Effect of moving chainplates closer to the mast

[ANDY_W]

For reasons that I won't go into now, I am examining the possibility of moving the cap shroud chainplates closer to the mast.

At the same time the chainplates would be raised to be at the same level as the foot of the mast, moving them from their position on the deck to the side of the cabin.

I am hoping that someone here may be knowledgeable enough, and kind enough, to work out how the forces on the rigging would alter. I suspect that they would be greater but have no idea how much.

The following is how the system is now:

Sail area 540 sq ft Standing rigging is 1x19 strand wire
Mast is coachroof mounted
Mast foot to cap 36ft 3ins
Foot to spreaders 18ft
Spreaders to cap 18ft 3ins
Spreader end to mast 2ft 11ins
Cap shroud chainplates located 18 ins below mast foot vertically under spreader ends.

Proposed changes:

Spreaders shortened by 8ins to 2ft 3ins
Chainplates moved inboard by 8 ins and upwards by 18 ins remaining vertically under ( shortened ) spreader ends.
Cap shrouds shortened to match.

I am looking at the feasibility rather than the desirability so if anyone can help it would be much appreciated.

As ever, any or all contributions gratefully accepted.

 

[LittleSister]

You will need significantly higher tension on the shrouds (and chainplates) to achieve the same lateral support of the mast. This may require an increase in wire size, stronger mountings for the chainplates, and perhaps beefed up support for the mast.

You will need a much better recollection of trigonometry than mine to calculate the additional tension involved.

You would probably need a naval architect's report on the proposed changes to satisfy your insurer that the modification is sound and doesn't increase their risk.

 

[Minerva]

I listened to a podcast where just this topic was discussed for a boat (with female skipper) entered into the Golden Cup race.

Benefits was it allowed a tighter sheeting angle.

Downside, needed considerable re-engineering work and a transition from a single to double spreader rig to handle the increased rig.

I can't remember either podcast or the skipper, but a search of google will find the information.


*** update - it was the "on the wind" podcast with Susie Goodal entitled "Surviving the Golden Globe Race" ***

 

[Bajansailor]

@ANDY_W what type of yacht do you have?
I presume that you are ideally looking for a tighter sheeting angle on the headsail, re moving the chainplates further inboard?

At the same time the chainplates would be raised to be at the same level as the foot of the mast, moving them from their position on the deck to the side of the cabin.

The only advantage really re having the chainplates at the same level as the mast foot would be if you intend to lower the mast. But you would ideally want to have the mast in a tabernacle for doing this.
I presume that the current chainplates are attached to a bulkhead under the deck - or are they attached to tie rods that go down to substantial brackets laminated on to the hull?
Re moving them to the side of the cabin, if they are currently attached to a bulkhead, then you should hopefully be able to attach them to the same bulkhead, further inboard.
But be very aware (as stressed above) that the loadings will be very much increased, the further inboard you move the chainplates.

 

[dunedin]

I may have missed it, but it looks like you are missing perhaps the most significant dimension - the width of shroud base as of today, that you are proposing to narrow by 8 inches (Circa 20cm). By basic trigonometry, huge difference if changing from 4m each side to 3.8m vs 1..2m to 1.0m, for example (and you may find it much easier to do the maths and geometry, which is ESSENTIAL, using metric units.

Also shroud plates are typically heavily tied into the rest of the boat structure - ideally bolted firmly to the hull sides, bolted to a structural bulkhead or with metal tie rods through the cabin.
Finally, many cabin structures are not strong enough, unless designed for this from the outset. Old Westerlies with shrouds above cabin windows being just one example of potential problems.

Your really need to be able to work through the maths and do the structural assessment very thoroughly before even thinking about making any such change. If you have to ask on here it is perhaps not for you.
Certainly sounds a high risk change which would almost certainly require a surveyor to inspect to obtain insurance cover.

 

[ANDY_W]

To cover some of the points raised:

The boat is a Westerly Longbow of which, like many boats of the same period, the fibreglass lay up is
considerably thicker than modern equivalents.

The object is, indeed, to improve the tacking angle.

The present chainplates consist of " U " bolts bolted through the deck and channel shaped brackets
immediately under the deck which are bolted to approximately 1/8 inch x 1inch stainless steel strips.
The strips drop down and sandwich the bulkhead ( under deck part of the main bulkhead ), are about 10 inches long and are through bolted with approximately 1/4 inch bolts. The bulkhead consists of 1/2 inch marine plywood.

