TRENTON – One of the sciences needed to make your boat perform to the best of its ability has to deal with the choice of a propeller. It is not an easy science to understand and some say it is nothing but black magic. With the advent of the computers and laser measuring systems this science is becoming much more precise. Now couple that with the knowledge of what worked on the same hull with different configurations (i.e. how it was finished, weight distribution, engine horsepower and gears) and one can be extremely accurate. So what do you need to know about propellers, well I asked Mark Dickinson of Nautilus Marine in Trenton to give me an understanding of the basics.
“Things that people should know about propellers and in particular, people who might work on their own boats or even finish their own boat,” explained Mark, “Number one, what do the measurements mean? What is the diameter? That’s how big around the propeller is. From tip to tip the blades are 26 inches so that is the diameter. What most people don’t know is, what the pitch really means? The pitch is the number of inches that your boat will move forward per one revolution of the propeller. So, 30 inch propeller in theory will move your boat forward 30 inches each revolution. It doesn’t really go 30 inches, because a propeller is always slipping in water. If it is a good efficient hull, light weight, slippery, maybe it’s going to move forward 28 inches for a 30 inch pitched prop. The rotation left hand or right hand, or you can call it clockwise or counterclockwise, simply is which way does the propeller turn when you put the boat in forward gear. Does it turn clockwise or does it turn the other way, left hand or counterclockwise. What difference does that make? It makes a difference in how your boat handles. Most of the lobstermen that we deal with, like most people, are right handed, so most of their boats are set up on the starboard side for the helm, docking and hauling gear. When you pull into a dock on your starboard side you put the boat in reverse and a left-handed propeller will help draw your boat into the dock instead of pushing it away. That is why most of the lobstermen have a left hand propeller. Cupping is rolling up the trailing edge of the blades of the propeller to a certain amount. There are all different degrees of cup. How high is the cup? Some people call it light, medium, heavy. Well, there’s numbers that relate to that. A medium cup in number form is a number five which in measurement form is 73/1000 of an inch of lip that you roll that lip up above the actual surface of the blade of the prop. What does it do? It is really designed to help control cavitation. I am not enough of an engineer to explain that well, but it has to do with redirecting the water around the propeller. Most Maine lobster boats, or lobster boat type hulls, benefit from a cupped propeller of some degree. Most of them have interrupted water flow to the prop because of the keel in front of the propeller and that produces cavitation, some a little, some a lot, and cupping helps that. That is why cupping usually helps performance and propeller longevity on a lobster boat type hull. Material, bronze and Nibral are the two most common ones. Once in awhile people ask us about stainless, it is ungodly expensive; it has very limited availability and it is unnecessary in our waters up here. Stainless props on big boats are used where there is a tremendous amount of silt and abrasive stuff in the water, like the Mississippi River and the Gulf of Mexico. Places like that. Here, the main two materials are manganese bronze and Nibral. Nibral stands for nickel, bronze and aluminum. It is extremely strong. It welds nearly as nice as stainless steel. It is much harder to bend, but it will damage. The damage usually doesn’t run as deep into the propeller as it does on a manganese bronze prop. Nowadays, with our type of more modern equipment for repairing with hydraulic presses we don’t really care if it’s bronze or Nibral as far as being able to straighten it, because we can straighten it. We like the Nibral better because it welds better and it straightens better. Costs more money initially to purchase a Nibral propeller, but most of the time it is well worth it in longevity. If you don’t really need it because of the lower horsepower requirements or something else manganese bronze is fine.”
“People talk about differences in 3 blade, 4 blade, 5 blade,” continued Mark. “Those are the three most common for what we use around here. A correctly sized 3 blade propeller with the right amount of blade area and everything will still almost always give you the highest top end speed, generally. They might make more noise in normal operation even as nice as we can tune them now. The 4 blade is kind of the standard that everybody uses. Mostly it is a great compromise between cost and performance. There are various options available for blade area and profile. Five blades, we are probably going to start to see more usage in some of the lobster boats than we use to. With these big boats, with big power, fairly low gear ratio and kind of limited prop diameter aperture size, you get to the point where you need more blade area than you can efficiently put on a 4 blade. We are starting to see more of those boats that benefit, not just in smoothness and quietness, but actual speed and efficiency with a 5 blade over a 4 blade because you have just got to have that blade area because you have so much power and it is turning kind of slow and in all reality you don’t have enough room for the diameter that you really ought to have to be as efficient as you could be.”
