Truth or Dare: trust no data
Miscellaneous Musings from the Technical Director

Monday, 20-AUG-2007 by Donald MacPherson - Technical Director

Remember the game Truth or Dare? You know, the game where you had to choose between the truth and or an embarrassing alternative? Well, it seems that Truth or Dare is a daily event for anyone who is in the propulsion business, especially application engineers and those sizing propellers for a living. While we find the best games occur when dealing with consumers (boat owners or builders), we also find that the largest multi-national manufacturers are offering us some real challenges. Consider this recent email to me from a propeller vendor and PropExpert user...

"A customer had me size a pair of wheels for [a well known motor yacht]. Was running 3 blade so he gave me sea trial data. We sized for [popular commercial propeller] 4 blade and PropExpert gave us a 26" D prop. We sized to be under pitched about 100 rpm. He got back to me after he ran his new wheels… couldn’t turn them. They were 300 rpm low… PropExpert has never been this far off. We re-checked all of the data and what he was telling me made no sense. Turns out that [major transmission builder] calls a 1.45:1 trans a 1.5:1 and so marked on the data plate. He should have had 24” D wheels with the 1.45! Put a set of 24’s on and the numbers were right on with PropExpert!" (Note: this was edited only to remove company names.)

This is not the first time that I have seen an incorrectly printed gear ratio, and it speaks to one of my guiding maxims - trust no data! It really doesn't matter whether the data inaccuracies are honest mistakes, overly optimistic expectations, lazy or careless acquisition of information, or intentional manipulation of data. They all ultimately affect your ability to provide an accurate answer, and it is also ultimately your responsibility to check and confirm that the data is truthful and correct.

Let's look at some typical ways where problems might occur. I have created five categories with one or two examples in each, and I have ordered from least-likely to most-likely (in my humble opinion).

1. Strategic marketing decisions

Ah, yes, corporate deception. (Well, perhaps the term "deception" is a bit harsh.) In years past, for example, marine engine manufacturers would define the margin of "actual-to-published" power output in different ways. One might use plus-or-minus 3%, while another would use plus 5% minus 0%. Let's say that both had engine models that tested in the range of 950 to 1000 kW. The former would therefore have a published rated power (975 kW +3%/-3%) that was a few percent higher than the latter (with a published rated power of 950 kW +5%/-0%). The latter, however, gave you better insurance of delivered power. Fuel rates were treated similarly. Fortunately, we really no longer have to worry about engine power, as the major players all follow comparable SAE and ISO specifications. But, it does illustrate how marketing data might not be the best technical data.

2. Bad measurement equipment

This is more common that you might think. It is always good practice to frequently calibrate your instruments. Analog engine tachometers in the wheelhouse can be off as much as 10%, and turbine-based speed meters can drift over time. If conducting a sea trial, you should independently check your shaft RPMs with a hand tachometer (e.g., a strobe tach) and boat speed with GPS.

3. Mistakes - plain and simple

"To err is human", etcetera, etcetera. I would consider the above anecdote with the gear ratio to be a mistake. No one was trying to deceive us, just they did not understand the need for a bit more precision. When quoting speed, for example, don't forget that many people mix up knots and miles-per-hour. If things don't make sense, you may need to question things like gear ratios or speed units.

4. Overly optimistic customers

We all want to be optimistic. We attend a ball game logically knowing that our team is greatly overmatched, but we still hope for some miracle. Where would the lottery be without irrational optimism? Many, many boat owners think their boat is capable of running much faster that it will ever run, that somehow they are immune to the laws of physics. They are not bad people, but like a young child who thinks he can fly and is willing to give you proof, it is our job to keep them out of trouble. And sometimes this means taking a hard stand and having them substantiate to you how they reached their conclusions. Believe me, you are doing them a big favor to do this before they "jump out of the tree".

5. And the winner is... Laziness

No need for examples here. Do the phrases "Well, it was about...", or "I think it was..." strike a familiar chord? Technical information is worth the effort that was put into finding it. A number that was casually "tossed off" should be "tossed out".
 

Adrián Sarasquete, Baliño SA, Spain, wrote on 29-AUG-2006:
First of all, Don, thanks for this nice article. This makes me think again about our periodic headaches related to ships not reaching expected speed and/or consuming too much. We have now solved this partially by means of a torque meter, which is saving us a lot of time and also provides good feedback. Of course you still need time to perform the tests and analyse the results, but knowledge is always welcome. I am now going to review a recent case where the gear ratio was suspicious but never checked, maybe we can blame someone else...:) I think even a propeller manufacturer has to live with these types of experiences.

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