Spreader Angle for External Chainplates

Discussion in 'Boat Design' started by Swiftsure33, Nov 14, 2017.

  1. Swiftsure33
    Joined: Oct 2017
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    Swiftsure33 Junior Member

    Hello everyone. In a recent post I talked about restoring the deck on my newly purchased 1960 Rhodes Swiftsure 33. During this process I will be removing the inboard chainplates. They are parallel to the hull, and are thru-bolted to the hull on to a backing plate that is glassed to the interior. Since the backing plates are already set up for this, I'm considering converting to externally mounted chainplates for ease of inspection, avoiding leaks that lead to crevice corrosion and deck rot, and adding strength to the rig. My main concern is that if I move the upper stay chainplates further outboard, this will affect the angle the stay makes to the spreader. The spreaders are aluminum with steel mounting brackets. Is the difference minute enough for it not to be of concern? I realize that ideally I should have a professional rigger take a look, and I will once it comes time to put the rig up, but I'd just like some testimonials from others who've converted to outboard chainplates to see what they did and how it's worked out so far to see what I'm up against. Thanks.
  2. Blueknarr
    Joined: Aug 2017
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    Blueknarr Senior Member

    In the late 90s a friend moved his inboard chainstays to outboards. No problem with spreaders.
    The hull started to crack while we were tensioning the rig.
    The hill's internal aluminum plate was too close to the outside. When chainstay on inside 98% of glass between chainplate and aluminum plate.
    The aluminum plate was rebedded and glassed over. A new SS plate added to the inside. Final bolting thru outside chainstay, glass hull with its internal aluminum plate and new inside plate.

    Vessel headed to South Pacific.
  3. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    Increasing the angle of the shrouds by moving them outboard will decrease their load, so it won't be a problem.
  4. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Agreed, the distance you'll move the plates is relatively small and you'll gain purchase angle, if ever so slight. Insure the backing can tolerate the loads, as the through fasteners will have a small cantilever on them, so maybe one size up, just to be safe. Also consider epoxy "bonding" these fastener holes, just to insure no leaks and offer a good bearing surface under the fasteners.
  5. Ajg2199
    Joined: Aug 2022
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    Ajg2199 Junior Member

    I’m also thinking about switching to external chainplates (maybe composite), but the original ones were bolted to the aft faces of knees and a bulkhead.
    I’m wondering what others might have done in similar situations.
  6. Rumars
    Joined: Mar 2013
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    Rumars Senior Member

    Usually with internal backing plates, specifics depend on actual boat build.
    The real question is why external? The old ones probably lasted several decades and it's easy to rectify any shortcomings. Switch to a material that doesn't suffer from crevice corrosion (bronze, titanium), properly epoxy backfill and seal the deck penetrations and go sailing.
  7. Ajg2199
    Joined: Aug 2022
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    Ajg2199 Junior Member

    Yes, you’re probably right. But I can never seem to leave things well enough alone.

    I like the idea of external to prevent leaks through the deck, reduce mast compression, clear the side decks, ease of inspection, etc.
    Composite would be interesting to avoid any kind of leaks, try something new, and, as I understand it, they’d be stronger, lighter, and last the lifetime of the boat.

    My question had to do with their location relative to the bulkhead and knees. If I keep them just outboard, but in line with the old ones, they’d be off center of the knees and bulkhead. I’m very hesitant to move them forward to center them with the knees and bulkheads because I think it would affect rigging, loads, etc. But the bulkhead is structural, so I wouldn’t want to move it centered with the new chainplates either.
    Maybe I should ask on a composites- specific thread, but I wonder about bonding to both the hull and bulkhead/knees compared to cutting out the bulkhead and knees, bonding entirely to the hull, and then tabbing the bulkhead and knees back in their original location.

    Like you said, probably not worth the trouble, but I’d still be interested in thinking this through.
    Thanks for any input.
  8. Rumars
    Joined: Mar 2013
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    Rumars Senior Member

    Sorry but I don't understand your problem. If you want composite chainplates you laminate them wherever you want on the hull and tie them in to the knees and bulkheads with more laminate. Given the way the individual uni strips must be fanned out it's mandatory to tie them in anyway, regardless if you take the knee and bulkhead out first or not, or if you move the chainplate hole position fore-n-aft.

    Removing the bulkhead is not necessary, if you want to first laminate onto the hull you simply cut a strip from it next to the hull to create a slot, then glue it back afterwards and tab back. If the builder decoupled the bulkheads initially you don't even have to cut into the bulkhead, there is a foam strip between ply and hull.
    Unless your boat is really large the total amount of longitudinal hole movement to have the chainplate holes over a bulkhead will be less then one inch, negligible from a load perspective even for swept back spreader rigs.
  9. Ajg2199
    Joined: Aug 2022
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    Ajg2199 Junior Member

    Thanks. So I won’t worry about moving the chainplate location a little bit. I’m assuming it would be better to have them centered and wrap equally around the bulkhead and knees. My concern was having half splayed onto the hull, and the other half over the bulkhead and knees. I wasn’t sure if that would cause some imbalance, but I guess it’s all connected as long as the bulkhead and knees are well tabbed in.

  10. Midday Gun
    Joined: Mar 2019
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    Midday Gun Junior Member

    I did this not so long ago on my boat. (28 foot sailing)
    My sail area isn't too far from yours, but righting moment probably a lot less.

    I paid a designer to do the calcs as I also wanted to change other aspects of the rig.

    The process involved putting 7 layers of biaxial cloth 600gsm laminated with epoxy in the area the chainplate bolted to, each on being bigger than the last up to approx 500x500mm. Then a plywood knee extending down as far as the top of the settees was made, it sat just FWD of the chainplate & followed the angle of a line drawn from the chainplates to the centre of the mast. The knee was was made of 12mm marine plywood sheathed in 12mm biax and glassed in place with 3 layers of 600gsm biaxial cloth.

    You should keep the angle of the spreaders the same, which means that if you have any angle on your spreaders the chainplates will move further aft relative to the mast.

    Remember that this will either limit your sheeting angle or you will be stuck with relatively small genoas, think like 105% or something, my boat has a tall fractional rig with a big main, if you have a masthead rig with a skinny main, you'll probably just make your boat worse.

    That's just an idea of one way that i can be done, please don't just copy it blindly as a naval architect made the calculations of my boat.
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