Re: [electricboats] Using online calculators - do they work?

I agree with Darin only in the respect that all those facets he listed are not currently being calculated and even if they were they will only get close to the reality of the situation.  However, I think with some input from some learned people we could have something that can give us a calculation that gets really, really close.  Take resistance of boat X's hull.  If you have the number of hulls, length at the waterline of each, displacement and draft I think you can come up with a good measurement of resistance through the water and resistance to acceleration.  However, it won't be perfect because the shapes and state of the surface of the hulls make a difference.  But let's say you get as close an evaluation, based on descriptions like this that you can reasonably obtain, it would be a good start.  If trial found that the number generated for size and pitch was low by 10% based on performance numbers like we want to collect, maybe its' a good factor to adjust the calculation by to get the best approximation we can.  The question to me for the differences between the ice calculations and our motors is what does the even torque do for us.  In a car you can feel that difference in the acceleration.  There's no lag.  So does that only mean that the prop on our boats can be large so we can spin them quickly and get good acceleration?  Most of us don't really care much about that factor.  We, and I'm taking a small leap here, are interested in best efficiency at a reasonable speed in comparison with our fellow sailors out there.  IE I don't need to be able to run around at 15kt passing every snail boat around me.  But I also don't necessarily want to creep along at 3kts while every sail boat around me passes me.  At least some number of us do outings with others and would like to keep up.  For those out on their own cruising the goal of 5kts is more to do with fighting currents and windage in certain circumstances.  But I think the reality is we want just enough to do what most ICE boats do now and keep using that amount of energy as long as possible.  If we propped our boats to do 15kts but we could only do it for 5 minutes with a ton of batteries we aren't interested.  But that is one of the values in the ICE calculator (what speed do you want to attain).  So does instant torque even help us more than a rabbit start from the dock?  Is it more efficient to spin a prop at 1000 rpm with a big prop or 2000 rpm with a small prop kw per mile?  And what is the right pitch for those? (big prop, small pitch, low rpm, or big prop, large pitch, low rpm, or big prop small pitch high rpm, etc)  The combinations should yield a power vs efficiency table that you could use in relation to the boat's resistance to get really close to ordering the right prop. 

Anyway, I think a calculator could be made really close if we can factor in the right elements of the equation.  It doesn't exist today so I went with my gut based on comments made here and what people are using.  I got a 15", 14 pitch folding prop for a 38' trimaran with a 3' draft and 9k lb displacement.  I just installed it a few weeks ago but still need to do some sea trials.  I'll share my data as soon as I get it.  BTW folding was a decision based on the incredibly high number of crab traps sprinkled around our local waters period.  I would have gone with fixed or if the wallet would allow, variable pitch if it were not for that issue.

On Tue, Jul 14, 2020 at 3:05 AM Darin <armyofpenguins@gmail.com> wrote:

Theo, good suggestion.  There have been some lengthy discussions about this topic in several forums, but the bottom line is this: You want to know what size motor to use for your specific application.  There's not really any good way to standardize that with a formula as there are so many factors such as prop diameter and pitch, gear reduction, water line length, beam, hull shape, etc, all factors which are part of the complex relationship between power and speed and which should be considered when choosing your motor.  In my case, I eliminated my transmission and installed a reduction gear, and changed my prop diameter and pitch.  All of these factors are different for each conversion and all affect each other.  There's some math for sure, but there's also a lot of educated guesses and experimentation necessary.  What I did was look for conversions of boats the same size as mine and try to learn from their successes and failures.  If you can, get them to share some data with you.  Ultimately, the most useful formula will be an empirical formula for how much power in kilowatts it takes to get their boat to a certain speed.  If you can develop that curve for their conversion, and if your boat is a close analog to theirs, you will then be able to select an appropriate combination of motor, nominal battery voltage, prop diameter and pitch, and gear reduction needed to move your boat efficiently and without burning up your motor or overdesigning (and overspending).  I've considered putting together a website where we can all submit our basic info and data to generate a power curve.  Alas, the older I get, the more it's about time.  Cheers and welcome aboard!  

Darin

On Mon, Jul 13, 2020 at 8:54 PM Theo B <theo.brillhart@gmail.com> wrote:

Hi all - first time poster. I think I have some insights to the calculator question, but maybe not a totally straight forward answer.

What's missing in traditional calculators like VicProp is the torque that the electric motor brings to the party. The electric motor can supply almost constant torque across its RPM range and near full torque from the time you turn it on. So there's a mathematical relationship that predicts this power discrepancy and uncovers the false assumption that electric hp can be treated like ICE hp.

Mathematically, horsepower equals torque multiplied by rpm. Or, torque of one newton-meter, twisting a shaft at an angular speed of one radian per second, requires a power of one watt. So power is torque times angular speed (with no extra constants or conversions if you work everything in radians and SI units). So for the same angular velocity (RPM) the electric motor has all of its torque available starting at zero RPM, an ICE does not, and will not until the RPM gets sufficiently high. Thus the exaggerated discrepancies seen at slower speeds.

Given enough time and brain cells one could likely develop a compensation factor for typical ICE hp, but it would have to be non-linear due to the non-linear torque of the ICE. I've seen a 5:3 hp ratio tossed around in the electric car communities, but this does nothing to compensate for exaggerations at slower speeds. Might get you in the ballpark though.

--

Larry Brown

S/V Trident

Palm Harbor, FL

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