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 ½” Milwaukee drill ( 660 watts or 0.88 HP) is geared for 0 to 950 RPM and I am pretty sure it can’t 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 it’s 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
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.