New Mast Tangs

[Roger Long]

I'm replacing all my standing rigging this year as well as pulling, inspecting, beefing up, and re-bedding the chainplates. All this is in anticipation of years of hard sailing to come.

I've decided to replace the upper mast tangs entirely rather than spending money to have them dye tested and still being unsure about fatigue effects. I'm also not too keen on the design which is as shown here on the left:

Note the unequal length of the tang legs. Under strain, the outer plate is able to flex more due to the bends, this will create strain cycles in the bends. If the outer plate should break, the rig would come down. It doesn't help that they were made so there is a sharp bend with an actual groove right below the bolt.

I'm going to have new, slightly wider, tanks made up of 3/8" stock as shown on the right and make them 1/2" longer so I will have the option of using mechanical jaw terminals at the top. I'll probably go with swagged jaws to start but rigging replacement in some remote spot will be easier down the line if I can fit a Sta-Lock or Norseman fitting at the top.

My 2011 winterprojects golb (blog in reverse since I like to keep it in chronological order) is now started at:

http://www.rogerlongboats.com/2011WinterProjects.htm

 

[Tim R.]

If your mast can handle it, why not convert to T-ball fittings?

 

[Roger Long]

If your mast can handle it, why not convert to T-ball fittings?

No compelling reason I can see for a cruising boat. It's a major reconstruction job at the mast head as opposed to just unbolting the existing tanks and bolting the new ones on. The slight weight and windage increase aren't something I'm going to notice. I couldn't even feel the difference with 25 pounds of radar dome and mount half way up the mast.

 

[kloudie1]

Good job, Roger. A better, more forgiving design. As you know, the metal brake marks from bending should be polished out for best fatigue protection. You're never going to have to worry about that piece being a problem !!

 

[tommays]

My 1970 Cal mast has the big ones in your second drawing as orginal which requires you to use the matching fitting which then becomes the weak point

 

[Ross]

Always better to have the bend with a small radius than to have a sharp bend. There shouldn't be much if any flexing on the tang if the alignment is correct.

 

[RichH]

I disagree with the design!!!!
The most vulnerable and quite 'unpredictable' part of the design will be the 'kinked' or bent section of the tang. Such will cause minute flexure at the 'kink' when the rigging stretches and the base dimensions change. This is what a structural engineer would call a 'stress riser' ... a geometric shape, etc. that causes the 'combined lines of stress' to abnormally and artificially come close together thus raising the 'apparent stress' to much much higher levels than if in 'pure simple tension'. Anytime you make stress 'turn a corner' you vastly reduce the load bearing capability of the component and accelerate FATIGUE.

It would be better and more 'stress pure', to weld angled 'tabs' onto the mast so that the 'pull' is all on 'one single straight line' and then simply 'direct connect' the rigging to the 'tab' ... that 'tab' would be rotated 90° to what you have now and the 'thin crosssection' of the tab would be parallel to the long axis of the mast. .... I dont know if your mast is 'thick enough' to withstand such weldment. Seldén masts often use these 'tabs' for high stress connections.

Tee Balls would be even better as they are not a 'constrained' join but are free to self-adjust (no flex).

 

[Ross]

With all of the boats that have been rigged as Roger's mast is has there ever been a failure of the tang at the masthead?
On wooden masts the method sometimes involves looping the shroud cable completely aroung the mast. This would achieve the straight line pull that is ideal but it is not very neat.
If the bend in the tang is a half inch radius the stress will be well distributed.

 

[RichH]

If the bend in the tang is a half inch radius the stress will be well distributed.

No it wont.
Ross, its the flexure AT the bend that leads to the vulnerability to fatigue failure. Such bends are quite 'unpredictable' unless you test them the rigorously to failure.
The 'simple' / classic beam formulas will show that typically such 'cantilever' structure can be a much as 1/4 as 'strong' in comparison to plain straight line stress.

 

[Ross]

No it wont.
Ross, its the flexure AT the bend that leads to the vulnerability to fatigue failure. Such bends are quite 'unpredictable' unless you test them the rigorously to failure.
The 'simple' / classic beam formulas will show that typically such 'cantilever' structure can be a much as 1/4 as 'strong' in comparison to plain straight line stress.

Then make it four times as heavy.
My question stands, has anyone ever heard of an upper tang failure?

 

[RichH]

Ross - rhetorical and anecdotal questions always stand.

Yes I had a tang failure on a wood mast on a small boat ... right at the 'bend'
Ive had cranse iron failures ... right at the bend.
Ive also had chainplate failure .... right at the bend.

I'd be quite sure to bet that many of the 'smiley toothed face' symbols you see on nautical charts are due to 'structural failures' where someone used cook-book values and directly applied them to complicated structure. <g>

;-)

 

[Roger Long]

I disagree with the design!!!!