How do you size a propeller? “Well, the basic information that we have to have is,” said Mark, “what hull is it? If it’s one we are familiar with then we are leaps and bounds ahead. It is great to have the finished weight with the fuel and everything if we possibly can. What is really the horsepower, what is the correct RPM that it’s rated at and exactly what is the gear reduction ratio. Those numbers even though they sound close can make as much as an inch or more difference in pitch or diameter of the final size of the prop. Engine manufacturers with the new computerized systems, all digital controls, it is critical to have the right size propeller for the correct engine loading to No 1: so that your engine is warrantied and No. 2: for the most efficient operation. We have to have the exact information. We have to know how big a propeller can fit under the boat with the correct clearance. So, the procedure is, give the prop shop all the information that I just spelled out, and there is a formula that can figure it all out. Beyond that, being able to know how a certain boat goes through the water, just kind of having the experience. In Maine, every boat we build is custom. Well I have got a book full of them and every one of them is different. This guy has 200 gallons of fuel, the next guy has 400, this guy wants his engine further forward because he doesn’t want an engine box. That makes the boat sail different. The next guy wants his boat to sail as fast as it will go, so they pull the engine back, adjust the weight, and add small tanks. Do they have a great big light stand up on the roof or those humungous LED lights and two radars and big stick antennas? Makes a huge difference in the amount of windage that the boat is pushing through. It could make an inch difference in pitch in the propeller or more. Is the boat going to have a cage on it? Two inches less pitch if it has.”
“The computer program that we use,” added Mark, “which is from Michigan Wheel, it’s simple, it doesn’t cover everything, none of them do. I have looked at and tried a few of the other ones, I have never seen much of an advantage over the really fancy, expensive ones other than the fact that they print off a really fancy looking report for you to look at. All of them, almost always unless you know what factors to put in, will over prop a Maine lobster boat.”
“Proper clearance,” stated Mark. “Correct clearance for a propeller. The kind of industry rule of thumb is 15 percent of the diameter of the prop between the blade tips and the hull and the skeg. We have found with most of these, again lobster type hulls with some cup, we can get away with 10 percent clearance and it works fine. Guys keep putting more and more power in these boats, but they are not making the boats any deeper. You have got to be able to get the proper propeller under it. Ten percent is kind of easy to figure out. If you have a 30 inch prop, 10 percent of that is 3 inches. So you need 3 inches of clearance from the tip of the blade to the underneath of the hull and to the skeg. It is not as important to have that clearance to the skeg. It can run closer, but the clearance from the tip of the prop to the bottom of the hull is very important. Propellers and air bubbles don’t get along well. The propeller loses everytime. Cavitation is the impingement of air bubbles caused by a low pressure area on the propeller blade and will actually eat away the metal. Appendages hanging down on the hull will create a stream of interrupted water or air bubbles if you will and all of those things create interruptions and induce cavitation in the propeller.”
Balance! “Prop balance is simply making sure all the blades weigh the same within a few grams,” said Mark. “One could imagine that if there is a heavy spot on a prop, once it gets spinning it would set up quite a vibration. It’s the same reason a tire is balanced.”
What is slippage? “I have done it for just a few boats this year,” explained Mark, “that seemed to be incredibly slippery for their horsepower and the numbers were quite amazing. Again, let’s go to a 30 inch pitch, it ought to be going ahead 30 inches for every revolution of the prop. You calculate however many revolutions that prop is turned when the boat has gone a mile. Well say it was only going five percent less than that speed, well there is your five percent slippage. Twenty to fifteen percent slippage, is pretty average for a heavy well-equipped boat. This rush to huge power, I get it, you are a working guy going I don’t know how many miles offshore, every knot makes a difference in how much time you spend going back and forth. It is kind of interesting when you compare your typical 42 foot lobster boat with 750-hp if everything is all balanced right and everything is nice, she will do 25-26 knots, maybe a little better. Now, spend another $100,000 on an 1,000-hp engine and it will do 28½-29 knots. The engine is a lot bigger, a lot heavier; the shaft is bigger and heavier; everything is bigger and heavier. A lot of time the gains aren’t as drastic as some think. You can put a humungous engine in your boat, but you are not going to be able to get enough propeller under it to take advantage of it. I have seen a shift, ‘we are building this boat, what do we need, what would work best for this gear so we get the right sized prop. Run some comparisons in this engine and that engine with this gear ratio, that gear ratio. See what is going to be the most efficient and that is great. It is a simple operation to at least get in the ballpark. Then we can fine tune the prop size later. I encourage people to do that.”
