Thrust to HP Calculator
Pounds of thrust and horsepower measure different things. Add a speed and the conversion becomes real — this works it out both ways, from speed and from electrical draw.
Trolling motors are sold in pounds of thrust. Outboards are sold in horsepower. Naturally everyone wants a number that lets them compare the two — and the honest starting point is that no single number exists, because the two specs describe different physical quantities.
Why Thrust Alone Cannot Become Horsepower
Thrust is a force, measured in pounds-force. Horsepower is a rate of doing work. Work only happens when a force moves something, so:
A trolling motor pinned against a dock is producing its full rated thrust and drawing full current, and its useful output power is exactly nothing. All the electrical energy is going into accelerating water backwards, churning turbulence, and warming the motor. It is precisely the same reason a torque figure without rpm can't become horsepower — see Nm to HP for the rotational version of the same problem.
Where the 375 Comes From
One horsepower is 550 foot-pounds per second. To use miles per hour instead of feet per second:
- 1 mph = 5,280 ft ÷ 3,600 s = 1.4667 ft/s
- 550 ft·lb/s ÷ 1.4667 = 375 lb·mph per horsepower
Thrust (lb) = (hp × 375) ÷ speed (mph)
Trolling Motor Thrust Reference
Useful output power at a realistic trolling speed, alongside typical electrical input. The gap between the two columns is where propeller and motor efficiency go.
| Thrust | System | Typical draw | Electrical input hp | Useful hp at 3 mph |
|---|---|---|---|---|
| 30 lb | 12 V | ~30 A | 0.48 | 0.24 |
| 40 lb | 12 V | ~42 A | 0.68 | 0.32 |
| 55 lb | 12 V | ~50 A | 0.80 | 0.44 |
| 70 lb | 24 V | ~42 A | 1.35 | 0.56 |
| 80 lb | 24 V | ~56 A | 1.80 | 0.64 |
| 101 lb | 36 V | ~46 A | 2.22 | 0.81 |
| 112 lb | 36 V | ~52 A | 2.51 | 0.90 |
Current figures are representative of the class rather than any specific model — always check your own motor's rated draw before sizing batteries or wiring.
The "72 to 75 lb per HP" Rule of Thumb
You'll see this everywhere, and it is worth understanding what it actually is. It is a market equivalence, not a physical conversion — a rough statement that a 75 lb thrust trolling motor moves a small boat about as usefully as a 1 hp gasoline outboard would.
It works reasonably at trolling speeds on small hulls, and falls apart quickly outside that. At planing speed a 1 hp outboard and a 75 lb trolling motor aren't remotely comparable, because the trolling motor was never designed to operate there. Use the rule for a first sanity check, not for choosing propulsion.
Thrust Horsepower vs Shaft Horsepower
Three different power figures exist for the same motor, and mixing them causes most of the confusion online:
| Figure | What it measures | Relative size |
|---|---|---|
| Electrical input | Volts × amps from the battery | Largest |
| Shaft horsepower | Mechanical power delivered to the propeller | Input minus motor losses |
| Thrust horsepower | Thrust × speed ÷ 375 — actual forward work | Smallest |
Propulsive efficiency between shaft and thrust power is commonly in the 50–70% range for small craft, and worse at very low speeds. That's why the useful-hp column in the table above is roughly half the electrical input column.
Related Tools
For estimating boat speed from engine horsepower, use the boat HP to speed calculator. For power in nautical terms see HP to knots, for force in SI units newtons to HP, and for electrical conversions watts to HP.
Frequently Asked Questions
No fixed answer — thrust is force, horsepower is a rate of work. The common rule of thumb is 72–75 lb ≈ 1 hp, but that's a market comparison, not physics.
hp = thrust × mph ÷ 375. A 55 lb motor at 4 mph produces about 0.59 hp of useful work.
About 0.24 hp of useful output at 3 mph, from roughly 0.48 hp of electrical input (30 A at 12 V).
Power = force × velocity. Hold the boat still and velocity is zero, so useful output is zero — even at full current and full thrust.
Volts × amps ÷ 746. A 12 V motor drawing 50 A is 600 W ≈ 0.8 hp of input. Shaft output is lower after losses.
Up to the point the hull can use it. Thrust matters most at low speed and for holding against wind or current — which is exactly the job trolling motors are rated for.
Shaft hp reaches the propeller; thrust hp is what the propeller turns into forward motion. Propulsive efficiency of roughly 50–70% sits between them.
Only loosely. Trolling motors are built for sustained low-speed control, outboards for planing. Thrust alone flatters one and understates the other depending on the job.