In preparation for installing a bow thruster during the next haul-out, I need to estimate the needed capacity. The box thruster must be capable of countering wind. The force applied by the wind onto the boat is determined by the factors including the wind speed, angle of wind attack, and lateral wind draft area of the boat. The wind pressure is given by the formula:
P = 1/2 × ρ × V² (lbf/sq.ft), where:
ρ (rho) represents the specific mass of air = 0.0752 lb/ft²
V is the velocity of the air in ft/s, where 1 knot = 1.688 ft/s
The wind-draft of the boat can be determined by multiplying the wind pressure by the wind draft area. The wind draft area is determined by the shape and the dimensions of the of the boat (its windage) and the wind angle, and the greatest wind-draft is created if the wind is at 90 degrees to the boat. An efficiency reduction factor is generally applied to account for windage, and a value of 0.75 is frequently used. To calculate the required torque to rotate a boat, we need to know the wind pressure and the length of the boat.
The Aegir-Ran has a LOD = 36 ft and the freeboard is about 3 ft, giving wind draft of 36 x 3 = 108 sq ft. Adding the cabin, hard dodger, and the sail pack adds perhaps 50 sq ft, which is why the efficiency reduction factor is often used. I will assume the lateral draft is 200 sq ft and I will not use an efficiency factor. Assuming we want to counteract a wind force of 25 knots, the wind pressure is:
P = 1/2 × (0.0752 lb/ft²) × [25 knots x 1.688 ft/(s x knot)] = 1.59 lbf/sq.ft
To counteract this wind pressure, we sill need the following amount of torque:
T = 1.59 lbf/sq.ft x 200 sq/ft x 36 ft/2 = 5,724 ft.lbs.
The thrust-force required is the torque divided divided by the distance from the bow thruster to the pivot point. In our case, we will assume we want to pivot about the stern, do the distance is essentially the LWL = 33 ft:
F = 5,724 ft.lbs. / 33 ft. = 173 lbf.
I have decided to install a 12 V Ventus bow thruster. Examining the specifications of the models, it seems the BOW7512D is the best model with a maximum thrust of 180 lbf.
