Tuesday, January 22, 2013

Re: [Electric Boats] Prop shaft torque

 

Carter, I've followed your project through Diane's writing for awhile now. It's been a continuing inspiration. Hope to cross paths with you guys someday.
     Roger L.  
      
 
----- Original Message -----
Sent: Tuesday, January 22, 2013 7:10 AM
Subject: Re: [Electric Boats] Prop shaft torque

The ruling "fysics" here is that Force=mass x accelleration. That's the water your prop moves times how fast it moves it and your boat then moves thru the water in reaction to this force. This force, created by your prop moving the mass of the water, is what determines the actual torque load on your drive shaft.
 
All the variable of each particular system will have some effect on this. I've done a lot of what you call "ballardpull" testing primarily to see how long my batteries will last under various loading. I was genuinely surprised when I saw that my motor amps where actually less at full trottle while tied up than they where when out in open water because intuitively I figured I would find just the opposite as it seems most people here do. Since I doubt seriously that my prop is cavitating, which is something else that could have a profound effect on individual results in this type of camparison, I suspect I am experiencing some kind of ground effect.  Because I'm moving so much water in a relatively confined space, this must be starving my prop for mass to accellerate while tied to the dock, thus creating a counter-intuitive result from the test.
 
So tied to the dock you don't necessarilly have the most resistance and aren't necessarilly moving the most water the fastest. I would also think that a bollardpull will tell you a lot about your prop choice and it has certainly taught me a lot about various aspects of my system performance even though I don't have a tugboat.
 
Carter Quillen, PE
 
 
 
 
From: Carel Ruysink <c.ruysink@hetnet.nl>
To: electricboats@yahoogroups.com
Sent: Sunday, January 20, 2013 10:44 AM
Subject: Re: [Electric Boats] Prop shaft torque



I see a lot of answers to an extremely simple question.
A lot of answers put in a bunch of extra variables that complicate matters beyond the question and all is correct but is it enlightening enough?
 
The question suggests to me only a basic understanding about the ruling fysics here.
 
Same REVS and same PROP is the starting point here.
 
Tied to the dock you have the most restraint thus the highest required power (Volt and Amps).
The more speed through the water the less power/torque/VA you need. If your boat goes as fast as revs*pitch, your motor can idle at no load conditions, if you go faster then that you are even regenerating.
So, yes there is a very big difference between torque/power/load tied down and when at speed.
 
If it is the motor that has to provide the speed you need a very rare kind of controller to keep a constant rev when accelerating. But then you will see that the power will reduce while picking up speed until you reach the revs*pitch speed (minus ca 20%).
 
BTW tests when tied down (bollardpull) tell you little about performances;
not about prop choice,
not about topspeed
not about hullperformance
not about systemperformance
 
The bollardpull is imho only of interest for tugs.
 
Carel
 
----- Original Message -----
Sent: Sunday, January 20, 2013 3:24 PM
Subject: Re: [Electric Boats] Prop shaft torque

 
Mike,
You and Eric have confirmed my suspicion. What was the factor that limited you at the dock? So far I have only run my system at the dock. I still have to get my steering system back together before I can do a sea trial. What I am seeing though is that I hit 125 amps on both motors a lot lower in RPM than I expected. Motor speeds are 1000 RPM, motor volts are 21. Input into each controller is about 55 amps. From this, torque and horsepower are relatively low, but motor amps are high.

Chris

--- In electricboats@yahoogroups.com, "Mike Gunning" wrote:
>
>
> I concur with Eric's assessment. I have personally test run about 10 boats tied to the dock for initial motor testing and we have seen a constant 25% more power required from the motor when tied to the dock as compared to open sea. Difficult to get to top RPM wen tied to dock. When in open sea, max RPM and max power reach their high point at the same time. Not so when tied in a slop.
>
> It does make a splendid display of water rushing from the stern but we were not able to get the motors to their top end until we got away from dock constraints.
>
> Be aware that until you do testing in open water, you do not know if your boat is geared and propped correctly. You want to reach hull speed at the designated max motor speed.
>
> Mike Electric Yacht Southern California
> --- In electricboats@yahoogroups.com, "Eric" wrote:
> >
> > 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
> > >
> > >
> > >
> > > ------------------------------------
> > >
> > > Yahoo! Groups Links
> > >
> > >
> > >
> > >     http://docs.yahoo.com/info/terms/
> > >
> >
>





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