Many years ago when you took a propeller in to be reconditioned it was done with pitch blocks and a rubber mallet, but that is not how they are done today. “I can’t remember when we built the first hydraulic press,” said Mark. “What has changed is how we work on them today. The advent of the computerized measuring systems, the MRI, I think the original one came out of Australia. That allows us to accurately measure and map out on a computer screen so you can see in dimensional form the dimensions of where the various sections if the propeller blade are in relation to where they are supposed to be. So it gives you an extremely accurate map so that you then can manipulate that blade to within a few thousandths of an inch of where it is supposed to be. It is kind of a blend of computerized wizardry, but there is still some blacksmithing involved. You push it this way and you push it that way and you are eyeballing it, maybe you are taking a quick measurement to see if you are close. Then you put it back on the computer to measure it and it tells you if you are close or not. That has been a massive change and a huge difference in performance. Let’s say it’s a 4 blade, you can have all 4 blades, for all intents and purposes, exactly the same. So every blade is taking the exact same bite of water on every revolution. Vibration comes from propeller blades that are not the same. One blade is trying to push your boat ahead 34½ inches and the other blade is trying to push your boat ahead 33 inches, which is more common than you think. Computers have allowed prop shops to really super tune props. Much more efficient, more fuel efficient, better speed, smooth, quiet, all of those things.”
“Now,” continued Mark, “speaking of computerization, the shift you are starting to see is a shift in propeller manufacturing. They are still casting. They are poured into a mold and then finished, but it used to be you poured it into a mould and that mould was made the old fashioned way. Now, a lot more of them are still cast in a mould but the mould has been made on a five or six axis C&C machine. That mould itself is dead-accurate. It still has to be finished on the outside and then the propeller goes into another C&C machine and every surface of the blade, front, back, edges, bore, hub is finished to exact specs. The price is more or less the same as any other.”
So as fall comes to end, and you have hauled your boat up on the hard what should you do to get the prop ready for next season? “Well, physical damage of course,” said Mark. “Look for little chips and chunks or bends; red, flakey corrosion which is electrolysis and/or galvanic corrosion; cavitation issues will just have a whole series of uniform little round pits in certain areas of the prop; and check the edges to see if they are eaten away. If there is any question take it in and have it inspected. We have had them come in and there’s no real obvious damage and maybe the prop has some age on it and you start to clean it off a little bit and where the blades come down and meet the hub there are stress cracks showing up and that is a recipe for disaster. Just the fact that you lost a blade off your propeller can set up a vibration and quickly you can bend the shaft and wreck your bearings all within a matter of a few seconds.”
“Zincs, there is nothing more important to the life of your propeller than keeping a zinc on the shaft,” said Mark. “Hull zincs, transom zincs that are connected through bonding and connected to the engine and all that, those are great for your electronics and interior metal parts. Your bronze and Nibral propeller is setting on a stainless steel shaft, which is bolted to an iron engine. You have got at least three different metals in a brine of salt water. The least corrosion resistant metal, which is the bronze or the Nibral will go away. So, the propeller goes away, the shaft stays there, and I get to sell you a new $3,000 or 4,000 or maybe a$10,000 propeller every couple of years or you can put a $15 or $20 zinc on the shaft and save your propeller. The other thing that happens a lot is electrolysis. The terms get used interchangeably but they are different. Electrolysis is stray electrical current from a bad ground. That current pulls the iron molecules out of the metal and puts it out in the water. The iron in propellers are a big part of the strength of the metal. The rest of it goes if the situation continues and then all that is left is the copper, which has no strength. It is all red and rotten looking, flakey, looks like a Ritz cracker when you break off the edges. How do you fix that? You have to check and make sure there isn’t any electrical current going down your shaft and if there is you have to find out where it is coming from and that can be a bear of a problem.”
What does the future hold for the propeller? Mark said, “We have seen different blade designs over the past few years. They were initially designed for use on more higher speed pleasure boats, off-shore patrol craft, oil rig tender boats, that kind of thing. The benefit we have seen is amazing with the relatively high powered, relatively light-weight boats that are made in Maine. They have benefited drastically with these more modern propeller blade profile designs. There is a lot of engineering that you could discuss, but to put it simply, a traditional propeller is a flat faced designed propeller or constant pitch, no matter where you measure the prop, Let’s say it’s a 30 inch pitch whether you measure it right in next to the hub or all the way up next to the tip. These more modern designs, call it variable pitch. I will just say that when you measure the propeller the graph line looks like a big S-curved cup. The whole blade, not just the lip where you put a cup in. The whole thing starts out as kind of an S-curve and then turns into a big gentle, gradual cup. They grab water and are efficient like no other. Some of these lobster boats racers have gone with these types of propellers and the improvement has been quite remarkable. As far as what else is coming down the line, I hope that they are not made out of composites any time before I retire.”
“MAKE SURE you tell your prop guy the right information,” concluded Mark. “It has got to be right. You can’t say it’s a 2½:1 gear when it’s really a 2.05:1 gear, that will not work out very well. It’s always good to check with your prop guy before you plunk that money down for your engine and gear that is for sure.”
