Beefing up shroud attachment points

MiddayGun

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As per the title.

Pretty standard setup, single spreader swept fractional rig, shrouds to the deck, tie bar to the hull. 

IMG_20180514_124318.jpg


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Problem is the hull has a tendancy to flex inwards as the shrouds are wound on, and since there isn't all that much sweep on the spreaders we do need to wind them on quite hard to get some mast control. 

What are the best options for reinforcing this to try and transmit the load out across the hull a bit more?

I'm thinking some sort of foam formers epoxy glassed in to try and get some stiffness, or mini bulkheads inside the locker? (Sorry about the rats nests, was rewiring) 

IMG-20140827-00067.jpg

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Zonker

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Ideally you'd have a full width bulkhead there, but that would sort of ruin the settees. 

You've got the idea. The bulkheads/knees shown can be slightly ahead/behind the shrouds. The knee can be in front of behind the beam or the beam can be split on either side of the knee, especially if wood.

Then a beam across the cabin top. Could be wood (several layers laminated together to take the deck curve) or lots of glass over a foam former. Top tip - hot melt glue the foam to the roof temporarily, remove and glass over while upright, including a bonding flange about 3-4" wide. Then stick it back in place and bond in place with thickened epoxy.

I'm going to guess 26-28' boat so knees/bulkheads can be 1/2" to get a bit more stiffness than typical bulkheads which might be a bit lighter on that size of boat. 

Beam should be ~3" deep if possible and 3" wide (minimums; more is better) if solid sturdy wood like a fir. If glass, similar dimensions of foam but about 3/8" glass (that's a lot of layers of glass)

Oh and watch the upper tie rod end - those are bad for cracking at the threaded entry of the cross piece.

reinforcement.jpg

 

MiddayGun

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Cheers for the write up. 

Good guess with the boat size, she's a Rob Humphries Contessa 27. (Though this one was built by Marine Glass) 

Unfortuantely any reinforcing will have to be limited to that area, while a beam across the cabin would probably be the best option, its not very practical. The boat has a moulded in liner which provides the structural strength. Voids between the liner & hull are filled with foam, though it was  never claimed to be unsinkable like some of the others built in the same vein. It would also go through the middle of a hatch. 

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Bulkheads aren't structural, they're just screwed into the recessed areas on the liner. 

Hopefully the photo of the area shows you why I don't want to put a beam across. 

What about a couple of mini bulkheads glassed in fore and aft of the attachment point (with access for inspection still) and maybe tie them into some fore and aft stiffeners to help spread the loads?

 

 

LionessRacing

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What about a couple of mini bulkheads glassed in fore and aft of the attachment point (with access for inspection still) and maybe tie them into some fore and aft stiffeners to help spread the loads?
Can you access behind/below the settee and but a built up mostly thwartship layup underneath, going to the turn of the bilge, potentially meeting up with a support that goes below/behind your mast step if you ran it slightly forwards ? think of it as a "bilge spreader" to hold the loads outward, much as the cabin beam would do. or, make the beam above deck, and use as a "wave shield" & place to mount turning blocks etc. 

 
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Zonker

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Yikes, Rob sure wasn't thinking about structure too much with that window there either.

Any deck beam is for the transverse loads, not vertical loads i.e. it is what is primarily preventing the sideways deformation you are worried about. So shallow beam will work.

Really any sort of deck beam will help, but with the windows there you've got nothing to grab onto. :(  

Two shallow deck beams fore/aft of the mast would get those loads spread out but you've really got to get them into the hull.

How about this - diagonal strut running from top of compression post - could be a piece of 1-1/2" aluminum (1.9" OD) pipe, slit and bolted into bulkhead. Paint it matt black and tell friends it's carbon. 

image.png

Or run narrow bulkhead up and split window. Do not glass bulkhead to window - it will look stupid :)  

I suspect  you can glass to the liner without too much grief. Is the deck single skin and the liner is therefore the inner skin of a foam sandwich construction?

Be tidy, use some masking tape and just white gelcoat over the taping.

image.png

 

MiddayGun

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Well that escalated quickly!

The liner isn't structural for the deckhead. Its a standard cored deck with the liner attached to it. So there's a gap between the liner and bottom skin of the deck. But that's by the by, big struts across the windows and struts across the cabin are definitely off limits!

Its not just used for racing, and besides I have to sell it at some point in the future. I realise that it might be the ideal solution, but I'm looking for something a bit less major!

To be clear when I say the shrouds are pulling in the hull, we aren't talking massive deformation, I just mean that it starts to pull in around the shroud attachment a little bit, so from the outside the curve of the hull flattens off, its very obvious when sat in flat water because of the light and reflection, not so much just to look at. I mean its held up OK for 30+ years so maybe I should just ignore it and sail it. 

I'm at work right now, but I'll try and make some sketches later. 

 

xyzzy

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Could you cut a channel in the liner and recess a beam into that?  So it wouldn't have to reduce headroom as much.

 

MiddayGun

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I'm not sure what a beam is going to achieve, the shrouds aren't inline so it wouldn't cross the mast compression post. As already stated it would go across the windows & hatch. 

I don't have whole boat bending issues (any more than any boat bends), just localised distortion of the hull. 

Hopefully this carefully prepared schematic will help! 

uq6njaI.jpg


Bottom bit shows the deflection, its not that much its just to demonstrate, its also over a wider area, top bit is my tentative idea, one of those on each side of the chainplate to try and give some stiffness to the hull there, possible tying into some longditudinal stiffeners.  Plenty of boats with a shroud anchoring system like this without big transverse beams across the cabin. 

