Bow thruster not producing much thrust

I see neighbors and online videos of people whose bow thrusters move their boats around with ease, but my Vetus 75 Kgf bow thruster has always been rather weak. It is unable to overcome much more than a light breeze and does little to aid turning the bow on our single screw boat. It makes noise and I can see water moving laterally off the bow when running, but it does little to move the boat. Vetus indicates that the 75 Kgf is "suitable for Yachts and Powerboats up to 15m (50ft)." Escape is a 2007 40' Mariner 37.

Our bow thruster is powered by the house bank which lives in the lazarette almost 60 wire feet away. Cables between the house bank and the bow thruster are all 2/0, though they connect through a 1-2-Both switch and a main busbar along the way. Other than the anemic output from the thruster, I've not had any electrical issues with the motor or controls and they all appear to be original equipment.



I can not say the same for the Lofrans Kobra 1000 windlass (above right). First the down terminal on the motor lost continuity and then the up terminal went dead leaving the motor unresponsive. The picture is old and the contacts have been cleaned to a shine. The power arrives at the terminals on the motor, but the motor does not run. The 1000 watt windlass motor is also powered from the house bank nearly 60 wire feet away. Lofrans explained that low voltage and the resulting increase in amperage is a common cause of electrical contact failure on their motors and controls, though living in the wet environment of the anchor locker is a much more difficult life than living in the dry space above the bow thruster tunnel.



Point is, both motors aren't performing like they should. Is it unusual for the batteries powering these significant loads to be so far away? Are new Helmsman 38s designed the same way, or do they have dedicated thruster and windlass batteries in the bow? I have not yet put a meter on the thruster when it's running to measure current draw or voltage, but might that be why it seems to produce so little thrust? Yes, it could be something in the thruster prop or tunnel, but output has always been less than expected.

The simple answer is to check voltage under operation. Hold a digital volt meter on the bow thruster lugs and have your wife give a 2 second pulse to measure voltage drop (while the boats is in the water). Both the bow thruster and the windlass motor are DC which is absolutely effected by voltage drop.

On my boat, changing the bow thruster from the house bank (30'+ away) to 24 volts 2' away, easily double the effectiveness and reduced pulse duration buy 75%.

Ted

Many discussions on why not to use house bank as starter batteries. Often I have said my preference is to have windlass on the Start battery while the engine is running.
Line loss and the fact that house banks usually do not output a continuous starter load (I consider a thruster as a starter load) from two feet let alone 30 feet away on 2/0 cable.
You see water flow comparable to a garden hose instead of a fire hose as a result.

O C Diver said:

On my boat, changing the bow thruster from the house bank (30'+ away) to 24 volts 2' away, easily double the effectiveness and reduced pulse duration buy 75%.

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Doubled the effectiveness and reduced pulse duration by 75% meaning that your thruster put out that much more flow? That is impressive. Did you replace the 12 volt thruster motor with a 24 volt motor?

Yes need separate batteries for thrusters load test them first. If that’s ok then voltage drop test when in use. If that’s ok it’s the thruster itself or something with prop or tunnel. You can get minor movement or misalignment which will drop effectiveness

SteveK said:

Many discussions on why not to use house bank as starter batteries. Often I have said my preference is to have windlass on the Start battery while the engine is running.
Line loss and the fact that house banks usually do not output a continuous starter load (I consider a thruster as a starter load) from two feet let alone 30 feet away on 2/0 cable.
You see water flow comparable to a garden hose instead of a fire hose as a result.

Click to expand...

I have read those discussions from a perspective of battery life and charging rather than from the perspective of starter (or thruster) motor speed, but it makes perfect sense. Suppose my 75 Kgf thruster hypothetically spins its propeller at 200 rpm when the motor is getting 12 volts. Does that mean it only spins the prop at 150 rpm when the voltage drops to 10 volts due to line losses?

Voltage drop is the most likely cause. Drop the voltage in half and the motor power output drops to 25%

Just looking at your setup I would guess the problem is cable size and voltage drop. Where do you pull out at? My boat is already pulled and is at Crossview Marine.

HeadedToTexas said:

Doubled the effectiveness and reduced pulse duration by 75% meaning that your thruster put out that much more flow? That is impressive. Did you replace the 12 volt thruster motor with a 24 volt motor?

