How to anchor a raft on a river

Question

The past couple years I've been taking a homemade raft out onto the Charles River in Boston to see the fireworks on July 4th (think an old boxspring mattress holding 4 people). One problem we always run into is anchoring the boat in the middle of the river (about 10-20ft deep, ~5mph current).

I'd like to have the raft anchored to stay in a specific orientation (so it's easier to view the fireworks), and also strong enough to not move downstream. What kind of anchors should I use, what weight, and how many? Also, is there a specific technique used to drop anchors?

Previously, I tried just attaching 2 10lb weights and dropping them on opposite sides of the raft, but that didn't work out well. We still seemed to be drifting and turning. Any suggestions would be much appreciated!

Answer 1

If you want to remain stationary, you need a solid anchor, and the best option is going to be to attach to a tree or something that you can securely fix to on the bank.

Not knowing that river, I can't say whether that would work or not, so I'll discuss anchoring to the bottom. This is the same procedure for anchoring in a tide or a river, except in a river you only need to worry about water in one direction, which is a lot more straightforward.

In the sea the rule of thumb is to use an anchor rode 8 times as long as the water depth. This way, you are never pulling the anchor up, but always along the bottom - which aids friction. 8 may be overkill here, but perhaps 4 times is a good idea.

Ideally, you'll want two anchors, each on a short length of chain (to help weigh down the rope nearest the anchor - to help further with keeping it on the bottom) with a nylon rope; and row so the two anchors are each 45 degrees off due upstream. Once they are both set, allow your raft to drift out to the full length of the ropes. With each rope attached at one corner of the raft, you should have a nice stable platform.

Answer 2

Interactions with other boaters

This is probably the top thing that I would consider when anchoring. Key questions include:

• Am I allowed to anchor here?
• If I am using a single anchor, and the winds or currents change where will my boat swing?
• Are other boats using a single anchor or multiple? You should do the same thing, so that your craft moves constantly with other crafts.

How many anchors?

I would recommend a single anchor. This allows you to:

• Avoid tangling multiple anchors
• Dedicate the proper amount of rope or chain to the single anchor. This will be much more productive that multiple anchors with insufficient scope.
• Be much easier to manage
• Be clearer for your fellow boaters where your anchor lines are running.

The only times I would consider using multiple anchors is when:

• The wind, waves or current are running in different directions and you have a very good reason to point into one of them.
• Boats around you are using 2+ anchors and you need to as well to avoid swinging into one another.

What anchor?

I have personally had good experiences with both personally Bruce Claw Anchors and Danforth anchors. More info.

The Claw works great in mud and sand, although on harder surfaces, I sometimes had trouble getting it to grab.

How much scope?

How much rope or chain do I need?

Typically, if you are using chain, you need to use 3 feet of chain for every foot of depth. In your case this would mean 30 feet, if the water is 10 feet deep, and 60 feet, if the water is 20 feet deep.

If you are using rope, you need to us a 7 to 1 ratio.

The key is that you don't want to pull up on the anchor, you want to pull it a long the bottom.

Setting your anchor

1. Drop it in the water where you want it to catch.
2. Slowly backup your boat, spooling out line/chain. Don't drop lots of rope or chain at once, because you risk tangling it
3. When you have reached a reasonable scope, stop letting out rope/chain and use your engine to reverse on your anchor. Your anchor should be as strong as your engine.
4. Enjoy Fireworks