T-Pain Is On A Boat: [Electric Boats] A bit off topic...

A bit off topic

--- In electricboats@yahoogroups.com, "epowermarine" <todd@...> wrote:
>
> I think when this `thrust' discussion was started a few threads ago we were comparing manufacturers published ratings of STATIC thrust between brands. My concern there was that the tests producing those numbers are not standardized, so it's not an apples/apples comparison. Now we have dropped the word STATIC and are just talking about thrust, in terms of propulsive force, and these are really 2 different animals.
>
> Think of this `in the limit' hypothetical situation. I have a perfect hull, a perfect motor, and a perfect prop with zero slip. My theoretical speed equals the prop pitch times the RPM. The prop is carving through the water like a screw into wood, and is bringing the boat along with it. Since I have zero slip I have no `thrust' shooting out of the back. This example is more like a car running on tires, which ordinarily would not slip as you drive down the road. We never use the term `thrust' when talking about cars, but we do talk about horsepower and torque a lot.
>
> Now, I have a real boat that will do 17mph with one Torqeedo Cruise 4.0 (1300rpm) w/optional `HP' prop (approx. 14" pitch), this calculates into a 1% prop slip, so this hypothetical situation isn't that crazy. Short video for the skeptical among us: http://www.youtube.com/watch?v=7inDPMHqlV8
>
> So I reconcile it like this: Low speed, low RPM, high slip, high thrust (this is closer to a STATIC thrust value and useful in displacement mode).
> High speed, high RPM, low slip, low thrust (this is propulsive force and more applicable to 'hull speed +' applications). So 'in the limit' the thrust output of any system is a sliding scale from maximum static thrust while tied up at the dock running full throttle, to approaching zero thrust when up on plane, if you get that far.
> Capt. Todd
> www.epowermarine.com
>
>
>
> --- In electricboats@yahoogroups.com, "Roger L" <rogerlov@> wrote:
> >
> > 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 Francis
> > To: electricboats@yahoogroups.com
> > Sent: Sunday, June 02, 2013 7:56 AM
> > Subject: Re: [Electric Boats] Thrust
> >
> >
> >
> >
> >
> > I 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 Francis
> > 1975 Newport 28
> > Port Clinton, Ohio
> >
> >
> >
> > On Sat, Jun 1, 2013 at 11:22 PM, Jason Taylor <jt.yahoo@> 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.
> >
> >
> > /Jason
> >
> > On Jun 1, 2013, at 19:41, "Roger L" <rogerlov@> wrote:
> >
> >
> >
> >
> > 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 Murray
> > To: electricboats@yahoogroups.com
> > Sent: Saturday, June 01, 2013 3:10 PM
> > Subject: Re: [Electric Boats] Thrust
> >
> >
> > Roger,
> > 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@>
> > To: "electricboats@yahoogroups.com" <electricboats@yahoogroups.com>
> > Sent: Saturday, June 1, 2013 10:53 AM
> > Subject: Re: [Electric Boats] Thrust
> >
> >
> >
> >
> >
> >
> > This one?
> >
> >
> > http://ia600301.us.archive.org/7/items/speedpowerofship00tayluoft/speedpowerofship00tayluoft.pdf
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> > ----------------------------------------------------------
> > From: Roger L <rogerlov@>
> > 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 Carmichael
> > To: electricboats@yahoogroups.com
> > Sent: Friday, May 31, 2013 11:38 PM
> > Subject: Re: [Electric Boats] Trolling motors ==> electric outboards
> >
> >
> > Ya, 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
> >
>