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I swapped motors but kept the same lower unit. While I didn't have an amp meter available to read the 12 volt motor, it was fused for 400 amps! The 24 volt was fused for 200 amps. I believe the cable was 3/0, but obviously suffered from the 30' (60'+ round trip) length.

I going to repeat this at the risk of being annoying, "YOU HAVE TO MEASURE THE VOLTAGE DROP"! It's not about percentage of RPM, there is a point (RPM) below which the propeller is spinning and the boat's bow isn't being moved. There's also a point above which (for the same size propeller) you can't move the boat's bow over faster. At the point slightly above the minimum where the bow is starting to move, an increase of 20% RPM will increase the speed far more than 20%. If your bow is slowly moving, increasing the voltage from below 10 volts to above 12 volts, may have a substantial speed increase, as a percentage of previous speed.

Regarding doubling the speed and reducing the pulse time by 75%. The reduced pulse time was a function of speed and momentum. It became necessary to get off the lever much sooner to let the momentum finish moving the bow as opposed to applying reverse thrust.

Ted

While on the subject, check the propeller. Think of the bow thruster as a kort nozzle propeller. The objective is to not loose any water to the sides, and push it out through the tunnel. Don't know if the propeller is original, the correct size, or if it's worn down at all. But clearly it's worth checking when you haulout.

Below is a picture of mine to indicate relatively small clearance. The blades look very odd until you realize there are two propellers.

Ted

O C Diver said:

I swapped motors but kept the same lower unit. While I didn't have an amp meter available to read the 12 volt motor, it was fused for 400 amps! The 24 volt was fused for 200 amps. I believe the cable was 3/0, but obviously suffered from the 30' (60'+ round trip) length.

I going to repeat this at the risk of being annoying, "YOU HAVE TO MEASURE THE VOLTAGE DROP"! It's not about percentage of RPM, there is a point (RPM) below which the propeller is spinning and the boat's bow isn't being moved. There's also a point above which (for the same size propeller) you can't move the boat's bow over faster. At the point slightly above the minimum where the bow is starting to move, an increase of 20% RPM will increase the speed far more than 20%. If your bow is slowly moving, increasing the voltage from below 10 volts to above 12 volts, may have a substantial speed increase, as a percentage of previous speed.

Regarding doubling the speed and reducing the pulse time by 75%. The reduced pulse time was a function of speed and momentum. It became necessary to get off the lever much sooner to let the momentum finish moving the bow as opposed to applying reverse thrust.

Ted

Click to expand...

My hypothetical numbers are only getting in the way of what I am after. I will measure the voltage drop under load at the motor. I am guessing it will be well below 12 volts, and I am guessing that is why my bow thruster is so weak.

And you're right about the round trip impact of direct current; my round trip distance is over 110 feet. Yeesh. It's been that way since the boat was built in 2007.

SteveK said:

Many discussions on why not to use house bank as starter batteries. Often I have said my preference is to have windlass on the Start battery while the engine is running.
Line loss and the fact that house banks usually do not output a continuous starter load (I consider a thruster as a starter load) from two feet let alone 30 feet away on 2/0 cable.
You see water flow comparable to a garden hose instead of a fire hose as a result.

Click to expand...

I could cut the line loss from 110 feet of 2/0 down significantly by moving the thruster feed from the house bank busbar to the main engine start battery. From there I would be down to about 24 feet round trip.

I would install a separate battery bank in the bow using starting batteries. When I install thrusters I use a dedicated battery bank right next to the thruster and still use 4/0 cable. Running 60’ with 2/0 cable is part of your problem. Also using the house bank which I assume is deep cycle batteries is the wrong type of batteries. As said above check the voltage at the thruster when the thruster is running. Clean every connection. You might do a test by temporarily installing a battery next to thr thruster and see how it performs. If it does better than install the dedicated bank and use 4/0 cable no matter how short a run.

O C Diver said:

While on the subject, check the propeller.

Click to expand...

The tolerance on my propeller looks okay, but I can see why it moves less water with only one prop with 3 blades. Big difference. Vetus makes a six blade 185mm prop (see inset). Wonder why mine has only 3?

HeadedToTexas said:

And you're right about the round trip impact of direct current; my round trip distance is over 110 feet. Yeesh. It's been that way since the boat was built in 2007.

I could cut the line loss from 110 feet of 2/0 down significantly by moving the thruster feed from the house bank busbar to the main engine start battery. From there I would be down to about 24 feet round trip.

