Mizzen Running Backstay Block Sizing
 

Yes, it's another one of those backstay rigging questions. I am trying to decide what size blocks to select for re-rigging the mizzen running backstays on my 44' Spencer Ketch. Previously, the backstays had been moved by a previous owner and oddly attached to large davits as fixed backstays. The main and mizzen are independently stayed on this boat (except for the stay that everyone erroneously calls a "triatic" which connects the mastheads) and the mizzen backstays never seem to load up much even in high winds when I prefer to use the mizzen to balance the helm. Also, the mizzen sail area is only 177 sq-ft.

In order to re-rig the running backs I performed the following calculations:


Calculation of max backstay load due to righting moment

The waterline length is 32'6" and using this, I arrived at an estimate for RM30 of approximately 40,000 ft-lbs (from Skene's and Rigger's Apprentice).

Ultimate Righting Moment of Mizzen = 40,000 * 0.5 = 20,000 ft-lbs

RMult / (1/2 Beam) = 20,000 ft-lbs / 5.5 ft = 3,636 lbs

Using 1.5 to 2 for a factor of safety for mizzen loads and assuming that the backstay will take ~25% of the load:

Max working load on mizzen backstays = 3,636 * 2 * 0.25 = 1,818 lbs


Max load capacity of existing mizzen backstay pennants

The existing pennants are 1/4" 1x19 316 S.S. wire.

The highest manufacturer's tensile strength value that I have found for 316 wire is 7,481 lbs so I will assume that is the largest breaking load that the backstays will see.

Assuming that the safe working load is: 55-65% * BL (I think this was from Rigger's Apprentice)

SWL of 1/4" 316 = 0.65 * 7,481 lbs = 4,863 lbs


So ...my question is: what load should I size blocks to - the max calulated working load of 1,818 lbs, the max working load of the 1/4" wire of 4,863 lbs or the tensile strength of the wire 7,481 lbs?

Sizing the blocks to match the breaking strength of the wire just seems a bit excessive in this situation. If everything is matched in breaking strength then you still can't predict which component will fail first.

Harken 75mm Black Magic blocks would seem to fit the numbers and Garhauer has some blocks in their 70 series that have 5,000 lb working loads. Lewmar's Ocean blocks seem pretty huge and heavy to reach a 5,000 lb working load and I can't even tell if Schaefer's blocks are suitable because the Series 12 weigh about 4 lbs each! I do want a rugged and durable rig but that much weight swinging around on a backstay pennant just seems like a real brain basher.

By the way, I'm just contemplating a simple 4:1 or 6:1 tackle. Any and all thoughts on this would be appreciated.

Cheers,
John

Whoa
 

Hi there,
You were good right up until the assumption that the (running) backstays would take something like 25% of the load. They don't. They just keep the mast from popping forward when sailing with the mizzen stays'l (you do have one of those, right?), and possibly when hitting a stiff chop. The weather shrouds will handle all of the mizzen loads, and almost all of the stays'l loads, so we're talking a tiny fraction of the total load here. Put up some 3/16" 12-strand Spectra. Hang a Colligo purchase system at each lower end (http://colligomarine.com/products/co...rchase-systems). Belay to a PXR cleat. All you need.
Oh, and 1.5 to 2 isn't much of a safety factor...
Fair Leads.
Brion Toss

Yeah ...I thought you'd say that
 

Well ...I believe that using 25% of the total load for the backstays came from the example in Rigger's Apprentice on the top of page 138 (since it was difficult to find any other rule of thumb in other design texts).

I'm assuming that you mean that 1.5 to 2 is insufficient for a factor of safety for shock loads in backstays but, again, this came directly from the "Mizzenmast Scantlings" inset on page 284 of Rigger's Apprentice. In Chapter 5 of Rigger's Apprentice the value of 2.5 is quoted a few times relative to main backstays but I'm still lacking any real good data on mizzen scantlings.

I do not actually have a mizzen staysail because the main has fixed backstays attached to the masthead that seem like they'd interfere even though my sail plan shows a staysail and Franz Schattauer might have actually made one for this boat years ago. The only stay that holds the top of the mizzen forward is the one that goes between mastheads (still don't remember what the correct term for that is). The mizzen has single uppers and double lowers for shrouds so, aside from the aft-led lowers the stick has nothing else keeping it from springing forward (and it's a real wet noodle to climb above the spreaders without those backstays).

The only situation that I can imagine where the running backs might take a decent load would be if I got pooped by a very large wave when running. It would have to be really really large though because this is a pretty dry boat with a lot of freeboard.

I believe that your description of a "tiny fraction of the total load" lines up with my experience with the rig Brion. It seems like I've seen at least one old picture of a Spencer with a ketch rig where it looked like they were using line that might have been as small as 3/8" and terminating it at either a cam cleat or regular cleat near the side of the cockpit. I do wonder if those Colligo system "blocks" actually run free well since they are decribed as "static". I don't really want to fuss around untangling things on the after deck on every tack.

John

Lone Ranger Shot with Silver Bullet
 

Hi again,
Yes, that is what I said in the "Apprentice." To be fair though, note that the backstay loads I refer to are for the mainmast, where the heavily-loaded jib is the opposing force. No such load can come to the mizzen, even with a mizzen stays'l.
I had quite forgotten about the low mizzen S.F. recommendation, I admit. And I don't think I would ever actually go with it now.
As for your current main backstays, if they are doubles, then yes, you won't be able to fly a mizzen stays'l. But if they are forked -- or you can convert them to forks, -- then you can. Recommended for this and other reasons.
If you don't fly that stays'l, then a truly teeny set of runners, made off to light belays, will be fine. Just consider function and load.
It's called a springstay.
Fair leads,
Brion Toss