Engineering a mast

[jarcher]

Originally Posted by Paul B

The 6040 is probably marginal as a single spreader rig (I have not run any numbers). Just a quick guess makes me think the lowers would be right at about 10 degrees with that CPW and a single spreader.

I didn't realize the vertical spreader angle was a factor. Is it more upward angle that adds strength or less?

The 5636 looks marginal for a double spreader rig, given that you are deck stepped.

Hm, because it is shorter? I thought a shorter spar would be more sturdy. Why does deck stepping reduce strength?

I've sailed on many boats that started life with babystays and then removed them (SC27, Olson30, IOR boats, etc). When those boats were originallt built the masts were beefy things and the sails were quite stretchy. You needed to be able to induce a lot of bend and a babystay was very helpful. Now sail materials don't stretch as much, so less mast bend is needed as you go through the range. Therefore babystays are no longer as useful.

That's very interesting. I'll keep all that in mind as people are telling me that my mast is going to pump itself into oblivion ;-)

This also makes me think I can afford (from a flexibility standpoint) to go with the 6040 instead of the more flexible 5636. My sails are nice D4 laminates. The 6040 would give me more of a safety margin, although it still means more overall weight and more weight aloft.

How much bending is needed with good (as in not old, blown out) sails?

Prebend is important. Having a deck stepped mast is problematic. However, you can wedge the mast step (like a J22) to help induce prebend. Be sure to discuss this with your rigger.

Wedged mast step. I'll definitely ask about that, thanks. This is good news also because I thought I was going to have to add more chain plates forward of the mast.

If the E32 rig is about the right length (I think it is) and the repair is done well that might be a great choice if the price is right. The E32 is a bit lighter than your boat but they stack the rail with 7 bodies or so and that really adds to the RM. So if it is working for them it could be OK for you. Just be sure to keep paying your insurance premiums.

LOL! I'll do that. Since the E32 is keel stepped and my rig is deck stepped I'll actually have to take about 3 feet off the bottom. The top is tapered and has a welded mast head. As for the price, well he is asking $1,000 which I think is too much for a older, repaired spar that needs paint. I think he will come down though.

If you are using a bolt rope mainsail (no slugs) then the 15" of missing track shouldn't be a problem.

No I have slugs. My sail maker didn't think it would be an issue, do you disagree? He said changing the slugs to smaller ones is not a problem.

When it comes time for resale some potential buyers might shy away from the boat if the rig looks too noodly.

Maybe another reason for the heavier 6040, if I can find one. I expect to have this boat for at least 6 to 8 more years and probably a lot longer though. My daughter goes to college in two years and that will consume all my surplus income that would otherwise be spend on, I don't know, a Summit 35? Nah, I still could not afford that!

Thanks again Paul! You're a great teacher!

[Paul B]

Originally Posted by jarcher

I didn't realize the vertical spreader angle was a factor. Is it more upward angle that adds strength or less?

The wider you can stay the rig, the lighter the rig can be. The narrower the stays the more the tension is compressing the rig. If the stays are too narrow and the rig starts to fall to one side (all rigs fall off to some degree, hence the slacker shrouds on the leeward side) the angle decreases even more, which gives less stability to the rig, more fall off, etc until failure.

Originally Posted by jarcher

Why does deck stepping reduce strength?

I'll let you read up on the theories regarding pinned ends and fixed ends in the books you've ordered. I will tell you a multiplier is increased by about 50% when comparing the calculation of the deck stepped rig vs a keel stepped rig.

Originally Posted by jarcher

That's very interesting. I'll keep all that in mind as people are telling me that my mast is going to pump itself into oblivion ;-)

Walk around the marina with the people who tell you this. Point at the J29s, J35s, 1980s/90s MORC boats, Beneteaus, Jeanneaus, ect and ask the experts why all those masts have not pumped themselves to death yet.

Originally Posted by jarcher

This also makes me think I can afford (from a flexibility standpoint) to go with the 6040 instead of the more flexible 5636. My sails are nice D4 laminates. The 6040 would give me more of a safety margin, although it still means more overall weight and more weight aloft.

I took a a couple of minutes the other day and ran your numbers through my spreadsheet. Using an "I" of 38, CPW/2 of 3.5', 400# RM@1, full Safety Factors, and three 200 lbers sitting on the rail, I came up with a required longitudinal moment of about 7.4. The 5636 is at 7.3, so it is probably doable, especially if you used checkstays.

In fact, we used this section on some semi-custom MORC 30s back in the '80s. However, we used 3 sets of spreaders. The athwartships moments were under for a double spreader rig. It looks like it would be a bit under for your boat as well. In the conditions you mention it could work, but it is marginal.

I think the 6040 size will bend just fine for you and will look "right" on your boat. In fact, about 25 years ago I put a 6040 double spreader rig into a Chaser 29 (another half tonner from about the same era as your boat). I just saw Dave a few days ago and he says the rig is still doing great. He has won a lot of races with that boat/rig over the years.

Originally Posted by jarcher

How much bending is needed with good (as in not old, blown out) sails?

