Knockabout Anchor Selection

One of the challenges of a knockabout type sloop is carrying equipment and supplies without overloading and hindering sailing performance. The follow is my thoughts on a reasonably light weight but capable main anchor. I have blatantly borrowed the methodology from "The Capable Cruiser" by Lin and Larry Pardey.

Anchor Calculations

Maximum Beam = 6 ft - 5 1/2 in
Maximum Freeboard at Bow = 2 ft - 9 1/2 in
Windage (Hull) = 18 square feet

Add 2/3 to account for spars and deck equipment
Windage (Spars) = 12 square feet

Total Windage Presented Head to Wind
Windage (Total) = 30 square feet

The Pardey's method assumes a maximum yawing factor of 2. So the total windage becomes:
Windage (Total, Max Yaw) = 60 square feet

The Dynamic wind loads are taken from page 355 of "The Capable Cruiser" and interpolated for 60 square feet of surface area.

Wind Speed (30 kts) --> Load = 180 lbs
Wind Speed (60 kts) --> Load = 756 lbs
Wind Speed (80 kts) --> Load = 1,338 lbs
Wind Speed (100 kts) --> Load = 1,944 lbs

The recommended chain size should be able to take a direct load of five times the maximum expected pressure without this figure exceeding the ultimate breaking strength of the chain. The following chain strengths would be need for these wind speeds:

Wind Speed (30 kts) --> Chain Strength = 900 lbs
Wind Speed (60 kts) --> Chain Strength = 3,780 lbs
Wind Speed (80 kts) --> Chain Strength = 6,690 lbs
Wind Speed (100 kts) --> Chain Strength = 9,720 lbs

Below is a summary of the minimum diameter chains and their weight per 100 feet.

BBB Chain
3/16 inch - Breaking Strength = 3,200 lbs - Wt = 43 lbs
1/4 inch - Breaking Strength = 5,300 lbs - Wt = 75 lbs

Grade 30 Chain (Traditionally referred to as proof coil)
1/8 inch - Breaking Strength = 1,600 lbs - Wt = 20 lbs
3/16 inch - Breaking Strength = 3,200 lbs - Wt = 39 lbs
1/4 inch - Breaking Strength = 5,200 lbs - Wt = 63 lbs

Grade 43 Chain (Traditionally referred to as High Test)
1/4 inch - Breaking Strength = 10,400 lbs - Wt = 73 lbs

For the Shields knockabout weight is a paramount issue. All of my sailing will be coastal during the Spring, Summer and Fall. I have decided to go with a combination of chain and nylon for the rode. I have selected 20 ft of 1/4 inch High Test chain and 150 ft of 3/8 in Samson 2-1 Supper Strong nylon anchor rode. I have matched this combination rode with a Bruce 16.5 lb galvanized anchor.


Main Anchor System Weight

Bruce Anchor = 16.5 lbs
20 ft of 1/4 inch High Test chain = 14.6 lbs
Swivel = 1.1 lbs
150 ft of 3/8 in Samson 2-1 Supper Strong nylon anchor rode = 6.0 lbs

Total System Wt = 38.2 lbs