These formulas are for a estimate only. If you like to play with numbers these tools can be a major help in starting a new boat building project.

Please keep in mind you can plug any number you like into the fields. It doesn't mean that it will work! You have to have a basic understanding of the limits of your parts.

For example, 300gram 2000kv Motor will spin a 45mm prop just fine in most 25-30" racing hulls on 14 volts. But that same motor will burn up itself or the esc in a 50" boat.

Its pretty simple to use the formulas. Anywhere there is the "field" shaded yellow, change the value to a numeric /  number value. Any letters or spaces in the field will cause a error in the formula. 

One of the formula's I have particular interest in is the "Estimate the top speed of a prop". Its amazing to see how much speed our hulls and props are wasting.

Click anywhere out of the field for calculation. Inches  Millimeters

Speed Estimate   Input your boats information in the "Yellow" fields below. Motor information Enter Manufactures State RPM/Volt "KV". RPM   Torque constant "Kt" in inch-ounces/amp Enter Motor Efficiency %    This is an estimated number. 70% for Brushed. 80-90% for brushless, Depending on the Motor quality. Neu, Leopard motors are in the 90% area, while less expensive Feigaos are in the 80% range.   Estimated RPM under load per volt. Battery Pack information Pack Voltage   Enter # of cells in series? Enter Volts per cell?(1.2v for Nimh, 3.7v for Lipo)   Estimated loaded RPM is       RPM @       Volts Propeller Information:  Total Prop Pitch is in theory how many millimeters forward the boat will move with each propeller revolution. Total Prop Pitch is: Enter Prop Pitch Ratio Enter Prop Diameter in Millimeters Millimeters Gear Box Info:  Most brushless fast electrics boats are running direct drive, so this area doesn't need changing. Enter Pinion Gear Teeth (enter 1 if direct drive)   Enter Spur Gear Teeth (enter 1 if direct drive) Final Gear Ratio is: :1   Propeller Slippage Info: Propellers are not 100% efficient. Certain hulls will be more efficient than others. This make some hull designs faster then others. As a guideline, estimated prop slippage for Mono Hull or Deep Vee is approx 25% : Full Bodied Hydro Plane appox 20% and Riggers approx 15% Enter Prop Slippage    %   Estimated Top Speed of:   Km/H MPH

Estimate amp draw   Enter battery capacity in mAh mAh   Enter run time in seconds seconds     Estimated Average current Amps

Estimate Runtime   Enter Cell Capacity in mAh mAh   Enter Discharge current Amps     Packs will last approximately Seconds   Minutes

NiMH & Nicd battery pack voltage and power output under load.     Enter number of Nimh cells Enter internal resistance of cell Milli-ohms     example 1.0 milliohms Enter discharge current Enter Motor Efficiency Amps %     Cell voltage under load is Power output @ motor efficiency Volts Watts Pack voltage under load is   Volts Horsepower Pack power output   Watts

 

Estimate your speed between two known points with a stopwatch

Distance between check points Feet   Time taken to pass between check points Seconds     Speed is MPH Km/h meters/second feet/second

 

Estimate the top speed of a prop. On MOST Octura Props, first two letters or numbers designate the pitch. Example of an Octura x438 propeller. The X4 tells us the pitch is 1.4 and the diameter of the prop is 38mm   Prop Diameter in MM mm       Total Motor RPM Pitch type Enter Slip % Total Pitch mm   Total Pitch inches       Pitch Speed No Slip Pitch Speed with Slip mm/min mm/min m/sec m/sec km/h km/h ft/sec ft/sec MPH MPH