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Old 22-08-2015, 14:58   #76
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Re: Electric Inboard

Originally Posted by TacomaSailor View Post
"I think you would be wise to look into the differences in torque that the electric motor provides, also the adjustable pulley reductions you can tune. I don't see you calculating the torque advantages of electric propulsion swinging large oversized props. "

Torque is only a force but it is power that moves the boat.

HP = (torque x RPM) / 5252
Torque = HP x 5252 / RPM

If you double the torque with gear or pulley ratios you slow the RPM and the horsepower stays the same.

Increasing torque by decreasing RPM (that is what gear ratios do) does not increase the HP and HP is what moves the boat.

Torque is measured in foot pounds (at least in the US) so if you move 550 pounds one foot you've exerted 550 foot pounds of torque. But, that says nothing about the time taken to move that weight that distance.

Horsepower measures the energy produced/consumed per unit of time and ONE Horsepower is the power needed to move 550 pounds one foot in one second. One HP is equivalent to 746 watts of electrical energy.

So I can move that 550 pounds one foot in one second while using 1 HP or I can take 10 seconds to move it and use 1/10 of a horsepower.

So - you can have very high torque, as shown in a stalled electric motor, but produce no horsepower because there is no shaft rotation and nothing moves - but there is still the torque (force) available.

You can change the gear and pulley ratios all you want but the power needed to move the boat will remain the same.

A boat has weight and drag and to move that weight and overcome that drag will always require the same force at any given boat speed. You can increase the torque (force) but if you do that only with gear ratios you will decrease the RPM and therefore the power (energy per time unit) will remain constant and the boat speed will remain constant.

If you increase the torque and maintain the RPM then the power required will increase at the same ratio as the gear ratio.

Using a bigger prop makes the prop more efficient but as you increase the prop efficiency you also let it put more power into the water thus requiring more power from whatever drives the prop shaft.

You can not gain boat speed except by applying more power thru the prop to the water and that power has to come from somewhere.

You can not make any additional power, just by changing gear ratios and prop sizes without adding watts (amps x volts) delivered from the battery.

Power and Force have always confused me and if you find something wrong in what I have written above - please let me know 'cause I copied most of it from some physics books.
I really do appreciate you taking the time to look all this stuff up. Yes, it is fascinating for sure. But how about trying this just for fun >>
I have an idea! How about instead of assuming nobody is getting the mileage and range the factory shows is happening in the real world, (in essence calling all those people liars), why not work your numbers and calculations backwards to see where your calculations are perhaps letting you down? That is done all the time in good science. See where a coefficient, or a factor, is simply wrong? Or no longer valid? You seem confused a lot so why not reverse engineer say a boat going 3.2 knots and drawing 20 amps? People are getting these numbers with their boats. It's being done and it is conservatively projected for my hull from the factory. Maybe give that a try? Why not? What have you got to lose?

You know not meaning to go OFF THREAD but; what's going on here is similar to other things being argued today. It kind of reminds me of the global warming crisis. The research numbers show it's happening, 95% of the world's scientific community says there is no doubt it's happening, but the nay-sayers say it's impossible, so they run around trying all sorts of "numbers" to prove it can't be happening. I guess this kind of thing happens a lot when science is approached with a preconditioned mindset that something new can't be breaking the rules or the concrete laws we have had forever. There are the same kind of heated arguments taking place regarding the use of LiFePO4 batteries versus lead acid batteries. You wouldn't believe those wars, unless you've tuned into them here. There are even people thinking I am installing a giant bomb in my boat! LMAO!.... Oh well, just some thoughts.
PS-I'm wondering when or if this thread will be moved to the Propulsion Forum?

s/v "Bamboleo" - Freedom 32 (Hoyt)
Farrier Trimaran -- Morgan 41 Classic,
Rawson 30
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Old 22-08-2015, 15:19   #77
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Re: Electric Inboard

Originally Posted by TacomaSailor View Post
Let me preface this post with a clear statement of my admiration and respect for those trying to advance the engineering and technology of electric propulsion in power boats. I love technology and love numbers. My questions and comments are an attempt to understand how the leaders in the new frontier of electrical propulsion plan to overcome what most of us thought were well established limitations based on solid physics and electrical engineering.

Please do not think I am trying to stop folks from moving forward - I just really want to know how you plan to overcome these limits.

"I hope the numbers below will give you some food for thought in your calculations. Here you go >> (it's from a graph so I will interpolate as best I can).
Motor: 10Kw PMAC @ 48 Volts
Shaft HP: 9 with 40:24 reduction
Prop: 15" x 12" (I have gone up to a 16.5" variable pitch Kiwi feathering prop)

Amps vs. Speed Graph
10 amps - 2.4 knots
20 amps - 3.2 knots (hoping to squeeze another .8 knots our of my variable prop?)
40 amps - 4.5 knots
65 amps - 5.0 knots
80 amps - 5.5 knots
120 amps - 6.3 knots
180 amps - 7.0 knots "

I am always glad to learn new things and play with new numbers. Some of what you report above is confusing.

The high efficiency motor you cite is 10 Kw and 9 shaft horsepower. But, 10 Kw is actually 13.3 HP at 100% efficiency. Your reported 9-shaft HP is only 67% efficient. What do I misunderstand here?

10 amps at 48V is 0.63 HP and you say that will move your 8,900 pound boat at 2.4 knots? How can that occur? My Milwaukee drill ( 660 watts or 0.88 HP) is geared for 0 to 950 RPM and I am pretty sure it cant move an 8,900 pound boat at 2.4 knots.

