Jock Strap Physics
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I spent a while looking at my mast step yesterday, trying to ascertain if I should invest in some straps. Now, I am not an engineer, but I have a pretty good handle on physics. I could not really figure out how the straps stiffen the system much at all, IF YOUR MAST FRAME IS SOUND. Here are my findings:
1) BOD installed and functioning as designed (no flex). BOD is strapped directly forward to mast and downwards to step.
2) bulkheads are in excelllent repair
3) The design of the boat has the bulkheads bonded to the floor about two inches aft of the lateral part of the frame. There is no indication of any failure along the length of the bulkhead or frame bonds.
4) mast frame is sound. In Old School, the frame is actually supported on the keel by a 1/4 aluminum plate mounted vertically on the sump bed and throughbolted to the frame.
5) the partners on my boat are pretty tight athwartships.
6) the deck is tensioned down to the mast by a turnbuckle anchorWhen the rig becomes loaded, the windward chainplate will have the tendancy to pull up on the system, and perhaps a touch inwards as well. This will also increase compression on the mast. If the BOD is doing its job, any inwards pull will be minimized. On Old School, I have intermediate shrouds as well. The leeward intermediate becomes taught when the rig is powered, so this will in fact help the BOD a bit, keeping the deck centered. In fact, the deck can not move much at all with being anchored down. With what I observed, the weakest part of the tension system has to be the small bit (2″ fore/aft) of hull between the bulkhead and the frame, assuming the frame is strong enough to withstand the compression loading. This little bit of hull is seeing pretty large tension forces, as the bulkheads are pulling up on it, and the frame is pulling down on it. BUT, of course, I am simplifying, and realize that these tension forces will actually be transmitted through the hull along the length of the bulkhead bond.
So, with all this in mind, is the only purpose of the jock straps to minimize compression on the step? Thus, if your step is sound and well reinforced, do you need straps?
As an interesting aside, if you use straps, it will effectively put all compression into the mast and remove it from the hull. For those of us with single spreader rigs, it is wise to eliminate as much mast compression as possible. This is one of the reasons I have a 2:1 main halyard, and my genoa halyard clutches are on deck, not on the mast. Could it be that straps might actually be contraindicated for single spreader boats?
IMHO: Jocks do not make sense. You cannot possibly offset the compression of the mast with them. In fact..you ARE succeeding in transmitting loads to the bulkhead/chainplates!! Think of a bow and arrow. If ANY movement of the base downwards happens..it goes to the chainplates..which will react as an inward movement. Here comes the BOD to resist that. Bandaids at best.
Let’s get real. The compression needs a solid base. The job is to spread the force across the frames. The frames have to be sound. The stock design puts 90% of the force on ONE frame. That frame has to be VERY strong. If you still have the original..you have a shaved 2×4 supporting a ton or better of weight. See the problem? A metal plate across the 3 frames will not work to spread the load. It will flex as the 2×4 fails. I will report when I can analyze what my frame replacement produces. I removed and replaced with a twice as thick (3 1/2″) and SOLID laminated mahoghany beam that reaches to within an inch of the bottom of the bilge. All tabbed and glassed. I say..forget the jocks..beam me up scotty.
Okay..I’ve added fuel to the discussion. What say you?
I favor using a jockstrap setup, but most of those I see are useless. The reason for the BOD/jockstrap structural stiffening is to prevent TINY flexure of the hull that will make big differences in shroud and headstay tension when you sail in gusty winds/big waves with lots of crew on the rail.
To do this the BOD needs to take an enormous compression load and the jockstrap takes an enormous load in tension with miniscule strain (compression for BOD and stretching of the jockstrap).
The shroud load on my boat is 1800# on each side after I set up the rig and at the dock. That compression load is on the mast no matter how your boat is rigged. If you have an oversize rod jockstrap, as I do, the boat does not flex appreciably when you initially set up your rig. The load is transmitted from the chainplates to the mast step via the jockstrap…and remember I use an incredibly stiff one. The tendency of the lowers to squeeze the hull is offset by the BOD. Whether you have a single spreader rig or a double-spreader rig is irrelevant.
