Here is a list of all the postings Gareth Jones has made in our forums. Click on a thread name to jump to the thread.
|Thread: Method for choosing the right prop and motor|
They were measured statically, either in the pond or the domestic test tank. See below for the Springer in the bath.
Heres the example I was trying to illustarte earlier.
Your child has got its first fan, I am sure it will be very useful.
I am not sure how best to illustrate this but here is an example and you will have to work it out and see for yourself. I have run a Springer tug with a Speed 500E motor, 7.2 volt battery and no gearbox (1:1 gearing). I ran it with three different propeller sizes and measured the performance at full throttle as below :-
Prop size (mm) 30 40 50
Voltage 7.8 7.75 7.75
Current 3.1 4.1 5.1
Prop rpm 4700 3250 1900
Thrust (grms) 229 234 236
My conclusion before seeing the Billysugger Motorcalc programme was that the 30 mm prop is the best of the three but the Motorcalc programme makes it very clear. Input the above motor parameters, i.e. Speed 500E motor, 7.75 volts, and 1.1 gearing. Look at the charts and using prop rpm as the reference datum, see where the three prop sizes sit relative to each other and it clearly shows how much influence prop size has on the efficiency.
As a guide I will provide the prop rpms I have measured for a few other motor/prop combinations and you can see whereabouts on the performance charts you would be operating for that size and get an idea where you would be with a bigger or smaller prop. I will try and put the data on this evening.
Edited By Gareth Jones on 11/10/2010 13:28:59
Edited By Gareth Jones on 11/10/2010 13:30:48
Edited By Gareth Jones on 11/10/2010 13:33:05
Maybe I am a bit odd, too as I have been experimenting over the past few months to try and achieve the same aim as you. I began building my first model boat about 5 years ago (a TID tug) and I was surprised there were no recommendations on matching propellers and motors. In the electric model aircraft world there seems to be much more information with advice on thrust available from various battery and propeller sizes for a given motor. My second boat was a three motored PT boat (Snogg as described in the Sept issue of Model Boats) and that turned out to be a bit of a disaster initially. Even the second attempt has not turned out to be very good after about a years running with ever higher motor temperatures. I have done some testing and research and now I am much more confident that I have a good working match of propeller and motor, at least for Snogg. I have already written to Paul Freshney and suggested a follow up article on the work I have done, Snogg the Sequel perhaps. I am also supposed to be giving a presentation on my experience to the club that I sail at, (Goole MBC), probably in November.
Essentially I have gone down two routes. I have compiled a spreadsheet for our limited fleet of models (I say our because my wife built some of them). This documents the build standard details, size, weight, type, battery, motor, propeller size etc. I have also measured performance using a wattmeter to measure the power into the speed controller, a tacho to measure prop speed and a digital set of fishing scales to measure thrust from the propeller. I have put this data into the spreadsheet and rated all the combinations I have covered, ranging from a 1:12 narrow boat through Springer tugs, Steam drifter, TID and PT boat. I have come to the conclusion that the best performance is with a propeller that lets the motor run at around 60-70% of the free running speed, i.e. somwhere between the max efficiency and max power points on the motor performance chart. I have also done some experiments with different propellers and found some surprising variations as you can see if you look on the forum under R/C and accessories of a few weeks ago. I am not an expert in electric motors but I spent nearly 40 years as a Flight Systems engineer with BAE Systems so I have a lot of experience with collections of equipment connected together and being forced to work together.
I would be interested in collaborating in some further work as you suggest. I thought it would be relatively easy to come up with some analytical tool to predict performance but as Colin Bishop has said its not as easy as it looks and there are more variables than is first apparent. If you would like to get in touch I could send you the spreadsheet as a starting point.
|Thread: Variation in thrust and propeller pitch|
|I have now done some more work on Snogg's propellers and here are the results. Firstly I measured each blade tip height as shown below using the 'penny washer'. For my best propeller, which was originally the LH side one, I took an average of all three blades, which came out as 10.5 mm.|
Then I carefully bent the blades on one of my RH propellers to match, setting all three blades as symmetrically as possible. I also made a minor adjustment to the LH propeller to match the blades to the 10.5 mm setting. Note, in bending the blades I dont think I am changing the pitch very much, just evening up the circle which is swept by each blade.
To avoid bending the blades again when I refit or remove them I made a tool from a couple of scrap ply disks, three small bolts and some heat shrink as below. The tool can be held in my fingers with the bolts up against the edge of the blades, reducing the chance of bending them.
