Correcting chain plate location

Hello esteemed boat designers.
I've discovered that the chainplates for my shrouds are improperly located and I'm reaching out for your assistance
The boat is owner built, steel and to my best investigative efforts, a Roberts 345 (Special). I have no idea if this was a one off from Bruce or a builder modification from the standard 345 design. 3 1/2" of length has been added to the standard design (LOA 37'11"). Regardless, the cap and V1 shrouds are too far outboard and possibly aft as well.

Rig particulars...
Isomat N-70 spar with NG2 masthead (47' 1" length)
Note: Cap shrouds exit mast on each side ~16" below top of masthead. See diagram below...

Isomat NG70 Mast https://www.rigrite.com/Spars/Isomat_Spars/Isomat_Masts/NG70_mast.php

Isomat BF-4 upper spreaders (39 7/16" base to shroud retainer [~5.5° upward angle]) with welded spreader base
Isomat BF4 Spreader https://www.rigrite.com/Spars/Isomat_Spars/Isomat_Spreaders/BF4_spreader.php
Isomat BF-5 lower spreaders (44 5/8" base to shroud retainer [~5.5° upward angle])with welded spreader base
Isomat BF5 Spreader https://www.rigrite.com/Spars/Isomat_Spars/Isomat_Spreaders/BF5_spreader.php
Spreaders are not aft swept and appear to extend perpendicular to the fore/aft centerline of the mast.

Outboard jib sheeting. Sheet tracks located on deck beginning about 12" aft and just outboard of the vertical between the lower spreader tip and the deck.

Boat particulars...

• LOA ~ 38'
• Beam - 12'
• Draft - 4' 6"
• Disp. ~ 20,000 lbs
• Ballast - unknown
• 1/8" steel plate throughout
• Long fin keel with 2" thick horizontal "triangle" shaped steel base
  
• Large skeg and rudder (NACA 0012)

The particulars of the issue...
Basically, before I removed them, the cap and upper (V1 I think) shrouds were located very near the toerail. At this location, Negative pressure is applied to the lower spreaders. The location of the cap shroud was about 5" outboard of a line through both spreader tips and the deck (where there would be zero pressure on the lower spreader). It was 13" outboard of the vertical between the lower spreader tip and the deck. See diagram below...

I'm no naval architect. But I know this is wrong.

The particulars of the geometry of chainplate location are not easy to find. I have found some guidance on this forum though. Chainplate location https://www.boatdesign.net/threads/chainplate-location.3334/
This thread was discussing aft swept spreaders and the chainplate locations. The pertinent quote from this thread read "...the drop of the V1 should not be vertical, it should be loading into the spreader tip a bit (chainplate slightly forward and inboard of the spreader tip)."

If anyone can shed some additional light on my issue I would be extremely grateful.
Settling the placement of the cap/V1 shrouds is first on my list. However, I'd like to follow up with questions about the fwd and aft lower chainplate geometry as well.

 

Welcome to the Forum Deepnit.

I have tried hard to understand what your problem is, and I am not succeeding. But there again, I am rather dense in matters like this.

You mention that you have a steel Roberts design - how old is she now, and how long has she been sailing with the current rig set up like this?
The spreaders are not swept back - are you worried about this, thinking they should be, because the rig is slightly fractional (re the cap shrouds, and I presume the forestay, being 16" below the top of the mast)?

You note that "The location of the cap shroud was about 5" outboard of a line through both spreader tips and the deck (where there would be zero pressure on the lower spreader). It was 13" outboard of the vertical between the lower spreader tip and the deck. "

Would it be feasible to acquire / fabricate a pair of slightly longer lower spreaders instead of trying to move the chainplates further inboard?
Generally, the further outboard your chainplates and spreaders are, the less are the loads in your cap shrouds.
Look at the Open 60 class, with outriggers to get the cap shrouds as far outboard as possible.
Apologies if I have gone off a completely wrong tack here.

Changing tack, re how you mention that the hull is 1/8" steel plate throughout, apart from the 2" thick base of the keel, does this imply that the sides of the keel and the hull bottom are also 1/8" / 3 mm thick? If so, then you do not have much margin for corrosion - I would have expected the hull bottom to be at least 4 mm, and the keel sides to be at least 5 mm (or even 6 mm / 1/4") thick.

 

Any chance you have the plans? If not you could contact the designer and ask for the plan sheets on the rig. It would be best to verify everything from the plans to be sure the chainplate location is the error.

 

Thank you for the welcome and time to respond!

