Props, Props and More Props
Want to know a dirty little secret about boat performance? Most of the time when a boat isn’t performing, it’s not the fault of the boat. In my experience playing around with just about every boat on the market and a host of outboard motor brands as well, when a boat and motor package are performing poorly, the blame can be traced to the propeller.
In this photo the boat is trimmed, yet is still plowing, and splitting the water forward of the console. This is an indication that more bow lift is required. After switching to a four or five blade prop we accomplished that goal.
By the time a fisherman or boating enthusiast has dumped a bank load of money into their new water toy, the last thing they want to hear is that now to make that rig run right you need to also purchase an expensive stainless steel prop. Stainless props these days start at about $300.00 and it’s pretty easy to spend upwards of $700.00 on a prop and another $100.00 on the required thrust washer and hub kit. Yikes!!
What’s worse is that unless you are clairvoyant, the ability to pick the right prop the first time is literally a shot in the dark. The marine industry is flooded with all kinds of charts designed to help fishermen and boaters pick the right prop for their rig. The problem is every boat and outboard combination is different in design, weight, width, dead rise, the hole the engine is mounted on, etc, etc, etc. It’s virtually impossible to create a prop selection chart worth the paper it is printed on.
The only way to figure out what prop will work best on a particular boat and outboard combination is to roll up your sleeves and try a few to see what works and what doesn’t. Thankfully a lot of the more experienced dealers have prop loaner programs that allow a guy to try a few before you buy.
Stainless steel props are the best choice for most fishing and boating needs. Compared to aluminum, stainless steel costs more but this cost is justified by the superior performance a stainless prop provides. Because stainless is more rigid than aluminum, under power the prop flexes less and delivers a better hole-shot, better mid-range RPM performance and also much better top end speed results.
The process of getting a boat properly propped is a matter of trying several different sizes and models to identify which one delivers the best collective set of performance goals.
Stainless props are produced in three configurations including three, four and five blade versions. Generally speaking the three blade props provide more top end speed and the four and five blade props are better at delivering mid-range performance and neck snapping hole-shots even when the boat is loaded heavy.
WHAT DO THOSE NUMBERS MEAN?
All props are rated using a set of two numbers. The first number is the diameter of the prop measured from blade tip to blade tip. The second number is the pitch of the prop and this number indicates how far forward the boat will move when the prop rotates one full turn.
Smaller in diameter props are designed for smaller outboards and larger diameter props are more suited to larger and more powerful outboards. Those “prop charts” we discussed earlier are useful in terms of selecting a prop diameter and pitch that is in the ball park for what should perform on specific size boats and horsepower ranges. Nothing more or nothing less should be gleaned from a prop chart.
The problem with these charts is they can only provide “general” information and they simply can’t guide an angler or boating enthusiast to prop perfection. That goal is only accomplished through trial and error.
MAXIMUM RPM LEVELS
Every outboard motor manufacturer publishes a maximum RPM level at which their product can be operated safely. The goal in selecting a prop is to pick one that comes close to the maximum RPM level without going over that critical line.
Of course for every rule there are exceptions to that rule. The Evinrude G2, E-Tec series of outboards feature a power curve that occurs normally between 5400 and 5600 RPM rather than 5800 to 6000 RPM. This occurs because of the exceptional torque provided by the G2 outboard. To get the best speed and performance characteristics from a G2 requires picking a prop that stays pretty close to 5600 RPM or a little higher.
TESTING REQUIRES A TACHOMETER
A lot of smaller boats do not come equipped with a Tachometer that can measure the RPM levels of the engine. If your boat doesn’t have a tach, one can be installed by a dealer.
A boat can not be propped without the help of a tachometer. This digital ICON Gauge produced by Evinrude not only provides the necessary RPM levels, but also provides a wealth of other useful information such as trim percentage and fuel economy.
Most boats feature an analog tachometer gauge at the console or near the back of the boat if we are dealing with a tiller model. Most manufacturers of outboards these days also produce digital gauges that provide critical engine information such as RPM’s, trim percentage, fuel consumption, etc. A digital gauge is a good investment for anyone serious about fishing, boating or outboard motor performance.
