Bow thruster not producing much thrust

[HeadedToTexas]

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.

 

[O C Diver]

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

 

[SteveK]

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.

 

[HeadedToTexas]

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%.

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?

 

[Hippocampus]

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

 

[HeadedToTexas]

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.

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?

 

[PierreR]

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

 

[PierreR]

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.

 

[O C Diver]

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?

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

 

[O C Diver]

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

 

[HeadedToTexas]

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

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.

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.

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.

 

[Comodave]

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.

 

[HeadedToTexas]

While on the subject, check the propeller.

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?

 

[ranger58sb]

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.

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

 

[Mac2]

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

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.

 

[HeadedToTexas]

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

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.

 

[O C Diver]

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.

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