Subject: Re: [harryproa] Re: 18m Proa Electric Thruster Test 3
From: Rick Willoughby
Date: 5/21/2012, 6:51 PM
To: harryproa@yahoogroups.com.au
Reply-to:
harryproa@yahoogroups.com.au

 

Roger

I spin my latest HPB props at 7X cadence.  At 10kts they are spinning over 700rpm:
http://www.youtube.com/watch?v=ONk3fUnHamI&feature=relmfu
By comparison the electric thruster prop will have an absolute maximum rotational speed of 800rpm - so not much difference.  The cause of shaft vibration in sailing boats is more often inclined shaft rather than imbalanced prop. 

The nylon prop has a taper and keyway so it is well connected to the shaft.

The unit ran smoothly with really nice control.  It is possible to set motor speed to just hold against the wind while leaving the mooring.  

The two old batteries have not been used much so they should not be slugged.  I suspect the active plate area is low so I am hoping a deep discharge and recharge will increase the active area. 

The test batteries are only 85Ah - one is the starter battery for the existing outboard.  Two were used infrequently as forklift starters and one is new so we could get 48V.  The target cruise power is 2kW meaning we want to pull at least 50A at a steady state eventually.   The existing batteries are hopelessly inadequate for long term use as burst current is 200A for each motor or 400A total.  I prefer a battery that can hold this without significant voltage droop for 5 minutes or so.  A decent set of batteries is a major investment and we need to be confident that we can engineer a reliable system that fits neatly into the proa operation.   I suspect from a power delivery point of view lithium batteries are going to look very attractive.

The thrusters have to be compact enough to fold up out of the way when sailing.  The existing outboard transom can only be lifted about 400mm clear of the water and acts as scoop/brake when the boat drives through waves.  Main objective with thrusters is to get steering leverage from two thrusters and mount them nearer the hulls so their immersion does not vary so much.  Existing outboard prop can be in open air when the boat is over a trough. 

Rick
On 22/05/2012, at 7:54 AM, Roger L wrote:

 

As you probably know, as wet cells charge/discharge they shed a conductive sludge which settles out and shorts the plates - I believe it is a function of the purity of the lead plates....with possibly some sulfur compounds involved....although I don't know that for sure.
 
I do know that it is possible to take most battery cells apart - some require removing a sealant or even cutting the top of the case. Then the plates can be lifted out, cleaned, and reassembled with fresh acid. I've done that with some success.
 
There are chemical processes where cadmium or magnesium salts are used to flush out the sulphation without disassembling the battery. I've had less success with that method.  
 
Yes, the large diameter props do need to be unusually strong. We made models from wood and plastic and had them cast in aluminum. Of course all that torque makes for a bit of a problem at the connection to the drive shaft, so additional thought is needed to make it very strong where the prop bolts to the shaft. Human powered props don't turn all that fast, and so perfect balance and geometry is not as critical as with faster turning outboard types. 
    Roger L.
 
----- Original Message -----
Sent: Monday, May 21, 2012 3:21 PM
Subject: Re: [harryproa] Re: 18m Proa Electric Thruster Test 3

Roger
The old batteries have sat around for years without any use.  We plan a couple of slow discharges to about 20% voltage from full voltage.  It may not work in which case we have to decide on what permanent batteries will be.

The first tests we did used a large model aircraft prop that was 32" diameter.  It was very efficient but it does not have the strength to handle the 250 to 300kgf that each thruster will be capable of.  The props we found are quite low BAR with blades aspects over 2 so they are quite efficient. In the final design at 5kts in calm conditions the props will be  achieving 75% efficiency.  The existing outboard prop is 45% as the same condition - just too small.

Rick
On 22/05/2012, at 12:47 AM, Roger L wrote:

 



Rick, I'm curious how you are going to restore life in the old batteries. I've tried restorations by cleaning the internal plates and also with cadmium salts - both with varying degrees of success.

What is the rpm & torque or ratio? I watched some years a go as a human powered prop was developed for a high torque/low rpm bicycle/prop drive. It was quite different from a gasoline motor driven prop. Apparently there are standard naval engineering formulas for prop shape. We used several reference books.
good luck, Roger L.


--- In harryproa@yahoogroups.com.au, Rick Willoughby <rickwill@...> wrote:
> Next step is to try to restore some life in the old batteries that
> have sat around for 7 years or so then do another test closer to >the 4kW motor rating or even into overload if the batteries can hold the voltage.
>
> Two thruster will not improve low end efficiency compared with a
> single unit but they will give heavy weather performance similar, or
> better than, the 60hp outboard. The tiny 14" prop on the outboard is
> cavitating quite heavily even at slow cruise around 6 knots. But
> worst situation is the ventilating that occurs in anything over 0.5m
> waves.
>
> Rick Willoughby
> rickwill@...
>


Rick Willoughby






Rick Willoughby




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