 
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Zonker

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Hey, nobody said you have to do it, especially if it's been ok for 30 years.

If you are only talking about localized deflection prevention then bonding a stiffener or two inside the locker seems like a good approach.

Just think about "where do the stresses go now" once you've added a bit of structure.

 

MiddayGun

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Hey, nobody said you have to do it, especially if it's been ok for 30 years.

If you are only talking about localized deflection prevention then bonding a stiffener or two inside the locker seems like a good approach.

Just think about "where do the stresses go now" once you've added a bit of structure.
If the boat was used exlusively for racing and didn't have any resale value then I'd probably do with something like that. But then if that was the type of sailing I did then I'd probably own a different boat. 

My use is the usual race on the weekends and a couple of regattas, one or two offshore crossing to Holland and a bit of cruising. 

The rigs it the one areas of the boat that's IMO in need of work, there's not a huge amount of sweep on the spreaders so its quite hard to control the tension of the forestay and mid-section which has a tendancy to pant in a seaway. I'm hoping that by stopping the localised deformation of the hull it will stop some of the tension being 'wasted' as it were. That and the fact that bending the hull is never good. 

I'll follow up with picutres for you lot to rip into when I'm done.

 

Raz'r

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I was thinking a "beam of destiny" that could be made removable. Put it on while racing. Stow it all other times.

 

Zonker

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Well to be useful such a beam has to be a tight fit. Bit of a pain to slacken the shrouds every time you want to remove it. I don't think this is quite what MiddayGun has in mind!

So more thoughts:

- the tie rod is just bonded into a bit of hull. Tis not a good idea because it's really a point load on the hull. So yeah, you're flexing the hull locally. Global sideways deformation of the hull is a secondary effect (for now)

- I'd try the 2 small bulkheads in that locker either side of the tie rod attachment as a first step to spread the load. If you remove the tie rod one piece of tape can go between both mini bulkheads. 

- it is important to glass the mini bulkheads to the upper locker lid (assuming it is glassed in place) and intersection with seat top. This spreads the load further into the structure.

image.png

- maybe tie the bulkheads together with a bigger longitudinal foam / glass beam. Use lots of glass if you do. But may not be required.

- then go sailing and see if you have reduced local deformation enough that mast is more stable. Use lots of mainsheet tension to keep forestay tight. Report back! 

- if this does not help then maybe global deformation of the hull sides is the main issue and you will have to think about stiffening the boat more to deal with the sideways loads from the diagonal shroud and mast base.

 
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Raz'r

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Well to be useful such a beam has to be a tight fit. Bit of a pain to slacken the shrouds every time you want to remove it. I don't think this is quite what MiddayGun has in mind!





 
Agree that its a bit of a bother, but a simple screw jack and the appropriate end fittings could deal with the install and remove. Just seems a simple way to address the squeeze problem. 

Does fuck-all to address the hull deformation issue...

 

MiddayGun

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Thanks for all the advice, I'll start with Zonkers idea and report back. 

This is the 3rd rig that this boat is on, I know it lost one racing in the Solent many years ago, this was partly down to a mast support failure, the rig was replaced like for like (Proctor tapered section), it was then lost a second time a couple of years before my ownership, beating into a heavy North Sea chop in around F7 gusting 8. Apparently she hit a wave particularly hard and the rig folded. 

Now to me, 2 losses of the rig suggests some inadequacy in the design, but unfortunately the PO again decided to specify a like for like rig, rather than taking the opportunity to improve it, his only concession to this was having some kind of stiffener rivetted in to beef it up athwartships around the shrouds (where it bent) while leaving the fore aft bend characteristics untouched. 

I find in heavy seas the rig pants a lot in the midsection pants a lot, as this happens the hounds move down & the forestay slackens, not ideal hard on the wind when its blowing. Winding on the backstay hard does improve matters, being 7/8ths it does have a good effect on forestay tension, but it still moves to much for my liking, club racing I can live with it, but when I'm mid North Sea its quite worrying.  

Unfortunately the only places I know with a few good of these boats are Poole &  Weymouth, both a long way from me & we don't have any kind of class association. So its possible that I'm having to reinvent the wheel here. Seeing some of the photos from their website it looks like they're sailed hard without the rigs coming down, so maybe I'm just worrying too much.

Anyway, I found a photo that illustrates the hull deformation, you can see it quite obviously here, photo was taken just before Scarborough regatta, if we backed off the internal supports to let the hull move out then the deck started to lift.  20170824_173533.jpg

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Grande Mastere Dreade

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Well to be useful such a beam has to be a tight fit. Bit of a pain to slacken the shrouds every time you want to remove it. I don't think this is quite what MiddayGun has in mind!

So more thoughts:

- the tie rod is just bonded into a bit of hull. Tis not a good idea because it's really a point load on the hull. So yeah, you're flexing the hull locally. Global sideways deformation of the hull is a secondary effect (for now)

- I'd try the 2 small bulkheads in that locker either side of the tie rod attachment as a first step to spread the load. If you remove the tie rod one piece of tape can go between both mini bulkheads. 

- it is important to glass the mini bulkheads to the upper locker lid (assuming it is glassed in place) and intersection with seat top. This spreads the load further into the structure.

View attachment 303405
how about between the two stringers, mount a new attachment point for the shroud off the hull ?    otherwise just glass in some horizontal stringers , which I'm guess those bumps are for, but not doing their job.. pretty short, wonder what they were thinking..

 

longy

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   Those 'bumps' are a ss ladder that the shroud attaches to. This allows enuff area for the glass overlay to hold the load without shearing apart.

 
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