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Old 15-10-2024, 12:24   #1
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How crazy is +8 feet of mast on my Spencer 42?



I have a 1966 Spencer 42 Sloop, and it has the original spruce mast.

A couple of months back I picked up a new boom for my boat, a 19' Sparcraft that was a takeoff for someone going to a roller furling boom and happened to be exactly the right size for my boat!



I noticed removing the old boom how heavy it was in comparison to the Aluminum, 3-5x the weight! It was all I could do with a friend to maneuver it down off the boat and that was using a winch, where as I was able to mount the Aluminum boom by myself!

This has me thinking as I know where there is a 56' mast that would fit my boat, looking up the specs on that section it is 5.58 LB's per foot which is pretty light.

Here is my sail plan:



My current mast is 48' above the deck, so standing this mast I would be a full 8' taller adding almost 17% to the height of my rig.

Which understanding that the rig is a lever I understand that difference is exponential.

What I am trying to discern however is considering that right now my boat will carry full sail through 20 knots before it starts dipping a rail, she has the 8,000 lb ballast keel and I will be going back with Dyneema Rigging which is 1/4 the weight of the 19 strand stainless, and that the Aluminum will likely be lighter than the spruce, I am wondering if it is wondering if this Mast would work with my boat?

The Dims on the current rig are:

I: 46.50 ft
J: 16.50 ft P: 39.75 ft E: 18.50 ft

Main: 414 SQFT
Genoa: 482 SQFT (150%)
Total: 896 SQFT

The proposed rig would be:

I: 54.50 ft
J: 16.50 ft

P: 47.74 ft
E: 18.50 ft

Main: 486 SQFT

Genoa: 581 SQFT
Total: 1067 SQFT

Thus the extra 8' of mast would pick up an additional 171 SQFT of sail area which is a gain of about 19%.

So it is not insignificant as it would give her a sail area to displacement ratio of 23.17, where as now she is at 19.46 (assuming 20,000 lbs displacement, although she is actually about 26,000 lbs in the slings according to the travel lift here anyways) Increasing the SA/D ratio about 19% so I can assume that under full sail it would drive her much harder.

I know right now in light air she is a bit slow, really the boat doesn't come alive till you are above 8-10 knots steady, and when you have two sailboats headed in the same direction in light air, I have found myself wanting more sail area!

Over 15 she turns into a rocket and at 20 I am swapping for my 85% yankee and throwing the first reef in by 22-25.

The other thought I have it to just cut 4 or so feet off the bottom of the mast and move the fittings up. Considering the shrouds would be near vertical I would think this could be a workable solution but I am unsure?

I am due for new chain plates and my sails are going to need to be done within the next year-2 so that is not an issue.

It is a lot to think about and figure out but before I can set out on any open ocean adventures this is something I need to have absolutely sorted.

Any insight here would be greatly appreciated!
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Old 15-10-2024, 12:49   #2
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Re: How crazy is +8 feet of mast on my Spencer 42?

The weight of the new rig shouldn’t matter as you are making it lighter, and you can always add an additional reef to reduce the sail area so that shouldn’t matter much either.

So, what remains is the 8’ height difference - I am not smart enough to answer your “lever” question, but, I am sure you will appreciate the additional sail area in light winds.
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Old 15-10-2024, 12:52   #3
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Re: How crazy is +8 feet of mast on my Spencer 42?

you can always reef. Just make sure it has plenty of reef points. Then it doesn’t matter.
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Old 15-10-2024, 13:29   #4
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Re: How crazy is +8 feet of mast on my Spencer 42?

According to "Sailboatdata", at 20,000lbs the SA/Disp. ratio is 17.03, which was considered decent for a cruising boat of that vintage, and that's actually right close to the mid-range, certainly not at the low-end.
Hint: the SA/Disp. ratio is figured upon a 100% foretriangle, no lapping headsails, it's what used to be called "all plain sail".
Once your pushing ~20 you're getting into what's considered "performance" territory.
Your boat at thousands of lbs. heavier would certainly be below that now.
I think I would speak to a NA and ask them: "How much higher in rig would I need to get my boat back to somewhere around its original SA/Disp. ratio of ~17 or thereabouts".
Another question the NA can answer is if 8Ft of additional height will require another set of spreaders and if so the placement of such.
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Old 15-10-2024, 13:34   #5
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Re: How crazy is +8 feet of mast on my Spencer 42?

