A technical discussion part 2

In this part of the discussion, we’ll talk about strain on a PWC engine.  This has to be the second most common “issue” when people discuss the use of a Waveboat with their PWC and why they are avoiding it.  Let’s take a look at what is going on under your seat in that tiny PWC engine.

Strain

First off, we should define strain.  Some people think of strain as a situation which causes an engine to work harder than it would in other situations.  This type of strain shouldn’t really be called strain, as such, it should be called having a need for higher power output.  Strain, in the mind of others, is defined as the accelerated wear-down of components over time due to increased operating workloads.  Lastly, strain can be defined as operating an engine outside of or close to the intended design limits for that engine, resulting in sub-optimal performance or reliability.  I hope to touch on all of these definitions of strain below.

A comment I’ve seen a number of times goes something like this, “Sure, hook your PWC up to one of those and your engine will burn out after a few hours.”  I can understand the fear, uncertainty, and doubt behind this comment, but a closer examination reveals it may be unfounded.  I haven’t yet run into any 4-stroke, reasonably modern (since 2003 or so) PWC’s that can’t operate in long runs at WOT (wide open throttle).  If you have such a ski, please let me know so we can recommend people avoid them!  The engines and all supporting systems are designed to sustain that level of performance without the engine sustaining damage.  Lubrication, cooling, fuel flow management, timing…every aspect is usually monitored and maintained to keep the engine from blowing up or otherwise self-destructing.  This means you can safely operate your PWC in a WOT mode for long runs.  I know I have, while cruising up and down the Columbia River, and I’m sure you have too.

Let’s assume for a moment that we operate our PWC at WOT while it is coupled to a Waveboat (Yee Ha!).  The same systems present in the PWC to keep the engine in check are still there, continuing to do their work.  If the engine begins to overheat due to high water and air temp, most ECU’s will go into “limp” mode, retarding the throttle to reduce engine temperature.  There are many aftermarket intercoolers and air intakes that are solely designed to reduce operating temperature to prevent the engine from going into this limp mode, but they are usually only needed if you are modifying your engine to extract more performance from it than it outputs stock.

Now consider that, in this WOT scenario, you compare the speed of your PWC through the water both with and without a Waveboat attached.  How different is the speed?  It’s not much if the Waveboat is lightly loaded, more if it’s heavily loaded due to the hydrodynamic drag we discussed in part 1.  Is this difference in speed through the water straining the engine components more?  I would say no on one level, because the power output from the engine remains constant.  It’s not spinning at a higher RPM in either scenario, so the power being generated and the load on the engine is the same.  Hence, the “strain” on the engine is the same.

This is a very important point to understand, as even many mechanics lose sight of this when discussing engine use in watercraft.  The crux of my point is this: Engine RPM does not directly correlate to the speed of the watercraft through the water. What?  How can I say that?  Consider tying up your PWC to a dock and running the engine full out.  What’s your forward speed? 0mph.  What is your engine RPM?  Ahh…now you’re starting to understand.  Let’s take this a step further by understanding why even mechanics make some mistakes on calculating engine load on watercraft.

Pull your car out onto the street and put it in drive, accelerate to 5mph and note the RPM.  Now tie a 1-ton block of concrete to the back of your car (I want to see a Youtube video if you do this).  Accelerate to 5mph and note the RPM.  If your car can even do that without the wheels spinning, you’ll be shocked to see that the RPM of both scenarios matches.  Why?  Simple, in a friction-honoring environment where your transmission stays in first gear, there is a direct relation between engine RPM and forward speed.  Now, is the engine working harder pulling the 1-ton-concrete-block-of-doom?  Yes, it is, because more power is required to drag that thing down the street at 5mph than is required to move the car without the block down the same street at 5mph.  The RPM, though, is constant because the transmission remains in 1st gear.

This mindset and mental image is haunting the mind of consumers and mechanics alike when they think about engine operation in a watercraft.  You can’t do that, though, because watercraft operate in a fluid.  The amount of water that your propeller/impeller pushes out the back of your PWC doesn’t have a direct relation to the speed at which you move forward through the water.  As a pilot, I’m acutely aware of this, because I have to plan for the “winds aloft” when I chart out a course to follow in the air.  If you’re in a boat and you’re headed into a stiff breeze, you’ll notice that your speed is not as high as it is if you turn 180* and cruise the other direction.  Why?  Your engine doesn’t have a non-slip grip with the medium through which you are traveling like a car has with the road under the tires.  As in my example above, a tied off PWC can push a lot of water around and go absolutely nowhere.

So lets take these principles and apply them to the situation at hand: a PWC pushing a Waveboat.  Let’s assume the throttle/RPM of the PWC remains constant and the only difference is whether or not the PWC is mated to the Waveboat.  In both cases, is the engine turning at the same RPM?  Yes.  In both cases is the same amount of water exiting the pump nozzle?  Yes.  Has the water changed in density, thickness, surface tension, or suddenly become compressible or more difficult for the engine to move?  Certainly not.  Has our speed changed?  Yes, the speed of the Waveboat-attached PWC is a little lower than that of one unattached due to the slightly increased hydrodynamic drag discussed in part 1.  That, my friends, is where the golden nugget of science lives.  The load or “strain” on the engine in this example is the same in both situations.  The only difference is the increased drag resulting in a slightly lower speed.

Eureka!  This means that the “strain” on the engine is not relevant to whether or not the PWC is pushing a Waveboat or not.  It is purely dependent on your “lead trigger finger” wanting to go as fast in the Waveboat as you do on the PWC.  Due to the slightly increased drag, you will need more RPM from the engine in order to expel more water out the pump nozzle to achieve the same speed.  Running at the same RPM’s in both modes, however; the strain, wear, load, and every other aspect of the engine will remain constant.  That should keep your mechanic happy.

As in part 1, please comment below if you think otherwise and can offer proof or at least a darn good explanation.  We’ll talk about fuel burn in part 3.