skiboat u-bolt "chainplate"

[Redbeard33]

This is intended for JC-II but please chime in if you have experience with this. John on your fathers design for the 33 the backstay splits and attaches to the hull flange with the u-bolts you described in the other thread, I like this arrangement but I have removed my toerail in favor of a wooden bulwark to be built at a later date, the hull/deck was joined with fiberglass tape and epoxy, I plan on keeping the same backstay arrangement however without the toerail it will need some backing plates. is the 1/4 u bolt you recommended for the 30 alright? is there a certain type of unbolt (hi test) that is needed? and what sort of backing plate is needed? thanks in advance

 

[DianaOfBurlington]

U-bolts for backstay

I found some u-bolts at West Marine that are 'technically' 5/16" (part no. 599217). This is adequate for an H25. Investigate the load ratings. I usually figure on half the rated number for the actual load. I have not done volumes of research on this but have been going by basic numbers for the components used. The halyard mast-base blocks are Schaefer 5-series; that's rated at 1500 lbs. Therefore all my running rigging (but for the outhaul, topping lift and pole lift) is rated at that or more. The u-bolts I got are rated at 2500 lbs each. With six of them as shroud chainplates doing half their rated work I figure I am all right. 'Where's it going to go?' my dad would say.

The toerail adds an immense degree of stiffness and strength as it is spar-grade 6061 aluminum and being in a T shape doesn't want to bend when the shroud u-bolts are bolted through it. The whole length of it assists in keeping them down-- imagine what of the toerail you would have to bend to move them. Every bolt through it helps.

I used the same u-bolts for the backstay. I was wary of this because it is at the very tail end of the toerail (I mean the very last hole and one I drilled beside it). It's easier to bend the rail from one end than from the middle. I used a piece of (scrap) 304 stainless angle, about 1-1/4" on a side, about 1/8" or more thick, as backing plates. The piece I found was about 16" long and I cut it in half. This got bedded under the flange with 5200. I pulled three bolts, total-- the last one was for the back leg of the u-bolt, I drilled one more for the u-bolt about 1-1/8" forward, and then I drilled for the next two holes (at 5-1/2" centers) as well. The vertical side of the angle is actually outboard because it made it easier to see and get access to the locknuts back there, but I really think I should have turned it around. I have not yet investigated if I can use the port one for starboard and vice versa to facilitate this.

I spun the hex nuts up to the top of the threads and set the u-bolts in upon SS fender washers. Under the washers I stuck a piece of electrical tape to isolate them from the aluminum of the toerail, but even so I was impressed how little corrosion existed under the head of the carriage bolt I removed from it. The installation looks nice with the hex nuts on washers above deck. Below I replaced the hex nuts with locknuts and used big fender washers against the SS angle-backing plate.

For my H25 this should be adequate. Barring any full standing jibe in 30 knots I don't know that I'll ever have cause to find out its limits (and keeping the adjuster at least moderately tensioned adds safety as well). I have upgraded the rigging on my boat from 35-year-old 1/8" wire to new 5/32" wire. If my memory serves you will have either 3/16" or 7/32" wire on your H30. Here is a link which will tell you the load ratings (where Hunter got your rigging): http://www.loosco.com/index.php?page=yacht-rigging-cable

I'm not the engineer I should be for this; but going by the proportions of my boat I would say to get u-bolts that are safely rated in the vicinity of 5000 lbs or half the weight of your 10,000-lb boat. Here is one: http://www.defender.com/product.jsp?path=-1|118|311635|312082&id=125214. I don't know if this is 316 stainless but it should be. Be sure to get 316-SS locknuts in the right size. If you cannot find them, see a machinist who can get you 4 of them (but make sure they're 316). I have a suspicion that mine are really metric though the 5/16-18 locknuts did spin on and seem all right. Sometimes it keeps me up at night. Sometimes I just drink more and forget about it.

You having no toerail will need bigger backing plates. I would not rely on even very big fender washers alone but get some kind of angle, maybe at 1-1/2", and make it as long as you can stand to have the weight of it. Bed it with 5200 from underneath as you install it, the more to keep the holes through the deck from egging (as the u-bolts will want to migrate forwards, with about 30% of a shear load and the rest in tensile). This plate could be aluminum or stainless (an angle of G-10 would be ideal); all that matters is that it is not prone to pulling through the deck nor to letting the locknuts dent, dimple and destroy it on their way past it. Remember an angled backing plate is less prone to distortion than a flat one.

I am not a fan of the 'waterproof' Wichard u-bolts because, ironically, the rubber boot can allow some moisture to be trapped against the metal which can lead to crevice-crack corrosion and failure. Impose for yourself about a 8-10-year maintenance cycle and keep to it. And caulk them adequately with 5200-- between the shoulder and the deck/toerail-- in spite of their 'waterproof' promise.

Another thing-- my backstay legs now pull at a weird angle. The u-bolts are in line with the toerail and the stay legs pull off the eye and almost at 90 degrees to them. This is not great-- a load should pull vertically in line (up) with a u-bolt. Eventually these will bend, work-harden (increasing their stiffness by a factor of 10 whilst increasing their brittleness the same amount) and I will need to retune the rigging a little. This is an issue any time you use a clevis pin and a hole (here the eye of the u-bolt) that are not concentric or of the same diameter. But we have never had a u-bolt-related rigging failure at Cherubini and it's an easy enough thing to keep your eye on.

This is only my advice as a fellow yachtie and of course I can't guarantee anything without having seen it in person. Do let me know how you make out with it.

 

[Redbeard33]

Thanks for your detailed response, I will source some quality 316 angle and make as long as reasonable, maybe 24"? And at about 50 bucks per pair its not a huge deal to switch these out every 8 years maybe even every 5 if i start feeling paranoid. I will be sure to get a lot of pictures and update this thread.

 

[DianaOfBurlington]

Redb, you may not even need 12" pieces of backing plate; but you are right in that every bit of surface area under the deck helps.

Pics would be nice, just so we are both seeing the same thing. If you can under the deck (like you'll have to when you install them), a few pics of that area might be helpful.

In any event I wish you the best with this modification. I thought a lot about this the other day and wondered why you disliked the transom-mounted chainplates. At least they are easy to have access to the mounting bolts and the bolts are properly in shear there.

 

[Redbeard33]

mine never had transom mounted chainplates (as far as i know) at least it didn't when i got the boat, it had the same configuration as discussed however the old U-bolts went through the toerail (now obsolete). So the location of the backstay attachment won't change just the material the U-bolts are attached to. its a bit hard to explain I will get pictures from a few different angles to better illustrate the issue

 

[Redbeard33]

Heres some pics of the situation, i put X's on the location of the U-bolts. The bolts will still pass through the deck and hull flange, just no longer through a toerail. So I'm really just looking to compensate for the absent toerail

 

[DianaOfBurlington]

Thanks for the pics. Your flange looks just like mine under there. Get a piece of SS angle and see where it will fit up against the inside of the flange. It should be fine if you clear out some of the sloppy 5200 and seat it against there properly. If you have to buy a new length from like McMaster.com, it's not too terrible to buy a full 36-inch length, cut it in half and let it extend forward. You can mount a mooring cleat at the forward end of the angle which will actually help the angle stiffen that back quarter of the deck there.

 

[Redbeard33]

Great, thanks for the help!