Ask the Experts: Propellers

Kruger Propeller Crew“The test of the machine is the
satisfaction it gives you. There isn’t any other test. If the machine produces tranquility it’s right. If it disturbs you it’s wrong until either the machine or your mind is changed.”

—Robert M. Pirsig, Zen and the Art of Motorcycle Maintenance

The quiet hum of the prop and gentle hiss of one’s wake on a calm day are just parts of living the dream on the water. Many of us, perhaps even most of us, take our propellers for granted. The designs of most props appear elegantly simple and rather foolproof. New props come in boxes and we bolt them on the shaft without too much thought, or we assume the prop on the newly purchased used boat is just fine. We’re getting from point A to B and having fun, aren’t we? Isn’t that good enough?

No, it isn’t. The truth of the matter is that when it comes to propellers, anything shy of a custom adjustment is going to cost you in wasted fuel and overexerted engines. Every boat needs her propeller to reach her full efficiency, which is a relatively straightforward process that could improve your boating experience by leaps and bounds.

A properly working propeller should operate quietly, and a roaring prop is often the sign of something wrong. Additionally, while many boaters will chalk up some vibration as normal, often shaking is a sign that something is wrong with the propeller blades. The money saved in terms of fuel economy is no laughing matter, and those dollars and cents saved per nautical mile add up pretty quickly over an active season.

To get the real insider perspective, we talk to Seattle-based propeller masters Doug Kruger and team of Kruger and Sons Propeller.

Q: So I’ve bought a boat and the propeller seems to be working fine. That’s good enough, right?

I think it’s important for people to realize the value of the propeller related to everything else aboard the boat: engine temperature, load, RPM’s, speed, etc. It all points back to the propeller, if the propeller isn’t tuned properly, you basically have a three-legged dog hopping around on the water inefficiently, burning more fuel than necessary. You can’t just buy a prop off the internet or from a buddy, put it on the boat, and expect everything to be perfect. Propellers are mass produced and generally inaccurate from the factory. It’s important to have specialists like us go through your propeller to check and adjust the pitch, track, and balance to insure accuracy and performance.

Q: What does tolerance mean?

Tolerance is a measure of consistency from blade to blade in terms of pitch, track, and balance. The tighter the tolerance, the better. All blades should be completely matched, but 90% of the time there are inconsistencies from the factory.

Your propeller is custom to your boat. You almost always need some adjustments for the weight of your boat, the shape of your hull, etc. There’s more to it than pulling the propeller out of the box, bolting it onto the shaft, and away we go. I’d say the biggest waste in efficiency arises when the prop isn’t pitched properly.

Q: So variation between blades is one of the worst things?

Yes, they need to be matched or you will feel a vibration. Vibrations cause unnecessary wear and tear on the running gear.

Q: Can’t I just take a peek at the prop and look for any signs of damage?

One of the most important things to know is that one cannot just look at a propeller and tell if it’s ok. We can’t even just look at it and say if it is ok. It’s important to haul the boat, take off the props, and have them checked for pitch, track, and balance. If there’s nothing wrong with the propeller, we recommend a clean and balance service at minimal cost.

Terms:

Tolerance: The consistency of parameter measurements between all blades of the propeller. Coast guard spec is Class 1. All propellers that leave Kruger & Sons are balanced and finished within Class 1 tolerance.

Pitch: The angle of the blade. A higher pitch is moving more water than a low pitch per revolution. Increasing the pitch increases the load on the engine, decreasing the pitch decreases the load on the engine.

Balance: The state of equilibrium in which centrifugal forces due to rotating mass do not produce force on the shaft, i.e. all blades weigh the same. A properly balanced propeller reduces vibration.

Materials:

Bronze: Relatively soft material suited for lower horsepower applications that is economical and abundant. Reacts negatively with aluminum hulls.

NiBrAl or Nickel, Bronze, Aluminum alloy: A strong durable material suited for high horsepower applications. NiBrAI is easily rebuilt even with a fair amount of damage and one the best at combating the corroding effects of electrolysis. Does not react negatively with aluminum hulls.

Stainless Steel: Tough, rigid material for props destined for hard work, perhaps in a commercial fishery or out in the Bearing Sea. Commonly found on commercial crab boats. Does not react negatively with aluminum hulls.

A propeller should be cleaned and balanced every couple of years. We check out props for free, so you might as well just bring in the prop for us to look at when you haul-out every year or two. A propeller, like a hull is inefficient when covered with barnacles and growth. Giving it a clean and balance is a good idea. With proper maintenance a propeller can last a lifetime.

We use MRI computer-aided technology, and a proprietary pitch table to provide accurate before and after reports for the customer. The initial report shows the condition your propeller was in when it arrived and the final report shows the condition it’s in when you pick it up. Keep the final reports onboard in case you lose a propeller or damage one beyond repair in the middle of a trip. They include important data like diameter, rotation, pitch, material, shaft diameter; the information we need to get you a new prop.

Treat your props like your tires. You rotate your tires, you take care of them so they are balanced and you get maximum efficiency. Propellers are the same way, get them cleaned and balanced.

Q: How much fuel economy is really on the line?

We recently sold a big propeller to one of our longtime customers, a commercial fisherman out of Fisherman’s Terminal. After eight years of us suggesting that he buy a new propeller, he finally sprang for one of our properly tuned, high efficiency propeller designs. He departed Seattle for Juneau (like he always does) and called us to say that his fuel savings alone just paid for his $8,000 propeller. He knows how much fuel he burns on that trip, but it can be hard for recreational boaters who are always going on different, shorter trips to really appreciate the savings. If the propellers aren’t done right, boaters are just throwing fuel overboard. It’s a big deal.

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