Converting Pounds Thrust To HP
Ok, this is quite a can of worms…
I suggest you settle down with a brew but hopefully we can shed some light on this for you!
How many ways can you skin cat? Certainly than there are ways of trying to make sense of this….
Why is this important?
Well petrol outboard motors are rated according to their Horse Power (hp), with small boats quite happy with a 2/3/3.5 2 stroke or 4 stroke outboard.
However, most electric motors are rated using lbs thrust which can be very confusing – how many lbs thrust are equivalent to 1 hp?
There are also many complexities including the fact that electric motors deliver 100% of their torque immediately meaning they generally have larger props and can run more slowly.
Lets look at how we can compare electric motors to petrol engines then!
Possibly the simplest method in some ways, but we can use a basic formula of:
HP = Thrust (lbs) * Velocity (mph) / 375 (as per here).
Immediate issues are that horespower and and thrust are measuring entirely different things!
- HP is a measure of power
- Lbs thrust is a static force measurement
The lbs thrust is a result of the power developed by the electric motor, the propeller shape, pitch, dimensions and speed in revolutions per minute!
To cut to the chase, they are not comparable, however we then remember back to physics class and know (or lookup as I just did) that:
Power = Force x Velocity
Ah ha! So, if we know the rated Thrust and the Speed achieved we can work this backwards to Horse Power!
Of course this bring up various issues…
- We need to be out in a boat
- With a motor rated at say 55lbs
- Measure the maximum speed we are doing
So say we measure the top speed at 5mph. This will mean that the motor is producing 0.73hp.Conveniently we will ignore the affect of tide and wind and assume an averaged two way run in identical conditions….
If take a moment to think through some of the variables that could affect this:
- Hull weight (inc you, motor and battery)
- Hull length
- Hull shape/efficiency
- Propeller efficiency
None of which are taken into account…
So, the exact same hull but with 2 people, a dog (and the battery and motor still) might easily have 120kg+ more in it and will certainly be slower due at the very least to more wetted area.
What about comparing an inflatable tender to a light rigid planing hull? The rigid hull might plane and pick up a big chunk of extra speed – if it gets to 7mph that would indicate 1hp.
Crikey, this doesn’t look so good now?
Also, what on earth is the 375 constant in the equation?
That is probably the easiest and effective part of this…
1 horsepower is defined as 33,000 ft lbs per minute – see here, so:
- 1 mile is 5,280 feet so 33,000 / 5,280 is 6.25
- There are 60 minutes in an hour (we measured 5mph) so 6.25*60 = 375
Seemples? Well not really.
If using knots for speed then we would have measured 4.3kn – but is your speed tracking device really that accurate? Would you just say 4 knots?
- 1nm = 6,076ft so 33,000/ 6,076 is 5.43
- 5.43 * 60 minutes is 325.87 so use 326 for the constant
This gives the same 0.73hp.
The difference between 4 and 5mph though is either 0.58hp or 0.73 hp.
Good grief Charley Brown!
Ok, this will be better!
It has to be doesn’t it?
1hp is regarded as being approximately 746 watts as it is a measurement equal to 550 foot pounds of work per second.
Oh yes, we’ve got it nailed now!
If we look at the motor we can know what voltage it runs at and if we can see the max amp draw we can work out watts using:
Watts = Amps * Volts
so Watts = 55 A * 12v for a typical 55 lb rated motor
so Watts = 660 in this case – this is 0.88 hp
What about the inefficiencies of the electrical motor?
Although extremely efficient at probably 90%, that is still not generally factored in. Compare this to typical petrol outboard engine efficiency of 30-35 percent, but lets not go down that rabbit hole!
Lets also not talk about the immediate 100% torque from the electric motor while needing to thrash your little 2 stroke 2 hp screamer….
Also (again), no taking into account prop slippage, hull shapes, weights, planing factors…
Dad, are we there yet?
Sorry son, we’re still on the drive.
This is a great article giving some bollard pull data from 2009 for a range of smaller 4 stroke engines.
- Mercury 3.5hp 90lbs thrust
- Tohatsu 3.5hp 90lbs thrust
- Mariner 3.5hp 90 lbs thrust
- Suzuki 2.5hp 83 lbs thrust
- Yamaha 2.5hp 78lbs thrust
- Parsun 2.6hp 70lbs thrust
- Honda 2.3hp 66lbs thrust
This gives some level of comparison for a bollard pull or comparing static thrust at least.
This still doesn’t translate into the actual moving of a variety of hull types with different weight/prop configurations though, as I don’t know about you but I generally don’t keep the boat tied up and gun it hard as I can!
Unless I am pretty ratted, in which case I’ve probably fallen in trying to untie so it is kind of a moot point.
A bit more detailed this time, if you’ve got this far you must be gripped. Hooked no less.
I’m reeling you in.
Yaaaayy! Or boo!
Depending on your mental state.
So, as we are now painfully aware, thrust and horsepower are not directly related….
