Question about loaded RPM

[jamie]

Im doing some SAW runs with my hydro and hit 71 MPH (GPS) but not having the benefit of any type of logging device i am using a speed calculator to determine aprox. loaded rpm.

My question(s) are:

Whats a normal cell voltage for highly loaded rpm?
My aproximate loaded RPM seems to indicate that the cells are only producing about 3 volts each but the batts are in good condition, temps about 110 /esc about 105 and came back showing 3.9 volts each after about 8 passes. Do i need better lipos?

In my calculations do i subtract motor efficiency before making my estimates?
Neu motors are supposed to be around 90% so would i use 2900kv less 10% making it 2610 kv or have i got this wrong?
I know if i want to get serious i should be getting an Eagle Tree in it, but i was just curious to see what my boat could do while waiting for parts for my FE30 and things kinda got out of hand lol.
Would love it if some of you guys can share some data about this stuff Thanks

My combo is Neu 1521 1D (2900 kv)
200 Swordfish with etti cap bank
2S 2S Turnagy 5000 40C lipos
V937/3 octura (2.8 pitch i think)
UL1 Hydro (Very modifyed)

Thanks

[Brushless55]

you can figure 3.7V per cell under load to start with
have you also considered prop slip in your figures?

[jamie]

I factored prop slip at both 20% and 22% being a hydro and even at 22% with 14.8 volts (3.7) it says it should be about 80mph with 90% motor efficiency. I know im either calculating this wrong or the lipos are draining more than i thought ?

[Boaterguy]

isn't a V9 series prop 1.9" pitch?

[Brushless55]

setup is a big factor to...

[jamie]

Sorry your right i was talking total pitch of 2.8 (1.9 Pitch by 1.48 Dia.) and yeah setup is critacle, i know if i move the strut 1 degree
it thinks its an airplane lol

[Shooter]

Sorry, I'm not smart enough to answer your question, but I found it interesting that you are running a 937/3 with that much motor.

Is there a "rough" rule of thumb relating pitch and diameter? i.e. "x" increase in pitch is equivalent to "x" increase in diameter?

Thanks.

[jamie]

Octura V9 props are SAW props. pretty aggressive pitch without the diameter to torque the boat around as much. I plan on trying the V940/3 but just wanted to test temps with something a little smaller first. Its only a little boat at around 27 inches, so im not sure if much bigger than that will work on something this small, the 45 mm dia. props i have i think will make it unstable but i havn't tried any yet.

[bwells]

The Octura v940/3 is a 2.983 total pitch but for SAW, I thought 2 blade, 3 blade for turns? I have that prop sitting in front of me that someone notched the blades at the hub to reduce pressure on the blades and still can't figure which setup it will burn the quickest! Edit: Sorry, I know the post did not address you original question of loaded RPM but 2/3 of spec sticks in my mind, seems like it should be lower.

[Chris Harris]

90% efficiency is probably a bit optimistic even for a Neu. Everything would need to be perfect for the motor to reach this level of efficiency. I would use 80-85% in your calculations.
The other thing is that is a small prop for that size boat, not just diameter but in blade area. If there is not enough blade area to sufficiently push the boat it can greatly increase the slip factor.
This can also make the hull run wet which is another variable that will reduce speed. These are a few things that could be the reason for the discrepancy between your speed calculations and your actual speed.

All that being said, 71mph with 4s sport hydro is cooking right along. Put a bigger prop on it that 1521 can handle a lot more prop.

Chris

[jamie]

Thanks for replying Chris.
I was thinking closer to between 80 and 85% but couldnt be sure, also i know the V937 was being a little timid with it but i was worried about heat. I will be trying the 40 and the 42 next.
BTW. i ran the calcs. again with 80% motor eff. 14.8 Volts and 21% slip and it was bang on 71 MPH ... at least i dont think i need to spend money on lipos for a while now
Thanks guys

[martin]

The way i work my calcs on the figures given i come up with 72.8mph, Like Chris has said i normally use 80% for motor + 20% slip on prop.Martin.

[jamie]

Thats very close to what i got too yeah ... as far as how wet it runs im not sure how wet is too wet LOL ... I wanted to video the run with my dodgy old stone age video phone, but its a bit hard driving and filming (just kidding) noone wants to come down to the lake at 7.30a.m in the middle of winter

[Chris Harris]

Thanks for replying Chris.
I was thinking closer to between 80 and 85% but couldnt be sure, also i know the V937 was being a little timid with it but i was worried about heat. I will be trying the 40 and the 42 next.
BTW. i ran the calcs. again with 80% motor eff. 14.8 Volts and 21% slip and it was bang on 71 MPH ... at least i dont think i need to spend money on lipos for a while now
Thanks guys

Hey, no problem,
If you are worried about heat I would limit your passes to just a couple when you start trying the bigger props and check your temps.
In my experience with SAW type setups the difference in heat between two passes verses four is huge.

Chris

[drwayne]

Jamie.
I have v942/3, v947/3, barrcut abc1616/3 , and an eagletree with widgets you can use.

Yes, 0730 is too early for us night owls.
Doc Wayne
Adelaide
Au