You can see that the whole arrangement seems to be quite lightweight but it has lasted over 40 years.

If I were to go ahead, I would have chainplates bolted through the cabin side and backed up inside with
angled backing plates which would be secured by bolts through the cabin side and chain plates, and through the main bulkhead. The bolts through the bulkhead would be staggered so as not to induce a shear line. All would be in 10mm stainless steel. The bulkhead under the bracket would be reinforced with
an extra layer of 13mm plywood glued to it.

The present mast support consists of a teak post which runs down to the bottom of the hull, and a stainless steel tube which is supported by a substantial internal moulding.

As to cost, although the rigging appears to be in good condition I have no idea how old it is so plan to replace it together with the roller reefing gear for the genoa. I have been given a ball park figure of
£ 3500 so the cost of new chain plates will be a relatively small addition to that.

I have discussed it with the surveyor who carried out an insurance survey and he thought it was feasible.

I was hoping that someone here might have the knowledge to be able to give an idea of the increase in loading so that I would know whether or not an approach to a professional would be worthwhile.

All the contributions have been useful, for which many thanks.

 

[flaming]

There are a lot of things that you could do to improve the tacking angle that I would think would be easier, cheaper and give a better overall result than moving the chainplates.

Bear in mind as well that just because you can sheet the sail in closer it doesn’t necessarily follow that you will sail closer to the wind. The Longbow has a pretty old school and “conservative” underwater profile, so won’t generate the sort of lift that a more performance minded boat would.

That said, if improving pointing is your aim I would start by buying a non overlapping blade jib made out of a laminate cloth and sheeting it inside the shrouds. I’d then have a reaching “code” sail built to plug the gap that giving up your Genoa would leave.
this would, I suspect, cost less overall than essentially re-rigging the boat and would also not add the extra weight aloft of the heavier wire you would undoubtedly need. Plus with a smaller blade jib you get a more versatile sail as the wind builds, almost certainly without sacrificing any meaningful light wind upwind performance.

 

[ANDY_W]

The idea of a smaller foresail plus a large down wind sail is an alternative that I have considered. The advantages include less cost and, I sail alone, an easier sail to handle when tacking. The downwind sail would ideally be used in conjunction with a retractable bowsprit and furling gear

The problems arise when looking at sheeting arrangements. It is almost certain that a track would need to be fixed on the coachroof requiring substantial reinforcement to take the loads. In addition the lead of the sheets back to the winches seem likely to create a trip hazard either on deck or over that part of the cockpit coaming which is the only access to/from the cockpit.
In addition, the Longbow is not over-endowed with sail area and decreasing it wouldn't help.

However, the idea is still under consideration as a possible solution.

A third idea would be to use a bowsprit to move the foresail further forward, increasing the distance from the sheeting point, which would reduce the angle of the sail to the wind. I'll have to give that some thought.

 

[Laminar Flow]

Moving the chainplates inboard will increase the compression load on the mast and may very well exceed its moment of inertia.
Shortening the spreader will have the same effect.
The example mentioned by a previous poster regarding a GGR boat addresses exactly this situation. It required shortening the panel length by converting to a double spreader rig.
Shroud angles of less than 9 deg to mast are extremely risky. Not only do they increase compression loads significantly, but as a masthead has the tendency to bend to leeward this may well shorten the angle even further which greatly increases the chances of failure.
To be clear: compression load on the mast is calculated as : RM (righting moment) x 2.78 (safety factor) divided by chainplate distance to mast (1/2 beam). Consequently, the shorter this distance, the greater the load.

As others have said, you would have better luck improving tacking angle by getting better sails. Not only will this increase speed and improve VMG, it should also be noted in this context that lift, the ability to resist leeway, is a function to the square of the speed. not only would you be traveling faster, but you would also be reducing leeway, no matter the shape of the underwater profile.
Being able to reduce sheeting angle alone will not necessarily improve your windwardability, increasing your boat speed will. In fact, as can be seen from any polar diagram, the closer you are pointing, the slower you will be going and the greater your leeway will be. Max pointing angle, on its own, does not correlate to best VMG. None of this is helped by the Westerly's full bows. Sailing her more open and faster may very well be the better deal.

