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Old 11-26-2020, 11:23 AM   #1
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Bow Thruster Performance

The marina we are currently in has a very narrow fairway. Over the last two months, I have watched probably 30+ boats of every variety come in and out - some with interesting 'entanglements' (my swim platform ladder included in one of them ).

What has struck me after watching them all, is the large variation in effectiveness of different bow thrusters - apparently having nothing to do with the thruster itself.

It appears, from my anecdotal data, that one of the primary components of effectiveness is the depth of the props from the surface of the water (DUH). I just watched a Beneteau 40-something trawler come in and while the thruster was wailing at full power and water was absolutely thrashing about at the surface, very little power was being imparted to moving the hull at all. It was obvious that the props were not very far into the water.

Other boats, some with proportional control operating at MUCH less than full power, barely needed any effort from the thruster motor to get the bow swinging around. The current at the water surface indicated that the props were indeed very low underwater.

If anyone is installing a bow thruster, making sure the props are WELL below the water surface would be a good parameter to consider - likely more so than just the raw kW of the thruster motor. Deeper is better, maybe to the point of not being worth anything if it is too shallow.
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Old 11-26-2020, 01:07 PM   #2
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I put a stern thruster on our boat. I went with the size larger than the chart called for. The incremental cost wasnít that much but it moves the stern very well. Deeper is much better. I donít have room for a bow thruster tube so I will put an Exturn pod thruster on the bow. They are much deeper than the normal bow thruster tube so that helpd as well. Many manufacturers put in the minimum size thruster and that doesnít help.
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Old 11-26-2020, 01:37 PM   #3
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My 24-volt thrusters move the bow smartly.
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Old 11-26-2020, 01:43 PM   #4
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When we bought Beachcomber J wanted a bow thruster. The manufacturer's charts called for (IIRC) 250 pounds of thrust in an 8" tube. The marina owner suggested going with the next size up which was around 280 pounds. He couldn't find one so he went up one more size to a 325 pound in a 10" tube.

It is able to move the bow swiftly against a current or wind and moves it well.

Nobody ever said "I wish my thruster didn't have so much power."
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Old 11-26-2020, 01:46 PM   #5
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Quote:
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When we bought Beachcomber J wanted a bow thruster. The manufacturer's charts called for (IIRC) 250 pounds of thrust in an 8" tube. The marina owner suggested going with the next size up which was around 280 pounds. He couldn't find one so he went up one more size to a 325 pound in a 10" tube.

It is able to move the bow swiftly against a current or wind and moves it well.

Nobody ever said "I wish my thruster didn't have so much power."
You got that right. Always go big on a thruster or go home...
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Old 11-26-2020, 02:13 PM   #6
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Installed by PO I am not sure the size, but mine is also quite effective. And as you can see it is WELL below the waterline.
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Old 11-26-2020, 03:35 PM   #7
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The problem with props too high is cavitation, that dishwasher sound and a less than effective propelling force. Due to how my boat is constructed with a hull wooden exoskeleton, plywood over that and fiberglass over that, the center line keel or whatever it is called, the internal backbone of the framing took up to much room to allow an effective tunnel bow thruster.

In the propaganda for the external thruster I had installed, it emphasized at how low it would sit in the water, below the hull and then some. So a smaller motor can have a greater effect because the propeller is not cavitating. I know my refit guy was impressed with how maneuverable and instant responding the boat was with the external thruster versus his Backcove 34's boat bow thruster.

I am happy with the external bow thruster, very happy, but I would still prefer a tunnel version if my boat had been able to accept one. Logs are a very real concern where I boat and in my first sea trial after an extended refit, in the first ten minutes I hit a log at 20 knots, fortunately up on plane so the thruster was out of the water.
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Old 11-26-2020, 05:07 PM   #8
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We never run faster than about 9 knots so hitting a log isnít as much of a concern, and we arenít in the PNW with all the logs in the water.
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Old 11-26-2020, 05:44 PM   #9
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We never run faster than about 9 knots so hitting a log isnít as much of a concern, and we arenít in the PNW with all the logs in the water.
Bye, bye, external bow thruster.
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Old 11-28-2020, 10:21 PM   #10
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Motor power selection and thrust most important ....contact thruster mfrs engineers with your boats specs to get expert tech advice and properly specing ...do not use "any good yard". This is an engineering task.
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Old 11-29-2020, 12:36 AM   #11
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I recently installed a new thruster. The documentation had specifications regarding the depth of the thruster.
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Old 11-29-2020, 01:49 AM   #12
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I also installed a Vetus bow thruster and took the next size up and fitted it deep to grip the water.
Our boat has a single engine, for single handed manoeuvring I fitted a Morse 1750 control with tilt switch which I converted to use as a bow thruster switch which means I can use the throttle/gear/bow thruster lever in one hand and the steering wheel in the other to make the boat go sideways.
Parks Masterson can supply you with one at a fair price.
If I had to do it again I would definitely fit hydraulic thrusters even if it cost a bit more initially.
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Old 11-29-2020, 06:41 AM   #13
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Before purchasing a thruster be sure you can live with its time allowed to be on.

Frequently buried in the maint info,instead of the sales brochure.
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