Crazy Run Time and Speed Expectations

[CraigP]

It seems we are getting a slew of people wanting to get crazy run times from their boats. Some of these numbers are just crazy! So I’d thought I would post the formula here and hopefully some folks can crunch some numbers. First, all batteries have their capacity listed on them, usually in MAH, which is MilliAmpHour. It’s easier to use the AmpHour value, since boats use big time amps, not milliamperes. So take that mah number and divide it my 1000. So 5000mah becomes 5ah. That is 100% capacity. If you run your batteries down to that each time, they will fail pretty quickly. You need to leave AT LEAST 25% in the batt, so 5ah Times 0.75 gives a useable capacity of 3.75ah.

Rule 1: Batteries wired in parallel add the capacity of the batteries together, to get the total ah. So two, 5ah batts wired in parallel gives you 10ah and with the reserve, 7.5ah.

Rule 2: Batteries wired in series do NOT add, you are adding voltage instead. So two, 5ah batts wired in series doubles the volts, but the capacity stays the same for each cell. So two, 5ah batts gives 3.75ah.

Now to calculate runtime, you need to find the time ratio number between the stated capacity, which is given as 1 hour, and the desired value. This time ratio will then give you a number that you can use to find out how many amps you can draw for the desired time. Let’s jump into an example:

I want to run 5 minutes with my boat. Getting the time ratio, divide 5 by 60 to get a value of 0.08333 Let’s assume I’m running two, 5.2ah batts in parallel, so I have 10.4ah combined and I can use 0.75 of that which gives 7.8ah So with that size of capacity, if I want to run 5 minutes, my average amp draw cannot exceed 7.8/0.08333=93A. This is a reasonable value...

So for you guys wanting to run 20 minutes with that same battery, which is pretty good sized, you would need to work with an average amp draw of 20/60=0.3333, then 7.8/0.3333=23.4A. I don’t know of too many boats 30” and above that is going to only draw 23A, especially with a crazy speed expectation.

So do the math before you post something crazy, because most of us are just laughing at some of these posts!

[Old School]

When one is running on water the speed observed is very deceptive, unless GPS verified. Same with runtime. Unless one actually clocks his actual time running then what may be just a few minutes of adrenalin rush can seem like a lot longer. One reads a lot of exuberant postings that seem unbelievable and not surprisingly the original poster seems to vanish without trace.

To me a boat doing just 50 mph looks like it is doing a much higher speed. Any claimed speed should be backed up with evidence and not just a visual approximation. How one achieves a maximum speed combined with maximum runtime must have the perfect running setup or likes to exaggerate.

[vinnyp]

Hey Craig,
Thanks for posting those formulas. While I never made claims of insanely extended run times, its nice to know how to make these type of calculations.

[R2315]

Thanks Craig, very helpful post. Spring is here so I'm sure there will be no shortage of.......high kv, big runtimes, big amps, fast fast fast boats and BBQ'ed electronics . This is by no neans a gripe, just a observation but unfortunately some that are new to this, sort of just jump on and ask without researching or reading through previous posts. There is a wealth of knowledge here and most of these questions have been answered before. I don't blame them but I've noticed the more experienced here have grown tired of answering such things now and are not even bothering anymore. Some give there advise with decades of first had experience only to have the OP turn around and basically say, what do you know.... I don't mean to ramble on but sometimes the answers you get are not going to be what you want to hear or the answer you are looking for but are proven by people who have already been there and done that. As someone who is still new to this my advise is, listen to these guys, read up before you ask a question and if we are lucky they will continue to help us out as Craig has done here.

[srislash]

Guess this calculation should get ‘stickied’. Good lesson Craig

[TRUCKPULL]

Here is a chart that was made up years ago.
Time in minutes down the left side.
Mah's across the top.

If you go out an make a timed run of lets say 2:00 minutes.
When you recharge your batteries you put 4,500 Mah's back into them.

