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Pick a Propeller - Try Again

Background

Propeller design is a complex topic which I am sure has been the life's work for many engineers. The difficulty is that a propeller has many parameters which are all interdependent. There are diameter and pitch (the distance the blades will travel through the water if the propeller turns once, like a corkscrew), the area of each blade and the number of blades. Once these are determined, and the blade profile chosen, the speed of the boat will depend upon the hull characteristics and the weight of the boat.


Let's get back to first principles. The whole point of Perseverance is to cruise quietly. Smoothly. Without fuss. Therefore I opted for a 4-bladed propeller as more blades give a smoother ride. I also found that standard propellers have a limited range of area ratios, so that reduces the number of unknowns further. I tried to use charts to find out the propeller performance, but some are fiendish like this one:

The wake caused by a large propeller moving more water more slowly will be less turbulent, and hence quieter, than a small propeller moving a little water more quickly. For this reason, I opted for an 18in propeller, and using a chart like the one above I settled on a 14in pitch.


As a sanity check, I asked lots of people for advice, used on-line and downloadable propeller sizing tools, and plotted all the information I could gather on a chart of diameter vs pitch. Fortunately my 18x14 choice was in the middle of the range which provides reassurance.


The actual propeller and sterngear can be seen in Sterngear Vlog, and Why Waste Money addresses the different types of power plant which could be used.


2022 Update

Well this didn't work out as expected. The propeller certainly shifted lots of water and was wonderfully silent in use. I did many measurements, and in fact the thrust for the bollard pull test was so great, it bent a steel bar used to connect to the tensiometer.


The snag was that by 660 rpm the motor reached its torque limit (82Nm) so we were only using about 6kW to move the boat from a motor rated at 12kW. Half the speed means half the power. This performance was good enough to see us complete the autumn 2021 cruise to the Odiham, Godalming, Brentford and back to Napton, but sometimes we would have liked to speed along a bit more.


Back to the drawing board, and I decided to start again from scratch, this time using a computer. From data gathered from the cruise and other tests I now know the drag charactersistics of Persy and adapted the standard form cubic drag equations to match. Then with some polynomials I encoded the Wagningnen chart as shown above. I blew the cobwebs off my old textbooks and set up an optimization process that fiddles about with the variables to find an optimal solution, and added motor performance boundaries to avoid demanding more power or torque than is available.


The end result allows you to type in the hull length and displacement and the motor limits. Then you can either optimize the propeller for the highest speed, or for the lowest power at a selected speed. This process has since been used to select the propeller for some of the newer Ortomarine boats.


My first stab at using this software was aimed at getting the highest speed out of the boat. The original propeller was 18 x 14 in, 4-blade, and the optimized one was was smaller diameter, and a lower pitch than the original, at 16 x 8 in and still 4 blades. The result was mixed; although the motor could reach maximum power and, even allowing for the less efficient propeller, the boat speed increased, the mechanical noise of the shaft spinning at higher speed and the more turbulent wash made the experience less pleasant.


Of far greater significance was that over the cruising range of propeller speeds, the propeller "sang" terribly. All propellers have mechanical resonances - just like a bell - and if one matches the input disturbances experienced on that boat, the prop will ring. We could stand it just long enough to shout "Quick! Get me another prop".


With this experience, I ran the program to find an optimal propeller aimed at a lower speed and (because I can't help tinkering) I chose a 3-bladed prop. A 3-bladed prop will be slightly more efficient than a 4-bladed design, and optimizing for a speed of 3 mph I selected a 18 x 10 inch prop. Here are the second (just come off) and third (just fitted) props.


The new new prop didn't sing, so we happily did some test measurements and went off cruising to Dudley and back. The midway selection of area and pitch does give a midway performance - higher power and faster than the original without the noise and excitement of the smallest prop (remember, excitement is a bad thing on Perseverance). One downside is that the 3-blade prop appears to be more prone to collecting weed, and its stopping power in locks is very poor.


