Thrust is a unit of measurement that manufacturers of electric trolling motors for fishing kayaks and other boats use to describe propulsion capability. Thrust is measured in units of weight. In the USA it’s usually pounds (lb.).
This can be confusing, since we often tend to think of propulsion in motion terms, or in horsepower (HP).
Before going further, we’d better clarify what weight and thrust have in common:
This (rather crude) illustration shows a small boat on the water. The boat is equipped with an electric trolling motor and propeller unit whose measurable output is 36 lb. The boat is attached by a line to a 36 lb weight that’s pulling it backward. Since the motor unit can provide 36 lb of thrust it will keep the boat in place: It would be strong enough to counterweight the 36 lb weight, but not strong enough to get the boat to move forward.
Once the battery gets weaker and/or the propeller entangled in seaweed the thrust achieved will diminish and the 36 lb weight will drag the boat backwards.
Similarly, if we lifted the propeller out the water it would still thrust the boat forward, but much less so, since it would be pushing against air that’s hundreds of times less dense than the water this propeller was designed to work in… In this case the 36 lb weight would easily win this tug of war.
Note that this simple model describes thrust without using speed terms.
There is no simple formula that can help you convert thrust to horsepower or vice versa, although the terms are closely related to each other when motorized boats are concerned.
In our case Thrust is the directional force resulting from the rotation of a propeller at a certain speed. Different propellers rotating at the same speed will generate different thrust. The same propeller will usually generate more thrust at a higher rotation speed (RPM).
Horsepower is a unit of measurement for power (it’s quite obvious isn’t it?…), which is the ability to do work. Power is described by weight lifted over a distance during a certain time.
1 HP is equal to the power needed to lift the weight of 550 lb over a vertical distance of 1 ft – in 1 second.
Just by looking at these numbers we can sense that not every human is capable of producing 1 HP – not even for a short period of time. Most of us can produce much less than 1 HP over long periods of time, such as when paddling, biking etc. Estimates vary from 0.2 to 0.4 HP, but that doesn’t mean much for us as individuals.
So, going back to our illustration, if we had a 1 HP gas engine on top of the dam, and that engine was attached with a pulley to the line holding the 36 lb weight, we would be able to lift that weight up at a staggering speed of over 15 ft per second (550:36 = 15….).
Apples to apples: How can we compare the 1 HP gas engine to our 36 lb electric trolling motor?
We need comparable, that is mutually convertible units of measurement. In this case it’s HP and Watt. To convert Watts (W) to a horsepower rating (HP) simply multiply the Watts by 0.00134
In other words, a 750 W electric motor (1:00134 = 746…) produces the equivalent of 1 HP.
In boating terms, Thrust would be the result of applying this power to move a boat through the water by connecting the engine to a suitable propeller and letting it move water… In order for such a comparison to make some practical sense we need to assume certain things about RPM, type and condition of propeller, boat size, boat speed etc… It’s really not that easy.
More specifically, when it comes to electric motors for kayaks you shouldn’t be tempted to get a strong motor that would consume your battery power too fast. If such a thing happens you’ll have to paddle your kayak back with a heavy battery and motor on board…
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