Propellers: Out of Sight and Out of Mind? They Shouldn’t Be!

Kruger & Sons Propeller Set to Launch a New Machine that will Set the Propeller World Spinning at a Whole New Pitch

Published: Monday, March 21, 2011 7:00 am
By: Kruger & Sons Propeller

In the ongoing quest for fuel efficiency on your boat, power or sail, the prop or props are often overlooked and usually neglected until the time of your next haul out. As we learned on a visit to the Kruger & Sons Propeller shop on the south side of the Ship Canal near the Locks, having proper, well maintained propellers is really the foundation to a more economical, more enjoyable and less problematic cruising season.

Before we go much further in this relatively unexplored region where metals react with water, you may be asking why Kruger & Sons? We had initially followed up on a rumor from one of our friends that something special was cooking at the Kruger shop. Well, that was correct in more ways than one.

When we first arrived at the shop, you couldn't help but notice a bright pink, pig-shaped barbecue sitting out front.Then when you got out of the car, the aroma of roasting meat was positively mouth-watering. Were we at the right place? A glance around the corner at a driveway stacked with hundreds of props in various shapes and sizes provided the positive response to that question.

The real purpose of our trip was to learn about a new machine that the gentlemen at Kruger & Sons had just patented that would greatly reduce the amount of time required to measure, balance and re-pitch a propeller. We looked at one another and said, well that's interesting but why is that important? They are, after all, just propellers. Well, we certainly got more than we bargained for with that question.

A Bit of History

Howard Kruger arrived in Seattle from Minnesota in 1940 fully intending to continue on up to Alaska to seek his fortune there. He did however make a stop at the corner of 4th Ave South and Horton Street and looked into the window of Doran Company: Makers of Propellers and as they say, the rest is history.

For the next 40 years Howard Kruger made propellers and made history as well. From 1950 to 1973, Howard was the number one person in Seattle or for that matter the United States when it came to making propellers for hydroplanes and not just the big unlimiteds, but everything from junior outboards up.

The list of unlimiteds he supplied props for pretty much reads like a who's who of unlimited hydro racing, The Slo-Mo-IV & V, Hawaii-Kai, Pay n' Pak, Oberto, Miss Bardahl, and most of the rest. It was Howard who first came up with the idea of running a three-bladed prop instead of the standard two-blade on the unlimiteds and that change was one of the things that made the Slo-Mo-V a legendary vessel to this day.

As Howard claimed, there was really no magic in making the hydros go faster, it was simply a matter of matching the propeller to the boat, balancing the propeller and calculating the correct rake and pitch for each boat. Even though Kruger & Sons no longer does props for hydros, the same principles are applied in the shop each and every day by sons Doug and Jim, for whom working with propellers has been their only job.

Some Basics

So what are the basics when it comes to propellers and fuel efficiency? When it comes to power boats, a three bladed propeller usually offers a higher top speed while a four bladed prop provides more thrust, smoother and quieter running. A four bladed wheel is more efficient than a three bladed prop and therefore will usually operate with better fuel economy.

All of this is well and good however you still have to match the prop to the boat and while most boat manufacturers will offer the same boat with the same engines, struts, and reduction gears, that doesn't mean the same props will work on the boat. The reason is simply that owners load and use their boats in different ways. So while your new boat zipped right along at the designed and advertised speed at the dealership, you may find that when you've added all your options, filled all the water and fuel tanks, you can no longer achieve the designed rpm's for those engines.

If for instance you have a standard Mythical 45 power boat with twin diesels that has a designed displacement of 28,000 pounds, and carries 500 gallons of fuel and 300 gallons of water and a 50 gallon holding tank, simply by filling all tanks you can change the displacement of the vessel by almost 5600 pounds, 2.8 tons! Not to mention what else you may added in options.

If that's the way you're going to run, you will need to re-pitch the props to match that kind of loading. If however you decide to run with only the amount of fuel you need to get to your destination and you know they have fuel and water, you can keep the weight down, the fuel efficiency up and while the fuel may be more expensive at the destination, the efficiency achieved by running light more than pays for the extra cost of fuel at your destination.

The Problems

The above is simply the start of achieving maximum fuel efficiency for your vessel. The rest comes from proper maintenance and proper propeller installation. As anyone who has spent anytime cruising the Great Lakes will tell you, there are only two types of boaters, those who have hit something in the water and those who are going to. We all know the feeling, first there's that clunk and then as you frantically try to throttle back, there's that crunch as that invisible piece of flotsam or jetsam interacts with your prop. As you first listen for problems and then look in the wake to see exactly what caused the dreaded noise, you start to throttle up and to see if there's a new vibration. If there isn't, off you go breathing a sigh of relief that nothing bad happened to any of your expensive running gear. Wrong! At this point, all you know is that you really don't know what actually may have occurred. The only way to find out is to have your diver check out the props and struts.

Cavitation Corrosion or Burn

This brings up the next step in preventive maintenance for your running gear. Anytime, you have the diver down to clean the hull make sure they give the props and struts a close visual inspection and while doing so, clean the props and struts of any marine growth. They should do so with only a scotch brite type pad, no files or tools. If there are any nicks or dings in the props you will want to get those fixed immediately especially after a close encounter with an underwater object. The reason is that while the only visual indication of a problem is the ding, in fact the collision may have altered the pitch of one of the blades as well which may not be visible to the eye, especially underwater.