How much bend was the luff curve cut for? Better talk to your sailmaker about that.

Originally Posted by jarcher

Wedged mast step. I'll definitely ask about that, thanks. This is good news also because I thought I was going to have to add more chain plates forward of the mast.

With the wedged step and careful placement of the spreaders you should be able to induce some prebend, I would want about 1.5" - 2" on a rig of your size, if I could get it.

Originally Posted by jarcher

No I have slugs. My sail maker didn't think it would be an issue, do you disagree? He said changing the slugs to smaller ones is not a problem.

I only mentioned it because when you flake the main on the boom with a bolt rope the luff is free, so the 15" of missing track is not problem. With slugs you need the track so the slugs can slide all the way down and stack above the gooseneck.

Originally Posted by jarcher

Thanks again Paul! You're a great teacher

I'm not a teacher. I'm just a guy on the internet with an opinion. Never trust what you read on chat boards. Most people who post on them have no clue what they are talking about. That is why I mentioned Hall to you. Even if they don't supply the tube to you their riggers are going to give you trustworthy, professional opinions. That's not something you can count on from the internet, or from most "riggers" out there either.

[jarcher]

Hi Paul, thanks very much for running the numbers! That's all great information! Sorry it took so long to reply, I had a little work emergency to deal with :-( Well I gotta pay for the new rig somehow.

The books are in. Understanding Rigs and Rigging, Richard Henderson and The Riggers Apprentice, Brion Toss. A quick glance through seems to indicate that the first will be very helpful, the second perhaps not so much.

I did finally talk with Hall, they have a AL spar section which is somewhere inbetween teh two Kenyon ones we have been discussing. I expect it will be a little pricy but we'll see.

Thanks again! I'll let everyone know how I make out.

best,

Jim

[kevinf]

I have been trying to figure out all these same problems for my own boat project. There is some really great information here. I wouldn't bypass "The Rigger's Apprentice" too fast, I definitely learned a lot from that book.

Originally Posted by jarcher

There are two places I know of which destroy boats. They hang on to the rigs and sell them. I plan to visit them this week to see what they have, but in order to do that I need an idea of what I can use.

Where are these boat destruction yards? I have only been able to find one in Florida, Don's Yacht Salvage, and I am looking for something like a Kenyon 6092 for my 48ft tabernacle stepped main.

[jarcher]

Originally Posted by kevinf

Where are these boat destruction yards? I have only been able to find one in Florida, Don's Yacht Salvage, and I am looking for something like a Kenyon 6092 for my 48ft tabernacle stepped main.

I'm sorry Kevinf, I never saw this note until now. I know this thread is ancient now. But for anyone who is interested, I checked out a few places in RI where I was told I might find a mast. Pleasure Marine was one, and there is a guy in North Kingstown, but I forgot his name. But its not worth your time. All they had was scrap.

[jarcher]

Hi All...

Well I didn't get this done last summer. Now I'm hoping to get it done before launch this season.

I bought a repaired Evelyn 32 mast, which is the Kenton 3656 we were discussing earlier. It was supposed to come with one set of spreaders, two spreader bars and the anti-chafe guard. Well it did come with the lower spreaders, bit thats it. On the plus side, I only paid $400 for it.

I sent a quote request to RigRite for the upper spreaders, a mast step, jib halyard sheaves, the sheave axes and the anti-chafe guard. We'll see what those prices come in at.

The mast needs to be shortened and have a bunch of holes filled, and needs sanding and paint desperately. For anyone who is interested in pics, there are a few here:

http://jims-scampi.com/drupal/node/352

I have not been able to find a rigger who is willing to assist with this project, but the machinist is very good. He told me if I tell him where to cut, he will cut.

I spent a lot of time reading Brian Toss's Rigger's Apprentice book. Fascinating stuff! I now understand everything you guys all wrote earlier in this thread.

So that's the long overdue update!

[jarcher]

Hi All...

I took the formulas from the Rigger's Apprentice book and did some math to see what size shrouds I need. Here is what i did:

RM@30 13,500 foot pounds
Safety factor: 1.5
1/2 chain plate width: 3.5 feet

The RM was posted here last spring, but that number is confirmed by a chart in the Toss book.

So...

13,500 * 1.5 / 3.5 = 5,786 pounds of shroud loading.

I then multiplied that by 2.5 as a safety factor and got 14,464 pounds. I am thinking 2.5 is plenty of safety factor. It seems I need to at least double the weight to make sure that I am safely away from the elasticity limit of the cable.

I have a double spreader rig, so the upper and intermediates will carry 30% of the load while the lowers will carry 40%.

So that all means that the upper and intermediate shrouds will need to each carry 4,339 pounds while the lowers will carry 5,786 pounds.

If that is correct, I can use 3/16 inch 1 x 19 316 cable for the upper and intermediate shrouds and 7/32 1 x 19 316 for the lowers.

This seems very light to me, if only because the existing rigging is all 1/4 inch. But it seems that everything on this rig is way over sized. I also know that the standard upper shrouds on an Evelyn 32 are 3/16, so it seems I'm on the right track.