Spreader Length/ Rig design

Hello fellow sailors,

I am a long time reader of this site and have pretty much exhausted the knowledge I could find using the search function. To make a long story short, I have a 31 ft Custom designed MORA boat from the Early 80s. It can be described as simply as what the baby would look like if a Moore 24 and a Mumm 30 had a baby. Beamy, Flat deck, Planning hull, fairly full bow. The original trapezoidal keel was having some major keel box issues (likely due to poorly repaired structural damage many years ago) that ultimately led to me just completely replacing the keel a year ago with a deeper bulb keel. As a result I have over 13% more righting moment. The boat was notoriously under canvassed its entire life, now that problem seems exacerbated.
I bought a used hall carbon stick out of a Quest 33s that had been totaled and plan to retrofit the mast into the boat this summer. This mast shares the same 15 degree spreader sweep as the current mast.
My main question is, what will be acceptable for the spreader lengths. I keep reading different numbers for the acceptable shroud mast intersection angle. Some people have mentioned they have used as small of an angle as 7 degrees, others are saying stick to 12 minimum. The current 2 spreader rig has a fairly short spreader base to accommodate overlapping headsails. 32" for the bottom spreader and 24 inches on the top spreader. Its currently 118 inches from the uppers to he hounds giving the existing mast a top cap shroud angle of approx. 11.5 degrees.
The new mast is much taller, and a 7/8 fractional vs 3/4 frac making the spreader size an even bigger issue. Here is a drawing of how I think it needs to be set up. I will be lengthening the top spreader by 6 inches and the bottom by 4 inches. This will make the bottom spreader wider than the chainplates by 4 inches. (New hall spar has very very long spreaders' that I should be able to shorten. They are only tapered at the tips.) I am hoping to keep the chain plates in the same location. They were significantly reinforced/ completely reengineered around 20 years ago with a ton of carbon and a structural beam so it would difficult to move and I am pretty sure they are plenty strong enough for additional sail area. Other food for thought- carbon mast is approx. 6 ft taller than the current mast, New dimensions will be:
I 40,
ISP 43.25,
P 39.25
J10
E 11.33
The Quest was a much heaver boat and the rigs are close to the same weight (approx. 10 lbs heaver for the larger carbon) Original section is a Kenyon 3656, with several doublers as I believe it was under spec for the boat. New rig is a 4.5, 6.5 inch hall section.
Boat weighs 4400
Any opinions will be welcomed.

Cheers- E

 

As I'm not in favor of narrow shround bases I'll not offer any advice.
But I am reminded of a writing in which the author wondered if the purpose of a mast was to support
enough rigging to strain the birds out of the air.

 

Are you racing or cruising?

 

You are putting a 41 foot mast, with two sets of spreaders on a 31 foot boat. That about what the ratio is on my Mariner 19 with a 24' mast. What was your mast height before? You are keeping the boom length the same?

This will move your CE forward a little. Is the headsail also larger, taller? I presume you now have a narrower fin keel. How's the balance look with the Center of Effort to Center of Lateral Resistance?

The wider spreaders may interfere with an overlapping headsail when close hauled. If your mast needs the extra stiffness from wider spreaders, that will help, but the baby stay will accomplish the same thing without lengthening your spreaders past the chain plates.

-Will

 

Current dimensions are
P 33.5
I 33
Isp 37.75
Sprit is going from 13ft jsp to 16. I'm most concerned
E and j will remain the same
All other dimensions remain the same

I would prefer to keep the spreaders the same as the original mast but I'm concerned that it will make the diagonal angles too shallow. If I keep the spreader lenth the same they are around 9 degrees. Going wider is strictly to get the angles higher.

As far as CE I'm less concerned. The new keels center of Effort is 4 inches further forward to compensate for the L shaped bulbs cg being further aft than the original trapezoidal keel. There was virtually no feel difference that anyone could tell.

 

I think the main problem you face is the hull/deck structure. A taller mast will generate much higher tension in the standing rig. Also, the keel is adding to the total stress. Are there other boats that have made similar modifications successfully?

 

The boat had a major structural upgrade when the keel was installed. There is over 100lbs of carbon fiber uni specked out by a naval architect in the keel box alone. The chainplates similarly were beefed up with g10 and carbon uni to support the load. Its very over engineered. The top laminate carbon layer for the keel box alone is nearly an inch thick, and this in addition to a structural grid to support the keel socket. The engineer went a little laminate crazy in my opinion.
The boat is a custom one off boat built of S glass in the 80s so nothing I can easily compare it to. It is very stiff. I am not very concerned about the additional load on the boat as most professionals look at the structure and comment that the boat is "built like a brick **** house." The boat has something like 8 longitudinal full length stringers in the hull and 6 in the deck and several structural ring frames. I have the mast dimensions over at the naval architects office but they are swamped and haven't gotten back to me about dimensions. I'm mainly concerned about spreader width in regards to supporting the mast. I saw a few posts on here from people that seemed to be pretty familiar with rig design. I was hoping some of them might give me their 2 cents.

 

If the structure is adequate, the next question is whether you are cruising or racing.

Overbuilt is actually under engineered.

 

Strictly a race boat

 

The big problem I see with this design is that the shroud angle from the deck to the first spreader is less than the angle of the shroud from the first spreader to the second one is. This does not look right, because the lowered shrouds take up a greater portion of the load than the upper ones do. This looks like a recipe for a dis-masting at the deck level, if the rig is ever over-stressed. The lower shroud will likely part first.

On top of that, the resiliency will be all wrong.

As load is put on the rig, it is supposed to give a little (the shrouds stretch some and the mast bends some). What one wants is for most of the giving to happen higher up, to reduce the sudden loads on the lower shrouds. Having it designed this way could save the rig from failing due to a sudden shock load. If the rig is stiffer up top, more load is put on the lower shrouds, which increases the chance of them failing.