Formulas and Data
*This information posted with permission from Carlos Andrade (Shark Racing)
((C x 9)/5)) + 32
((F 32)/9)) x 5
on air 1700 fps
on water 1000 fps
1 inch=25.4 mm
1 foot=12 inches=30.48 cm
1 mile=1.609 km
Boat velocity according to propeller rotation:
(RPM x P x S)/1056
where RPM = engine rotation, P = propeller pitch and S = slippage factor (varies from 0.8 to 0.9)
(RPM x P x S x 1.609)/1056
1 gallon = 128 oz
1 gallon = 3.78 L
1 L = 33.86 oz
29.53 cm3 = 1 oz
203.16 = 6 oz + 1 gallon = Mixture 22:1
206.71 cm3 = 7 oz + 1 gallon = Mixture 19:1
25:1 = 961.5 gas + 38.5 oil
20:1 = 952.4 gas + 47.6 oil
Length of a tuned pipe
Lt=(Eo x Vs)/N
Lt=length, in inches
Eo=exhaust duration, in degrees
Vs=sound speed in feet/second (1700 at sea level)
N=crankshaft speed, in RPM
L=R x .01745 x Z
D2=root(D1² x 6.25)
D2=cone exit diameter
D1=cone entrance diameter
6.25 =exit/entrance ratio (constant)
Lr= medium point of reflection on convergent cone
Le=length of convergent cone
Determines the speed range of the engine.
Reduce the exhaust duration to increase power at low RPM and v.versa.
More exhaust duration means higher rotation range of the engine
D=K x root (C x N)
D=diameter, in mm
K=constant (aprox. 0.65 to 0.9, derived from the carb diameter)
C=displacement for the cylinder, in liters
N=RPM at maximum HP
Conversion site: (converts any metric numbers to American standard and v.v.) Thanks Don Betz
Gas Mixing Ratios
There are two ways of establishing the gas/oil proportion on the fuel mixture used on R/C boats engines: the American system and the metric system, used in Brazil.
On the American system, volumes are measured in gallons of gas and ounces (oz) of oil. On the metric system, gas and oil are measured in cubic centimeters (cc).
Besides this obvious difference, there is another, not so obvious but equally important. On the American system we calculate the volume in ounces (oz) of oil to be added to 1 gallon (128 oz) of gas. This way, a mixture of 6 oz results in 134 oz of fuel: 128 oz of gas + 6 oz of oil. Usually, this is the way we refer to this mixture: 4 oz/gallon, 6 oz/gallon, etc. In the metric system, we calculate the volume, in cubic centimeters (cc) of oil to be added to gas to reach 1 liter of fuel (gas + oil) and this mixture is indicated as a proportion: 22:1, 25:1, etc., this meaning that, for instance, in 26 parts of fuel, 25 are gas and 1 is oil.
What system is better? Is there a better system?
Of course there is a better system. IT IS THE SYSTEM YOU ARE USED TO. Use the one you get used to. Paragraph.
To make the understanding more easy, the bellow table shows the equivalence between the two systems, being understood that we rounded the figures.
|American system (oz)|| Metric system (cc)
Copyright © 2000 - Carlos Andrade