Snatch Block Sizing

[ErBrown]

For various reasons, I'd like to pick up a couple snatch blocks. The highest load task I can imagine using them for would be something like attaching to a sea anchor to adjust bow angle when hove to. Though I hope to not need them for this, if I'm going to purchase a couple I'd like them to be capable here.

So for the sake of this discussion let's assume the boat is 25,000 lb displacement. How much of this would I need to account for when sizing a snatch block for the task and any recommendation on make/type?

Thanks,
EB

[Stumble]

According the the JSD website the attachment loads on your boat can run as high as 16,000lbs. This is far to high to reasonably be controlled with a snatch block, it can be done but you really won't like the price.

I am partial to Karver blocks, and the right one for this job would be the Kbr105 with a swl of 10.5 tonns. The problem is that at £2,500 a piece....

Harken has a high load soft shacks block that's new to the market, at 12t MBL they run about $1,500 each, but I have no experience with them. You might get away with the 8t at just $1,000 a pop, but it's there won't be much safety factor.

The only reasonable way to attach a sea anchor or drogue is with chainplates added to the structure of the hull. The loads are just to high otherwise. And even if the snatch blocks can handle the loads, you need to be very sure whatever you attach them to can as well.

[ErBrown]

I'd say that pretty much sums up my concerns on scaling he block. Would I better better off with a rolling hitch on the sea anchor line led to a Genoa winch in the cockpit, to adjust bow angle?

I know this is a recommendation for stopping roll at anchor, to point the bow into the swell. Seems it might be an option here too?

[Hydra]

For a similar purpose (redirecting the traction of a rode to a primary winch), I went to friction rings: much cheaper than blocks, very high working load, very light.

The only negative point is that basic rings don't open. Some do but they are not strong enough: soft links-antal

IME, rigging a bridle for pointing the bow (or the stern) into the swell works well for reducing roll, IF the conditions are sufficiently stable. A small change in wind direction is often sufficient for causing the boat to "tack". Then, the risk is to catch the bridle or the rode in the keel (don't ask how I know).

Alain

[Stumble]

Winches are not strong enough to handle this type of load. The only thing that should carry a drogue or sea anchor is either chainplates built for it, or cleats wth massive backing plates. No normal deck hardware will come close to being able to control these loads.

Any type of drogue needs to be carried by a bridle, with each leg at as shallow an angle as possible. Then the two legs can be adjusted to center the boat. But in storm conditions nothing is going to be perfect. The wind is constantly changing and the boat will just have to be left to hunt some. This is why the predicted loads are so high.

Friction rings do work, but they require uncovered dyneema to work well, they rely on the low friction bearing surface of the rope to reduce friction, use standard nylon and they will chaff.