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.


  1. I just found your blog; beautiful work. I have been rebuilding a steel Dutch trawler, 56 ft, for the past 12 years. I have integrated the ship's hydraulics with the auto pilot since I need 2.5 gpm flow rate. To get around the clutch problem I went with a variable displacement pump which requires almost no energy to turn it when there is no demand. And that one pump also serves the windlass and bow thruster.

    My real question goes to your paravane stabilizer design. I am currently installing them on my boat. Beebe's book says that a bird can develop about 10 psi of resistive force. So over a 200 sqin bird you get a downward force of 2000 lbs. And that force in turn over a 20 foot moment arm creates other really big forces. I am using 3.5 inch schedule 40 steel pipe for the outriggers and at 2000 lbs bird force expect a compressive load of over 9000 lbs.

    Since the paravane design all grows from the resistive force of the birds, what resistive force have you assumed? What size birds you you use?


  2. Hi Mosby,

    Thanks for visiting. My new hydraulic pump is also variable displacement with a load sensing feature to adjust flow and pressure to demand. I chose to drive it from the main engine crankshaft via a clutch anyway so I could take the system offline completely (and still keep the main engine running) in the event of a hose rupture or some other issue with the hydraulics. Probably overkill, but it kind of makes sense to me.

    Our paravane design and installation was all done a few years ago. We did some load modeling at the time to determine pipe size and schedule. Some info is located here:

    For the fish, we went with 26" plywood fish from Englund Marine in Oregon. They are plywood, so a lot lighter to handle, and easier on the topside paint job.I seem to recall we used 5000lbs as max loading on the downriggers (this is the breaking strength of the weakest link in our paravane rigging from the ends of the downriggers).


  3. Hi Peter,

    I just finished my hydraulic system after two years of handling the boat with an installed bow thruster just sitting there doing nothing. What a huge difference the thruster makes...I love it.

    I have the same type piston pump, but I drive mine via a live PTO straight off the transmission. I have to cool it but was able to tap into the engine keel cooler and am able to keep the pump at 155 degrees. I'm thinking of welding another cooler for hydraulics into the hull on my next haul out.

    I was blown away by how big a job getting the hydraulic system installed was. It's on my last blog post.

    That hydraulic anchor winch you turned me on to works like a dream...I love it.


  4. Hey Conall.
    Sounds like we're doing the same project at the same time again. This time your turn to go first...
    I'll be putting in all the hydraulic circuit components this week and measuring for hoses. It takes a crazy amount of fittings and other bits and pieces. I'm seeing this stuff in my sleep...
    I'll check out your blog and see if you have any good ideas I can steal.
    Glad the anchor winch is working out. Your kids must be pretty strong now from hauling that anchor by hand for the last year!