Dyneema core on a preventer?

[Jdege]

I'm just watching the current issue of Mads' Sail Life, and among other projects he's installed new lines for his main boom preventers.

He's using Dyneema-cored rope, and that had me wondering.

Dyneema is pricey, and I'd always thought that it's great advantage was lack of stretch. And while lack of stretch in a halyard is much to be desired, I'd thought you'd want some stretch in a preventer.

Or am I wrong?

[markxengineerin]

There may be debate, but the majority I've read believe stretch is a good thing in a preventer. Think of catching the sail in a wave- the force on the boom and everything else is proportional to the stiffness of the system, and a stretchy line just might be enough to prevent broken metal. On the other hand, I can think of no disadvantages of stretchy line in this application, as long as it's strong. I think a good size is ~the same as is used for a mainsheet, if there are good chances of it being used in anger someday. Most preventers I've seen around the marinas are smaller diameter, including mine, but I'm planning to upgrade.

[fxykty]

Originally Posted by Jdege

I'm just watching the current issue of Mads' Sail Life, and among other projects he's installed new lines for his main boom preventers.

He's using Dyneema-cored rope, and that had me wondering.

Dyneema is pricey, and I'd always thought that it's great advantage was lack of stretch. And while lack of stretch in a halyard is much to be desired, I'd thought you'd want some stretch in a preventer.

Or am I wrong?

Can you ask him in the comments why he chose a Dyneema line? His reasoning would be interesting.

On a monohull a preventer is there to hold the boom in the case of the sail getting backwinded. It has a secondary use to keep the boom from bouncing around due to swells and lighter winds. In both cases it can be adjusted very tightly against the mainsheet to fix the boom in space, in which case less stretch is better.

However, if you cannot pretension the preventer such that a backwinding won’t move the boom then stretch would be useful to prevent shock loading. In that case a line with stretch, or a line with no stretch but with an added mooring line shock absorber, might be useful and/or safer.

We’re on a cat, so don’t use a preventer to protect against backwinding - we use it as a vang when the end of the boom goes beyond the traveller. We use a non-stretch line and add a shock absorber to prevent shock loading the fittings. Here’s what that looks like: