Water Pump Repair: Cheap is a Mistake

Impeller housing
In this diagram you can see the rib that supports the impeller as it crosses over the intake port. What we didn’t know is that the rib had broken on our housing, which trashed the new impeller in just a few days.

Sooner or later, you’ll find yourself down in the bilge of  boat, repairing a shredded water pump impeller. Then, a couple of days later, you could find yourself back in the bilge, fixing it again.

Not long after I bought a spiffy new-to-me boat — also known as used —we were on a road trip, boating on different lakes, when the overheat alarm sounded. After we limped it back onto the trailer, we figured out the impeller had gone bad, which, given the hours on the engine, made sense.

I pulled it apart. The Bravo One drive uses a belt-driven pump to draw water up from the pickups in the lower gear case and up into the motor, but it’s largely the same impeller design as an Alpha drive. The impeller in the Alpha drive is driven off the main vertical shaft, but it fails in the same manner as the Bravo impeller. The rubber gets hard and brittle, and biased in the direction of rotation, at which point it either, a) fails to draw water, or b) disintegrates into chunks you have to find and fish out of the cooling system.

Naturally, ours disintegrated, so I had to pull off all the cooler hoses to find all the missing pieces, which can clog coolers and lead to other problems.

Impeller kit
Save yourself the headaches and buy the whole body and impeller kit from MerCruiser. Most late model Mercs take Part No. 46-807151A14.

When I was at the marine store, I had a choice. I could buy the impeller and housing, or buy just the impeller. Our housing didn’t look bad, so I bought the impeller and saved myself $60, which I could spend on something else. Like beer. That would become false economy.

Turns out the rib in the black plastic housing that supports the impeller as it crosses over the water intake side had broken off, but I didn’t notice that. So, when the new impeller would cross the water intake opening, it would catch on the remnants of the rib, which shredded it in a couple of days.

When I was trying to flush the unit with the “muffs” over the lower gear case, it wouldn’t draw water. That meant I had to remove the water pump again — which is no picnic — and buy a new impeller and housing. Long story short, my penny pinching cost me more in the long run.

The moral of the story is, at least when it comes to Bravo drive water pump repairs, sometimes cheap is too cheap. Buy the impeller and housing, and consider that the extra expense goes toward peace of mind. And that especially applies to a boat you just bought, because the guy who just sold it might have gone cheap.


Sterndrive Engines: Big Block or Small?

The 454 Mag big-block inboard, mated to a MerCruiser outdrive.
The 454 Mag big-block inboard, mated to a MerCruiser outdrive.

I might be overthinking this, but let’s just say for the sake of simplicity that the price difference among all three is negligible, the hours are comparable, and that they all have a Bravo One drive. One has a 310-horsepower 454 big-block engine, another a 300-horse small-block, and the last one a 385-horse 454. How do you know which one is right? This is a good example, because the used market is well stocked with boats equipped with all of these engines.

Well, a lot of people might just shoot down the middle and go for the 310 horsepower big-block. No so fast. At 1,177 pounds with the drive, the big block is pretty heavy. The small-block propulsion package isn’t much lighter at 1,025 pounds, but it’s the better choice. Why? More power per pound of weight. The big-block in this example only has 10 horsepower to push around that extra 150 pounds or so.

It doesn’t sound like much, but the performance would be superior and the fuel economy would be better, too. I was never a fan of the 310 horsepower big-block, anyway. The internal components aren’t strong enough to handle any power upgrades.

That leaves the choice between the small-block and the 385-horsepower big-block, which is usually known as a 454 Mag MPI, and it’s where big blocks start getting good. For my money, I’d go with the big-block, but that’s for personal reasons. Every engine in every boat I’ve ever run has always become less and less satisfying the longer I’ve had it. With the 454 Mag MPI, it has the beefy internal components you need when adding power later. The 454 Mag MPI has the forged crank and rods, and higher flow heads than the 310-horse big block, so you can add a blower without fear of scattering the bottom end.

On the other hand, if 300 horsepower is plenty for you and your family, the small block is the way to go. It’s lighter and smaller, which also might mean larger stowage compartments under the engine hatch. Parts might even be cheaper because the small blocks were produced in greater numbers.

It might seem like I’m picking nits, but you’re going to have to live with the boat you choose for a while. It’s best to pick nits now rather than later. Of course, I might be overthinking it.