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Chocks on the bow

4991 Views 36 Replies 13 Participants Last post by  Stumble
So, are chocks on the bow of a boat:

1. To guide a mooring line from an inboard cleat as cleanly as possible in order to cut down on chafe.
2. A shock absorber to protect an inboard cleat, an essential part of any mooring system
3. Another point to chafe through a mooring line because the boat designer/builder couldn't figure out, or couldn't be bothered, to put a cleat on a rail
4. Something that could, and should be eliminated by putting a cleat on the rail instead.

Answers on a postcard. Or here.
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Bow rollers can take some load, but are not typically meant to take the potentially extreme shock loading of a choppy anchorage or the whipsawing of a bow that sails back and forth across the wind. That's why a snubber is recommended.

I confess, I will sometimes send the snubber down the bow roller, but only when I know the anchorage will be fairly smooth and I'm just trying to unload the windlass.

As for the chock, a simple piece of chafe protection at that location and you're good to go. I believe they are designed to keep your lines from sawing over your toe rail. I don't think you can get sufficient backing plates under the cleat, if it was immediately on top of the toe rail.

Some use old fire hose for chafe protection. Companies like Chafe Pro make neat alternatives for different size lines. You can either let them run free to be adjustable or put a few stitches through the line to secure them in a permanent location.
..... Many boats with combined rope/chain rodes (on which the rope section provides the snubbing function) run their rodes over the bow roller all the time. Given a properly designed bow roller I can think of no reason to do otherwise. The stem is among the strongest portions of a boat.
I may not follow. Are you saying you can see no reason not to run a snubber over the bow roller?

I've seen plenty that wouldn't take a good side load from shifting winds very well, let alone a serious fetch.
.......since a chock is supposed (though likely isn't) to be designed to handle twice the load of the cleats the structual issues should alreadybe resolved.
This must not always be true. Our chocks are simply through bolted with washers behind them. Our cleats each have a quarter inch thick piece of ~4in x 8in stainless steal backing plate underneath them.

However, the chocks take a substantially different and lighter load, as best I can tell.
Depending on the lead, chocks can take substantially higher loads in shear than cleats.
I'm trying to visualize why. From memory, the studs holding our chocks are 1/4". Those holding the cleats must be 5/8". Nuts, obviously, even larger.

I can picture the chocks are actually taking a lower shear load, but can't figure how they would be able to take a larger one.
A forward spring line for instance, the line comes forward, hits what amounts to a turning block (the chock) the goes to the cleat. The actual force it carries can be worked out, but a 180 degree bend equals double the face on the chock as the cleats.........
I'm still having a hard time visualizing this. Not all spring lines make a 180 deg bend through the chock, but one of mine does. That doubles the load from the cleat? I'm not engineer here, but that's hard to intuitively imagine.

Nevertheless, what I'm really trying to get to is that the chock is actually designed to take a larger load. As I mentioned, our chock is held by washers, our cleats are held by back plates.
My point is that your chocks should be designed for greater loads than they probably are.
That wouldn't surprise me.

Is the engineering true that a chock, with a 180 bend through it, is taking a greater load than the cleat? Best I can imagine, the friction around the chock is reducing the stress on the cleat, but I don't see how it multiplies. Particularly from a load that goes directly to the cleat, with no load on the chock.

I've also just considered that our chocks are sandwiched into the toe rail, therefore, there is some addition shear protection than the small studs and washers.
A 180 degree lead through and around a chock can be thought of as a tackle (blocks and line). The 2:1 (or 4:1 or whatever) force multiplication doesn't just happen..
Isn't a 180 deg bend around a chock like a 1:1?
Depends on what you are counting. The tension in the line is the same going in and going out. The load on the chock is twice that, just like a working end on a flying block.
Very helpful. I think I see it now. The cleat on the deck and on the dock are both pulling against the chock simultaneously.

I'm fascinated that mine aren't fully torn from the boat by now. One spring line does a 180 through the chock and doesn't show the slightest bit of strain on what seems like a pretty weak mounting. It's purely coincidental that I even know this, as I was tracing a deck leak and re-bed these.
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