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Friday, July 28, 2017

Preparing for the bow thruster install

The past couple of months of boat work have been focused on doing the design and prep work to install a bow thruster at our next haul-out. I have been driving boats of all different types and for over 20 years now and consider myself a pretty decent boat handler. Kama Hele is a very predictable, easy handling boat for a single screw, having a full deep keel, big rudder, and a 3.5:1 reduction gear turning a big prop.

But... there is no doubt that being able to spin the boat in her own length in a narrow fairway in an unfamiliar marina, or holding the bow tight to the dock against the wind while sorting out what we can tie off to would be a benefit when cruising. Of course we could have installed one for a lot less money and headaches while we were designing and building the boat, but I remember being completely out of funds when that time came and went. So, now the effort will require a lot of additional engineering and surgery on the boat to make it happen. So it goes...

We chose to go with a big, heavy, high-powered hydraulic bow thruster because our boat is also big and heavy, and there is no sense in going to all this trouble for a marginally performing installation.
After a lot of shopping we chose a 12" dual prop counter-rotating design by ABT (American Bow Thruster,
aka TRAC/Arcturus Marine). They are located in Rohnert Park (local) and do almost all of their fabrication work in-house. Excellent design and build quality from everything I've seen out there.
Here are some pics of the prep work on the hydraulic tunnel (12" sch40 steel pipe) and the fit-up of the gear leg and hydraulic motor for the thruster. We plan on welding this tunnel in place at our next haul out later this summer.

The engine PTO (power take-off) has been the most challenging part of the project so far. In order to provide the flow rate necessary to power this monster bow thruster (close to 25 gpm at low engine rpm) during docking, I had to upgrade the little 8gpm gear pump that we installed for the anchor winch to a 100cc axial piston variable output pump. This can pull hp in the 20+ hp range from the engine so the only option for driving it was off the front crankshaft of our Cummins 6BT main engine.

Even though the pump output is based on demand (it will happily spin at no load when no power is being demanded of the hydraulic loads attached to it), I really wanted the ability to clutch out the hydraulics completely when I'm not using the system. It was a big challenge to fit up: crankshaft adaptor-torsional coupling(to absorb vibration)-spline drive-BIG electric clutch-SAE C mounting pad-BIG hydraulic pump, and hang it all off the front of the engine in the limited space available. This involved some modifications to the front engine mounts and the framing for the deck plate in front of the engine.

Here is the result, which I am very happy with so far:

The big hydraulic pump just fit...

This pump also required up sizing the hydraulic tank. Pretty much the same deign as my old tank, but with a 12 gal capacity and up-sized fittings, including a 2.5" supply port to feed the new hydraulic pump. I am hoping that with the ability to clutch out the hydraulic system completely when not in use, I will be able to dispense with the complication of a cooling circuit for the hydraulic oil.

Since the new pump blocked access to the electic bilge pump in the bilge below, I moved the bilge pump off center, to the outboard side of the port side engine rail support. This puts it a little higher in the bilge, but it will kick in before any fluids in the bilge would reach the engine or the gear, so I can live with that.

This is all a lot of work, which hopefully will result in a high capacity robust hydraulic system that is big enough to also drive additional deck winches, pumps, and any other hydraulic gadgets we can come up with.