We want to chat about thruster tunnels. We’ve mentioned these before, but this series of photos will help us to elaborate.
This straight on the bow photo allows the eye to follow the lines of the hull. You can just make out the aft edges of the bow thruster tunnels. The intersection of thruster tunnel and hull are faired to reduce turbulence.
The aft edge of the tunnel has been move inboard, reducing the edges exposure to the streamlined flow coming off the bow.
Even with this complex fairing job there is substantial drag associated with a disruption in the waterflow created by this ten inch (250mm) tunnel. Is the ease of maneuvering the bow thruster brings worth the drag penalty? On most yachts the answer is easy, since they are impossible to maneuver without a thruster.
Our situation is different. The combination of the hull shape of the FPB 64 and its enormous rudder, together with 40-degree rudder angle, means the FPB 64 can be worked into very tight spaces without its thruster. Once you get the hang of “walking” the boat using engine and rudder, the thruster will rarely be required.
Posted by Steve Dashew (December 9, 2009)
December 9th, 2009 at 3:12 am
Hi Steve,
Wouldn’t a retractable bow thruster be an option?
Regards,
Peter
December 9th, 2009 at 10:21 am
Hi Peter:
We briefly consider a rectractable thruster but this cuts into the bottom structure in an unacceptable manner and adds a further layer of complexity to this system.
December 9th, 2009 at 8:13 am
Steve,
Will you put a thruster on your own boat? Have you tried to calculate the impact of the drag in gallons per hour or knots per mile at an efficient RPM level?
Finally, I have seen many thrusters with slight bulbous protrusions forward of the opening with a similar “Indented” fairing aft of the opening… Did you model different options or have an opinion?
Finally, if docking were only an occasional activity and cruising and anchoring were the commonplace, I would think aside from the drag, there would be no desire for added complexity in systems that would have to be frequently run so it didn’t foul.
thoughts
thanks
Scott
December 9th, 2009 at 10:19 am
Hi Scott:
We did not take the time to model the thruster drag in CFD (a very big project) but at cruising speed on the FPB 64 this is probably on the order of one or two percent. There are many ways to try and reduce the drag of the thruster tunnel.The approach to this is dependent on the distribution of volume in the forward part of the hull. In our case we are so fine that the slight indentation aft is optimum. Our own experience with bigger and less maneuverable boats than the FPB 64 is that for our style of cruising a thruster is not required. We’d rather pay the occasional penalty in tight quarters, and anchor out if we felt we could not handle the situation, and gain the efficiency and extra storage space.
February 2nd, 2010 at 8:39 pm
Hi Steve, what hp is the bow thruster and is it hydraulic or 12/24v?
February 3rd, 2010 at 7:46 pm
Hi William:
The thruster is 24VDC, fed off the main batteries and wired for a three percent voltage drop. It has 11HP/8kW rated thrust.
March 1st, 2015 at 2:05 pm
I just got a semi-crazy-late-night idea: How about having round plastic plugs with foam edges for tight fitting to thruster tube for long passages, installed by swimming or snorkeling? Could have a release cord on deck to quickly pull these off before next use. Would cut drag by percents, without added complexity of retracting mechanism…
March 1st, 2015 at 9:40 pm
Great minds think alike, Andy:
Most folks think this is a stupid idea, but it is on the list for FPB 78-1.
March 2nd, 2015 at 6:12 am
Steve: Thanks for kind words! I bet many/most folks regarded the whole FPB-concept as a pretty stupid idea when you were developing it, but as always it is the price pioneers have to pay. But back to this thruster tube issue, maybe the tube could be designed to have slightly inwards narrowing geometry for tight fit for the plug, and the plug parts could be tied together with a piece of rope maybe fitted inside a garden hose type of plastic tube for very easy fitting through the tube. With a tight fit this plug could have very smooth and flush profile outside for I would guess pretty substantial reduction of drag. Fitting this into place would be about five minute swim job, and with even few percent drag savings would save large amount of fuel on a long passage, or using same fuel would take you there many hours before.