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Old 08-07-2017, 05:42   #1
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question for you mechanical engineers

My boat has (had) a fixed boom vang, not to be confused with a rigid boom vang. This vang is nominally a fixed length vang, though the length can (could) be adjusted but not while sailing. The vang consists of a 3 inch diameter aluminum shaft with a 1.5 inch hole drilled about 8 inches deep in the boom end and threaded. The boom fork is machined out of 2 inch diameter aluminum bar and the end turned down and threaded to match the internal threads on the shaft. The boom fork piece is threaded into the shaft and set at a fixed length. Over the years salt water has gotten into thread and corroded the aluminum and the threads have now stripped. Needless to say these parts are not made anymore. I propose to drill a hole through both the outer and inner pieces and fix them in place with a clevis pin. The issue is what size clevis pin? The ends of the vang are fixed to the boom and the mast bracket with 1/2" diameter bolts labeled S30400 which I understand to means 304 stainless steel. Since both of these bolts are in double shear I don't think my clevis pin needs to be any stronger than these. All of the clevis pins I can find are 316 stainless which I believe is weaker than 304. I can't seem to find any information on the strength of these clevis pins in double shear. I can get information on graded bolts in double shear and obviously could go with grade 8.8 and be plenty strong but I've had issues with using steel bolts in aluminum and the threads cutting the aluminum hence I would rather use a pin. First question I have is can I drill a 1/2 or 5/8 hole in the 1.5" shaft and still have as much strength as 4" of aluminum on aluminum threads which originally carried this load? Secondly, Since clevis pins don't seem to be available in grades what size would I need to match the 1/2" S30400 bolts? Am I correct in assuming that this pin does not need to be any stronger than the bolts in the forks?

Thanks in advance for any input.
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Old 08-07-2017, 07:18   #2
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Re: question for you mechanical engineers

Since all the loads pass through a half inch bolt at either end you should be fine adding a 3rd same sized bolt as you described. You are taking a big amount out of the threaded rod section by drilling out a 1/2" hole across the widest part of a 1.5" diam rod however. If the high loads are mainly in tension rather than compression, which I think they will be, then you run the risk of fatigue cracks starting at the drill hole edges and growing until it suddenly fails. That's what the original threaded connection avoided, by spreading the forces over all the threaded length. Being an anal engineer I could give you a better answer if I knew the exact loads, maximum and minimums, Yada da.

A) Is there any chance a small machine shop can cut you a new fork from billet?
B) since you'd be ok with a fixed length vang, clean out the threaded hole and rod end preferably by sandblasting then wiping clean with acetone. Then find Devcon epoxy, and coat rod and inner threads and reassemble to previous depth.
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Old 08-07-2017, 07:33   #3
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Re: question for you mechanical engineers

You don't say how far the 1 1/2" fork shaft extends into the vang body, but if you have room to drill 2 half inch holes at 90 degrees to each other, an inch apart and an inch from the end on both pieces, I'd guess you'd be good to go.

If you only had room to drill one hole, maybe bump it up to 5/8".

If the threads are already stripped, what do you have to loose? Monitor the holes for elongation; if none is evident you should be safe

The strength of the steel is not an issue, 304 or 316 is far stronger than the aluminum, so within reason (don't expect a 1/4" bolt to last long against 1 1/2" aluminum in shear) any appropriately sized steel or SS pin (or cut off bolt) should work.

Put it all together with some kind of anti corrosion agent, even Teflon pipe dope is better than nothing.

Alternatively, a good machine shop could make a new fork and rethread the existing body, next size up...
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Old 08-07-2017, 08:16   #4
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Re: question for you mechanical engineers

For all intents and purposes the 304/316 is the same in that application. Either may, or may not, be work hardened way beyond the specification of raw material, so you have no idea what the reality is on a specific piece. I wouldnt worry, except maybe having a small pin that breaks instead of breaking your boom would be a good idea on a rigid vang!
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Old 08-07-2017, 09:44   #5
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Re: question for you mechanical engineers

Look at the longer term. A fixed boom height may be adequate for the current sail at most points of sail. Sort of. Will it be correct when you put that new main up? I doubt it. Then do you drill another hole? There is a reason itís threaded and I would not settle for the compromise of one setting.

If I visualize this correctly it does not sound like a complicated shop project.

There are so many possible options: Drill out the mast side and rethread to a slightly larger size and just replace the boom side. You could machine out the boom side and press in a sleeve. None of this is complicated to a decent machine shop. You donít say what the original thread is (UNF, UNC, ISO, proprietary, et al) is but there is nothing in the rules that says this had to be a standard size thread. You make both ends, just make to fit each other.

That is simple machining and may allow the reuse of the boom side. I have taken pieces like this and simply chased the existing female thread a little deeper until there is a full thread profile once again. If you only cut .050 off the inner wall, a chunk of two inch stock will still be big enough to make the male end.

