Replacing an Impeller

(Andy’s note: Most of this entry may be of interest only to boat nerds and people troubleshooting engines.)

Early Saturday morning, we struck out from Bimini to explore Gun Cay and North Cat Cay. All of that was lovely until our starboard engine (a Yanmar 57) signaled an overheat alarm about 10 minutes into our trip. It had been chugging along at a very modest 1600 RPMs, which should be fine, but I still had the port engine, and that had plenty of power to take us the 10 nautical miles or so to where we were going.

No big deal, right?

Wrong! I was trained as a private pilot, and losing an engine made me highly nervous, and it made Karen even more nervous. When an engine overheats, this is serious business.

We made it to a beautiful, calm anchorage outside of Cat Cay and took the dinghy over to Bu’s Bar at the Cat Cay Yacht Club to have some lunch and to take stock of our situation.

Taking a moment…

After talking it through, Karen and I came to the conclusion that we were probably in a tight spot due to the following facts:

  1. Really bad weather was bearing down on us (less than 72 hours out), and we had been planning every leg of our trip around finding good shelter.
  2. Our preferred shelter spot (Chub Cay Marina) was about 12 hours away via Gratitude.
  3. We were suddenly and unexpectedly down to one engine.
  4. In order to get our starboard engine working, we (thought we) needed a part we could not locally source. More on this further down.
  5. The marine store where we could source the part was 18 hours away (on one engine) in Nassau (we could return to Florida in less time, but that would be sailing into the bad weather).
  6. If we went to get the part, we would likely end up with two good engines but no good place to take shelter from days of heavy rain and very high winds.
  7. If we made the trip to our originally preferred shelter spot, we would have to limp there on one engine. If the port engine failed, we could end up in a more serious situation.
  8. If we reached our preferred shelter, I might not be able to safely dock this boat using only one engine, as I use the port and starboard (left and right) engines to maneuver into a slip. Only having the port engine functioning would not allow me to do what I was trained to do. I may get to the point where I could dock the boat in a slip with one engine, but I’m not there yet. We would have to source help from someone who could do this (assuming it’s even doable).

So while we sat and discussed our dilemma, we decided the most likely culprit for the engine overheating was the water impeller.

Now, if you are still awake (or maybe you found this in a web search, and this is why you started reading), one of the ways many marine diesel engines keep cool is this:

  1. Raw seawater is drawn in through an intake pipe
  2. The raw water is strained through a plastic, raw water intake filter to keep out seaweed and ocean critters
  3. The filtered seawater is drawn through the system by a rotating impeller (similar to a propeller, except that it sits in front of the hose to move the water along)
  4. The water is circulated back and forth around engine components, so that the water heats up while the engines maintains an optimal temperature
  5. The now-heated sea water is expelled through the exhaust back into the ocean, and fresh, cool seawater is drawn in to continue the cycle

Once I disassembled the impeller housing, I confirmed that we guessed correctly. We definitely had a bad impeller.

Now, I will not inflict upon you the entire story of how we located the spare impeller. Suffice it to say that this took the entire day. After trips ashore and an excursion to another island, we located a spare on our own boat late in the day. Spare impellers are like spare tires. You should always have one, and we did, thanks to the previous owner.

The impeller is a moving part that is subject to wear

Once we located a spare, this turned out to be a straightforward repair. Here were the steps involved:

  1. Folding myself into the engine compartment
  2. Closing the thru-hull seacock to seal off the raw water intake system
  3. Removing the four bolts and cover that house the impeller
  4. Gently prying out the old impeller (I used two sets of pliers to extract the old one)
  5. Lubricating the new impeller with grease and installing it
  6. Replacing the cover and the four bolts that house the impeller
  7. Priming (filling) the raw water strainer with water (an essential step that I missed the first time)
  8. Starting the engine
  9. Immediately opening the thru-hull seacock to allow the system to function
  10. Rejoicing as water spilled out of the side of the boat, indicating that the engine was circulating and cooling

More on step 7 above (priming the raw water intake). When I first fired up the engine, no water circulated. This was a very unwelcome development, as I wondered if the whole problem might be more serious, but once I primed the system and tried again, everything worked as designed.

Voila! Now we are back up and running with two engines and plenty of time to get to shelter ahead of the approaching storm.

4 thoughts on “Replacing an Impeller

  1. Courtney Krueger says:

    You may know that Lea’s family used to have a lake house. I once decided to take her out on a “romantic” ride in the little john boat during the winter when that was the only boat in the water. About 100 yards or so offshore, the motor died. Lea got out the oars and began paddling us back, while I worked on restarting the engine. Within feet of the dock, I got the engine going again and we were off again – for another 100 or so yards. We did this at least once more until Lea insisted that we end our romantic adventure. The problem was that the engine was overheating and then cooling down as she rowed! I’m glad Karen didn’t have to row y’all back once and certainly glad she didn’t have to do it more than once!

    • Andy Crowe says:

      Interestingly, I did not have one (or could not locate one). I was very careful with the old one, which seemed to be in great shape. I put something akin to Vaseline on it, and it fit tightly and did not leak a drop.

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