Still awake at he back?
Lets recap but rephrase from earlier:
The dictionary defines thrust as a force or pressure exerted on an object, and it is typically measured in units of pounds (lb) or newtons (N). Power, however, is a measurement of work, which is defined as the amount of motion a force creates when it is exerted on a body over a certain amount of time. Power is typically measured in units of horsepower or kilowatts (kW).
We can define the relationship between them as follows:
P = F * Distance / Time
- P = Power
- F = Force
- D = Distance
- T =Time
how can we show a simple example?
Lets assume you have to move a heavy box 3m (10 ft) across the office floor (I’ve just done that with some batteries).
So you press against it with 90lbs (400 Newtons) of force and it laughs at you. It simply doesn’t budge.
Force has been applied, but as the box remains resolutely in the same place mocking you, you didn’t actually perform any work! But you say, to the boss, I clearly did do some work! technicality, lets gloss over it).
So, you call Big Dave over to help you. Big he might be, but hard worker he is not. So Big (but vaguely useless) Dave pushes on the box with the same 90lb force as you do, so your combined force of 180lb (800 Newtons) moves the box in 30 seconds to its new place of rest.
Big Dave of course now wanders off to his “usual place of rest” (somewhere hidden away with a cup of tea, all the biscuits and the Racing Post) as that is more than he has done all week…
Based on our formula above though, you and Big Dave have performed work of 180lbs in 30 seconds so 60 foot pounds per second (or 88 newton metres per second) on that pesky box.
As a watt is defined as a newton meter per second, the power it took to move the box is 88 W or 0.088 kW.
A horsepower can also be defined as 550 ft-lbs per second so 88/550 gives us 0.16hp.
So in this case, and this case only, we can say, that a force of 180 lbs converts to 0.16hp, but as we see, this depends on both distance and time!
If instead you had taken 15 seconds between you, the power would be doubled 120 foot pounds per second (175 newton metres per second).
There are myriad of variables and issues going on here, honestly I give up!
Almost Final Thoughts
- Trolling Motor vs Outboard
- We will be doing an in depth article but don’t get hung up on this distinction. A trolling motor can perhaps be regarded as a lower powered and generally slower motor for smaller craft while an effectively 2hp+ electric motor will give more power and speed with all day performance
- More thrust or horsepower does not always mean faster!
- There are major prop design differences that will affect the maximum speed of your boat as well as the shape and weight of your boat as discussed above. Increasing the thrust or horsepower means you need a correctly sized and shaped prop to make the most of that power eg a 120 lb trolling motor with the same prop as a 50 lb trolling motor will not probably not push a boat any faster. We will be looking at prop differences for specific motors and boats to see what might be helpful for a given set of circumstances
- A great article on www.woodenboatforum.com if you require further reading!
This is horrific!
However, we are going to be posting lots of videos with different motors on different boats in different conditions so you can try to gauge what will suit you, your boat and your conditions.
We are also going to be documenting some bollard pull tests so we can see what lbs thrust a motor is actually generating for a particular boat / weight etc.
With some of our really powerful EZ Outboard 48v motors we will need to make sure the pontoon is in good shape I think, as I fear for it…
In the meantime, hear is a table summarising our various motors!
|Pounds of Thrust (lbs)||Max Amps||Voltage||Watts/Horsepower||Example Product|
|20||17||12||204||Haswing W20 Kayak Pro|
|30||30||12||360||Haswing Osapian 30|
|40||40||12||480||Haswing Osapian 40|
|55||55||12||660||Haswing Osapian 55|
|65||50||12||600||Haswing Protruar 1hp|
|85||40||24||960||Haswing Protruar 2hp|
|55||50||12||600||Cayman Bow and Transom Mount|
|160||105||24||2,520||Haswing Protruar 5hp|
|110||35||29.6||1,036||Haswing Ultima 3.0|
|165||60||48||2,880||EZ Outboard L03|
|195||90||48||4,320||EZ Outboard L05|
|260||135||48||6,480||EZ Outboard L10|
|128||90||48||4,320||EZ Outboard S06|
|172||135||48||6,480||EZ Outboard S10|
|260||220||48||10,560||EZ Outboard S20|
|130||65||48||3,120||EZ Outboard X05|
|185||138||48||6,624||EZ Outboard X10|
|225||240||48||11,520||EZ Outboard X20|
Feel free to guess the hp from the information above!
So what should I do?
Probably the best thing to do is to actually try some of our motors on your boat!
We can quite happily set you up here at various locations from Calshot to Christchurch even using our batteries (or your own if you want to bring them).
Alternatively we can often do a try before you buy if you are further afield, where you effectively buy the test kit at full price and we will charge a restock fee if you don’t keep it. There would be postage each way and you can either opt to keep the test motor or return it when you will be refunded once it has been returned in undamaged condition.
Drop us a line to find out more now and find the perfect electric motor for you and your boat!