By keeping your props clean and in top shape not only do you improve fuel economy, you also prevent what can become an even more expensive problem for your props.

The reason is that any ding or barnacle on one blade or the even the strut constitutes a problem for all the other blades as well. This is because any imperfection, no matter how small, on the edge of the blade can create cavitation. The longer you run at higher speeds, the sooner a problem known as cavitation corrosion or burn can occur and it doesn't just occur on the dinged blade, it occurs on ALL the blades. The longer you let it go, the more it eats into the blades, the more inefficient your props become and the more your fuel efficiency drops.

Galvanic Corrosion

The other item most divers check on is zincs. Most props being bronze or an alloy thereof are susceptible to galvanic corrosion or electrolysis. The point to remember here is that it's very rarely, if ever, the marina that's the problem. It is usually your neighbors and how their boats or shore power systems are wired or should we say improperly wired. If your diver tells you your zincs are shot and your bronze props have developed a reddish tinge, your props may already be toasted beyond recovery.

While at Kruger & Sons we saw two props from a boat that had been temporarily tied up in a boatyard next to an aluminum boat that was having some heli-arcing done to the deck, both props were shot from the stray current that had leaked into the water.

The Solutions

Most owners haul their vessels once a year and that is the best time to get your props checked. Besides, the great news here is that it doesn't cost you anything to get a report on your prop from Kruger & Sons, that's right, it's FREE! A boater's favorite and least heard word. Sure you may have to hire someone to take the props off, however if a problem is found, that's really a small price to pay and the money you save from improved fuel economy and reduced damage from prop vibration to the rest of your running gear will usually more than cover that cost.

The report you get from Kruger & Sons, or any prop shop that uses the Hale MRI (measurement recording instrument), will tell you very precisely the diameter and consistency of pitch measured at three different points on each of the blades of your propeller. Obviously, the closer the match, the better the fuel economy and the lower the vibration that may becaused by one or more of the blades being even slightly out of whack from the others. The tolerances we're talking about here are around 60 thousandths of an inch. The higher the rpm's you're running the more critical this becomes.

The Static Balance Test

The next step in the process is the static balance test which is part of the standard package. Here the prop is mounted on the precise arbor that matches the taper of the prop. The arbor is then placed on four nearly frictionless wheels. If any blade is not the same weight as the others, the prop will rotate it on its own. This test is so sensitive that when a small feather was added to one of the blades, the prop rotated.

Prop Installation

The last step takes place beyond the control of any prop shop and that's the reinstallation of the props. It is also a step where vibration can be induced into the drive train through a very simple error. When the prop is installed on the shaft for the first time once it returns from the shop, it should be installed snugly but without the key.The hub of the prop and the shaft should then be marked with a felt pen. The prop is then removed and reinstalled with the key. If the prop does not return to the mark, the prop is riding on the key and will then be out of balance on the shaft, inducing potentially damaging vibration into the system.

The Kruger Hydraulaform Work Station

So you've got the report, what next? This is where the newly patented device that the Kruger Brothers invented really comes into in its own. Let's say for the sake of peak fuel economy you decide that your props should be brought up to ISO Class 1 standards or S Class, the best you can get. In the past, the technician would place the prop on the MRI table and measure and mark each blade. Once it was determined which blades had to be changed and by how much, the prop would then have to be removed to a very solid working surface where the blades could be adjusted either by controlled impact or by mechanical or hydraulically controlled bending systems. You have to do this because the Hale MRI is not designed or intended to take the loads involved with blade adjustment. Doing so on the Hale MRI would do damage the sensitive bearings on which the prop is mounted.

Once adjusted, the prop has to be removed from the work bench and then remounted on the MRI table for re-measurement. This process is usually repeated two or three more times until the desired level of accuracy is achieved. You can see how this could easily amount to one and a half hours or more of work done by two technicians.

The Kruger Hydraulaform Work Station eliminates this costly and potentially damaging procedure by keeping the propin one place the entire time. It also means one person can control the entire process. The Kruger Hydraulaform Work Station not only simplifies the entire process by saving time, it also saves the vessel owner money in the short term and in the long term through improved fuel economy.

In addition, vessel owners receive a very detailed report on each prop that should be kept with vessel records. That way if a prop is damaged while the vessel is cruising just about anywhere in the world, the prop can be taken to any prop shop using the Hale MRI and the prop can then be restored to precisely the same numbers. Even if the prop records are lost or misplaced, Kruger & Sons maintains an archive of reports, a copy which can be sent at the owners request either electronically or by fax.

Even before the Kruger Hydraulaform Work Station was publicly announced, the first system was sold to a prop shop in New York. Just as the Hale MRI is recognized as the industry standard in electronic 3-D propeller analysis, there is little doubt that it won't be long before the revolutionary and remarkable Kruger Hydraulaform Work Station acquires the same status when integrated with the Hale MRI. All for the simple reason that any problem or imperfection with the props can get transferred up the drive train to the cutlass bearings, transmission or the engine itself. So preventing the problem with absolute certainty becomes a total no brainer, and the key to more economical and fun cruising.

For more information about Kruger& Son's Propeller, visit www.kurgerpropeller.com or call (800) 762-PROP (7767).