Click to expand...

Seems that would have been a reasonable installation option from the git-go.

And then it may give you useful test info. After you measure current voltage drop (sorry for that pun!) make the switch and consider results... and measure again, etc.

Could well be best solution is battery right next to the thruster, but either in the meantime... or instead of, if the trade from house to starter bank is useful... knowing what's available should help.

-Chris

HeadedToTexas said:

The tolerance on my propeller looks okay, but I can see why it moves less water with only one prop with 3 blades. Big difference. Vetus makes a six blade 185mm prop (see inset). Wonder why mine has only 3?

View attachment 158826

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I wouldn't get hung up on the number of blades. My understanding is the more blades, the more resistance in water. They add more blades for noise reduction in tunnels, so I'm guessing they do the same for a thruster tunnel. Maybe someone can clarify.

SteveK said:

Often I have said my preference is to have windlass on the Start battery while the engine is running.

Click to expand...

While my old Lofrans Kobra 1000 only draws about 85 amps, it may also be suffering from the line losses of 110 feet of 2/0 round trip from the house bank. While the thruster may only run for a few seconds at a time, the windlass may run for a couple minutes at a time. Longer than a starter, but still a similar load, yes?



I am not sure whether my main engine start battery is a start battery or a deep cycle battery. The three house batteries and the main engine battery are all the same: 12 volt 8A4D, 1110 CCA, 198Ah.

HeadedToTexas said:

While my old Lofrans Kobra 1000 only draws about 85 amps, it may also be suffering from the line losses of 110 feet of 2/0 round trip from the house bank. While the thruster may only run for a few seconds at a time, the windlass may run for a couple minutes at a time. Longer than a starter, but still a similar load, yes?

View attachment 158828

I am not sure whether my main engine start battery is a start battery or a deep cycle battery. The three house batteries and the main engine battery are all the same: 12 volt 8A4D, 1110 CCA, 198Ah.

Click to expand...

If the engine battery is connected through your wiring to the windlass, and the engine is running as you wind in the anchor rode, then you should benefit from the alternator raising the voltage to the windlass.

Depending on how much effort you want to invest, a high idle RPM and momentarily disconnecting the house bank from the alternator, could have most of the amperage (and significantly less voltage drop going to the windlass from the alternator. Obviously some of this is predicated on alternator amperage rating and the RPM needed to reach meaningful output. On my boat, the windlass was tied to house battery distribution panel along with the 220 amp alternator. At 1,000 RPM the windlass was basically running off the alternator at 14 +/- volts.

Ted

Our bow thruster (8" Wesmar) has a 12 volt battery of its own within +/- 4'. However, we are in a marina where growth (mussels) is significant. Our thruster worked great, but within 3 months the mussel growth in the tunnel reduced its effectiveness to almost zero. Still ran, you could see water movement, but almost no actual thrust. Cleaned by a diver, back to full strength. Another factor to consider, but voltage seems the likely culprit in your case. Good luck!
Regards,
Scott

It could be growth. One thing you can do is stick something like a round fender into the tunnel on each side when the boat isn’t in use. It will pretty much stop the growth if you can block the water flow into the tunnel.

HeadedToTexas said:

The tolerance on my propeller looks okay, but I can see why it moves less water with only one prop with 3 blades. Big difference. Vetus makes a six blade 185mm prop (see inset). Wonder why mine has only 3?

View attachment 158826

Click to expand...

Maybe because the thruster doesn’t have enough power to turn a prop with more blades? Maybe as Vetus if they have a 5 or 6 blade prop that will work with your thruster. But then you will need more power to the prop so we get right back to voltage to the thruster and that long run of fairly small cable.

Endeavor GB 32 said:

Our thruster worked great, but within 3 months the mussel growth in the tunnel reduced its effectiveness to almost zero. Still ran, you could see water movement, but almost no actual thrust.

Click to expand...

Sure sounds like my situation. The picture I posted above is after the haul out guys pressure washed the hull. I didn't see it before the pressure wash, but I will see it again in a month at the end of this season.

The difference is that I have always thought the thruster was weak, at least compared to what I see in videos and from a few others in my marina. My thruster does little to "get off the dock."

Comodave said:

Maybe because the thruster doesn’t have enough power to turn a prop with more blades?

