Wednesday, August 5, 2009
My current boat is a single screw ( one propeller, one engine), and the trawler I'm building will also be of the single screw variety. While my current boat is much smaller than what I'm building I've never had much of a problem maneuvering her around yet I'm choosing to install a bow thruster on the trawler. A bow thruster is basically a propeller in the bow of the boat driven by either an electric motor or a hydraulic motor that aids the captain in maneuvering the boat.
Choices for powering a bow thruster are either electric or hydraulic. While the electric models are more affordable and a little easier to install, I've chosen to utilize hydraulics as my power source. I going the hydraulic route for a few reasons. Hydraulic power is much more reliable than electric ( component corrosion is a non issue). Hydraulic units have a continuous duty rating unlike electric thrusters that are only able to be used for a few seconds at a time( the electric motors over heat, and the electric motor consumes the battery's charge quickly due to the huge amperage draw). By the time I figure on a separate battery bank, separate charger, cable runs, buying new batteries from time to time, reliability and failure when one needs it most, I feel hydraulics will be cheaper in the long run. I own and operate some excavating equipment that is 20 years old. I've never had a hydraulic failure on any piece of equipment other than a hose breaking, and it is safe to say that this equipment takes a beating. I can also tell you that breakdowns due to electric components, while not an everyday event, do happen and tend to cause things to come to a grinding halt.
After searching high and low I decided upon using a hydraulic thruster engineered and built by Key Power Inc. Key power is a Canadian company and I must say that all my dealings with them have been great. I've yet to purchase my thruster, but they gave me good advice on sizing the unit and have been helpful with the hydraulic system as a whole. As much as I wanted to keep my purchases in the states, Jim at Key Power was the only vendor who actually acted like they wanted my business and wanted to sell me components. Good people.
The actual thruster is housed in a 10" sch. 40 tube welded into the hull. I built a sled to rest the tube upon so I could scribe the cut out on the hull and scribe the shape of the cut on the tube. The sled made this job go pretty smooth. After I cut the hole and scribed the tube, I slid the tube into the hull and marked the other end and cut the exit hole. Once the holes were cut I fabricated doubler plates to weld to the hull on the outside of the hull. I slid the tube back through the now doubled hull and marked the tube for final cutting. A few hours of grinding and cutting and I was happy with the fit of the tube. I blasted the mill scale off of the tube before the final install with the hope of saving some blasting inside of the hull. I had welded some gussets inside the hull that would be on the bottom of the tube once the tube was installed with the end result being a water tight bulkhead below the tube. Once the tube was installed the final welding took another day to complete. I did not have to cut any frames to install the tube, but I did have to cut two longitudinal stringers and weld them back to the thruster tube. I've been told that the deeper the tube is in the water the better it will perform. Keeping with that thought I was told that the minimum depth on the tube was one tube diameter below the water line to the top of the tube. I think I'll fabricate a grill of some sorts to keep trees and heavy drift away from the prop.
The bow thruster tube cut one of my ballast boxes in h half so I'm going to have to be careful on maximizing space for my ballast. I measured the cubic footage of the halved ballast box and by using lead shot, I should be OK in regard to the amount of ballast in that area. The bow thruster motor will be housed inside of a water tight box welded around the tube accessed under the forward sole. I don't know if the water tight box is over kill, but if something grabs a hold of the thruster prop and rips things apart, the water tight box might come in handy.
Looking at the completed job all I can think of is how I just cut a big assed hole in the bottom of my boat. While the money I'm going to spend on the thruster is not small change, I'm looking forward to the added maneuverability my little steel trawler is going to have.