Testing various props is simple work, but time consuming. The first step is to make sure the prop is properly installed on the outboard. Different manufacturers of outboards and props produce different hub kits for their props and also thrust washers that ride on the motor shaft and act as a spacer or bushing between the prop and the motor housing.
Put a little grease on the prop shaft and then slide the thrust washer in place. Next put the plastic hub into the prop, flip the prop over and install the metal center of the hub kit. This metal part is designed to match up with the groves in the prop shaft, so it is critical to have the right prop hub kit for your outboard. Prop hub kits range in price from $20.00 to more than $100.00 depending on the outboard in question.
Most prop shafts are 7/8 of an inch in diameter. The Evinrude G2 is a full one inch in diameter and of course requires it’s own specially designed prop hub kit.
Once the prop with the hub kit installed is in place, the prop nut is added and tightened down using a prop wrench or torque wrench. I use a block of wood between the prop blade and the cavitation plate of the outboard to tighten down the prop nut without the prop spinning.
A prop wrench and a small chunk of wood is the best way to tighten a prop nut without fear of cutting yourself on the sharp edges of a stainless steel prop.
The final couple steps involves placing a collar over the prop nut and installing a cotter pin in the prop shaft to keep the prop nut from backing off. On some outboards a zinc sacrificial anode is installed over the prop nut. For prop testing the anode does not need to be installed.
The only way to test a prop is to lower the outboard to the lowest trim position and to quickly hit the throttle to the wide open setting. Watch the tachometer closely to make sure the engine is not exceeding the maximum RPM level.
A properly propped boat should come out of the hole quickly, getting the boat on plane in a matter of a couple seconds. If the boat nose comes up, but plows and don’t plane out quickly, the prop is likely too large and the outboard doesn’t have enough power to spin the prop at the necessary RPM’s to achieve a good hole-shot.
If the boat comes out of the hole nicely, the next step is to start trimming the outboard, monitoring the tachometer closely to be sure the engine doesn’t exceed the maximum RPM level. If the prop is too small the RPM levels will reach maximum levels long before the engine throttle is fully engaged.
If the prop is too large, when fully trimmed out the RPM’s will be well below the recommended maximum at full throttle. Ideally, the RPM’s should be about 200 to 400 RPM’s below the maximum posted by the manufacturer.
Once a first test is conducted, note the maximum RPM when the engine is fully trimmed and at full throttle. This is important information that will guide the second test run.
The pitch of a prop controls the RPM level. To change the RPM level simply replace the prop with one featuring a different pitch. Each change of pitch impacts the RPM’s by about 200 RPM’s. Using this as a guideline, it normally takes two or three prop changes to get a boat running properly.
Outfitting a fishing or recreational boat with the perfect prop is a bit of a fantasy. Like most things in life, picking a prop is an act of compromise. No prop is going to provide the best hole-shot, mid-range performance and top end speed. The reality is that the angler or boat owner must decide which compromise is best for the way he or she fishes or the conditions the boat is normally operated in.
As a fisherman, we strive to prop our boats so we have great hole-shot even when the boat is loaded with fuel, gear and also three or four man-sized adults, plus a live-well full of water and fish. The mid-range performance is also important as we fish big water and find ourselves dealing with 2-4 foot waves almost on a daily basis. A boat’s ability to handle rough water boils down to the prop’s ability to control the nose of the boat at low to mid-range RPM levels.
If the prop can’t keep the nose up in these situations, the boat will plow and ultimately the risk of sticking a wave and flooding the boat become a real possibility. Top end speed is nice, but in the big picture a boat that performs perfectly at hole-shot and mid-range is far better than having a raped ape speed demon that can’t get out of the hole or run smoothly at moderate speeds in rough water.
Getting a boat set up with the right prop is essential to enjoying a great day of fishing or boating. There are no short cuts to getting a boat propped correctly. The process simply requires testing a few different props and settling on the one that delivers the best collective set of performance goals.
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