Sizing a bow thruster

I am considering putting in a bow thruster.  This is a small propeller in a tunnel that is mounted in the nose of the boat.  It allows you to push the bow to either side.  Since this tugboat only has a single screw, it is going to be tricky to dock, especially with a wind or a current.   I found a nice site at Thrustmaster that details how to size one and how to power them. On their site they say, “As a rule of thumb, a bow thruster used for docking and undocking should be capable of producing thrust in pounds of force (lbf) equal to twice the lateral cross sectional vessel area below the water in square feet or two to three times the lateral cross sectional area of the superstructure above the water, whichever is greater.”

Considering just the below water area, I would estimate that the lateral cross sectional vessel area below the water would be the boat length x the draft.  This area is a rectangle, which is bigger than the actual underwater lateral cross section, but a good approximation. For New York Central No. 13, that would roughly be 90 feet long x 9 feet deep = 810 square feet.  As per the guideline, we double that, we get 1620, so 1620 lbf of thrust needed.  Looking at bowthrusters that create that much thrust, it looks like one that is powered by a 80 hp hydraulic motor is in the range. An 80 hp hydraulic motor needs 60 gallons per minute at 2350 psi passing through it. That is a lot of flow. What is that in pipe or hose size? 1″ perhaps?

The Thrustmaster site also says that two smaller units are better than one big unit as there is redundancy and the overall cost is cheaper.

Surprisingly, there are 2x nice 40hp Wesmar DPC-25 units on ebay right now that I am looking at.  These are the commercial series models and a single unit is recommended for boats from 50 to 85 ft in length.  So by installing two of these units, I think I am doing pretty good as NYC 13 has a larger draft than a comparable fiberglass vessel of this length.  These bowthrusters have two counter rotating 12 inch propellers per unit.  There is no hydraulic or electric motors to power them in this ebay sale.  They need to be installed into 14 inch steel pipe tunnels.  I could weld in the tunnels and install these units, get the boat back in the water and then come back later to work out the motors and hydraulic lines.  And the seller is located in Brick, NJ which is very close to the boat. Very interesting…

Update Aug 25, 2009 – after careful inspection, it turns out that one of the units is missing some parts.  Both units need the bolts that connect the bronze saddle to the tunnel pipe and the bolts that connect the unit to the saddle.  But one of the units is missing the unique tapered bronze nuts that hold the propellers to the shaft, the keys and the washers.  I have written to Wesmar and they say they can supply these parts.  I am particularly wondering if there is a gasket between the bronze saddle and the pipe and between the unit and the saddle?  Or does it just get bedded with something like 3M Marine Sealant 101 or 5200?