bowthruster blown

[RDB]

I have a quick question about bowthrusters. We have installed a BT and continue to blow a T-class 300 amp fuse. The question is could the problem lie with the grating we've installed over the hole? Turbulence, thus load, thus blown? Any thoughts would be greatly appreciated.

[jehardiman]

What type of vessel? What type of thruster unit? Does it blow while the vessel has forward velocity? Does it blow on startup or after running a while? What is the tube diameter to length ratio? That does the grate look like?

[RDB]

Vetus bow 75, the tube is supplied with diameter 185mm, length 14.5". We put a 2 bar by 2 bar grate. Problem is we're blowing after 30 seconds at roughly %250 of the fuse rating. Thus the bt is drawing 700+ amps. ???

[RDB]

Oh, and thanks for the response, the vetus guys are all in FLA

[jehardiman]

12V or 24V? Sounds silly, but did check your wiring, run length, and wire gage?

[RDB]

12V, 33ft complete circuit, 4/0 cable

[jehardiman]

Hummm...

VETUS product information (http://www.vetus.com/products/bowthr...ters_bow75.cfm) says the 12V version of the Bow 75 draws 550 amps with a max 2 minutes per hour. I assume you are using their supplied fuse (ZE355)? Wire size is as specified but is pretty close to the 36 ft max, Is it point-to-point wiring or are there terminal blocks/splices giving losses?

The shedder bars could be causing the fuse to blow if you are close to the electrical loss edge, but if you're that close there are half-a-hunderd things that could cause the same effect. How much of the tube bore is blocked by the shedder bars? should be less than 5%. I'd check the resistance of the motor also. Is the motor getting hot? Does it have enough ventlation?

[RDB]

Ya, the ZE355 is what we have to put in. That %5 stat is a good one though, going to have to abide by that in the future. Thanks again for all your help.

[jehardiman]

My full opinion on simple shedder bars and thruster tubes is that shedder bars should have about a 3:1 to 5:1 elipitical section (cord in the flow direction to thickness). If you are using bars, they really shouldn't be crossed (unless the boat is really slow), but rather be parallel to the flow direction. The bars should be flush to the outer hull with the inlet to the bow thruster tube a radius of ~20% of the tube diameter. This is about as good as it gets without using a retractable or Whitegill.