missing RPM's

[Crash]

hello men, thanks for the great forum.

I have a new set up and seems i am missing 12 k in RPM's

set up :

8 s lipo ( Turnigy HD 5000 4s x 2 )

TP Power 4050 1700 kv motor ( 7D / 166 amps. max / 35.5 V. max / 2350 watts continuous / 5000 watts max / 60 k max rpm

Swordfish 300 X

x645 propeller from Alex at Doctor Prop

TFL Pursuit hull

data log reads as : 33849 rpm at 100 % throttle / voltage drop was high but still does not account for missing rpm 26.5 volts / 236 amps max / 41 degrees C. / 6500 max watts / 68 mph on gps

whats your thoughts guys? I am new to Fe be nice ...........lol

[Peter A]

Well 68 mph is a pretty nice speed. With your set up the estimated speed in the speed calculator is 91 mph! Quite frankly a Pursuit is not going to do that! You are also pulling over 6000 watts, which is over the manufacturers designed capacity, so I would suggest that perhaps the motor has lost efficency somewhere. In fact changing that one variable in the speed calc from 90% to 70% puts the estimated speed at 71 mph, so in that may lie your answer. It could be that the prop is inefficient at high rpm on a mono, whereas with a cat hull it would not be and would do the business.

[Crash]

Thanks Peter.
Are you thinking the prop is loading up the motor and preventing the motor from reaching it's proper rpm range?

[NativePaul]

Yup, either the prop is too big, the voltage is too high, the KV is too high, the motor is too small, or the hull is too draggy, depending on your point of view.

Also as your voltage is dropping down to 3.3V/cell under load, your battery is too weak for your existing setup.

[Crash]

Thanks Paul.
so, if the hull is running too wet could that also be preventing the prop from reaching max rpm expected?

[Fluid]

....so, if the hull is running too wet could that also be preventing the prop from reaching max rpm expected?

Yes, exactly. A more realistic expectation for a loaded rpm is 43,000 rpm. For additional speed - although your current speed is pretty good - try one of several things:

1) lower the strut by 1/8”, check performance then try lowering it another 1/8”
2) move the CG back a 1/2”
3) try a slightly smaller pitch, like an x445, to allow the motor to reach its best efficiency

You should realize that this is a SAW setup and you shouldn’t run for more than 2-3 passes before bringing in the boat to cool off.


.

[Crash]

Excellent, thanks Fluid.
Yes, it's just for point and shoot passes..
I will adjust c.o.g.
I do have it set a little wet to keep it from blowing over...
Thanks to everyone for the advise.
Back to the work bench then take it for a couple more pulls at the pond.
Dan

[Speed3]

You battery is too weak for that setup. You may gain a little more with better batteries.

[Crash]

Yes, you guys hit it on the head. Today one of the lipos lost a cell and puffed up, lvc kicked in so no 🔥
Ordering fresh set of packs .
Thanks