I then repeated the performance measurements as before. The figures below show the way in which a fishermans digital scale is used to measure thrust and the Wattmeter measures the power into the speed controller.
I ran the tests twice, firstly with the propellers in their original position. Secondly, with the props swapped over side to side and the motor wiring swapped to provide the correct rotation.
Just to remind you originally before matching the blade heights I had:-
LHS motor 8800 rpm, thrust 900 grm RHS motor 9000 rpm, thrust 550 grm
After resetting the blades
LHS motor 8700 rpm, thrust 930 grm RHS motor 9400 rpm, thrust 740 grm
After swapping props side to side
LHS motor 9350 rpm, thrust 870 grm RHS motor 9300 rpm, thrust 1065 grm
My conclusions are that matching up the blade heights has improved the performance of the propeller. The penny washer method seems quite effective as a way of measuring the differences between blades.
I think I probably bent the blades myself originally when fitting or removing them and the tool should help prevent this happening again in future.
There is still a significant difference between the two propellers, even after they have been matched and swapping them from side to side shows the effect persists so its not the motors or prop shafts.
Overall changing from a 7.2 volt Speed 600 motor to a 9.6 volt speed 700 has roughly doubled the thrust and the motors run much cooler and more efficiently. The current at max power has increased from about 17 amps to about 20 amps so the original speed controllers are OK.
The propellers are now fitted so that the RH side rotates anticlockwise, when looking from the back, LHS rotates clockwise. I think this is the opposite of what is normally recommended but since it seems to give the maximum total thrust they can stay where they are.
Its been an interesting few weeks. Maybe if I can find some good looking 30 mm propellers at the Blackpool Show I might have a go at repeating the tests to see if they are better matched or more efficient.
I thought about this a bit more last night and I dont think that the pitch of the blades is that much different. I think that when the propellers have been fitted or removed the blades have been bent along the line of their roots which is the weakest point when you are holding the propeller and tightening up the lock nut. It would be very difficult to twist the blades and affect the pitch. Consequently its perhaps the efficiency of the propeller thats been affected somehow. I will stick to my original plan and try and set one of the RH side propellers (Left hand pitch I think, always get confused over this), so that all the blades are set at the same tip height.
I am thinking of writing 'Snogg the sequel' as the test results from various different motors (and propellers) has been quite interesting and enlightening.
I have been experimenting with Snogg, the PT boat featured in the September edition of model boats. I decided to try fitting a Speed 700 motor to replace one of the Speed 600's and this showed a significant improvement in speed and ran much cooler so I purchased a second Speed 700. I retained the original 30 mm propellers but increased the battery voltage from 7.2 to 9.6 volts. I measured the speed controller input volts, amps and watts using a Wattmeter, the prop speed using a laser tacho and the propeller thrust using a set of digital fishermans scales. (all very nerdy but I like to be methodical!)
The interesting point that came from these tests was that I had much more thrust from the left propeller than from the right. Both were doing near enough the same speed, LH 8800 rpm, RH 9000 rpm with similar input power, LH153 watts, RH 142 watts at full throttle. However the LH propeller gave 900 grammes of thrust, the RH only 550 grammes. I tried an alternative propeller on the RH side and it gave about the same thrust as before, around 580 grammes. I then put the propeller from the LH side on the RHS, reversed the motor direction of rotation and got around 850 grammes of thrust.
The only logical explanation is that the pitch of the propellers is different so I decided to try and measure the pitch. The photo below shows the original LH and RH propellers.
I fitted a penny washer over each one and used that to try and measure the height at the tip of each blade
The photo below shows the best of my propellers, the one that was originally on the LH side. While it was slightly more evenly pitched than the others, it still varied quite a lot and on average seemed to have less pitch than the RH blades. I plan to try and set one of the RH propellers to a similar but even setting to see the effect.
Has anyone noticed anything similar or got any ideas, I assume all three blades should be symmetrical or is the design more complicated than that?
|Thread: Lightships I want to build one|
The Maritime Museum in Hull have a print of the Spurn Lightship drawings in 1:24 scale. There are two sheets showing a side elevation and a plan view but no hull cross sections. They can supply the drawings as a jpeg file or you can go in and trace the originals if you wish - Bill Jefferson who built the model you saw traced the drawings in the museum.
I also have a long term plan to build the Spurn lighship but have not got round to starting yet.
|Thread: Speedline 1:16 Severn Lifeboat|
I have seen a part built example of the Speedline 1:16 Severn Class lifeboat at a recent model boat show. I have only built woden model boats up until now and I wonder if anyone has built the above model in perspex and has any information or advice on how it went?
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