Vetus Bow thruster deifying physics. BOW6012d has more thrust and draws less amps than the BOW5512d

[Beekeepergreg]

Shopping for a thruster and noticed...........
BOW6012d has more thrust and draws less amps than the BOW5512d.
Their data:
The BOW6012d, 4hp,132 ftlbs, max amps is 280a with a 185mm tunnel, suggested fuse is 200a.....Even at 10.5 volts....still seems low.
The BOW5512d, 4hp,121 ftlbs, max amps is 350a with a 150mm tunnel, suggested fuse is 250a..... seems about right for amp draw but fuse seems light.
I spoke with Vetus tech support.....no real explanation, but will get back to me.......not holding my breath.....tech guy seemed confused by my questions.
Anyone want to jump in on this?

Thanks
Greg

 

[rslifkin]

The props on the larger thruster are likely more efficient. And being larger, they're likely geared to turn slower as well. So the bigger thruster likely gets more thrust relative to input power because it's doing a better job of converting motor power into thrust. Keep in mind that the boat is never moving sideways all that fast, so larger, slower turning props generally do a better job. They'll cavitate less and slip less in the water, so they produce thrust more efficiently. Plus there's less resistance to water flow from the larger thruster tunnel, so less loss there for a given amount of water pumped through.

The fuse recommendations do seem way too small for both, however.

 

[Beekeepergreg]

My mind went there as well at first .....but comparing to Sleipner's SE60 Thruster rated at 4.1 hp with a 185mm tunnel produces 132 ftlbs and draws 370 amps at 10.5 volts suggesting a 325a fuse.
More effect props only get us so far. Sleipner probably has just as good props.

 

[Fleming]

Fuse size is determined by the locked rotor current (similar to starting current). Has a lot to do with the motor design and construction, and less to do with the normal operating current of the motor.

 

[twistedtree]

They also quote "max amps", which might just be a startup peak as opposed to a steady current which is what does the work. HP and thrust are nearly the same, so I would expect overall power consumption to be about the same. That said, I agree with others that the larger tunnel and prop size could have an advantage.

 

[Beekeepergreg]

Note: a 4 hp dc motor at 12 volts @ 100% efficiency ideally would draw 248a but realistically 80% efficient would draw 310amps.
4 hp dc motor at 10.5 volts @ 100% efficiency ideally would draw 284a but realistically 80% efficient would draw 355 amps. In rush would be much higher.

A side note...the Sleipner tech guy explained why they don't recommend Lithium Batteries....the voltage would stay constant and the "12 volt " motor would produce too much HP and over drive the gears....possibly causing failure. They figure 12v as max and 10.5v is expected for operation.

 

[Beekeepergreg]

I got an official response for my question from Vetus:
The thrust difference comes down to differences in the gear ratios in the tailpieces and the pitch of the differently sized propellers in differently sized tunnels. In terms of the amp draw differences, understand that horsepower amp ratings are different to circuit protection amp ratings.

Still uncomfortable with the fact that, Sleipner's SE60 Thruster, draws that much more power for the same thrust......they are a good company and would have the best prop performance engineering could produce.