Horsepower vs. Torque

RotatingAssembly02If there is a subject that bears much importance to the powering of a boat and yet is so misunderstood, it has to be this one. So let's tackle because it explains where a lot of the Medusa superiority comes from.


Horsepower determines the maximum amount of work an engine can achieve and torque the energy with which it can perform that work, or in more simple terms, horsepower determines the speed an engine can reach, and torque determines how quickly the engine can achieve that speed. An engine’s pistons move a crankshaft which in turn moves either a transaxle connected to the wheels of a car, or a shaft connected to a propeller on a boat.  Horsepower is achieved through RPM, the faster the pistons of an engine can rotate, the more horsepower it will generate.  Which is why peak horsepower is always reached at the very top of the RPM curve. Torque is achieved through the stroke of the pistons, or the length of travel through the cylinders, the longer the stroke, the more torque. Which is why peak torque is reached towards the low end, when the pistons are still not moving too fast and completing their stroke fully.

So, what does a boat need more of, torque or horsepower?

Both are interdependent in that the engine needs torque to generate enough rotational power to move the propeller while carrying the weight of the boat, and once momentum has been established, horsepower will allow the engine to move the boat faster. So while a high horsepower/low torque engine can move a car with ease, given how relatively light a car is and how easy is to roll over the pavement (a person can push a car), that same engine will have trouble getting a much heavier object to move through a denser medium, like a boat in the water.

Modern hulls are of the Planing type, where once a certain speed is reached, parts of the hull will come out of the water and glide over the surface instead of cutting through it. This allows the planing hull to reach high speeds with low fuel consumption, but on the other hand, a planing hull is very inefficient at low speeds, raising the bow, or nose, up, while trying to climb out of the water. If the boat engines do not have enough torque, then the boat will take a long time to reach planing speed and spend huge amounts of fuel doing so, all the while providing an uncomfortable, even dangerous ride, since the elevated nose prevents the driver from seeing what’s in front. In fact, many underpowered boats never even reach planing speed. Ever seen a boat ride with its stern almost sunk in the water and the nose high up in the air, all the while the propellers are churning up massive amounts of foam? It is quite a spectacle in inefficiency in fluid dynamics, and you're close enough to the boat, you will be able to smell the almost molten metal of the engine blocks, working tirelessly without respite. It is a familiar sight, especially with heavy, deep vee fishing boats, and it is even more of a painfully familiar sight with outboard boats, which are the engines with the lowest torque of all.  Unsurprisingly, most collisions happen when boats are trying to reach planning speed. So, without proper torque at the low end, all the high end horsepower in the world will not be able to move a boat.

This subject is particularly important because the large majority of marine engines nowadays are sourced from automotive companies like GM or Ford, and though they do a fine job adding marine specific touches like cooling and exhaust systems and urethane paints, the marine engine companies cannot escape the fact those those engines have been designed around an automotive architecture, which is to say, to favor high end horsepower with very little low end torque. No amount of water-proof enamel and catchy automotive names on the cover of those engines like "Corvette" or "Raptor" can hide the fact that, from their very foundation, those engines are not suitable for marine use.

At Medusa, we design and build engines that have exactly the right amount of torque and horsepower exactly where they are needed, for effective, easy and reliable in-water propulsion. This balance of torque and horsepower is what allows our engines to achieve higher efficiency with much smaller displacement than other gas engines, or to achieve far higher power outputs at comparable displacements.