I'm thinking that if we simply knew how much thrust it took to move your displacement hull at the speed you want, we could work backwards and produce the electrical requirements for doing that with various available props. That's assuming that the available props have a published thrust curve - and I can't see why they wouldn't.
Maybe you already have an idea of the thrust required for your boat? If it's a professionally designed boat then the designer probably already has an idea of those numbers. How much extra is reasonable for acceleration, windage, or currents?
Roger L.
..
..............
----- Original Message -----From: John FrancisSent: Sunday, June 02, 2013 7:56 AMSubject: Re: [Electric Boats] ThrustI understand about the thrust-per-RPM rating being more a prop issue than anything else.But, the electric motor can't be forever in a stressful state, building up enormous amounts of heat to maintain a given speed.Same goes for an I.C. engine. No one wants to run an I.C. engine forever at 3500RPM to maintain a speed of near hull-speed.I'm personally willing to accept 3knots(plus or minus), as my base speed under power. Since a displacement hull begins wasting energy at a very much ever increasing rate about something like 3-4 knots regardless of the size(or oversized), motor, I have little interest in thise speeds, except, maybe, for a short... very short period of time.I have always been known(even when gas was $0.25 per gallon), as a very frugal user of energy. It has always literally hurt me in a very personal way to waste energy.So, for me, I'm interested in my minimum thrust needs, along with a "little", more, and I'm interested in not taxing the limits of my electric propulsion.Maybe my needs are too nebulous?John Francis1975 Newport 28Port Clinton, Ohio
On Sat, Jun 1, 2013 at 11:22 PM, Jason Taylor <jt.yahoo@jtaylor.ca> wrote:
Thrust 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./JasonJoe, 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
------------------------------------
Yahoo! Groups Links
<*> To visit your group on the web, go to:
http://groups.yahoo.com/group/electricboats/
<*> Your email settings:
Individual Email | Traditional
<*> To change settings online go to:
http://groups.yahoo.com/group/electricboats/join
(Yahoo! ID required)
<*> To change settings via email:
electricboats-digest@yahoogroups.com
electricboats-fullfeatured@yahoogroups.com
<*> To unsubscribe from this group, send an email to:
electricboats-unsubscribe@yahoogroups.com
<*> Your use of Yahoo! Groups is subject to:
http://docs.yahoo.com/info/terms/
__._,_.___
| Reply via web post | Reply to sender | Reply to group | Start a New Topic | Messages in this topic (11) |
.
__,_._,___
No comments:
Post a Comment