And I agree with your reasoning. If I were doing this at standard safety factors for minimum weight in a racing rig, I'd do it differently. However, with the thickness and some extra width, the stresses will be low enough they should stand as many cycles as I have left to give it in this life time.

Welding isn't practical on this mast and my experience with aluminum and it's fewer fatigue cycles will be in the back of my mind every time I'm pushing the boat hard in a tough spot. I could perhaps still be talked into T Balls but don't want to get into a major masthead rebuilding. I just did that last year.

I've got the data to get these tangs bent to exactly the right angle and will put toggles on them.

What do you think about using two layers of 3/16"? There is the possibility of crevice corrosion but they are way up above the salt in the rain and just one would be enough to hold the rig up in most conditions. A crack would leave me with some redundency.

 

[CaptJake]

Just thought I'd chime in:
NO! one single piece of 3/8" will be perfect. yes it will stress but you will play hell stressing it to failure (the bolts would fail first). DO NOT "sandwich" 3/16" together as it will have the same effect as putting your thru-fit in a shear.

 

[tommays]

I gotta snap a picture of the Cal mast the thing is so overbuilt you could pick the whole 8500# boat up with ONE the SIX badly designed tangs

 

[Johan S.]

Sounds like a prudent or smart move to make for the32' Endeavour ( ref: white hull green stripe ) Do you keep her on a mooring? Beefing those up will provide more strength from back and fourth wave action. Btw, whats her name? Mighty fine boat you got there...

 

[RichH]

If you are committed to the tang arrangement then I Agree w/ jake.
SINGLE piece of 3/8 will better resist flexure than 2 pieces of 3/16".

Also agree to help with flexure resistance a near perfect alignment angle is needed.
;-)

 

[BobM]

I like Rich's idea, but rather than welding it to the mast, how about using the existing tang connection holes? In either case, Rich's or tangs (which I have), I much prefer having a standard toggle / marine eye/ jaw set up vs. T-ball fittings. The T-balls weren't still available for my last boat and having a solid toggle on the mast would enable you to change the more fatigue prone part of the rig easily and more often.

 

[Ross]

The bend will likely be around 12 degrees, Just the slope from the spreader to the masthead. rather easily determined and measured during fabrication.

 

[Roger Long]

I like Rich's idea, but rather than welding it to the mast, how about using the existing tang connection holes?

Yeah, since I know a very good welder and am going to all this trouble, I suppose there is no excuse for not making it bulletproof. The single tang will last as long as I will but someone, maybe one of my sons, will own this boat someday. So, something like this:

It would be wide enough to take a jaw terminal between the lugs so the pin won't be experiencing any significant bending stress. I'd probably skip a toggle with this design since the spreaders on this boat don't swing and have a fairly rigid attachement.

I'll have to lay it out pretty carefully to get the rigging lengths to come out right since the pin hole won't be in exactly the same place.

 

[RichH]

Roger
I really really do not prefer 'bolt connections' on masts or other 'thin sections' for the following reasons:

My design preference would be to establish a 'minimum torque value' of the bolt so that the tang to mast interface surfaces are/is a 'friction-compression' join ... the bolt there solely to provide the force to drive the mast and tangs together and with sufficient friction of the two faces to do all the 'work'. This will spread the lines of stress over a much larger area than letting all the load hang on a bolt and the bolt being driven down into that small mast bolt hole thickness (projected 'saddle' area of the hole).

The problem with bolts in thin sectioned 'saddles', etc. is compressional failure of the load bearing surface area of the bolt hole in the mast section - the 'fancy' name is bearing surface projected area 'tearout'.

So, if any of the currently drilled bolt holes in the mast show any 'deformation' (including any very small 'smiley faced' cracks) at all and especially in or near 'the bottom half' (half circumference) of the hole .... then I suggest a much deeper analysis for the selection of how to do this.

Simple speak - If the interior of the mast were accessible, I'd have a stainless, etc. 'thick' ferrule between the boltholes on either side of the the mast so that I could full torque the bolt without crushing the mast, the resultant induced friction from the torque would hold it all together, all the loads would distributed over a wide an area as possible and would not allow or minimize any 'side loads' (shear) on the bolting - the sole job of the bolt would be to deliver the 'force'. If the torque later released or failed, then the bolt would redundantly be able to carry the side-load due to its 'material strength' - a backup.
Just hanging a side-load on a bolt or 'pin' leaves no 'backup'.

Of course, if the original bolt holes, etc. show 'absolutely' no deformation ('egging', etc.) then you have a 'personal historical scantling-rule' that indicates that the original connection was valid. If there is *any* deformation of the current bolt holes (especially at the 'bottom of the holes' in the mast, then .......... .