Jeff Seigel, founder of ActiveCaptain, writes…
As we pulled into the anchorage north of the Lake Worth inlet this week, in our trawler Red Head, we found ourselves in a sea of sailboats. It made us realize some of the issues to think about when picking a spot for both sailboats and trawlers especially when both types of vessels are in the same area.
There are a couple of things that might not be obvious about anchoring but are critical to careful planning. First, the amount of rode you let out after making a scope decision includes the high water level and the height off the water where your anchor rode is attached to your boat.
Ignoring a situation where you're anchoring in very deep water, many cruisers pick about 5:1 scope for anchoring. 7:1 allows us to sleep better. 10:1 would be used if a bad storm were expected.
So if you decide to use 5:1 scope in 10 feet of water when there is an additional 3 feet of tide expected, and your anchor rode is attached to your boat 4 feet off the water surface, the amount of rode to let out should be:
((10 + 3) + 4) * 5 = 85 feet
The second thing that many people don't realize is that a bridle does not reduce the attach point of the rode to your boat unless the bridle line is attached to the boat at a point lower than where your chain exits the boat. It is most common that the hawse holes/cleats where the bridle attaches are at the same height as the anchor chain exit so in reality, a bridle does nothing to reduce the rode based on scope. This is often argued but it's true. When the wind picks up, your bridle will raise out of the water and form a straight line to the anchor – it has to. It's easy to prove with some string and a toy boat.
So here's the potential problem when sailboats and trawlers anchor together.
We all want to use similar scope, yet it's often the case that trawlers put out a lot more rode. It isn't because they're being selfish. It's because a trawler's rode attach point on the boat is generally at the bow which can often be 8 or more feet off the water.
Here's a real example:
- A sailboat and a trawler are anchoring at low tide in 6 feet of water where a 3 foot tide is expected (the Lake Worth anchorage).
- Both boats will put out 7:1 scope because the strong winds are expected
- The sailboat's rode attach point is 4 feet off the water at the bow.
- The trawler’s rode attach point is 10 feet off the water at the bow chain lock.
- Sailboat rode: ((6 + 3) + 4) * 7 = 91 feet
- Trawler rode: ((6 + 3) + 10) * 7 = 133 feet
In reality, we put out 125 feet of rode because I did the calculation in my head. I believe the sailboat who anchored in front of us put out 75 feet of rode.
In the middle of the night, there was a major wind change with a big storm. As we swung around, we'd now be in front of the sailboat. And it would feel like we were 50 feet closer to the sailboat since we had
50 feet more rode out than they had. We'd end up being a lot closer even though neither boat dragged an inch.
These are the things to take away from this:
- If you're a sailboat, be very careful when anchoring in FRONT of a trawler with a high bow. Consider the difference between your bow heights times the scope to estimate the distance they'll move closer to you when the wind shifts. So in the example above, you should move an additional (10 – 4) * 7 = 42 feet in front of the trawler than you expect.
- If you're a trawler, you have a similar responsibility to consider your extra rode distance especially if you're anchoring BEHIND a sailboat with a big wind change expected. You will move closer and you must account for that before you pick your spot using similar bow height estimates.
Trawler owners can correct a lot of the extra rode needed by having a bow eye placed near the water line and having the anchor attach there.
That will be the subject of another newsletter….
Also, don’t forget to use and understand my app for your tablet/phone – DragQueen..
- For Apple here
- For Android here..