Slow top speed

[Newboater]

I have an impulse 31. running 4s (two 2s) Boat is stock except for X642 propeller. Motor is 1750Kv 4 pole and Dynamite 120esc. Top speed on 4s is 33mph based on Sky RC GPS. Motor temps and esc are both 90* degrees. Is this the correct speed on the set up or is there another issue? I though I had read that the boat should be closer to mid 40's with that prop. Any help on what to check next unless 33mph is normal. Thanks

[Fluid]

You do seem low on speed. What radio are you using, and has it been bound to the ESC? You may not be getting full throttle. Other causes include weak packs, driveline binding, poor boat trim, or a damaged motor. Based on you temps I suspect a radio issue. Have you checked the calibration of the GPS?


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[Newboater]

Radio has been bound to the ESC, radio system is the stock one that came with the boat a spectrum DX2e. Battery packs are new SMC 7200 60c packs. Boat trim seems pretty good maybe running a touch wet, and GPS maybe a touch low on the reading. Compared to the truck speedo it did read 2-3 mph slower. Went back out and ran a set of 3s packs and clocked on the same GPS a reading of 47.5 mph. I am not sure what the speed difference would be between a stock prop which is the same pitch but 40mm vs 42mm of the x642. Another members stock boat ran 46.8. Not really sure where I should be at speed wise.

[Fluid]

Running "a touch wet" can shave 5+ mph off easily. The x642 should be more efficient = more speed. Re-bind the ESC. Has the motor ever gotten over 140*F?


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[fweasel]

Has the motor ever gotten over 140*F?
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What's special about that threshhold? Damage to magnets beyond it?

[Newboater]

Running "a touch wet" can shave 5+ mph off easily. The x642 should be more efficient = more speed. Re-bind the ESC. Has the motor ever gotten over 140*F?


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No never gotten over 140 everything is new. This was the second run on the boat. Didn't realize a little wet would be shaving 5 mph off of it. Figured I start out safe it looked like about 30% cog. Boat was flat out stable no chine walking or anything. I'll start raising the strut angle and recheck. I think a nice even 50 mph with great stability is what I will shoot for in the beginning. Not to far off the mark but looks like it has to be done on 6s.

[785boats]

You have bound the receiver to the tranmitter, but after that, did you set the throttle calibration as per the instruction manual?
It needs to be done.
Here's the section out of your manual if you haven't done it. As Fluid said, the ESC may not be seeing full throttle.

ESC Calibration Procedure
Ensure proper ESC function by calibrating the ESC to your
transmitter inputs.
1. Power OFF the ESC.
2. Ensure your transmitter is powered ON, the throttle is
not reversed, the throttle trim is neutral and the throttle
travel range is at 100%. Disable any special functions
such as ABS, etc.
3. Pull and hold the Throttle Trigger to full throttle and
turn the ESC on.
4. After 2 seconds a “-B-B” will sound. This means the
full throttle position has been confirmed.
5. Release the throttle trigger to the neutral position, a
steady long “B––” will sound. This means the neutral
position has been confirmed.

[Newboater]

You have bound the receiver to the tranmitter, but after that, did you set the throttle calibration as per the instruction manual?
It needs to be done.
Here's the section out of your manual if you haven't done it. As Fluid said, the ESC may not be seeing full throttle.

ESC Calibration Procedure
Ensure proper ESC function by calibrating the ESC to your
transmitter inputs.
1. Power OFF the ESC.
2. Ensure your transmitter is powered ON, the throttle is
not reversed, the throttle trim is neutral and the throttle
travel range is at 100%. Disable any special functions
such as ABS, etc.
3. Pull and hold the Throttle Trigger to full throttle and
turn the ESC on.
4. After 2 seconds a “-B-B” will sound. This means the
full throttle position has been confirmed.
5. Release the throttle trigger to the neutral position, a
steady long “B––” will sound. This means the neutral
position has been confirmed.

Yes I also did that procedure. I don't think the boat has the capability to run mid 40's on 4s with the factory 1750kv motor and x642 prop. According to the speed calculator here on the links, the boat would only hit 39mph on 4s. I was at 33mph, so I probably just need to adjust the rear strut more to get closer to the 39mph. I have seen a few videos where guys run mono's at fast speeds but they are almost out of the water and chine walking all over the place, I don't need to hit top speed if that is what needs to be done. At the 47.5mph on 6s the boat was rock solid and handled great, same handling as on 4s. I will continue to adjust the rear strut till it just becomes loose and then put some negative back in and see where she lands speed wise.

Thinking about it now my boat may also be a little slower as I added about a pound to it's factory weight as I reinforced the hull with 2ounce fiberglass all the way around the seams and on the bottom of the hull as well as strengthening up the canopy. I also removed the brass pins from the canopy and installed longer ones into the deck figuring that would help prevent the deck getting damaged in a crash like what happened to another members Impulse. Maybe the extra pound adds the stability but hurts top speed.

[785boats]

Oh well. It was just a thought about the throttle calibration.

[wuzofoz]

is it just my rigger orientated thinking that says a x642 is a small prop for 1750kv on 4s?
a x440 3 blade would create more thrust and guys at our club run them on monos with 2200kv / 4s setups

[Heath M]

I'd say an M447 would be better suited to that motor.