This now tells you that your average AMP draw is 135 AMP's

Feel free to copy and print.
I keep one taped to the inside lid of my charge box.

Larry

[CraigP]

I am hoping that new people will search and see this, and maybe read. But I’m getting the feeling they don’t know what or where the search bar is! When I got back into this sport, I spent (no joke) about 50hours on this site, just searching and reading. About a month long, before I even developed the purchase list for my first boat. It was reassuring to me knowing such knowledge was available! I want to thank everyone that helped me!

Larry, excellent chart! I recommend that you guys try the formulas out a couple of times, you’ll see how the chart derived the values. Just one note on the chart, the new user MUST apply the batt de-rate before finding the value or you will over discharge the batts. Larry’s describing using the chart as a means to see what your average running current is, based on mah put back into the batts when recharging after a run. Apply the de-rate to check your runtime expectation when figuring out what batts to buy. It will be interesting to see if any new guys actually read this post, I’m probably preaching to the choir, lol!

[CraigP]

When one is running on water the speed observed is very deceptive, unless GPS verified. Same with runtime. Unless one actually clocks his actual time running then what may be just a few minutes of adrenalin rush can seem like a lot longer. One reads a lot of exuberant postings that seem unbelievable and not surprisingly the original poster seems to vanish without trace.

To me a boat doing just 50 mph looks like it is doing a much higher speed. Any claimed speed should be backed up with evidence and not just a visual approximation. How one achieves a maximum speed combined with maximum runtime must have the perfect running setup or likes to exaggerate.

Time is indeed, deceptive while white knuckle steering your way through 6 hot laps! If your boat is frisky fast, your adrenaline is ON, which is one thing I love about the sport. There’s something about the uncertainty of the water surface that I think magnifies this rush!

[R2315]

Maybe there should be a "start here" section and we can all add important stuff like this to it as we go, charts, guides ect..

[CraigP]

Great idea!

[NativePaul]

Meanwhile outside America there are many people racing for 15 minutes, plus in lap, on 174watthours of energy or 840g of battery.

[properchopper]

Maybe there should be a "start here" section and we can all add important stuff like this to it as we go, charts, guides ect..

That IS a great idea - actually there are some charts now here :

https://www.offshoreelectrics.com/conversion.php

I have a collection I could add as well.

[CraigP]

Meanwhile outside America there are many people racing for 15 minutes, plus in lap, on 174watthours of energy or 840g of battery.

Yes, a BIG battery! But the math still works. I wonder at what point that the weight penalty starts to become more (hull friction) than what you can really get? Interesting math challenge for some wintry day!

[CHowarth]

I'm reading. I'm reading!

[CHowarth]

I am hoping that new people will search and see this, and maybe read. But I’m getting the feeling they don’t know what or where the search bar is! When I got back into this sport, I spent (no joke) about 50hours on this site, just searching and reading. About a month long, before I even developed the purchase list for my first boat. It was reassuring to me knowing such knowledge was available! I want to thank everyone that helped me!

Larry, excellent chart! I recommend that you guys try the formulas out a couple of times, you’ll see how the chart derived the values. Just one note on the chart, the new user MUST apply the batt de-rate before finding the value or you will over discharge the batts. Larry’s describing using the chart as a means to see what your average running current is, based on mah put back into the batts when recharging after a run. Apply the de-rate to check your runtime expectation when figuring out what batts to buy. It will be interesting to see if any new guys actually read this post, I’m probably preaching to the choir, lol!

What is the " batt de-rate"?

[srislash]

Yes, a BIG battery! But the math still works. I wonder at what point that the weight penalty starts to become more (hull friction) than what you can really get? Interesting math challenge for some wintry day!

Mmmm...

[CraigP]

What is the " batt de-rate"?

Keeping 25% capacity in the battery. You are de-rating the battery capacity. It’s explained in the first post.

[CHowarth]

Keeping 25% capacity in the battery. You are de-rating the battery capacity. It’s explained in the first post.

Got it.