I might be wrong (not unlikely) but I thought the Spence 42 had a keel-stepped mast. Thus, the 8 "extra" feet would be below deck.
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Old 15-10-2024, 15:17   #6
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Re: How crazy is +8 feet of mast on my Spencer 42?

Quote:
Originally Posted by Bowdrie View Post
According to "Sailboatdata", at 20,000lbs the SA/Disp. ratio is 17.03, which was considered decent for a cruising boat of that vintage, and that's actually right close to the mid-range, certainly not at the low-end.
Hint: the SA/Disp. ratio is figured upon a 100% foretriangle, no lapping headsails, it's what used to be called "all plain sail".
Once your pushing ~20 you're getting into what's considered "performance" territory.
Your boat at thousands of lbs. heavier would certainly be below that now. [/I think I would speak to a NA and ask them: "How much higher in rig would I need to get my boat back to somewhere around its original SA/Disp. ratio of ~17 or thereabouts".
Who is NA?

Recalculating based on just the size of the triangles, currently looking at 751 SQFT with the new rig it would be 891 SQFT

Meaning the current SA/D is 16.30
The new rig would be SA/D 19.35

So an increase of 18.7%.

It seems to me that would be well within the performance envelope of this boat. could handle

Reading up on this:
Quote:
CRUNCHING NUMBERS: Sail-Area/Displacement Ratio
By: Charles Doane

Interpreting the SA/D Ratio
The basic guidelines for interpreting an SA/D ratio are as follows: below 16 indicates a slow underpowered boat; 16 to 19 indicates reasonably good performance; 20 to 22 indicates high performance; and anything over 22 indicates super-high performance.
Based on this at 19.35 that would put my boat at the cusp of being a high performance boat.

It feels doable...

Quote:
Another question the NA can answer is if 8Ft of additional height will require another set of spreaders and if so the placement of such.
My current rig is a single spreader, this new mast already has the duals so should be good to go there.
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Old 15-10-2024, 15:23   #7
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Re: How crazy is +8 feet of mast on my Spencer 42?

Quote:
Originally Posted by jamhass View Post
I might be wrong (not unlikely) but I thought the Spence 42 had a keel-stepped mast. Thus, the 8 "extra" feet would be below deck.

They were both, mine is deck stepped, the keel stepped version was a cutter.


The sloop was technically a slutter with a removable inner forestay about 6' back from the bow on the anchor locker bulkhead.
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Old 15-10-2024, 15:30   #8
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Re: How crazy is +8 feet of mast on my Spencer 42?

You'd have to make sure the shroud angle on the lowers was still adequate. Any angle less than 12 degrees (IIRC) has greatly increased loads. Raising the lowers without widening the boat will lessen the angle.
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Old 15-10-2024, 15:50   #9
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Re: How crazy is +8 feet of mast on my Spencer 42?

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Originally Posted by Benz View Post
You'd have to make sure the shroud angle on the lowers was still adequate. Any angle less than 12 degrees (IIRC) has greatly increased loads. Raising the lowers without widening the boat will lessen the angle.

Should be good there as installing it as is and having the 56' mast would be exactly how she was designed to be rigged, not modifying it other than cutting the base off to make it stand on my deck rather than to be keel stepped, and I have conformed that I can do this.
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Old 15-10-2024, 16:14   #10
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Re: How crazy is +8 feet of mast on my Spencer 42?

Quote:
Originally Posted by Renegde_Sailor View Post
Who is NA?
In marine parlance "NA" means "Naval Architect".
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Old 15-10-2024, 17:31   #11
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Re: How crazy is +8 feet of mast on my Spencer 42?

The increased heeling from higher sail area is not the only issue. Changing the mainsail can also alter the center of effort of the sail -- possibly giving the boat much more weather or lee helm. Setting up the shrouds and stays for a taller rig is an engineering problem. The mast needs to stay up when loaded under sail, not just at the dock. To stay up the taller mast might need more highly loaded shrouds and stays. The cabin top and compression post might not be strong enough to withstand the increases. These are reasons why a Naval Architect should be asked about this change.
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Old 15-10-2024, 18:27   #12
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Re: How crazy is +8 feet of mast on my Spencer 42?

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The increased heeling from higher sail area is not the only issue. Changing the mainsail can also alter the center of effort of the sail -- possibly giving the boat much more weather or lee helm. Setting up the shrouds and stays for a taller rig is an engineering problem. The mast needs to stay up when loaded under sail, not just at the dock. To stay up the taller mast might need more highly loaded shrouds and stays. The cabin top and compression post might not be strong enough to withstand the increases. These are reasons why a Naval Architect should be asked about this change.

