Orest
that is excellent news! Problem solved I think.
A static test, aka test for bollard thrust, will definitely increase amp draw by up to 30%. If you have a scale you can push into against a wharf or put it in the tether and pull it will give a good indication of max thrust generated.
One useful thing is you now have an idea of the effect on power demand if the prop is partially fouled; Running on a lake or at slack water and no wind will give you a baseline on the optimum performance of the system; and latter a test in a swell will show you the spiking that pitching and aeration can cause; the effect of headwinds and currents will also be interesting. Forewarned being forearmed. Sail assist should be excellent, you might need a taller gear!
I am hoping the static test explains all of why the power system performance seems to stop at 60%. Looking at the graphs of the props performance and load I thought the prop must have been too big. Looking at the static amp draw it needs to fall by 34% or 14 amps - which is an interesting match to the increase in load a static test can create. Those recs for a 13+”pulley would probably see the load while tethered to the dock drop to close to 14A but at sea the motor would be revving out to very low amps and not producing the speed over ground.
My guess is that once underway in clam conditions the setup up will be on the mark. The prop will turn faster, the slip will persist – and the performance graph shows how the amp draw will drop as rpm rise – the boat will run faster and amp draw fall until equilibrium between thrust and resistance/drag is reached.
Best of all, at intermediate throttle settings, amp draw will drop by a similar amount or more, reducing stress on the controller. At 100% throttle it should be able to run more efficiently and the heating problems should be virtually non-existent
Here is to you seeing 4 knots on the gps.
Looking forward to hearing what results you get on an open water run.
Yours,
Andrew Gilchrist
fastelectrics.com
Australia
0419 429 201
From: electricboats@yahoogroups.com [mailto:electricboats@yahoogroups.com] On Behalf Of Orest Iwaszko
Sent: Thursday, 18 April 2013 3:52 PM
To: electricboats@yahoogroups.com
Subject: Re: [Electric Boats] Illusive gear ratio THE RESULTS ON PAPER
Andrew Im blown away by your analytics. My appologies , I was double checking the gear ratio because the tack results came to 4 :1 and sure enough the big pulley is 10 "
Was the fact the boat was tied to the dock during all the tests accounted for ? Humbly Orest
On 2013-04-17, at 7:18 PM, Andrew Gilchrist wrote:
According to the performance graph the maximum amps draw, at full throttle should be 13.6 amps, where the rpm will be 1788 this will output 8.8lbs of torque.
14”pully 5.6:1 is a straight reduction to allow the motor to run at 1788 rpm; however at 14 amps and 14”pully the torque to the prop is only 47lbs whereas in the present setup torque to turn the prop at 275 rpm is 41lb and the torque rpm relationship is liner so maybe 290?
Go for 13.5”first it may be the best compromise and if you can borrow or rent a few options
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