Tuesday, March 18, 2014

Big Prop Calculation

This is a picture of a 5 bladed prop assumed to be used on a Maersk Triple E class cargo ship.  It measures 34 feet in diameter.

I thought it would be interesting to run this prop through a prop design simulator to see what kind of numbers fall out.  Using the program available from Victoria Prop Ltd., publicly available numbers for the Triple E, and a bit of guesswork:

Waterline Length of Vessel:   1212 ft. (subtracting 100' from total length for waterline length)
Beam at the Waterline:            194 ft
Molded Hull draft:                     48 ft  (used basic draft number)
Vessel Displacement: 363,762,733 lbs (converted from 165,000 metric tonnes)
Number of Engines:                     2
Max HP per Engine            85,000 HP
Max Engine RPM                      90 RPM
Gear Ratio                              1.25
No. Shaft Bearings                      1
Desired Max Speed                  25 knots

The Triple E is the latest in super sized cargo ship design, nearly 1/4 of a mile long.  All of the parameters are taken from the ships details except for the gear ratio and number of shaft bearings, which were determined by using the known size of this prop (34ft, or 408 inches) and working backwards with the known figures for max engine RPM (90) and shaft RPM (73) to calculate the gear ratio.  Shaft bearings introduce performance losses, but assumed to be optimal at 1 for this exercise.  With these numbers we get the following recommended prop size for various numbers of blades (the simulator only goes up to 4):

2 Blade492.7" diameterX615.9"
3 Blade469.2"X609.8"
4 Blade441.1"X597.6"

Allowing for a little play in pitch, and extrapolating for a 5th blade gets us very close to the actual size of this prop at 408", meaning it is reasonable to assume our parameters are close and the following ratings apply.

Total torque at props: 11842531 ft/lbs
Total HP at props: 162350.0 HP
Total HP at props for max speed (25kts) : 111046 HP (about the equivalent of 28 modern locomotives).
Total losses from gearbox and shaft bearings: 7650HP (a small percentage loss of a big engine is a lot of power!)

It looks like Victoria Prop has a pretty good calculator and the prop may indeed be headed for use on the Triple E's.