T-Pain Is On A Boat: [Electric Boats] Re: Electra Glide Data and updates

Wednesday, December 16, 2015

[Electric Boats] Re: Electra Glide Data and updates

Hanna,

Your hypotheses that a high pitch, many bladed prop with a high swept area will be more efficient is correct, in my experience. Here's a copy of another post that describes my direct observations concerning that change, specifically on the topic of prop slip:

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ewdysar
Mar 12, 2014
Hi Luke,

Here's something that I posted most recently about prop slip last March:

While I have used a general prop sizing calculator that use 50% slip factors, I know that the propeller shop that sold me a new prop for my 5 ton, full keel, 30' ketch with the prop in a aperture used 55% slip to find the appropriate pitch. I found the following information on the Michigan Wheel propeller website:

Q: What are normal slip percentages for various craft?
A: With propellers correctly selected for the operating conditions, the slip percentages would be as follows: racing hulls 10 to 15 percent, planning runabouts 15 to 25, planing cruisers 25 to 35, displacement cruisers 30 to 40, sailing auxiliaries 35 to 40 and work boats 40 to 80.

I also found this reference on the Castle Marine Propeller Repair Specialists website: "The pitch is the distance a propeller would in theory move forward in the water in one revolution if there was no slippage i.e. a 21" pitch propeller would in theory move forward 21" in one revolution. In practice most propellers do slip the amount being dependent on the boat and as a general guide slippage would be between 8% for a race boat with a planing hull to as much as 55% for a full displacement craft. A 10" pitch propeller on a displacement craft with a 50% slip ration would only move the boat forward 5" per revolution of the propeller. The pitch is the second number in the description i.e a 13 3/4 x 21 propeller is 13 3/4 diameter and 21 pitch."

But enough with secondary research sources, let's discuss a primary data source. Here's the objective prop slip data that I've posted numerous times in this group, Feb 2012 (twice), Dec 2011, Oct 2011 and originally on Aug 22, 2011 (Post #19469). Surprisingly, nobody else here has measured and posted their actual slip numbers. It seems like other reports of primary data sources would be helpful during these conversations since the topic comes up a couple of time a year.

"I purchased my 13 x 15.5 four bladed prop from a local prop shop to replace the 13 x 10 two blade that was already on the boat. You can see both props side by side in "Eric's Serenity" photo folder in this grouphttp://groups.yahoo.com/group/electricboats/photos/album/1967401930/pic/list With the old prop and the diesel engine with 2:1 reduction, engine rpm was about 1200 (shaft speed of 600) at 3kts and 3200 engine rpm at 5.5 kts. With the new prop and electric motor with 2.55:1 reduction, the motor speed is 830rpm (shaft speed of 325rpm) at 3kts and 1900 motor rpm at 6kts. Doing the calcs, the old prop showed 39% slip at 3kts and 58% slip at 5.5kts. The new prop calcs to 27% slip at 3kts and 37% slip at 6kts."

Finding the proper gear ratio is often trial and error. If you hit max RPM without hitting max amps, then you are under-propped or your gear ratio is too high. If you hit max amps without hitting max rpm, then you are over-propped or the gear ratio is too low. With my new prop at full throttle and a 2.55:1 gear ratio, I'm hitting my target RPM and 5.3kW (106A) at 95% of hull speed. That's pretty close to hitting all of my marks and the next gear ratio, higher or lower, would move the balance away from this sweet spot. My performance numbers are pretty good for a 10,200 lb sailboat.

Generally speaking, larger diameter props are more efficient. But typically we're limited in size by the hull design. So many members here have looked for heavy pitch to go with their max diameter prop. In my case, the pitch is greater than the diameter and is considered "oversquare". Oversquare props are considered to trade acceleration for speed, for a sailing auxiliary, acceleration isn't as critical. This prop has given me good "watts to knots" specs and the slippage is much lower than the old prop that was on my boat.
But the optimum prop specs and gear ratios will be somewhat different for each boat. That said, your boat will go fine with the stock prop and a reasonable gear ratio. But higher efficiency (greater speed and/or range) is available by matching the prop and ratio to your motor and your boat.

I hope that this helps.

Fair winds,
Eric
Marina del Rey, CA