3 blades prop, how can i calculate the speed?

[boilo56]

I would like to know how to calculate the speed on three blades props. The OSE Speed calc doesn't give the option. Is there a multiplying factor to be used on the two blade results to get the 3 blades prop speed?
thank you!

[dasboata]

Theoretical speed,,,,, in a solid surface which water is not , rpm X pitch - 15% threw 35% or so slip,,, a 3 blade will have less slip

[boilo56]

Theoretical speed,,,,, in a solid surface which water is not , rpm X pitch - 15% threw 35% or so slip,,, a 3 blade will have less slip

Thank you, so a 3 blades prop slips 15% to 20% less than a 2 blades, am i right?

[dasboata]

I don't have any figures to me slip is a good thing it lets you spin larger blades my rule is 3 blades on a mono to help handling & 2 blades on a cat for top speed ,, but this is not always true,, Keith K. like to run 3 blades on his cat and he has record breaking boats

[boilo56]

I guess it is to the pilot liking then!

[Luck as a Constant]

Forget calculating speed and try the prop. Then gps it with prop.



Sent from my iPhone using Tapatalk

[boilo56]

Forget calculating speed and try the prop. Then gps it with prop.



Sent from my iPhone using Tapatalk

That is exactly what i did!https://forums.offshoreelectrics.com/...t-)#post630457

[Meniscus]

It is important to remember to be careful when you go from 2-blades to 3-blades. The amp draw increases which, in turn, means that the voltage may sag more in addition to the possibility of more heat or stress on a given ESC. Although it may be great theory that a 3-blade will slip less, it doesn't mean that it will equate to a faster speed. In my experience, I often use a 3-blade prop when I need to keep the transom up. You are able to do this with a 2-blade prop as well, but everything depends on what the model likes.

Regardless, please be sure to take into account that many 3-blade props have higher pitch (or a different progressive pitch) which means that you would need to go down in diameter. It's needless to say, but always try a prop with less diameter when going to a 3-blade and measure temps closely on short runs before you increase the diameter.

Another aspect that is not addressed here is the condition where you may have loading and unloading of the prop if the prop lifts out of the water for any reason (rough water, wake, skipping, etc.). These swings in current and load may be exaggerated on a 3-blade prop and may ultimately become detrimental to your ESC. Happy testing!

[boilo56]

Hi Meniscus, i noticed the increased amp draw, as my runtime on 3-blades as shorten by almost 30%.And i did noticed an increase in temps also. My batteries are the one thing that suffer the most, the temps were s bit high after the run on the x442/3.

[martin]

If going from a 2 blade to a 3 blade I generally reduce the prop size by around 10%, ie if im using an x442 2 blade which is 42mm diam x 2.31" pitch. If I go to a 3 blade I will use x438 3 blade which is 38mm diam x 2.09" pitch, theirs a lot more blade area on the 3 blades to push the amps much higher.

[boilo56]

If going from a 2 blade to a 3 blade I generally reduce the prop size by around 10%, ie if im using an x442 2 blade which is 42mm diam x 2.31" pitch. If I go to a 3 blade I will use x438 3 blade which is 38mm diam x 2.09" pitch, theirs a lot more blade area on the 3 blades to push the amps much higher.

How can i know the amp draw without logging?

[martin]

You cant know without logging, one thing for shore if you go from a given size 2 blade to a 3 blade of the same size you will see an amp draw increase just because of the extra blade & increased blade area.

[boilo56]

Ok, thanks, i'll keep this in mind! Sounds like a good habit!

[TRUCKPULL]

How can i know the amp draw without logging?

There is one way to calculate AMP draw.
But this is average Amp draw over time.
Lets say you put your boat in the water and run an oval coarse for 2:00 minutes,
when you recharge your batteries you put 4,000 mAmp's back in the pack.
This calculates out to 120 AMP's average draw.
Remember that when running an oval the Amp draw will be a little higher pulling out of the turns,
and a little less the second half of the straight away.

Print the chart below and keep it for your records.

Larry

[boilo56]

Thank you very much, this chart will be very useful!!!