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Perhaps, but Vetus has that 6 blade prop listed as a replacement part for the same model I have (or at least for the current version of that model; mine is a. 2007).

HeadedToTexas said:

The picture I posted above is after the haul out guys pressure washed the hull.

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Are you kidding? That is what mine looks like before I start the pressure wash

HeadedToTexas said:

My hypothetical numbers are only getting in the way of what I am after. I will measure the voltage drop under load at the motor. I am guessing it will be well below 12 volts, and I am guessing that is why my bow thruster is so weak.

And you're right about the round trip impact of direct current; my round trip distance is over 110 feet. Yeesh. It's been that way since the boat was built in 2007.


I could cut the line loss from 110 feet of 2/0 down significantly by moving the thruster feed from the house bank busbar to the main engine start battery. From there I would be down to about 24 feet round trip.

Click to expand...

The house bank unless it is getting charged will drop voltage under load, the start bank is getting charged as the engine is running, and at a higher voltage which may actually cancel out line loss. JMO
Try swapping too start bank and if that does not work as Dave and Ted said install a battery as close as you can to thrusters.

HeadedToTexas said:

I am not sure whether my main engine start battery is a start battery or a deep cycle battery. The three house batteries and the main engine battery are all the same: 12 volt 8A4D, 1110 CCA, 198Ah.

Click to expand...

Maybe these?


If so, Deka says deep cycle 4Ds. Reasonable cranking amps anyway, though, so should be decent for starting.

-Chris

I’m in the same situation with my older vetus thruster, but my cables are bigger and shorter. It’s just an old, underpowered unit. Talking to Florida thruster, they say a boat my size (40 feet) should have a 95kgf thruster. That’s what my stern thruster is, and it’s amazing.
The 185 mm tunnel will take a larger thruster if you want to go that route.
You can try the 6 blade prop too, but there are mixed reviews on that move.
Eventually I will upsize the thruster, but for now I just try to keep it as clean as possible.

I have asked Vetus about replacing my 3 blade prop with the 6 blade version they have on their current 75 kgf model. More when I hear back from them.

Adding a battery up near the thruster to serve it and the windlass doesn't sound bad, but rewiring them to the main engine start battery sounds even better. A couple of busbars and new crimped ends are within my scope. Pending the results of my volt meter testing, of course.

I don't have a bow thruster but I agree that you need to check the voltage AT the thruster terminals. Untill that is done all the discussion in the world is not going to fix it and may cause an even worse problem, buying an even more expensive unit and having the same problem.

Do the V testing first , then open up ALL THE POWER CONNECTIONS from battery to the thruster and CLEAN them well. ANY sign of corrosion needs to be removed,
Crimp tightness needs to be checked. They MUST be clean and tight. Once you start reassembling use a coating such as NoALox or a dielectric grease and coat the wire , the terminal contacts and battery post contact areas to prevent corrosion buildup. Use glue lined heat shrink tubing to further prevent any corrosion between the crimp and the wire.
I use only brass or bronze nuts and washers for clamping it all together.
Wire size or lack of it can also cause Vdrop.



And yes, Corrosion, poor crimps, of ANY type will cause Vdrop. V drop can cause poor performance of any and all electric motors.

HeadedToTexas said:

While my old Lofrans Kobra 1000 only draws about 85 amps, it may also be suffering from the line losses of 110 feet of 2/0 round trip from the house bank. While the thruster may only run for a few seconds at a time, the windlass may run for a couple minutes at a time. Longer than a starter, but still a similar load, yes?

Click to expand...

For reference, your thruster motor is 4.4 kw. I think a typical diesel starter is < 2 kw.

Are your three house batteries connected in parallel? The thruster will draw more than 400 amps. I agree that the cable length is problematic, but essentially using the thruster is a load test on the battery, and weak performance may also point to battery health issues. Measure voltage at both the battery and the thruster, and watch it while running the thruster for 30 seconds or more.

In any case a couple of dedicated starter batteries next to the thruster will make a big improvement. I found an online calculator that shows a 25%/3v drop for your cabling.

Took me a bit to come around to the current draw issue, but see that now. Vetus' page for this model includes an asterisk that notes "this model is design to be a compatible replacement for older Vetus BOW75 4.7kW units." Mine might draw more than 500 amps. No way that should be on 110 feet of 2/0 cable.