The amount of water and air that can quickly pass through a net will depend on the openness (open area / total area) of the material. Openness should be the first consideration in choosing a netting material as it effects the seaworthiness of the boat. With catamarans, the forward nets will get pounded from underneath by water forces. With trimarans the windward bow net will catch wind pressure while the leeward net will catch the water.

Generally, this is more of a concern for offshore sailing where heavy weather may be encountered. Boats with larger net areas require greater openness than those with smaller net areas. High preformance boats require greater openness than cruising boats. Nets closer to the ends of the boat need to be more open than nets near the center. The following chart shows the openness percentage for several of the different materials on a typical sized net.

Openness Precentage
for a 50 sq ft net area with 3" lacing gaps

Openness Chart 2
Polypropylene Mesh SquareHole Mesh Coated Polyester Mesh 1-1/2"
Ultra Pro
1" Webbing Net w/2" Avg Gaps 1" Webbing Net w/1-1/2" Avg Gaps 2" 3Ply Webbing Net w/2-1/2" Avg Gaps


The following table lists some very general guidelines for the minimum net openness for offshore sailing. This is a guideline only, based on typical cruising multihulls, and individual boat requirements may vary considerably from this.

Boat type, and net size
(total size, port and starboard)

Minimum net openness
Catamaran fwd nets under 50 sq ft 30%
Catamaran fwd nets 50 to 120 sq ft 40%
Catamaran fwd nets over 120 sq ft 50%
preformance cruising Trimaran bow nets 70%
cruising Trimaran bow nets under 100 sq ft 50%
Trimaran wing nets under 60 sq ft 10%
Trimaran wing nets 60 to 120 sq ft 30%
Trimaran wing nets over 120 sq ft 50%


Openness figures for all netting materials. calculated for a 50 sq ft opening including borders and lacing gaps

Polypropylene Mesh


Square Hole Mesh


Coated Polyester Mesh

Offshore 3/8" Polyester Open Net 61%
Light Duty Nylon Open Net 73%
Ultra Pro 1-1/2" Polyester Open Net 75%
2" Webbing Net, with 3" Avg Gap
2" Webbing Net, with 2-1/2" Avg Gap 39%
2" Webbing Net, with 2" Avg Gap 35%
1" Webbing Net, with 2-1/1-2" Avg Gap 56%
1" Webbing Net, with 2" Avg Gap 51%
1" Webbing Net, with 1-1/1-2" Avg Gap 44%
1" Webbing Net, with 1" Avg Gap 35%