I have made lots of calculations for you to look at, the rpm is the MAX you will see with the size:
23x22 – 500rpm = 55% eff, 2150N thrust
23x17 – 600rpm = 53% eff, 2160N
21x15 – 700rpm = 51% eff, 2024N
20x13.5 – 800rpm = 48% eff, 2001N
19x12 – 900rpm = 46% eff, 1980N
18x11 – 1000rpm = 45% eff, 1921N
It really is your call where you want to prop this, but seeing as load is not really an issue, I would get the most out of the system and prop the most propeller thrust (in N) and most efficient.
- It seems that I/we have got it right with the prop, or at least on the way.
- As I am running 6 pole motors that have a natural speed of 1000RPM, what will happen if I take the 23 x 22 prop above 500RPM, will I get more speed or will I get cavitation?
- I guess it will depend on whether I'm underpowered or not?
- All fueled up with a full larder and a couple of tubby eager crew she will probably come in at 10 ton, but with 22KW that should still see me well under the 1KW/ton rule of thumb.
- Even the great Fisher Panda say that tubby craft for their length (which mine is) "
For larger displacements 2.5kW per tonne is a very safe rule of thumb to achieve hull design speed in worst case conditions ie taking into account tide, current, high winds and manouvering power. If hull design speed is not a particular requirement, as is sometimes the case with sailboats & catamarans then you can use a factor as low as 1.0kW per tonne of displacement.
Hi Julian,
Our typical recommendation for an electric repower is 1kW per ton to achieve 90-95% hull speed in displacement hulls. Basing your conversion on existing ICE size means that you believe that the original building sized the engine correctly. Since many boats have had different size engines over the length of production, you can see that opinions about ICE sizing are all over the board. Looking at the Catalina 30, the factory installed a variety of engines from the ubiquitous Atomic 30, to an 11hp diesel twin in the 80's, and all the way up to a 28hp 3cyl diesel today. Based on that info, how much power does a Catalina 30 need? We know that the answer is that many successful electric conversions using 5kW electric drives which fit perfectly with the 10,200 lb displacement. Base on HP alone, a Catalina 30 owner might think that they need an 11kW drive to replace their 28hp diesel, and that is obviously overkill.
With 22kW for your 9 ton boat, you should be fine; the good part is that you shouldn't have to run at full throttle for cruising at 6kts. From what I've heard, V-drives are not particularly efficient, so you may see some losses there, but again, you've got some "extra" capacity available, you just may end up with shorter range.
Your main question was about props. You're already running 23" props, so you know that they will fit. You big question will what pitch. The prop pitch will affect your speed (prop slip) and motor load. Let's discuss slip first.
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). Suprisingly, 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 group http://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 underpropped 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.
But you asked for performance numbers, here's a copy of a previous post about mine:
I did another set of performance trials yesterday. I started at 12:00 noon, after the last rain shower passed. The process took about 90 minutes and I covered 6.35nm during the tests. I ran the same process as before, 1 pass in either direction at a particular throttle setting, progressively climbing through 6 settings and then repeat the process descending through the same settings until done. There was some wind by the end of the trials, but that should be offset through the pass in each direction. The results were more consistent than the last trials which had apparent tsunami surge that skewed some of the numbers. As a reminder, the boat is a 5.1 ton 30" ketch, with a 5.5kW Propulsion Marine drive.
Here's this week's results:
495W = 3.0kts
1000W = 3.9kts
1500W = 4.5kts
2450W = 5.0kts
3905W = 5.7kts
5165W = 5.9kts
Slightly better at the low end and a little worse at the top end, but fairly close (+/- 0.2kts) to the previous tests.
Because we had a conversation in the interim about motor temperatures, I recorded winding temps during the second half of the trials to see how quickly the motor would recover during the lower power settings. With a max observed winding temp of 74C at the end of the fourth pass at full throttle, the motor dropped to 52C by the end of the last 500W pass.
Right after the trials, I set the throttle to about 2500W and headed out of the marina, around the detached breakwater in open seas and then back in. 45 minutes later, at a constant power setting, the motor was stable at 60C and I had covered an additional 3.85nm. Apparently, the speed boost downwind is greater than the penalty going upwind resulting in an average speed of just over 5.1kts.
After messing about for 30 minutes or so at slower speeds, I brought her back to the slip. The entire day was 10.9nm with a 4.0kt average. The battery meter showed 107.7Ah (5.4kWh) consumed. After 15 minutes rest, the battery bank was resting at 52.35V. 4 hours on the charger, and she was fully recharged.
Even though there is no easy answer, I hope that this info helps.
Fair winds,
Eric
1964 Bermuda 30 ketch, 5.5 kW Propulsion Marine drive, 8kWh lithium batteries
Marina del Rey, CA
--- In electricboats@yahoogroups.com, Julian Webb <julian.proto@...> wrote:
>
> Hi All
>
> - Does anyone have actual real-world speed figures for an electric drive
> vessel they have built or been on please?
>
> - Just simple overall "she's 6 ton, 38', and my 10KW electric pod pushes
> her along at 4 knots comfortably" will be great thanks.
>
> - That's the short story/request on how you make me sleep better tonight
> with some examples of figures, and you don't have to know the whole gory
> story to help me feel better, but if you want to know the logic/story
> behind my plea, read on:
>
> - I am just finishing a Diesel/Electric canal cruiser and it has gotten
> down to the props.
>
> Her specs are: Built 1964 - 32' w/l - 35' o/a - 11' beam - 4' draft -
> displacement hull - 9 ton.
> She had 2 x 45 HP Fordson Diesels, original and never overhauled, running
> through 2:1 V drives that pushed her along at 8 knots cruising and 11 knots
> top speed spinning 2 x 23" props, borrowed, mismatched and both LH!
>
> - I thought I had it all figured, read Lynch and others on rule of thumb
> "electric drive can be 1/3 the HP/KW of an ICE and still be close to the
> same performance" but now I'm meeting doubt and silence at every turn!
>
> - I have installed 2 x 11KW/15HP 6 pole 1000RPM @ 110Nm and of course can
> use 2:1 reduction and get 500RPM @200Nm (including losses) which I would
> have thought would be enough to push the old girl down a quiet canal at 6
> knots?
>
> 1) I emailed 8 prop "makers" (from Italy to the USA) and when they saw the
> words "electric drive" most ran for cover.
>
> 2) 4 bothered to answer and in the end only 1 would guesstimate any thrust
> but even he has steadfastly refused to estimate boat speed, despite the
> fact that I have given him (and them all) figures, displacement etc and
> even a primitive hull plot.
>
> - After MUCH pushing (although I couldn't do it too hard or maybe scare him
> off completely) I found out that although they say the software they use
> has provision for calculating props for an electric motor, they work on HP
> or KW and the torque is "included in the power figures" as it were an IC
> engine.
>
> - When I said to them I can reduce speed 2:1 and double the torque they say
> they can't take that into consideration and it's based on HP/KW so the
> whole thing is looking shakey to me! HELP - HELP - HELP!
>
> All input kindly received, Cheers, Julian
>
> --
> www.proto.eu.com ltd
>
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