One important thing to consider. You say corrosion ruined the thread. Was it just corrosion or corrosion combined with being a little undersize? If the current set-up is engineered properly stick to that or make it a little stronger when machining.
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Old 08-07-2017, 09:45   #6
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Re: question for you mechanical engineers

Dymaxion is entirely correct about any holes through rod/piston and cylinder may quickly compromise.Cheapest solution I think,would be 3/8ths grade 8 bolt with 1/2 inch delrin bushing to avoid galvanizing.Either way,not knowing shock psi stresses vang encounters working life will diminish.So-again-cost over expediency or vis-a-versa.
Seems to me the cylinder may fail sooner.So the delrin "bushings" would help best here



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Old 08-07-2017, 10:40   #7
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Re: question for you mechanical engineers

The difficulty with "pin" solutions is that the weakest point, where the new cross pin goes through the old threaded 1 1/2" shaft, will be hidden from inspection unless you disassemble it, and the mast shaft with the 8" deep hole is perfectly placed to fill with salt water. That water corroded your threads, which might have been sealed to keep it out but apparently were not. If you effectively seal up your work this time, you lose any way to inspect the shaft/pin for egging or cracks. I think I'd go with one of the machine shop/thread alternatives rather than pinning it.
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Old 08-07-2017, 11:57   #8
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Re: question for you mechanical engineers

To be honest I would just use this as an excuse to upgrade to a rigid vang. If you send this stuff off to a machine shop the bill for that would make a nice down payment on one, and the adjustability is hard to argue with.
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Old 08-07-2017, 13:23   #9
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Re: question for you mechanical engineers

To answer a few questions:

The fork piece was originally inserted about 4 inches into the shaft.

The option of going to a rigid vang is definitely an option though an expensive one. Not only is the initial outlay expensive it appears that I would need to grind off the current tangs and replace them with ones supplied by the vang manufacturer. The cost of the mounting hardware appears to equal or be nearly equal to the vang itself not to mention the labor. The current tangs and pivot are 1/2" thick. This appears to be either to wide or too narrow for the off the shelf rigid vangs. The two candidates I see as most promising are the Forespar Yacht rod or a Garhauer 25-1. This is a Cat and the boom is quite heavy. It appears to be made out of the same extrusion as the mast so I would likely need to go up a size.

I'm trying a quick fix because I want to get underway again.

My quick fix is to position the boom where I want it and drill through both pieces at once using a drill guide to keep the bit centered while drilling the pilot hole. I would move to progressively larger bits until reaching 1/2" or 5/8". I would place the center of the hole about 2 inches from the end of the outer shaft or about half way down the inserted section. From the point of view of the inner aluminum shaft I would prefer going to a smaller pin size to keep as much aluminum intact as possible. I don't think that the size of the pins used in the rigid vangs need to be as big because the spring (metal or pneumatic) does not carry the sail tension load which is carried by the line/block system. In my fixed vang the pin would have to carry the tension loads from the sail and the compression loads from the boom weight. The problem is the more I think about this the worse the idea looks to me. A crude calculation of the surface area of the threads would suggest that about 1.3 square inches of aluminum are carrying the load and I won't have any where near that much left in the inner shaft if I put a 1/2" hole through it. I may be forced to have a new fork piece made and the outer shaft re-threaded or go to the rigid boom vang.

Like I said my calculation of surface area of the threads was crude and I assumed that each thread was a circle rather than part of a helix. I used the formula Pi x Major diameter x depth of thread x threads per inch x inches inserted. Am I at all close with this formula or have I neglected something?

Just in case does anyone have any recommendations on rigid boom vangs? Any positive or negative experiences with either Garhauer or forespar vangs?

Also has anyone got any suggestions for a machine shop in the Oriental NC area.
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Old 08-07-2017, 13:57   #10
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Re: question for you mechanical engineers

If a rigid vang is cost prohibitive, why not a block and tackle vang? Personally, I would probably rather sail with no vang at all (which I have done), than the unadjustable vang you describe. At least I could use the mainsheet to control of sail shape while sailing upwind.

Maybe as a compromise, make a non adjustable vang with some extra line. So you can control shape sailing upwind, and downwind still have the boom under some control - though not adjustable.

I just don't like the idea of not being able to flatten the sail while sailing upwind.
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Old 08-07-2017, 14:02   #11
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Re: question for you mechanical engineers

Bill,

Give Gurhauer a call and ask them about a custom end on their rigid vang. One of the real advantages to working with them is they tend to happy to do custom stuff, for a minimal if any price increase. Obviouslyyou would need some precise drawings and pictures but they aren't hard to formulate.
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Old 08-07-2017, 14:54   #12
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Re: question for you mechanical engineers

Quote:
Originally Posted by wholybee View Post
If a rigid vang is cost prohibitive, why not a block and tackle vang? Personally, I would probably rather sail with no vang at all (which I have done), than the unadjustable vang you describe. At least I could use the mainsheet to control of sail shape while sailing upwind.

Maybe as a compromise, make a non adjustable vang with some extra line. So you can control shape sailing upwind, and downwind still have the boom under some control - though not adjustable.

I just don't like the idea of not being able to flatten the sail while sailing upwind.
The rig has no topping lift for the boom and the rigid vang keeps the boom from crashing down into my solar panels. This boat is really set up for a rigid or fixed vang. If Garhauer can make me custom ends for not too much money and I don't have to make major mods to my boom and mast I can probably afford one of theirs.

