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    Prop size and speed
    by StuartE πŸ‡¬πŸ‡§ ( Petty Officer 1st Class)
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    15 Posts 14 Replies 4 Photos 31 Likes
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    samc
    Leading Seaman
    πŸ“ Prop size and speed
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    Thanks for the information, will read your blog
    i think i found a method to line up the prop shaft with the motor. Getting the right angle.
    Measures the motor shaft then prop shaft tube. Got the difference 1.550 in, then measurer the motor dia.
    Took the difference and came up with .620, so I put a piece of .625in wood 6-inch-long, on the tube and put a ruler (scale) on top of that. Put the motor in line with the rule and it all linen up, so I got the angle right, and made adjustment to the motor mount, shimmed it till it was right. Then measure that and make a piece to fit. The motor helped with the magnets in side holding on to the ruler.
    I have tried several way to get this angle and measuring didn’t work of me, or just a piece of tubing over the shaft,,, just not getting the right angle.
    Maybe this will help someone in setting up there motors.
    Now I’m going to finish up the placement of the batteries fuse, power disturbing boards, ESC and wire everything up and see if it WORK’S…
    2
    pmdevlin
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    πŸ“ Prop size and speed
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    I did a lot of amp/watts/rpm and gps speed testing with my twin screw 4 foot fireboat, you might wnat to take a look. The quickest and easiest, and by far cheapest route is change from s prop to x prop thus increasing the pitch. They are real cheap to buy (plasic ones)so get a couple of sizes, start with the smallest. You can buy cheap inline amp meters, so using this you can see how the amp draw alters
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    RNinMunich
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    πŸ“ Prop size and speed
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    Hi Sam,
    Yes, by all means mount the motor on an angled block of wood to align the shafts correctly.
    As you can see in my Sea Scout engine flat.
    I used a standard 400 size motor mount (from Krick) screwed down to the block.
    To align the shafts I used a chunk of brass rod drilled on my mini lathe to match both shaft diameters. Once you get the angle just right you can glue down the block, tack it first with cyano gel or 5 minute epoxy. When set you can remove the motor and mount fit the correct coupling. I try to avoid universal joints and use the flexible spiral types - again see the Sea Scout pic. You can also see that I took the opportunity during the refit to fit an oiler pipe as well πŸ˜‰
    Epoxy the block all round. When set reassemble motor and mounting bracket and check that the shaft rotates freely without straining / bending the coupling or binding anywhere.
    Good luck. Cheers, Doug😎
    2
    Young at heart - slightly older in other places πŸ˜‰ Cheers Doug
    samc
    Leading Seaman
    πŸ“ Prop size and speed
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    Ok i have to go back to school college to do the math in chapter 2.... but it looks like a 2 blade prop might be just as well as a 3 bladed one... just a question i wanted to ask to see if anyone got a big difference in speed or battery life...
    I'm a scale speed guy also... just want to model to look like the real thing in the water... on 50 MPH that is why i got small eng. Now i hope they will be big enough to power the boat...
    now on to mounting the motors..
    any suggestion???? so far i have a plywood plate sitting on an angle on the bulk heads.
    think i should put them on blocks of wood and match the angle of the shaft. or ????
    thank you guys...
    samc
    1
    RNinMunich
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    πŸ“ Prop size and speed
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    Agreed Martin, one thing at a time!
    BUT:
    Stuart; I believe we are discussing your 24" Commander here.
    In which case your results mirror mine with my 24" Sea Scout. Except that my prop shaft did not bind!!πŸ˜‰
    It was also reluctant to plane with a 2S LiPo (7.4V nominal) but went very well and planed easily with a 3S (11.1V nominal). I also was using 4000mAH batteries.
    I published my results using various batteries here in the Media - Video section.
    Go to the Media Gallery and search for Sea Scout.
    So, I would leave the motor and prop alone and (after fixing the prop shaft!!!) try a 3S battery.
    I'm sure you will be satisfied πŸ‘
    Prop technology is complex as Martin said, our old friend Bernoulli raises his head for instanceπŸ€” But, essentially bigger is better for more speed or same speed with lower shaft revolutions when applied to real size ships.
    BUT 2: with our electric powered models bigger props often simply increase the load on the motor causing it to draw more current for a fractional increase in boat speed if any.
    In essence props with fewer blades are inherently more efficient as you hinted. Minimum being 2 of course πŸ™„ Two bladers are popular with the Fast Electric guys for that reason.

