# Inflatable electric boat calculation question

Discussion in 'Boat Design' started by Stigla, Mar 18, 2021.

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### StiglaNew Member

Hi, I am trying to calculate the motor and batteries needed for a small inflatable boat (3m, max. weight 200kg)

I need to find out what kind of electric motor is able to push the boat between 6 and 11 knots and the size of batteries needed to have autonomy of at least 5 hours.

I am at a loss how to start calculating this and any help is greatly appreciated

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### bajansailorMarine Surveyor

Welcome to the Forum Stigla.

What type of inflatable boat do you have (or what type are you hoping to purchase, if you have not already acquired one)?
For example, there are 'round-tail' types, that can only take very small O/B motors, and there are inflatable dinghies with inflatable floors and solid transoms, and there are RHIBs which have a rigid fibreglass or aluminium hull with inflatable sponsons/ tubes.

Last edited: Mar 18, 2021
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### BlueBell. . . _ _ _ . . . _ _ _

3m long
max 200kg - 80kg person = 120kg for boat and batteries
6 - 11 knots for min 5 hrs...
Can't be done.

Well, maybe in a sit on top kayak.
No, not 6 knots.
Can't be done.

Why not just sail and forget the electrics?

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### KJL38Senior Member

If you are ok with 6 knots and can stretch the length to 6m you would be operating in displacement mode and would be possible with a canoe shaped boat.

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### portacruiseSenior Member

If "atonomy" means robotic boat, there is no passenger weight. If this is a student assignment to solve, use this info: The world record for a propeller pedaled boat is 245km in 24 hours, which is close to 6 knots average speed, and if you find the number of watts an athlete can sustain for 5 hours ( my guess 300 watts?), that can get you the watt hours for the battery for a very efficient boat design (inflatable boats are not as efficient) and with a passenger but no battery and motor weight. That's just the theory minimum needed for a near perfect boat of lower TOTAL weight. Increase in length means more weight and more wetted surface area friction which are negatives, but less flow disturbance losses which might help a bit overall if you want 11 knots. Just a rough idea.

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### sharpii2Senior Member

My best guess is that you will need around 15 kw hours. 3 kw to get the boat on a plane times 5.0 hrs operating time.

Slower speeds will give you much longer operating time for the same battery. If you settle for 4.0 kts, you may be able to get as much as 50 hrs of operating time out of the same 15 kwhr battery. But a 15 kwhr battery is not likely to fit on your tiny boat. Maybe a 2.0 to 3.0 kwhr one will. This would possibly give you six to ten hours of operating time.

But the inflatable boat has to be designed for non-planing speeds. It cannot have a deeply immersed transom.

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### BlueBell. . . _ _ _ . . . _ _ _

Not 300 watts, more like 100 watts sustained.
The athlete can do it longer that's all.

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### Will GilmoreSenior Member

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### gonzoSenior Member

3 HP is equivalent to 2,200 W not 1000 W.

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### BlueBell. . . _ _ _ . . . _ _ _

Stigla,
Why not just sail and forget the electrics?

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### StiglaNew Member

Thanks for all the replies and the really good info! Sorry about the slow response on my part, I was collecting some more info.
I don't know exactly what type of boat it will be, it still might change based on the budget and available options but my best guess is something similar to this one.
Sailing is probably not an option because it needs to be agile and follow a square wave pattern plus it will operate in areas with very weak winds.
The canoe is also an interesting idea.
Will keep you guys posted with the progress.

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### keroseneSenior Member

TLDR: not gonna happen.

longer. you talk about planing speeds, 3 meter length and electric power with several hours of run time. ugh.

speed is very hard to achieve with such small boat on electric power. you need several horsepower which will eat batteries very fast.

also 6 and 11 knots are world of difference in a boat. one is close to displacement speed the other is way into planing.

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### keroseneSenior Member

not correct. indoor rowin marathon record is 2h21min this comes to 347 watts. elite athletes can probably hold fair but over 200 watts for 5 hours.

Tour de France numbers are bit lower for the steady state but it's good to remember that they don't do even effort. hills and take outs hit higher numbers which need recovery from.

edit:
just checked the 24h indoor rowing record, ~160 watts. that includes some breaks for nourishment and toilet.

naturally these are elite numbers but 200w for a few hours is not crazy for a larger person.

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### keroseneSenior Member

canoe shaped hull, and as long as reasonably possible is radically superior to that inflatable.
Even making one that is quite "ok" is not that hard using 4mm plywood for example. Good birch is fine. epoxy inside and glass+epoxy outside.

with custom prop on a 1kw'ish trolling motor like 24V hasswing you might get near the 6 knots.
But on that inflatable not gonna happen with the same power.

Last edited: Mar 22, 2021
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### BlueBell. . . _ _ _ . . . _ _ _

But you didn't say elite athlete. And why would an elite athlete be powering such a craft?

Those rowing machines are ridiculously inaccurate catering to the egos of participants.

Lol, large individuals are heavier so require higher outputs to compensate for the added weight.

Also, there are drivetrain losses to consider.

Kerosene, you seem a little overinvested in this topic, why? Do you really believe everything you read?

Lets try and help the OP vs getting caught up in numbers.

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