Pit Talk, (Bulletin 3) Tunepipes, Mixture settings and propellers?

Pit Talk, (Bulletin 3) Tunepipes, Mixture settings and propellers?

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Baldwin Racing

Many people are always looking at optimizing the Tunepipe, propeller and mixture settings to get their boat to perform at its best. Well hopefully this article will enable the new comer in this sport to get there much quicker and who knows, using that process, you may identify your own little tricks of the trade.

I decided to write this article in a question - answer type format to get a little structure, as there are so many variables on this subject. It must also be understood that I do not say that this is the only way, but I can say that it works for me to get the results that I do. I am not going to get too technical, but I will tell you a little theory to ensure that you work in the correct direction during your optimization process.

Take note that all the measurements and recommended sizes are based on 6000 ft altitude and would be different if done at the coast. The process should however not change. Setting up a boat at high altitudes is a bit more sensitive and thus more difficult.

1) Where do I start with my settings when I put my boat on the water for the first time?

If I set up a boat for the first time and the motor, pipe and carb are all different to what I normally run I start as follows: Firstly I install a smaller propeller than I would normally expect to finally run with onto the boat. At least 2 sizes smaller to ensure that the motor can spin it. I then make the pipe the length that the manufacturer states it should be for the motor size it was made for. If you do not have a clue, then you can make them the following distances as indicated in the list below. Remember that these lengths are by no means the optimum but will get you started. Look at the measurement points used in the picture.

It is difficult to say where you should set the carburator but if you are really starting from scratch then I recommend that you screw the mixture needle totally in and then screw it 2 complete turns out. This should give you a rich mixture however that is subject to each needle arrangement. The boat is now ready to start and put in the water. Once I start the motor, I determine whether it's rich or lean while trying to rev the motor up, by either pinching the fuel line to the carb and seeing if it revs up or by choking the carb in short bursts to see if it needs more fuel to rev up. I then adjust the main mixture accordingly. I normally make the mixture a little rich on the jetty. The idle mixture is then set by bringing the engine revs down on the throttle and then pinching the fuel line again. If the motor stops then the idle is too lean. If it first revs up a little and then stops its too rich and I adjust accordingly. The boat is then put on the water and should go. I would then lean it off a little at a time until it starts revving smoothly on the water. I am not yet worried about speed at this stage but only checking that the motor sounds smooth. I then take the boat out the water and repeat the idle mixture adjustment as indicated before. Note that the Idle mixture effects the main mixture slightly. If you make the idle mixture leaner then the main mixture will also get slightly leaner. If the main mixture was correct then you need to make it a little richer. Note that it is not in the same proportion. The main mixture will only need to change a fraction. This is a reiterative process but gets you set up.

2) How do I know what prop to put on my boat.?

The propeller that you use is subject to your pipe length, exhaust port height and gearbox if you use a gearbox. If you run a big prop you should have a longer pipe length, if you run a small prop you should shorten the pipe. The mixture does not really play any part bar that you need to adjust it depending on the load on the motor. If you have no idea here is some guidance to start off with. Note that there are many variations but this reference table should put you close.

Reference TABLE
MOCOM Propellers
MOTOR Gearbox Recommended prop Desired prop
3.5 cc 1.8 : 1 2023 2024
3.5 cc 2 : 1 2024 2026
7.5 cc 1.4 : 1 2024 2026
7.5 cc 1 : 1 *1924* 1924
15 cc 1 : 1 2026 2127

* The 1924 in straight drive 7.5 cc is not an original but a cut down prop as I have not yet found the ideal prop for my motor yet. At the coast I do however turn a straight 1924 prop. I ran this prop in the French International event during July 97 with great success. Look at the Model Boats Magazine Vol 47 No 562 page 67

3) How do I optimize my performance with the pipe and prop?

Every motor has its ideal point of performance and that is the difficulty to identify where your motors performance is. Without a test bench you will not be sure that you have reached it but try this and see. You must first set up buoys in the water so that you can time your boat with a longer than usual pipe and get a reference. Then keeping the same prop make the pipe 5mm shorter. Get a reference again and see if its better. Keep on making it shorter until the boat cannot stay on the pipe if you turn a sharp corner. Plot all the results and see where the best performance is. Write down that measurement with the prop that was used. Now take a bigger prop and start all over. Make the pipe longer again and slowly move the pipe shorter as indicated above while taking readings. Chart all your results. Take note not to put a too small prop on and over rev your motor. If your motor is not designed to rev then do not try that range of props.

Make sure that you get an average speed to ensure that you get a fair result. I normally ensure that the fuel mixture is set correctly before I take a speed reading, as the mixture will need to be adjusted slightly if you change the pipe length and prop. Also make sure that you test from a small prop to a big prop to ensure that you cover the entire range. Also note that the more props you test the clearer your result becomes.

Within a certain range you are able to decide what prop was the best for what pipe length and you are then able to set up your boat to run with this prop. Advanced mods are available but this however would require that you start working with the motor timing and does get a little technical. As a general rule the higher you make the exhaust port the higher your revs can go the smaller the prop and the shorter the pipe and vise versa, the lower you make the exhaust port the lower your revs will be the bigger the prop can go and the longer the pipe should be. This is all to a certain limit of course.

4) When do I know that I have reached the ultimate performance?

YOU DON`T !! Thats why boating is such a great challenge. I have been boating for many years and still today I am finding new things that make it go better and better every day. My problem is that I do not get the time that I would like to spend to optimise and improve performance. But then again everyone has the same problem. Remember that I have just spoken about your pipe and prop but there are many, many, many combinations of hulls, props, pipes, carbs, shaft angles, engines, gear ratios, port timings and so on to keep you experimenting for the rest of your life. -ENJOY ! and keep up the boating.

If you would like to get a little more detailed info do not hesitate to E-Mail me. I also would like to know if you have better Ideas that you would like to share with me as we are never too old or too good to learn from others.

Compiled in December1997 by Gary Baldwin
Last Updated on 29 December 1998

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