    I don't do speedboats, I'm a scale guy and my maxim is; If the real vessel had an XYZ screw then so will my model!
    Summary; first fix the propshaft; lubrication, alignment with motor shaft, ensure a few thou of clearance between prop hub / locknut and the end of the shaft tube. Preferably with a thrust washer in between.
    Second, fit a 3S battery preferably crosswise above the C of G of the boat. As you can see in the pic of my Sea Scout 'engine room' in your Commander thread.
    For those with a mathematical mind, undying curiosity and an afternoon to spare I attach a paper on the Principles of Ship Propulsion from MAN, the supplier of small to huge diesels to probably a majority of ships (commercial and naval) built today.
    You should find Chapter 2: Propeller Propulsion particularly illuminating! Happy reading πŸ˜‰
    Just remember the paper deals with real ships and real water, we have scale ships but not scale waterπŸ€” Basic dynamics still apply though.
    Cheers, Doug 😎
    4
    Young at heart - slightly older in other places πŸ˜‰ Cheers Doug
    Martin555
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    πŸ“ Prop size and speed
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    The only problem is that if you change several things at the same time you will not know what parts had the desired effect.
    It may well be that changing motor and prop will be the answer.

    Martin.
    If it looks right it probably is.
    StuartE
    Petty Officer 1st Class
    πŸ“ Prop size and speed
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    Country: πŸ‡¬πŸ‡§ United Kingdom
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    Hi all,

    Many thanks for all the fantastic responses and info.
    I currently have a 7.4V S2 LiPo 4000mha, a 15A Motor and ESC with a 1000kva Motor. The prop I'm using is a 2 blade S type 35mm.

    Apart from the Motor there is no weight up front in the bow, so can't reduce that much, but I am using a "heavy" servo at 68g in the stern, so will use a smaller less weighty servo.

    I'll try changing the motor to 30A 1000Kva with a 42mm S type 2 blade to see what difference that makes. I also need to re-grease the prop shaft as suspect that it was running hot at the high rpm and caused a bit of speed loss.

    I'll post a comparison video so you can see the results!
    3
    Martin555
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    πŸ“ Prop size and speed
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    Hi,
    If you have a left hand and a right hand propeller then you will need to reverse one of the motors.
    As for propellers I think that at the and of the day you will have to experiment to find out what suits your needs as you must remember that you are dealing with scale models and that the water it is on is not at the same scale.

    A 2-blade propeller produces two pressure pulses per revolution, whereas a 3-blade propeller will produce three smaller pulses per revolution for the same amount of total thrust. As a result, the 3-blade prop will be inherently smoother and therefore quieter.
    A 4-blade propeller can improve all those characteristics that make for practical, all-around boat performance.

    Four-blade props usually have a lower pitch to keep the rpms the same as a 3-blade.
    But are 4-Blades Slow?
    So, why might a 4-blade generally be slower than its 3-blade counterpart? To be honest, many 3-blade/4-blade speed comparisons are simply not fair. That’s because the respective propellers in question are simply different styles, designed with different purposes in mindβ€”different diameters, rakes, cupping, and blade shapes.
    If however, for comparison purposes, we take two propellers, identical in design (blade shape, diameter, rake, cup, etc.) that are appropriate for a given application, and simply add a propeller blade, we get a truer representation of just where the difference lies.

    The addition of the extra blade causes increased drag, which, in turn, requires more horsepower, in order to achieve the same rpm. Since the horsepower is limited, the rpms drop, and the speed will tend to drop with it. This is why, when going from a 3-blade to a 4-blade, the pitch is dropped an inch, or more, in order to keep rpm parity. It is this difference in pitch that causes any potential speed differentials between the 3-blade and the 4.

    I don't know if this will help you but this is a complex subject and you could get your self totally bogged down with all of the theory.

    Martin.
    3
    If it looks right it probably is.
    samc
    Leading Seaman
    πŸ“ Prop size and speed
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    Hi doug and anyone out there....
    you mentioned you tried 2 & 3 blade prop
    what was your conclusion?? did the boat go faster or did it accelerated faster or is there advantage
    2 blades are for speed,? and 3 are for power?
    i have a dumas CC commander 36 inch long.
    wondering if i should us 3 blade prop. comes with a 2 bladed and going to run twin motors. also should i revers one of the motors, for better control???
    thank you samc
    RNinMunich
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    Agreed MartinπŸ‘
    But don't forget that there was a reason why the company was called The Fairey Power Boat Company! They won race after race after race back thenπŸ˜‰
    There's always the 'throttle limiter' on the TX anyway! AKA Operator.
    Doug 😎
    Archimedes? EUREKA!! (Murmur from background "You don't smell so good either"!😁)
    2
    Young at heart - slightly older in other places πŸ˜‰ Cheers Doug
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