Jason, I'm still turning it over in my head, but so far your towing idea looks real good to me. Maybe someone can point out downsides I haven't seen. Measuring towing force could cover variables nearly impossible to deduce or model. Do it against wind or current, at various speeds, and with a stop watch from a standing start too. You could even find a fudge factor for clean vs dirty bottom.
Wow! Good thinking!
Back when I was trying to decide which way to electric power the tri-canoe, I knew it didn't require much thrust, but I had no idea what "much thrust" actually meant for that boat. I finally decided to do what most of us seem to.....just install a system that would cover about half of what I could foresee and expect to modify it until it is right. But it sure would have been nice to have some idea of what to expect. As it was, the first time in the water was a revelation.
I wonder if we are ahead or behind the curve here? Recording towing force is a good enough idea that I'm wondering if this is already a normal service at some marinas? Anyone know?
Roger L.
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----- Original Message -----
From: Jason TaylorSent: Saturday, June 01, 2013 9:22 PMSubject: Re: [Electric Boats] ThrustThrust is important pre-aqcuisition. Once installed and in operation, the only important figures to watch are pack-voltage, amps consumed and shaft RPMs.Thrust should be able to compare directly to tow-tests. If you tow your boat with some sort of strain gauge in the town line, you will get a reading in pounds. Take readings at each knot up to a bit past hull speed and plot the data. I would think that a trolling motor producing 50lbs of thrust should be able to get the boat to the speed reached during the tow-test where the strain gauge read 50lbs, and the same for the other trolling motors. It just may take a bit of time to get there... This make sense to me but I've been known to be wrong on many things before.What's nice about watts though is that you are able to track your power consumption and project at what point you'll be paddling back... Thrust doesn't let you do that directly or indirectly.If you get a fouled prop, your thrust will plummet. You will know your prop is fouled how? Your current consumption will go way up (and your speed will drop).Personally I think the more critical thing to watch out on the wateris the watt consumption since that is the limited resource.Now, a thrust-per-RPM rating for a prop could be a good way to compare props for their relative efficiency. But that is a rating for the prop, not the motor./Jason
Joe, I can't make a better argument for using thrust to compare things than what John Francis just said:"My problem with Watts & HP, is that neither has anything to do with the amount of power actually exerted at the propeller, which is the actual power that moves a boat."Although I'm noticing that John made the case in one sentence whereas it took me a page to say the same thing.....basically that Watts and HP rotate shafts, but thrust is the force that pushes boats forward :-).Agreed that simply comparing thrust only works if the boats are similar.It might be best to start by finding all these performance measurements for one particular boat and inch out from there.Roger L.----- Original Message -----From: Joe MurraySent: Saturday, June 01, 2013 3:10 PMSubject: Re: [Electric Boats] ThrustRoger,
Why not use thrust as a basis for comparison? The reason is right in your reply. Thrust is dependant on too many variables. It's just simpler too stay with watts and horsepower.
Joe Murray
Different Drummer #364
Panama City, FL
From: Kirk McLoren <kirkmcloren@yahoo.com>
To: "electricboats@yahoogroups.com" <electricboats@yahoogroups.com>
Sent: Saturday, June 1, 2013 10:53 AM
Subject: Re: [Electric Boats] Thrust
This one?
From: Roger L <rogerlov@ix.netcom.com>
To: electricboats@yahoogroups.com
Sent: Saturday, June 1, 2013 4:56 AM
Subject: [Electric Boats] Thrust
I agree that the electrical numbers are helpful, but thrust is what moves the boat through the water.Imagine that instead of a pitched propeller, what we are spinning in the water is a propeller without any pitch, or even more simply that we have tangled up the prop in a canvas tarp and now what we are spinning is a big wad of cloth.We can measure the volts and amps required to spin that wad of cloth at any rpm, and change that into HP and torque.....but the information isn't helping much.... And the boat still isn't moving forward.To push the boat we need for there to be a prop with some pitch to the blades. Talking about props means we are also talking about thrust.For me, calculating thrust in fluids has always been more difficult and complicated than the fairly straightforward calculation of volts, amps, and battery capacity.....For a while I tried to think of pitch and prop diameter as the fluid equivalent of volts and amps - and that works to a degree....But a propeller works in a fluid so its thrust has to take fluid viscosity into account. The various energy losses due to fluid viscosity can be huge. They vary with the diameter, pitch, and speed of the propeller..... as well as the speed of the boat.Roger L.----- Original Message -----From: Craig CarmichaelSent: Friday, May 31, 2013 11:38 PMSubject: Re: [Electric Boats] Trolling motors ==> electric outboardsYa, I wish these people would cut the "#s of thrust" nonsense and
tell us the watts, or at least volts _and_ amps so we'd have some
idea and a basis for comparison. I think they want to hide it from us
so we don't realize how little power they have.
Craig
=====
>A trolling motor that produces, say 50# thrust at zero boat speed
>would produce zero thrust at 6 or 8 mph. Double the prop pitch and
>thrust at zero speed will drop while thrust at 8 mph will increase.
>Motor power is the same regardless.
>
>On Friday, May 31, 2013, danbollinger wrote:
>
>To a physicist, they are very different concepts. Thrust is a
>rotational unit of force. Power is a unit of work.
>
>Energy / Time = Power
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