Bow Thruster

 

Bow Thruster Issues

 

The bow thruster on LIBERTY seemed a bit anemic to us.It was difficult to judge how well it should work as we know the boat has a rather large amount of windage, especially with the fly bridge and weather cloths on the boat deck.The thruster was replaced four years prior to us purchasing the boat with a larger (7.6 hp, 95 kgf), Vetus modelBOW 9512.

 

The final straw was leaving the dock with about a 15 knot beam wind.The bow wanted to head downwind even though the bow thruster was full on.Fortunately we had room to swing that direction but something needs to be done about this as the worksheet that Vetus has on sizing thrusters says this model should be good for about 20 knots, given all our windage.

 

Investigating the wiring found that the thruster was wired in-line with the windlass which had #1 wire coming from the house bank, about 25 away.Vetus specifies 4/0 wire for that long of a run,so the voltage drop must be considerable.†† We put a meter on the wiring and found that it pulled 365 amps and that the voltage at the thruster was a miserly 4 volts.No wonder this thing seems underpowered; it is by a factor of 3.That needed to be fixed and we will do that by mounting two batteries under the sink in the stateroom and run 4/0 wires to the thruster.The wiring run will get cut down to about 3í in one direction.Something will need to be done about charging those batteries.

 

Vetus now makes a real nice 6 blade prop that is claimed to be slightly more efficient and has a reduced cavitation.Most of the noise of a bow thruster is due to prop cavitation, so reducing that noise alone is worth the price of a new prop.We purchased one and went to test fit it and lo and behold, it didnít fit.The blades ran into the side of the tunnel.†† It turns out that the replacement thruster was mounted off center requiring that the blades of the original prop be cut down.

 

So, the thruster is inefficient because a) it is not aligned correctly in the tunnel, b) the prop blades were cut down and c) the wiring is woefully undersized.

 

The thruster must come out (not that big of a job) and re-aligned.The image below shows how badly it was off center.

 

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After taking the thruster out we examined the holes used to mount/align it to the tunnel.It doesnít look too bad.

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We cleaned out the bedding compound around the edge of the holes and this is the result.Who the heck drilled these holes and did they think that bedding was going to keep thing from moving?Maybe what is more surprising was that this didnít leak.

 

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It looks like some glass work is in order.

 

After re-building the area with glass and then shimming the thruster correctly in the tunnel, thickened epoxy was used to build a new base for the thruster to properly seat to the top side of the tunnel proper.This insured that when it was bolted in that the blades would have the correct clearance in the tunnel.

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The thruster leg was rather ratty looking and the zinc pretty much gone.

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It did clean up quite well however.

 

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No trimming of the blades necessary and the new style 6 blade prop is a great improvement as far as the cavitation issues goes (much quieter).The green around the edge of the thruster tube is an indication that there is some moisture present.That will be on the list to investigate during winter haul out.

 

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If you are replacing the thruster on your Pilgrim with a newer (larger) model, be advised that it is taller than the original and another cover (one with a step in it) will have to be fabricated to protect it.

 

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To solve the low voltage issue, two batteries and a battery charger were installed beneath the sink in the forward cabin.This would only power the thruster, the windlass still being powered from the house bank.This location eliminated the sink but in our case the sink had been disconnected from the pressure water system years ago so we didnít lose any functionality.Somebody had thought to leave the old hoses for us to remove and there was still some pretty skanky water in those water hoses.We couldnít charge the batteries from the main invertor/charger as that was already split for two banks (house and start) so we elected to use an AC charger.Using the invertor to convert to AC and then back to DC is mildly inefficient but the only time we use the thruster is leaving the dock when we will be motoring for a while, or entering the dock where we will be tied to shore power so this charging method isnít an issue.

 

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The batteries are actually shimmed in the box to prevent any movement.

 

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The thruster now functions quite well after spending more than a few dollars on cables, electrical components and the new prop.The project also consumed a fair number of man hours to do the job correctly which might explain the less than perfect job that was done when the original thruster was replaced.

 

 

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