the casing. The disadvantage of increasing the power of the tackle would be more rope in the cockpit, and longer to wind it in.

If you have the original design of F33, with the American cabin layout, where does the lifting pennant go when it leaves the Centreboard casing? The UK one takes it up above the coachroof, through another sheave, then aft to a central winch, which would be where the companionway is on the US model. Have you a diagram of the layout?

Regards,

Gerald
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Re: Replace centerboard control line

Postby Erik » Thu Apr 12, 2018 3:59 pm

Thanks to you guys for the info. I just got a look at my centerboard through the top of the trunk. The arrangement I have is one single block attached to the board with two single sheaves located on the front wall of the trunk.

To increase the mechanical advantage I could attache a fiddle block to the board .
Then, attaching the line to the upper trunk support run it through the fiddle block and two stationary sheaves.

What do you think.

Again thanks
Erik
33 Freedom Cat Ketch # 74
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Re: Replace centerboard control line

Postby Erik » Thu Apr 12, 2018 5:05 pm

Gerald

The pennant goes up to the cabin top and then back to the winch which is a Harken 56. I definitely need to use the slow speed to lift the board. Will use a double block with 12mm line instead of the fiddle block. Will take some measurements this weekend and do a better inspection, am currently replacing the leaky fuel tank what a PITA.

Sorry about the double post still getting familiar with the website.

Thanks again
Erik
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Re: Replace centerboard control line

Postby Castaway » Fri Apr 13, 2018 5:59 am

Hi Erik,

Just looking through some of the stuff I have about centreboards; there's a really good post by Phil Downey (search this board under phildowney ) dated Sept 13 2009 about replacing blocks on the c/b. She built a double block with becket from two stainless Harken blocks bolted together,then used 10mm Dyneema for the pennant, and also replaced one of the fixed blocks, which was binding. She can manage to raise it 'one handed' (must be very fit). I think her suggestions are perhaps the best solution, though neither cheap nor simple; she is obviously very experienced in boatbuilding, as well as sailmaking.

From your description, there is no essential difference between the US and UK versions of the c/b layout, though the access may be a little different, since the aft end of the case is more accessible on the UK boats.

I think I'll take some Dyneema out to the boat when we go to Sweden in June; it might make life easier.

Regards,

Gerald
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Re: Replace centerboard control line

Postby andygc » Sat Apr 14, 2018 6:25 am

This is the thread http://www.freedomyachts.org/viewtopic.php?f=64&t=10028 and this is the post
phildowney wrote:hi when i bought kusi the surveyor recommended i checked the lifting gear since he couldn't check it as part of the survey.

when i did i found that the double and becket block on the centreboard was badly distorted made of tuffnol and very worn so i researched a replacement. since it had failed under load

i couldn't go like for like so i worked out the strength of the loop on the centreboard (10 mm stainless 3 - 5 tons )and the sheaves at the other side, on the front of the case(3*1.6 tons.) and of course the line(10mm dyneema 5 parts= 15 tons). this revealed that the weak link was the block , by quite a margin. i soon found nobody sells double and becket blocks of about 4 ton break load in the uk i could pay an arm and a leg for a shaeffer block a ali black magic harken 75 mm with balls and rollers or..

two harken stainless 75 esp blocks bolted together ( 3.1 ton each thus combined breaking load 6.2 tons) as that was the only way i could get a plain bearing block , double and becket in stainless not aluminium, strong enough.
it felt like sacrilege putting two fine hand polished and expensive blocks in the case but the only other way to get the strength was aluminium roller and ball blocks. below the waterline and hard to get at is no place for ali racing blocks. i might check them but future owners wouldn't know what they were.

while i was at it i found the turning block at deck level was pulling through the deck and the sheave didnt turn. i filled the deck with epoxy filler mix and put a big thin ss backplate under the deck and changed the sheave for a harken hi load sheave a 727 i think which was an exact replacement and it has side load balls and a teflon bush. the fairlead on deck i replaced with a harken roller block (part 1963 ) i picked up cheap

thus you can raise the board with one hand, before it took two.
also i can sleep better knowing it is unlikely to break.

it pays to reduce friction as much as possible im only 55 kilos 9.5 stone so i love harken blocks!!
and i have made kusi easier to sail purely by making things run freely.

powered winches sound good too , but better sheaves will make it more likely to work and save battery power

phil kusi f 35 ck
The discussion of the strength of components needed seems to have some logical errors. The weakest parts in the list are each of the three upright blocks at 1.6 tons (although where does that figure come from?). They don't have a combined breaking load of 4.8 tons. Similarly two blocks side by side do not have double the breaking load - they each have the breaking load they had before they were bolted together, so 3.1 tons, not 6.2. The breaking load of 10mm dyneema is 4,650kg, so its swl is 2,300kg. A breaking load of 1.6 tons gives a swl of 0.8 tons, and 3.1 tons a swl of 1.55 tons.

Essentially, every component in the lifting tackle phildowney describes is specified so that it will never approach its swl.

The Selden plain bearing block I'm using has a swl of 2,000kg and a breaking load of 4,000kg - a huge safety margin in a 5-part tackle on a keel that weighs just under half a ton, and which is also supported by a pivot pin. 12mm polyester braid on braid has a breaking load of 2,900kg (swl 1,450kg), so the swl of the block is about 500kg greater than the swl of the lifting pennant. I chose the block on the basis that it needed to be a plain block (no balls or rollers), I was going to replace 12mm rope with 12mm rope (the pennant fitted was too short!), and that the sheaves were a similar size to the bronze sheaves on the upright blocks fitted at the front of the trunk.

I can raise my board with one hand using my Lewmar 40ST in low gear, but I wouldn't do so by choice!
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Re: Replace centerboard control line

Postby andygc » Tue Apr 17, 2018 12:06 am

I thought again about this and just realised what phildowney meant by adding breaking loads together. Two blocks together can accept a total load of twice the load of one block alone. Sorry about that. :oops: However,it's still true that the safe working load of all parts of the tackle is way above the actual working load. Thinking about this, I think I could have safely replaced my old 12mm braid on braid polyester with 10mm.
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