From all I've been able to learn of the boat, she's seen little sailing. She's papered in 2004 but I believe construction began as early as 1985 as there is an etching at the bottom of the mast saying "1985 Roberts Special"
I am not concerned at all the spreaders are not swept. Just noting the fact.
Also, I am not interested in replacing spreaders to accommodate the chainplates. The chainplates have already been removed, welded to a doubler plate and are ready to be welded to the deck. They were previously through bolted. This is a no/low cost option. Buying new spreaders simply isn't in the picture.

To be clear about the issue, the position of the cap shroud/V1 chainplates effected negative or outward pressure on the lower spreaders. They were effectively trying to pull the spreaders away from the mast.
Those Open 60's don't have any spreaders on the mast. Thus the crazy wide external shroud arrangement. I'm not talking about a drastic change. I just want to eliminate the negative pressure on the lowers and locate the chainplates as they should be.

I'm not worried about corrosion. I've already sandblasted, primed, faired & re primed the exterior of the hull. Almost all areas of interior corrosion have been identified and addressed as well. The only real corrosion was around hull penetrations like the previously through bolted chainplates for example.

 

Thank you for your response!
I do not have the plans and am not even sure the boat was not altered from the plans as nearest I can tell this is a custom alteration of a Roberts 345 design.
I'm positive the previous locations were incorrect. Having outward or negative pressure on the lowers just violates the entire concept of spreaders in the first place.

 

How did you determine that the spreader length is correct according to the original plans?

 

Gonzo,
I have not determined the spreader lengths are correct. As far as I know, this is a one of one construction based on a Roberts 345 (Owner built).
I don't have any plans for any aspect of the boat. Nor can I find anything online. However, I can tell you the angles created by the cap shrouds. If, from these angles, you can determine there is a problem, I would be willing to investigate spreader replacement.
top of mast to cap shroud ~12.5°
Tip of upper spreader to tip of lower spreader ~2°


The fact that this boat was owner built is why believe an error was made in construction. The error could very well have been in the ordering of the rig. However, since moving the chainplates to accommodate the existing rig is free, that is the route I want to take.

Thanks for the question. I hope I'm being understandable.

 

All,
Restating the issue more succinctly. My cap and upper shroud chainplates were located outboard of a straight imaginary line from the upper spreader tip, through the lower spreader tip and ending at the deck. At the point where this imaginary line terminates at the deck, there would be No pressure on the lower spreader. So, the previous locations (5" outboard of this imaginary point) exerted outward force on the lower spreader effectively trying to pull the lowers away from the mast.

 

In general, 14 degrees is considered the minimum for cruising boats.

 

That's not good news.
Would you suggest shortening the upper section of the mast?
(Remove masthead, cut mast & re weld the masthead.)
Also, you mentioned the "minimum".
Is there an optimal?

 

I would suggest making the spreaders longer, if possible, rather than shortening the mast.

Do your spreaders have an aerofoil shaped cross-section, or are they simple round aluminium tubes?

Re a 'minimum angle', the larger the angle the better really (within reason of course) - as the angle increases, the loads reduce, and the structure becomes more effective (with a higher factor of safety, as the loads are now less).

The disadvantage of having a larger angle would be that you might not then be able to sheet the headsail in as tight, and hence you cannot point as close to the wind as a vessel with a rig that has a smaller angle.

When you are 'setting up' your spreaders, you ideally want the upper spreader to bisect equally the angle with the stay - ie the angle above should be the same as the angle below.
I presume that you also have intermediate cap shrouds coming down to the chainplates from just underneath the top spreaders?

 

Aerofoil shaped. Link to the exact unit is in the original post.
I'll look at the cost and give it a think.
Yes. Intermediate cap shrouds are also present.

 

Righty ho - it might be possible to buy longer spreaders from Isomat 'off the shelf'.

If you have a keen interest in learning about rig design, this is an excellent book.
https://www.amazon.com/Sailing-rigs-spars-Installation-maintenance/dp/0854297537/ref=sr_1_2

 

Thanks for the recommendation on the book.
Just checked it out. That looks just like the Haynes manuals I used to learn how to work on cars!
I'll pick it up.
Anyone else have any good literature/learning suggestions?

 

This talk of angles and in particular the upper shroud or intermediate cap made me do the math on that as well.
The angle of the intermediate to the lower spreader is ~16.9°. If that helps anyone help me, there it is.
See below and make any recommendations for replacement of spreaders if necessary.
Thanks everyone for all your help!!!