That makes sense and was what I was expecting to find as well but not what I measured.
It probably has a lot to do with the prop and hull geometry as well as the dock you're tied too. I have twin keels with a 3' draft and am tied up in about 3 1/2" feet of water right against the shore in a small basin. The prop wash sets up a pretty good current past the boat which would kind of simulate moving thru the water so maybe that would account for some of the difference. I have a 3 bladed prop with a fairly aggressive pitch and run at relatively low rpms on the shaft, around 500, and it moves a LOT of water at a low speed. Works great at dredging out my mooring btw. If you're moving smaller amounts of water at higher velocities, the dynamics would be totally different. I also have a very low power to weight ratio at .6 kW/ton which would make a difference in the loading of system while under way. Apparently, depending on all these factors you can get a wide variety of results.
But an amp is still an amp so if you're just testing your system to see how long your batteries last or how hot your motor gets at various speeds, it won't matter if you're moving or not.
Carter
From: Eric <ewdysar@yahoo.com>
To: electricboats@yahoogroups.com
Sent: Saturday, January 19, 2013 3:19 PM
Subject: Re: [Electric Boats] Prop shaft torque
To: electricboats@yahoogroups.com
Sent: Saturday, January 19, 2013 3:19 PM
Subject: Re: [Electric Boats] Prop shaft torque
Hi Chris,
My measured results have been different than what Carter describes. At WOT, on open water, my boat drive pulls 106A (about 5100W). At the dock, the same throttle setting can pull around 130A (about 6500W). Since the boat is restrained the motor is working harder and the RPM is lower as the prop has to move the water (100% slippage) rather than grip the water while the boat is passing through (37% slippage for my boat at 6 kts). My old 2 blade prop when my boat was diesel powered had 58% slip at 5.5 knots.
So my observations have been that static pulls (like tied to the dock) use more power and give lower RPM which make me believe that the torque demand is significantly higher. Let's do the math. My observations were 6.8HP (5100W) at 1900rpm in open water and 8.66HP (6500W) at 1800rpm at the dock.
Torque = HP * 5252 / RPM
6.8 * 5252 / 1900 = 18.8 lb-ft of torque
8.66 * 5252 / 1800 = 25.3 lb-ft of torque
So the math works out that for my boat at WOT, it generates 33% more torque tied to the dock compared to motoring at 6kts. I would expect that most of our boats are similar.
Fair winds,
Eric
1964 Bermuda 30 ketch, 5.5kW Propulsion Marine drive, 8kWh Lithium batteries
Marina del Rey, CA
--- In electricboats@yahoogroups.com, Carter Quillen wrote:
>
> Chris,
>
> It will depends on how fast you are running the motor and the dock you are tied up too, how deep it is, how close you are to the shoreline and to the bottom. I have been doing a lot of system testing by running at the dock. I am right against the shore in a small basin and the propwash actually sets up its own little current. I don't have a tackometer yet but at full throttle I can't quite load the motor as much as when in open water so I think it is safe to say the torque requirment are a little less at the dock at higher speeds but it's pretty negligable, (less than 10%). I would say at lower speeds there is even less difference in the relationship between rpms and torque. I think at the higher speeds the prop can get starved for water to push and there is not as much torque required but at lower speeds, since the boat isn't moving in response to the the trust it might actually require slightly more torque per rpm. But either way, I
> have observed very little difference between the trottle position and the amp draw on the motor.
>
> Carter
> www.ArchemedesProject.blogspot.com
>
>
> From: ChristopherH
> To: electricboats@yahoogroups.com
> Sent: Saturday, January 19, 2013 8:25 AM
> Subject: [Electric Boats] Prop shaft torque
>
> Same prop, same RPM, will the torque required be different tied to the dock versus at speed? If so, how significant?
>
> Regards,
> Chris
>
>
>
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