There are all sorts of calculators available, ones to plan retirement, manage our weight, make home improvements, etc. etc. All of these calculators are based on known values and assumptions for the values that are not known, put into formulae that calculate the interaction of the data input and produce an answer to whatever question that we asked . We use these calculators to predict future performance of a new system or changes to an existing system. This allows us to try different components or actions and see how those changes might influence the outcomes. These calculators are limited by both the quality of the data, i.e. accuracy of the known data and validity of the assumed data, and the quality of the formulae in modeling results in the real world.
Simple calculators using only known data work very well, like converting standard measurements to metric, there are single constants and they always work. Calculating miles per gallon is done with measured input, but the results are dependent on how accurate the distance (miles) or consumed resource (gallons of gas) are measured. Estimating your retirement wealth 20 years in the future is harder, because the assumptions of available income, investment returns, and inflation are guesses at best.
We can validate how well a calculator works by checking the predictions against objective measurements of the system after implementation. A 59” board will measure to 150cm every time (149.86cm to be more precise). If my car gets 39.6mpg (2014 VW Jetta Sportwagen TDI) and I plan a trip of 500 miles, I should use 12.7 gallons. But traffic, terrain, and how the car is loaded, influence the outcome, so the real answer might be +/- up to a gallon (36 to 42 mpg) and I would find that acceptable. Alternatively, I could make assumptions about the additional influences and add them into the formulae in an effort to make the prediction more accurate, but at some point there are diminishing returns for the additional effort and complexity.
Ok, so what about the calculators for our boat conversions? They predict power needed, suggest components (like props) and predict performance of your completed project. Some of these calculators are based on very complex models of hydrodynamics and some assumptions of how power is translated into movement. These calculators work fairly well for their intended audience, boats being powered by internal combustion engines operation near 90% throttle (look, we’ve already made some specific assumptions). And we know that they work fairly well by checking the predictions against the thousands of boats than meet that criteria. We can tell if a boat is under-powered, and nobody is particularly concerned about an overpowered installation. But even then, picking a propeller is still more of a fuzzy art, than a science, and dialing in a propeller selection when you don’t have an identical example installation is still often trial and error. Still, thousands of boats are re-powered with diesel or gasoline inboards and outboards every year and the owners are usually satisfied with the outcome, i.e. the resulting performance is close enough to the calculated predictions.
Unfortunately, these calculators don’t work very well for our projects. Most of these calculators today are based on the work of Dave Gerr who published “The Propeller Handbook” almost 20 years ago. It is an excellent reference and many of the guidelines and suggestions are as true for us as other boats. But, in my experience, the “power needed” predictions are not even close for our use case. In the case of the Vicprop calculator, it predicts that my boat needs 15hp at the propeller to motor at 6kts, and that 7hp will only push the boat to 4.5kts. You can see my measured performance results in other posts. The energy used is measured as it comes out of the batteries, before any losses in the cabling, controller, motor, gearbox, or bearings, therefore, the power available at the propeller must be less than the power coming out of the batteries. Let’s check the predictions against my measurements. I’ll be using the simple conversion of 1hp = 746W to translate between the two values.
my boat at 6 kts:
prediction: 15hp (11,190W) at the prop
measured: 7hp (5200W) at the batteries
result: calculator predicted 115% more power needed and any losses between my batteries and propeller makes that worse.
My boat at 4.5 kts:
prediction: 7hp (5200W) at the prop
measured: 2.1hp (1600W) at the batteries
result: calculator predicted 225% more power needed and again, my system losses would make the error greater.
We can see that as we slow down, the calculators get more inaccurate. I did have a conversation with Dave Gerr about these discrepancies, and he and I agreed that his formulae work for what they are intended for, and that they appear to not work for for our specific use case. We did not come to any conclusions about the cause of these discrepancies or adjustments to make for our use case.
I know that in this age of information, we want straightforward answers to what seem like straightforward questions. But, recognize that we are outliers in the world of naval architecture and marine engineering. For some of it, we’re on our own. Which is why we’re in this group, right? We’re breaking new ground together. Which is why I believe it is so important to go beyond “it works!” or “I’m happy” and really measure our results, share those findings and work together to figure this all out.
Whew! that was more than I had intended...
Fair winds,
Eric
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