The rig on this boat is very non-traditional, but it works pretty well. It has no traveler and the mainsheet is more like a traveler. Because the fixed vang allowed no up or down movement of the boom sail shape control is limited to the outhaul and luff tension. It doesn't sound great, but oddly enough it works fairly well. I've done the Georgetown round the island race 4 times and won my class three times. The only loss was when a couple of custom catamarans entered the race and were put in my class. In the in harbor race I have come in third once and second twice. The third place finish was to two trimarans and the two second place finishes were to another Endeavourcat 44 with the exact same rig. I beat all of the catamarans in real time, not corrected. Believe me it surprised no one more than me. I had entered just for fun as I had never raced before. so it wasn't a matter of my superior skill and experience.
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Old 08-07-2017, 21:07   #13
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Re: question for you mechanical engineers

Quote:
Originally Posted by Captain Bill View Post
To answer a few questions:

The fork piece was originally inserted about 4 inches into the shaft.

The option of going to a rigid vang is definitely an option though an expensive one. Not only is the initial outlay expensive it appears that I would need to grind off the current tangs and replace them with ones supplied by the vang manufacturer. The cost of the mounting hardware appears to equal or be nearly equal to the vang itself not to mention the labor. The current tangs and pivot are 1/2" thick. This appears to be either to wide or too narrow for the off the shelf rigid vangs. The two candidates I see as most promising are the Forespar Yacht rod or a Garhauer 25-1. This is a Cat and the boom is quite heavy. It appears to be made out of the same extrusion as the mast so I would likely need to go up a size.

I'm trying a quick fix because I want to get underway again.

My quick fix is to position the boom where I want it and drill through both pieces at once using a drill guide to keep the bit centered while drilling the pilot hole. I would move to progressively larger bits until reaching 1/2" or 5/8". I would place the center of the hole about 2 inches from the end of the outer shaft or about half way down the inserted section. From the point of view of the inner aluminum shaft I would prefer going to a smaller pin size to keep as much aluminum intact as possible. I don't think that the size of the pins used in the rigid vangs need to be as big because the spring (metal or pneumatic) does not carry the sail tension load which is carried by the line/block system. In my fixed vang the pin would have to carry the tension loads from the sail and the compression loads from the boom weight. The problem is the more I think about this the worse the idea looks to me. A crude calculation of the surface area of the threads would suggest that about 1.3 square inches of aluminum are carrying the load and I won't have any where near that much left in the inner shaft if I put a 1/2" hole through it. I may be forced to have a new fork piece made and the outer shaft re-threaded or go to the rigid boom vang.

Like I said my calculation of surface area of the threads was crude and I assumed that each thread was a circle rather than part of a helix. I used the formula Pi x Major diameter x depth of thread x threads per inch x inches inserted. Am I at all close with this formula or have I neglected something?

Just in case does anyone have any recommendations on rigid boom vangs? Any positive or negative experiences with either Garhauer or forespar vangs?

Also has anyone got any suggestions for a machine shop in the Oriental NC area.
In the interest of the bolded goal above.

Wouldn't put too much stress on the 'capacity' of the threaded portion of the vang, would instead would look at the weakest link.

I'm betting that that is going to be the tang where the 2" wide fork connects to either the mast or the boom...and further guessing that the 'tines' of the fork are going to be 1/2" and the tang is going to be 1".

So for a 1/2" dia pin 1" long, the total surface area is 1.57 square inch; since the force is going to be either tension of compression, let's assume that the effective area is halved for either force, so the resisting area is .785 sq in.

The tensile yield strength for 6061-T6 aluminum is around 40000 psi so it seems that the fork will take around 30000 lbs tensile before failing...

Same calculation for the pin (bolt) through the fork shaft or vang body gives 1.1775 sq in, for a capacity of 47100 lbs.

Ultimate bearing strength for 6061-T6 is 88000 psi, so the commensurate capacities are around twice larger in compression.

Given that your boat only weighs 20000 lbs (and being aware that forces can be multiplied by different leverages), I'm thinking your original plan has at least some validity.

Again, in the interest of your original goal (you can always buy what ever vang you find appropriate), I would get a piece of 1" aluminum EMT, cut it to the correct length to fit into the body of the vang to hold the fork at the correct extension and absorb compression, buy two 1/2" x 5 1/2" or so ss bolts, drill two holes through the vang body and fork shaft (there are, after all, two 1/2" pins holding the vang in place...), fasten the bolts with nylocs, cut off the extra bolt length, and go sailing for a total cost of about 10 dollars. But that's just me...

I have no doubt that the real engineers here will fill me in on how I've misused or perverted some principle(s), but I'm open to the education.
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Old 09-07-2017, 02:34   #14
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Re: question for you mechanical engineers

Greetings and welcome aboard the CF, wholybee.
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Old 09-07-2017, 09:09   #15
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Re: question for you mechanical engineers

316 and 304 are about the same strength. But 316 is more corrosion resistant to salt spray. Both are pretty ductile unless cold worked. While ductile materials are not as strong, they are more forgiving.
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