I have yet to do the sums to quantify these differences, but already I can feel prop 4 coming on... After all, this boat is a big boy's toy, and I like playing. Oops - that should read "I enjoy the engineering challenge" (sounds more grown up!).





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Dave Jesse
Dave Jesse
Jun 26, 2023

Here are my current thoughts about prop selection, which I may tidy up into a better blog post later.

The force taken to push a boat through the water at a given speed depends upon hull shape (we can't do a lot about that, but the stern swim is key, and much more important than the bow), weight (lighter is better) and length (longer is better). Also important is the canal width and depth, so we all need to keep pressing the CRT to do more dredging.

If you want to go faster, the forces to slide through the water and make waves go up by the square of the speed, and the power by the cube. So, twice as…


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Ian Dedic
Ian Dedic
Jul 04, 2023
Replying to

Found out yesterday that my boat will have a 16 x 12 x 4 prop, will be interesting to see how that goes once it's in the water -- I believe the Engiro motor is higher torque and lower rpm than your Tema one... ;-)

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Interesting you're finding that the 3-blade collects more weed than the 4-blade did; is that compared with the original 18" 4-blade, or the smaller 4-blade? I found on my current (diesel) boat that moving from an 18" to 17" saw a massive decrease in the number of times the weed hatch is up; I wondered if that is something to do with the 18" having less blade clearance under the counter and if your findings might be the same. Interestingly on my new boat that I just ordered which will be serial hybrid Lynch are suggesting a 19x12 prop - that's not on direct drive like yours though, so the reduction ration is playing a part there.

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Dave Jesse
Dave Jesse
Aug 18, 2022
Replying to

Weed collection comparison was between two 18in props.

The clearance issue is interesting - there is a "20%" golden rule that comes from somewhere, but I am tempted to go closer as this, combined with more blades, will give weaker tip vortices. I did think about sealing the weed hatch to see if that reduces the slight noise I get from the prop at higher powers.

You are right to say that a slower, larger prop is advantageous, but from my point of view the noise of a toothed belt drive combined with the screech of rubbing commutators makes the Lynch unsuitable for silent cruising. Couple that with the need to do routine maintenance on the motor, and I went…

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Ian Dedic
Ian Dedic
Jul 05, 2022

I'm surprised you have a problem with mechanical noise from the motor/shaft -- is the motor rigidly mounted or on flexible feet, and do you have any flexible coupling between it and the prop shaft?


Actually, I've just spotted from other blog posts that you have both. What's your maximum motor rpm?


One other suggestion -- going by the photo your prop looks rather close to the hull and a long way from the rudder. For a 3-blade prop *minimum* (more is better) recommended space from the nearest point on the blade to the hull is 0.27D which is >4" for your new prop (looks less), and minimum space from the nearest point on the blade to the rudder is…


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Dave Jesse
Dave Jesse
Jun 26, 2023
Replying to

Hmm. Where does your 0.27 figure come from? I found 0.2 in a couple of articles, but I have never seen 0.27.

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Peter McLaren
Peter McLaren
Nov 06, 2021

Great to read about your boat. I had my 57 ft narrowboat converted 2021-21 winter by Wharf House from conventional diesel to serial electric propulsion (Fischer-Panda generator and Bell Marine Motor). At present I am still using the 172 x 13" propeller originally supplied with the Isuzu 42 diesel engine, but I can't help wondering whether a larger propeller would make better use of the torque available, say 18" x 14". However I read that you are having second thoughts on propeller size. I have not been able to find any guide of how to calculate the optimum and my naval architect contacts don't have any ready answer either. Sometimes propeller design is described as being as much art as…

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Ian Dedic
Ian Dedic
Jun 26, 2023
Replying to

That's correct, but the other side of the equation is the torque absorbed by the prop, which is roughly proportional to rpm^2 so rises rapidly with engine speed. Maximum rpm will be where the motor and prop torques curves cross; at any speed below this the motor will have excess torque available, regardless of the type of engine, so will be operating at part load. See the curve here:

https://www.hybrid-marine.co.uk/index.php/hybrid-info/hybrid-efficiency

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