Different gear ratios and unbalanced propeller thrust

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Different gear ratios and unbalanced propeller thrust
Miscellaneous Musings from the Technical Director

Monday, 30-APR-2007 by Donald MacPherson - Technical Director

We had a call this week from a PropExpert user (a propeller distributor) who was working on a project for a potential customer. It seems that the owner of a twin-screw boat was repowering and wanted to use different reduction gear ratios between the port and starboard engines. The question to us was regarding how to best size the propellers, as well as the implications of using different ratios and propellers.

This is something that is asked of us at least a couple of times every year. The motivation for using different gear ratios is typically to save a few dollars on the purchase of an available gearbox. Sometimes it is to save money with different engine models. On at least one occasion, the motivation was to reuse an exist dissimilar propeller and purchase gearboxes to suit the different propeller pitch. While it may be technically possible to use dissimilar gear ratios, it is not a particularly good idea. In fact, I would submit that it is really short-sighted and to use a cliché, it is "penny wise, but pound foolish".

Let's consider the case of a typical 20 knot inboard-driven production boat, something like 14.5 m (48 ft) length with a pair of 200 hp diesel engines. What are the implications if each of the two gear ratios are arbitrarily chosen as 10% higher and lower respectively, such as 2.00 and 2.50 versus the optimum ratio of 2.25?

The sizing calculation will suggest a propeller P/D of 0.95 for the low gear ratio and 1.25 for the higher gear ratio to match the specified design power and RPM. So far, nothing is too exotic or unreasonable. However, even though operation at the common rated RPM may indeed draw equal power from the engines, the unseen issue is that the slightly different efficiencies of the propellers means that they do not produce equal thrust. OK, so what? Why is unbalanced thrust a problem? The short answer is that anything that is functioning off of its equilibrium condition is wasting thrust, power, and subsequently, also wasting fuel.

A more complete answer must also consider the effect on the boat of unequal asymmetric thrust. There will be a tendency for the boat to pull to one side, requiring active rudder to maintain a straight track by countering the "thrust moment" created by the unbalance. Applying even a few degrees of rudder adds drag, which in turn requires more thrust, power and fuel to maintain speed. These effects become even greater at cruising speeds and as boat weight increases over time. Just to give you a sense of proportion for our example boat, the thrust imbalance would be approximately 5% at top speed and more than 8% at an 8 knot cruising speed. Additional fuel usage due solely to the imbalance (not considering the added drag of applying the rudder) would be about a percent and a half. This would be expected to double if you included the effect of rudder drag.

You also have to consider the different required shaft diameters for the two gear ratios. Do you use the calculated minimum shaft diameter for each side, or the largest common diameter? In our example, the shaft sizing calculation indicates that one shaft must be 13% larger than the other.

And, of course, we cannot forget about the legal implications. Are you accepting liability in the case of an accident by installing a propulsion system that creates operational imbalance and a requirement of active helm control to maintain a straight track? I don't have an answer to this question, but I would not want to defend such an installation in a court of law.