Computer model of rode behavior

[pdqaltair]

Rode - Dynamic Behavior

Rode - Static Behavior

Rode - Dynamic Behavior

Not my information, no one I know, but thought provoking. Though I don't agree with all of his conclusions and it is clear that the model is too simple, he makes a good case for a good mix of chain and fiber. He also put a lot of work into this.

But let's trade thoughts.

[AdamLein]

It echoes a lot of the opinions I've read here, but with some formal reasoning to back it up.

One thing I noticed he did, though, when analyzing the dynamic load on all-chain rode vs. all-nylon rode. For the all-chain rode he assumed that the force on the rode was sufficient to lift the entire rode off the bottom, and gave the force. He made the same assumption for the nylon rode but didn't mention the exact force. This would occur for a much higher force for chain than for nylon, so the comparison isn't totally fair. If the force that lifts the all-nylon rode were applied to the all-chain rode, then presumably the catenary could absorb the extra tension due to a sudden gust, and the lack of elasticity of the chain would not be an issue.

Other than that, very rigorous treatment. I'll have to let it set in my brain and then uh, back down on the engine, metaphorically speaking.

[billyruffn]

Very interesting (not that I could understand the math)..... but last time I checked chain didn't chafe either at my bow nor on stuff on the bottom, nylon does. Chafe has a tendency to disconnect the anchor from that which is being anchored, in which case most of his curves flatline pdq.

IMHO, case closed.

[pdqaltair]

Originally Posted by billyruffn

Very interesting (not that I could understand the math)..... but last time I checked chain didn't chafe either at my bow nor on stuff on the bottom, nylon does. Chafe has a tendency to disconnect the anchor from that which is being anchored, in which case most of his curves flatline pdq.

IMHO, case closed.

In which case, you are enjoying the best of both worlds... no? The snubber damps wave impacts, and the chain catenary damps the gusts. You see, he neglected all of the damping effects - for example, the model showed your boat bouncing on the chain, which it clearly does not - so what he really showed was that a rope DOES NOT damp gusts longer than the absorption period of the rope. It makes them greater by allowing the boat to slide back.

The real story is that you need JUST enough snubber to manage the waves. And that squares with your experience, I think.

[billyruffn]

Do I use a snubber? You bet. And I really don't care if it parts due to chafe because the chain is wrapped around a cleat. The snuber does as you say....absorbs the initial shock of the waves and the chain then does what it's susposed to do -- keep the anchor in the mud and,... this is very important, attached to the boat.

All mathmatics aside, chain rodes are much more conducive to a good night's sleep.

[pdqaltair]

Originally Posted by billyruffn

Do I use a snubber? You bet. And I really don't care if it parts due to chafe because the chain is wrapped around a cleat. The snuber does as you say....absorbs the initial shock of the waves and the chain then does what it's supposed to do -- keep the anchor in the mud and,... this is very important, attached to the boat.

All mathmatics aside, chain rodes are much more conducive to a good night's sleep.

However, there are smaller boats that cannot tolerate the weight, and don't generate the chafe either. A chain rode would have been absurd on my first "cruising" boat, a 1300# Stiletto catamaran. 10% of the boats mass, all in ground tackle and all in the bow. Very different - different answer.

The interesting thing about the math, after adjusting for damping, is that it suggests that a fiber rode is only really useful if light weight is a factor, and so, unknowingly, I think it supports the conventional wisdom that cruising boats are well served by chain with a longish snubber. Lighter boats? There is a cross-over point somewhere between your boat and the Stiletto; I don't profess to know where. As my current boat is ~8000#, I wish I knew. In a few years I will need a new rode.

A good nights sleep is the goal.