Deck Cleats - Four or Two Bolts? SS or Aluminum?

[sailingdog]

True... I was oversimplifying. Of course, loads that are perpendicular to the cleats are going to be loading them far more than ones that are in line with the cleat. Four-bolt base cleats are obviously better at resisting side loading due to their wider base.

Mr. SailingDog,
Thank you for the correction about the area.
I wouldn't say the cleat is being pushed downward however. Rather it is being sideloaded with a force vector that has an angular component in the negative Y direction. This is putting a bending moment on the base of the cleat. I think Scott said it was a two bolt cleat so one side of the base is pushing down on the deck while the other side is pulling up. In short the screws are being bent so the nut is digging into one side of the washer while the other side is unloaded. This concentrated load, coupled with a possible wet core could explain the catastrophic failure. Using a four screw cleat as Scott did with his replacement provides a much larger base to resist this loading as the reaction moment on the base resisting the bending moment on the cleat will be much better. I am concerned as on my O'day my cleats are all of the two screw variety and possibly the hollow aluminum version that Maine Sail showed fractured. Pearson and O'Day were pretty close companions just before they both went backrupt. At one point Pearson owned O'Day, O'Day also ran Cal for a while and the last few years of the Cal brand looked just like an O'Day, but my year was still under the Bangor Punta banner along with Smith and Wesson. My stern cleats are arranged in the direction of loading however so that is better (lines apply loads in direction of the long axis of the cleat) for stress distribution.

 

[Bill Roosa]

Design loads

Back in engineer school, dinosaurs roamed the earth then BTW, we always started with the design load then built the structure to handle it. You should not design the structure based on one random part because you don't even know if that part is strong enough to handle the design load.
So while the cleat designers will, no doubt, have specified bolts that are strong enough to hold the cleat on, you don't know if that cleat is strong enough to break the line. An undersized cleat would be a problem no mater how "strong" you made the deck support.
Course if you buy the cleat that is specified to handle a certain size line you should be OK until you double up on the lines...... and the loador use a larger line.

 

[Bill Roosa]

Resisting loads

a wider base does not necessity make the part stronger. It could transmit higher point loading to the deck and make it fail due to increased contact pressure. If you use a smaller base the load could be transmitted through the bolts (not to the deck) to the backing plate and then to the deck (with a larger resisting area).

You really have to trace the load from start to finish and analyze all the failure modes to "design" a cleat.

 

[sailingdog]

The general rule of thumb for sizing cleats is they should be 16x the line diameter. This usually allows you to use two lines of that diameter on the cleat.

Back in engineer school, dinosaurs roamed the earth then BTW, we always started with the design load then built the structure to handle it. You should not design the structure based on one random part because you don't even know if that part is strong enough to handle the design load.
So while the cleat designers will, no doubt, have specified bolts that are strong enough to hold the cleat on, you don't know if that cleat is strong enough to break the line. An undersized cleat would be a problem no mater how "strong" you made the deck support.
Course if you buy the cleat that is specified to handle a certain size line you should be OK until you double up on the lines...... and the loador use a larger line.