Best o' Luck, LF

 

[flaming]

The problems arise when looking at sheeting arrangements. It is almost certain that a track would need to be fixed on the coachroof requiring substantial reinforcement to take the loads. In addition the lead of the sheets back to the winches seem likely to create a trip hazard either on deck or over that part of the cockpit coaming which is the only access to/from the cockpit.

[/QUOTE]
Inhaulers on the jib using low friction rings. Uses the same track and has the advantage that you can open out the sheeting angle again when you're just off the wind.

 

[TLouth7]

The heeling moment generated by the sail will be the same as before, so the restoring moment achieved by the shrouds must also be the same. The effect is linear so if you half the separation between chainplate and mast you double the tension generated.

Additionally the new setup will be less stiff (inversely proportional to the cube of chainplate-mast separation).

 

[DJE]

Spreaders shortened by 8ins to 2ft 3ins
Chainplates moved inboard by 8 ins and upwards by 18 ins remaining vertically under ( shortened ) spreader ends.
Cap shrouds shortened to match.

Looks to me like the lever arm between the mast and the chainplate will be reduced from 35in to 27in. So - for a first approximation - the shroud tension and mast compression will both increase by 35/27 which is about a 30% increase. You need to be very sure that both the rig and the hull structure can cope with this.

 

[anoccasionalyachtsman]

The effect is linear so if you half the separation between chainplate and mast you double the tension generated.

No, it's a sin/cosine relationship.

 

[Blueboatman]

It is completely daft
Before even running the numbers.
Condider spending the money on a folding propellor and a decent headsail or two and ADD value to the boat and enjoyment instead perhaps ?
The op could try sheeting inside the caps with barber haulers and play with the curve of the sails foot but/and see the VMG unchanged , if you don’t like what I’m writing !
And btw , in the GGR Ms Goodall was the only R36 to lose a rig , and subsequently the whole boat. Rotten awful luck in rotten punishing weather, blow after v blow by account.
So, not the rigs fault per se.
And that was a massive , keel stepped, new Selden thing, ripped out of the boat!
But…..
Why go looking to create engineering weakness?

 

[Tranona]

Could not agree more. If one wants a close winded boat then a Longbow would be way down the list. Plenty of scope for improving overall sailing performance through better sails without messing about with the rig just to get a closer sheeting angle and the illusion of getting closer to the wind.

 

[dunedin]

As Flaming and others have said, you can’t instantly change a Longbow into an upwind speedster (other than with a large Volvo Penta !).

And narrowing the rig base would be bottom of my list to consider. But high quality sails, and potentially as Flaming says a narrower upwind sail and a downwind sail on a bowsprit and furler might be better investments. And ones which are less likely to invalidate your insurance and risk serious structural issues in an old boat

 

[TLouth7]

No, it's a sin/cosine relationship.

The angle between shroud and mast is small enough that we can make a small angle approximation, which yields a linear relationship between shroud base and tension. For a reasonably flexible mast and chainplate directly below spreader tip we don't even need to make that approximation for the cap shrouds; the relationship is exactly linear.

 

[ridgy]

Not to mention making it impossible to sell. Few things would terminate my interest in a boat quicker than DIY rig modifications.

And 3.5k for new rigging and a furler sounds pretty steep on a boat like that.

 

[Laminar Flow]

To be honest, I'm not sure what the point of the (mental) exercise is.
As is, the Longbow is not all that beamy, additionally , the shrouds are already set further inboard, which means that the sheeting angle cannot be all that terrible in the first place.
In this context, it is almost ironic that contemporary design does everything to widen shroud base, taking the chainplates as far out as possible, even using outriggers, all to reduce mastload, not to increase it.

 

[ANDY_W]

OK let's put this to bed. It's clear that you all consider moving the chainplates is a bad idea, and I would be foolish to ignore the accumulated knowledge and experience that says so.

I will, instead, look deeper into using a smaller head sail for upwind and a larger sail for off the wind set on a retractable bowsprit. The idea of a folding prop is good.

Just for the record, I am not attempting to turn a sows' ear into a silk purse. I just get mildly brassed off being overtaken by boats of similar size and speed but which can sail a bit closer to the wind. I know that's life but there's no harm in trying improve matters.

For all your contributions, my deepest thanks.