How do you guys calculate prop size

[Froggy]

Is there a formula that doesn?t take a mathematician to figure prop size for a given boat motor ESC combination.
I have a 30 in cat, a Leopard 4082 1800kv motor and Seaking 180 ESC what size starter prop preferably ABC Or Octura prop

[tbonemcniel]

Yeah, it's just multiplication, no advanced math. RPM = kv * voltage under load * motor efficiency. Then RPM * Prop total pitch * (1- prop slippage) = mm or in (whichever units your total pitch were in) per minute. Convert that to mph. Done.

For example: Let's say you're running on 6S on an Octura X445. 1800kv * 22.2v * 85% = 33,966rpm. 33,966rpm * (45mm * 1.4pitch ratio) * (1-20% slippage) = 1,711,866 mm/min = 63.8mph. Easy peasy multiplication.

OSE even has a handy online calculator if you have trouble with multiplication. https://www.offshoreelectrics.com/conversion.php


What the math can't tell you, is what diameter prop is in the happy zone for your boat. Too little diameter and it'll have trouble hooking up, too much diameter and the prop will drive the boat for you. Somewhere in the 40-46mm range I would think we be good for a 30" cat. It also can't tell you the amp draw and if you have the electronics to support it. You could set it up with a prop for 120 mph, but just because the math says it'll work, doesn't mean you have the power system to support it. It also doesn't take into account the "tune" of the boat. If it's running really wet, the amp draw will be high and the speed low.

Tons and tons of variables, but it can help you get in the ballpark.

[Froggy]

Thanks so much for sharing your wisdom, the thing I really love about this hobby is the challenge of it all, so much to learn, I want to get to the point where ai have enough knowledge to understand and build my own boat.

[oscarel]

If you don't want to do that math, there's a calculator on ose, again it won't help with amp draw.

https://www.offshoreelectrics.com/conversion.php

Sent from my SM-S908U using Tapatalk

[Panchothedog]

A good way to tell is to get a selection of props then test for your self. Checking temperature and visually seeing how your boat handles. Graupner
makes a pretty good selection of plastic and glass fiber props that are about
1/3rd the price of metal props. Using those is much easier on the wallet. Once you find what your boat likes, buy the real one. Keep increments small. 2mm can be the difference between success and a burnt motor. Not with the plastic, but once you get a good prop there is a very large selection of pitches available. Staying with the same diameter but increasing the pitch can sometimes yield good results. You will probably end up buying a few before you get it dialed in.