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…

Read more about motorizing fishing kayaks >>

Thanks!

Al

One more question: I can’t seem to find information on watts in electric trolling motors but some places talk about Amps.

Al

If you know the Amps and Volts it’s easy to find the Watts (power).

To convert Amps to Watts you can use the equation

Watts = Amps X Volts.

For example, if your fishing kayak’s electric trolling motor is powered by a 12 Volt battery (typical), and its manufacturer rates it at 20 Amps, then 12 X 20 = 240 Watts.

In other words, your fishing kayak would have a 1/3 HP (750/240 Watts) motor onboard.

This is very little compared to gas engines, but it’s not too bad compared to human power, except for the fact that the battery and motor can add almost 100 lbs to your kayak…

Would 36 lbs of thrust be enough for a W kayak?

Most experts seem to agree that for a small boat such as the W kayak an electric motor with less than 30 lbs thrust would be enough.

It’s important to remember that the more powerful the motor the faster it drains the battery, and a battery without power is not just useless, it’s also about as heavy as a child, which means you need to work harder paddling…

An electric trolling motor system consisting of Battery + Motor + Mount can weigh as much as a small, lightweight adult, but unlike such a passenger this system can’t ‘paddle’ if the battery is empty.

😀

Few thoughts:

– What would you consider to be the maximum weight a person can be to even engage in Kayaking? The answer would coincide with my next question:

– Knowing the maximum weight a kayak enthusiast can be, what would be the ideal type of engine to install in such a floating device?

– If it’s electric, would it be feasible to link an extra battery to compensate for the weight in the kayak (i.e. if the kayaker is about 300lbs).

– If not electric, what size gas-powered motor would be feasible to put in such a floating device?

K Thanks!

In ordinary kayaks, the heavier the kayaker the more likely he is to feel both unstable and uncomfortable, mostly because of back problems.

I guess 300 lbs is a ballpark number above which I wouldn’t recommend kayaking.

Back pains are not a problem in the Wavewalk kayak, but for very tall and heavy people stability might be a problem, especially at high speeds the boat can reach with a gas engine.

Therefore, I’d recommend outfitting the W-kayak with at least one outrigger when using a 0.5 HP or more gas engine.

With electric trolling motors I’d recommend using the weakest motor possible, so that you don’t drain the battery too quickly and be forced to paddle back with a 50 lbs battery and motor onboard.

It’s possible to take two batteries onboard the W-kayak, one at the bottom of each hull, but you’ll have to take into consideration the extra weight. Some batteries are lighter for the same power.

This guy from the UK outfitted his Wavewalk with a 2.5 HP gas engine: http://www.wavewalk.com/Motorized_Kayak_01.html

I’d recommend placing the motor further to the side, preferably with one outrigger: https://wavewalk.com/blog/2008/03/13/outrigger-for-w-fishing-kayak/ for various reasons, but mainly better ergonomics when it comes to using the tiller.

Yoav

Generally, an easy to understand, well writen article. With one exception; in your third paragraph, air is not ‘hundreds of times less dense than water.

In fact, water is slightly over 3 times the viscousity of air. Take a fish finder and shoot the transducer at an object out of water and note the ‘depth’ to prove this.

The density of air at sea level at a temperature of 20 C is 1.2 kg per 1 cubic meter, or 0.0012

In comparison, the density of water is 1.0, which is 830 times the density of air.

See: http://en.wikipedia.org/wiki/Density_of_air

Chuck,

You may be getting those odd readings from your fish finder simply because it’s designed to collect data from water, not air.

Pete

Here’s a new article about small, gas outboard engines for fishing kayaks:

https://wavewalk.com/blog/2010/12/09/small-outboard-gas-motors-suitable-for-fishing-kayaks/

Hello,

I am glad to have found a blog on motorized kayaks for seniors.

There has not been a response to this blog in a while. Most trolling motors are made for large boats.

I think a trolling motor would be awesome on a WaveWalk Fishing Kayak.

You are absolutely right about battery weight vs time of use.

For fishing, I recommend using a Walmart 55 amp marine battery (38 lbs – 3+ hours), but it also runs on well a 20 amp (8 lbs – 1+ hour). My friend uses a 105 amp (5+ hours) and travels 8 miles to his fishing spot and back.

I have a 25′ pontoon boat (about 3500 pounds) with two pontoons and am going to outfit it for fishing a fresh water lake. It has a 90HP (gas) and a trolling motor (electric). I want to go with a gas motor (plenty of gas on board) instead of electric for pulling outrigger boards. The speed should only be about a slow walk. Any suggestions would be helpful as to HP rating of motor. Thanks, Dave.