Dave Gerr is a very well respected Naval Architect and has published a great deal of technical professional information about calculating power to move vessels. Here is a link to an article about the formula and its application in the professional journal published by the Westlawn Institute of Naval Technology.

Gerr says you need 2.06 Kw (2.76 HP) or 43 amps at 48V to move the boat at 2.4 Knots rather than the 0.63 HP your data reports.

You claim to move the boat at 5.5 knots using 3.84 Kw (80 amps) but the Gerr formula says you need 7.11 Kw.

How do you propose to overcome the formulas used by professional boat designers? Those formulas predict you need more than four (4) times the shaft horsepower to move the boat as you are citing.

I am not making up any numbers! I am just applying the numbers you provide to standard formulas. Are you saying that you and the other electric boat advocates know how to overcome the power issues that have plagued naval architects since the first propeller driven ship?

I have reviewed the E-Boat modeling spreadsheet used to calculate power needed to move a boat thru the water. I see that sheet predicts the same power requirements you cite above.

Looking at the E-boat formula for hull drag I see a simple linear function that seems to ignore the exponential characteristic of drag as a function of speed.

So a fundamental difference I find in our positions is the calculation of power requirements.

A big difference I see in the formulas are probably the cause of our vastly different estimations of required power.

The Gerr formula is exponential in respect to drag increase with speed increase. The E-boat formula for drag is linear and specifically excludes calculations below an S/L of 1.0 or 5 knots in your case.

The E-boat spreadsheet also assumes a fixed 55% efficiency factor the transmission of power to the water by the prop. But, we know that most prop curves are cubic in nature and the prop puts very little power into the water at low RPM. For example, a 20HP Yanmar 3GM makes 9 SHP at 1800 RPM but the prop only puts 2.5 HP into the water for an efficiency of 28%. At 2200 RPM those numbers are 12, 3.5 and 29%. The engine has to be turning 2900 RPM for the prop to put 50% of the 14 SHP into the water.

As far as I know electric motors turning a propeller in the water suffer the same cubic prop curve. The E-Boat model ignores the inefficiency of the prop at low RPM compared to total RPM.

The results of the spreadsheet equate shaft horsepower in a direct and linear fashion to propulsion power. I am pretty sure than is not the way it works.

Is there something unique about the new motors and controllers that allow them to move boats thru the water with less power than all previous boats powered with diesels?

I have applied the Gerr and Bebe formulas to my Caliber 40 and find them, when using the Yanmar prop curve charts, to be very accurate at predicting boat speed in still water. That is - the Gerr prediction for a given HP needed agrees almost exactly with the prop HP delivered at a particular RPM and a measured speed. I would be totally gob-smacked if someone told me the Gerr, or Bebe, formulas are incorrect.

Those formulas also work very well with a Tartan 42/Perkins 4-108 that I operated for several years and thousands of miles.

That is the reason I am so curious about the predicted HP needed to move electrically powered boats - it differs by a factor of 3 or 4 from what I measure in my real life.
I have a GREAT idea. You are in San Diego, I am in Mission Bay. When I get this Electric Yacht Systems 10Kw PMAC/Sevcon package installed and the monitoring completed (Victron 700, Junsi Cell Log8's x2 data logging with alarms, and Electric Yacht CAN buss readout at the helm), then I will be able to dial in the pulley ratio and the prop pitch as best I can to see if I can hit my numbers. Then you come over to the boat and we can take 'er for a spin and see some REAL numbers! This will either make me very happy and satisfied, and you going away scratching your head, or you very happy and satisfied, and me still very happy and satisfied even if I don't meet my numbers exactly...
Sound like a plan? I'm up for it. How 'bout you?

s/v "Bamboleo" - Freedom 32 (Hoyt)
Farrier Trimaran -- Morgan 41 Classic,
Rawson 30
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Old 22-08-2015, 15:28   #78
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Re: Electric Inboard

Originally Posted by Stumble View Post
SolarWave is actually a pretty good example of how good design can change the numbers.

Drag is proportional to the L/B ratio (leingth/beam) or more technically the froude's number. Basically as a hull gets longer and thinner it produces less drag. On a monohull there is a practical limit to this since the hull also has to contain the living space. On a multihull the living space can be pushed to the bridge deck and the hulls can be made very long and very narrow.

This is exactly what SolarWave has done. Where a typical monohull has a L/B of about 2.5:1 SolarWave is about 7:1. This means the hulls are incredibly easily driven compared to a monohull, thus needs less power for the same speed. To use it as a comparison we would need a 45' monohull that is about 6' wide at the waterline. You could certainly design such a boat and it would take very little power to move, but it would be a pain to sleep on.

This is why all the prop-speed formulas have a disclaimer that they don't work for catamarans BTW. but there are separate formulas that do.
Actually solarwave is more an example of trading off load carrying ability and accommodations to reduce the drivetrain power needs. Take that same boat and put in a couple 5hp diesels and limit the throttle setting to 4kts and you are probably getting 30-40 MPG negating most of the assumed fuel saving benefits. 50gal tanks is ocean crossing range.

They are willing to sacrifice comfort (relative to more comfortable boats of the same length) to reduce the force needed to drive the boat. That comes with it's own set of costs but doesn't prove the viability of electric drivetrains for typical cruising boats.

Also don't forget the replacement cost of batteries. I've yet to come across a battery that doesn't wear out after a few years and when you are talking thousands of dollars for a battery bank, that's a lot of fuel that could be purchased.
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Old 22-08-2015, 16:06   #79
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Re: Electric Inboard


1st rule of yachting: When a collision is unavoidable, aim for something cheap.
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