Without the jockstrap the hull will flex as the compression load on the mast changes (under sail). That flexure is an enemy of your hull for durability and an enemy of consistent sail trim when racing.
That’s my two cents.
AlAl – “Without the jockstrap the hull will flex as the compression load on the mast changes (under sail). That flexure is an enemy of your hull for durability and an enemy of consistent sail trim when racing.
But this is my point…I have a very difficult time seeing the hull flexing any significant amount IF the bulkhead tabbing and step are sound. Now, if you have broken tabbing and a soft step, I totally get it.
And, I do believe that reducing mast compression is important in any sailboat. This is why so many high end boats have gone to masthead halyard locks instead of deck clutches. With our boats, the benefits of reducing compression would be more significant in the single spreader rigs, as the double spreader setups are inherently more stable.
BTW, I believe that the additional longnitudinal bulkhead between the main bulkhead and the forepeak bulkhead that some boats have would have a much larger effect on the perfomance of the boat than jockstraps would.
Halyard locks are a good idea because you gain better control of luff tension. They do reduce mast compression by an amount equal to halyard tension. Unless you put 1000 pounds of halyard tension on your sail, this will be small compared to the roughly 2 tons of compression from the shrouds, plus whatever you put on the headstay with your 50:1 backstay. Again, single or double spreader rig is irrelevant. Most folks put only a small amount of tension on the intermediates.
If your cabinet work has not pulled away from the hull yet, then you are getting significant stiffening from it, but it will not keep the hull from flexing to the degree that a proper jock strap will.
AlThe BOD keeps the hull from coming inward IF the mast is pushing the bottom of the boat downward. I believe the turnbuckle deck connection to the front of my mast holds the deck down and is doing the same job as the BOD. If the deck can’t rise, the sides are not coming inward. Boats that have cabinet tab problems..you have to look at how you have been setting up the boat. The tabs are breaking for a reason..most likely the deck rising. Do you have the deck secured to the mast or to a keel bolt? If not..there is the problem. I’m thinking a vertical ROD with a turnbuckle installed from the deck to a keel bolt is probably the ultimate fix. Replacing the 2×4 frame is mandatory in my book. It was a ridiculous joke to think it could hold up a mast.
Fixing the deck to the mast at the partners is structurally helpful. I use an aluminum channel cut precisely to length. It keeps the the deck from going up OR down. Again, the goal is to keep the hull from flexing. The halyards pull up on the deck at the partners and your crew presses the deck down. Flexing reduces the life of the hull. But this does not do the same thing as the jock strap.
AlHmmm. My doghouse/coachroof is a stepped configuration with paper thin glass that I can depress with my finger and likely kick a hole through. How does this flimsy piece of glass resist perpendicular loads athwartship. I’m not an engineer but the argument that the deck resists water, let alone the compressive loads of the rig under tension, is weak at best. The deck is similar to a trampoline on the Olsons I have stepped on. How this would replace a BOD eludes me.
Remember the rig loads are carried fore and aft, also. The bow and arrow is fore and aft. We raced Cal 25 ‘s in the seventies and the way we knew the backstay was tight enough is when the rudder bound up and wouldn’t turn then we backed off ever so slightly. If you can suspend the side stay rig loads on the Olson and not compress into the hull at the mast step, it seems to me that the hull would avoid the fore aft bow and arrow effect and carry the forestay load better. To better carry the forestay load is the reason for the longitudinal bulkheads, approved, spanning the cabinets and v-berth. I see this as the benefit of the jockstrap. The benefit being to deminish the fore and aft bow and arrow effect. I won’t be sailing without one.
If your deck is that soft..you have a core issue. Mine is a rock.
The fore and aft bow and arrow is a compression on the deck at the chain plates. Limiting the ‘rise’ of the deck will be the best solution (if you have a solid deck). A weak deck had better have the BOD or the deck will explode. :)
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