A Dumas Chris Craft 19' 1940 Barrel Back Runabout & Propulsion Design Study

Built in 1998 this model boat is 28 1/2" long x 9" Beam and planked in mahogany and I believe the most popular of the Dumas Chris Craft range of kits.
The running gear consists of a Mabuchi 550SH motor, Robbe 30A electronic speed controller, Overlander 7.2v 3.3A/hr Nimh battery and a Dumas Dia' 38mm x 50mm pitch brass propeller.

Calculations,

Output power = current draw × battery voltage x efficiency

Output power = 8.5A* x 8.4v** × 55% = 39.3w = 0.0393Kw

*under full load

**fully charged battery (nom 7.2v)

N.B 540 type motors are more efficient at high rpm. A 2:1 reduction geardrive is better than direct drive as per this model.

Torque = output power × 9550 ÷ shaft rpm

Torque = 0.0393Kw x 9550 ÷ 4750rpm (est) = 0.079Nm

Run time = battery capacity ÷ current draw × 60 ÷ correction factor

Run time = 3.3A/hr ÷ 8.5A x 60 ÷ 1.75 = 13 min's

N.B The higher the discharge rate the higher the correction factor required.

Theoretical speed = propeller pitch × shaft rpm ÷ 60

Theoretical speed = 0.05m x 4750rpm ÷ 60 = 3.96m/s = 8.8 mph

Actual speed = theoretical speed - slippage

Actual speed = 8.8 mph - 20% = 7 mph

In most cases propeller slippage is between 10% and 30%

N.B The propeller shaft sits at around 10 Deg' from the water line. Which will reduce the calculated speed. This is a marginal loss which I have chosen not to quantify.

Propeller mechanical advantage = propeller Dia' × 3.142 ÷ pitch

Propeller mechanical advantage = Dia' 0.038m × 3.142 ÷ 0.05m = 2.38:1

Force at propeller tip = torque ÷ radius

Force at propeller tip = 0.079Nm ÷ R0.019m = 4.16N

Force = force at propeller tip × mechanical advantage

Force = 4.16N × 2.38 = 9.9N = 1.009kg (theoretical static thrust)

Displacement = 3.3Kg (mass)

Nominal acceleration = force ÷ mass

Nominal acceleration = 9.9N ÷ 3.3Kg = 3m/s/s

A typical small car accelerates at 3m/s/s

At full power this model shifts 3.5 litres of water per second.

Also of interest is the General Thrust Equation.

Velocity(m/s) = flow rate(Kg/s) ÷ (density of water* × thrust area**)

*Kg/cubic metre

**metres squared

As this is a planing boat acceleration is presumed non linear primarily due to the variable wetted area of the hull and slippage is expected to be higher during acceleration.

Conclusions: A static thrust test has now been carried out in the DMTF and a normal thrust of 1kg was measured and a thrust of more than 2kg was initially measured due to the battery being re-peaked. The only fully unknown in these calculations is the efficiency of the motor and transmission, therefore the efficiency has been back calculated to suit a thrust of 1kg.
A surprisingly powerful little boat with an equally little 550 motor.

Thanks to my wife for the use of the Domestic Maritime Testing Facility 😉.

Calculations courtesy of A1 DRG Ltd

Recorded at the Mill Pool, Looe, Cornwall.

Thanks for watching.