Those are good questions to ask, I am going to have to see if I can find a Naval Architect to have look at it.

Although I would be inclined to think it is strong enough as the bulkhead the mast sits on is 2x layers of 3/4" plywood and could probably be further reinforced fairly easily.



The chain plates are due for replacement so that is already a given...
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Old 25-10-2024, 09:26   #13
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Re: How crazy is +8 feet of mast on my Spencer 42?

Our Passoa 47 was designed for a 17 metre aluminum mast but we installed a 20 metre carbon one
The overturning moment of our rig due to mast plus rigging weight is somewhat lower than the original design
It has worked well Canada-Cuba-Spitsbergen-Marquesas over a 20 year span.
I don’t think balance will be affected significantly by a taller mast.
Light air performance is much better than her sister ships
Of course we reef early, usually about 14 knots close hauled
Discuss with a naval architect, particularly for standing rig sizes
One reason to change is that your aged wooden mast must surely have a wee bit of rot somewhere by now, or soon.
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Old 25-10-2024, 16:37   #14
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Re: How crazy is +8 feet of mast on my Spencer 42?

Quote:
Originally Posted by psk125 View Post
The increased heeling from higher sail area is not the only issue. Changing the mainsail can also alter the center of effort of the sail -- possibly giving the boat much more weather or lee helm. Setting up the shrouds and stays for a taller rig is an engineering problem. The mast needs to stay up when loaded under sail, not just at the dock. To stay up the taller mast might need more highly loaded shrouds and stays. The cabin top and compression post might not be strong enough to withstand the increases. These are reasons why a Naval Architect should be asked about this change.
Increasing sail area or mast height does not increase the load on the rig, you just heel more at a given wind speed. Increasing righting moment, that is resistance to heeling, does increase rig load.

Increasing sail area can increase transient rig loading in the form of shock loads from gusts, but that is not do to more heeling.

Raising the mast won't change the center of effort because, presumably, both the main sail and the fore sails will be taller.
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Old 25-10-2024, 23:08   #15
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Re: How crazy is +8 feet of mast on my Spencer 42?

I don't think you're crazy, but it's definitely a big change that requires some thought.

The first question I have that I don't see being addressed is what size and type boat did the 56' mast come from? (See reasons below.)

Contrary to popular opinion here, I think the load in the stays will almost certainly increase. The shrouds are much less likely to increase, especially since it is going to double spreader.

Let's say, for example, that your current forestay has a 1000 lb of tension on it. (The number doesn't really matter, the % change will be the same.) The angle of the stay is roughly determined by I and J. So the current angle (from vertical) is atan(16.5/46.5) = 19.537, say 19.5 deg. Then the forward force on the mast is 1000 sin(19.5) = 333.81, say 334 lb. But the new forestay will be at a different angle: atan(16.5/54.5) = 16.844, say 16.8 deg. To get the same forward force as before, the required tension in the stay is: 334/sin(16.8) = 1155.58, say 1156 lb. So there will be a 15.6% increase in the tension in the forestay. Note that the compression force into the mast (from only the forestay) goes from 943 lb to 1107 lb; so a 17.4% increase. This is before we've considered any other changes in the system, like for example, does the new larger mainsail require more sheet tension and therefore more forward supporting force from the forestay? And then there's headstay sag, where the longer stay would result in more sag, but that stay also has higher tension (as we've seen) which would reduce sag, so...

The shrouds aren't quite as straight forward. For example, are the spreaders swept back (old and new) and if so how much? My only further comment here (because of all the variables) is what was the beam of the previous boat for the 56' mast? It was presumably a larger boat, so if say it had a 13' beam and you're putting it on your 11.33' beam that could be an issue. Lowers would be similar analysis to the forestay, but what are you doing with the spreaders? Are they wider than your beam? Do you shorten them, but then you'd be reducing the angles again?

Final comment, if the mast is going to a smaller boat with lower stay and shroud angles, the compression in the mast itself will increase too, and the strength will need to be checked for buckling. If the boat was a racer I'd be more concerned, but if a cruiser, then maybe not as much of an issue. (And yes, this compression will go into your bulkhead below.)


Quote:
Originally Posted by Renegde_Sailor View Post
Those are good questions to ask, I am going to have to see if I can find a Naval Architect to have look at it
You'll definitely want a NA to look at this first. I'd expect that's a small cost compared to new mast, rigging, sails and labor (even if your own). There's some guy named Perry out your way, but local isn't a requirement these days and there are many others.
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