How to choose the right propeller for your new Carolina Skiff.
By Bob Reeves
Let me start out by saying that this guide will be a combination of material and links that I have found useful and my own personal experience. I am NOT a propeller expert, and welcome other's input into this FAQ.
Choosing the right prop for your skiff may be one of the most important things that you can do to improve performance and maximize efficiency and fuel economy. Unfortunately, choosing the right prop is as much an art as a science. No two skiffs are configured exactly the same, so the right prop for your buddy's skiff may not be the right prop for your skiff, even though they are the exact same model, year, and engine.
First, some definitions:
Zero trim - this is when the propeller shaft is parallel with the main portion of the bottom of your hull. As the anti-venilation plate (frequently mistakenly called the anti-cavitation plate) is parallel with the propeller shaft, an easy way to measure zero trim angle is with your rig on the trailer, use a simple level to level the boat by adjusting the tongue jack, then place the level on the anti-ventilation plate and tilt the motor until it is level. If you are lucky to have an adjustable trim gauge (I do not) then set this point for zero. Some motors will allow negative trim - tucking the bottom of the motor in further than zero degrees
What the numbers on the prop mean. If your prop is a 10 x 11, it is 10" in diameter with an 11" pitch. In theory, it will move the boat 11" for every revolution of the propeller. So if you want to go faster, just increase pitch, right? Wrong - it's a little more complicated than that as we will go into shortly.
In general the prop pitch has more effect on propeller performance than diameter.
Factory propellers are usually made of aluminum. Other materials used for the engines on our skiffs are stainless steel and composite (plastic). A stainless steel prop is often the first upgrade that a user considers. Generally, a stainless prop will have a higher effective pitch than the same pitch in an aluminum prop. The reason for this is that stainless is a much harder material, and aluminum tends to bend more under stress, effectively reducing pitch, where stainless is harder and maintains its pitch.
I personally did not consider a stainless prop for my rig because I boat in an area that contains many underwater obstructions (stumps, logs floating just below the surface, etc.) and was concerned that a stainless prop would transfer more energy to the gears and shafts in my lower unit should I come in contact with an obstruction than an aluminum prop would.
You will need a speedometer (conventional or GPS) and a tachometer to evaluate props. What we are trying to obtain is a propeller that will allow the engine to reach the high end of the manufacturer's recommended RPM range as this is where it produces its maximum horsepower. In my case, Yamaha has a recommended range of 5000-6000 rpm, so I wanted to get as close to 6000 rpm as possible. I use my rig primarily alone, so that was how I tested. It's probably a good idea to test at minimum load (passenger-wise) so that you don't wind up with a prop that over-revs your engine. Example: if you do all your testing and tuning with you and your fishing buddy aboard and subsequently run your rig at WOT with just yourself aboard there's a good chance that you will exceed the maximum recommended rpm of your engine at wide open throttle (WOT). You can obviously tune for as many aboard as is typical for your use - just keep an eye on the tachometer and don't exceed recommended rpm when you have a light load.
This is how prop pitch affects engine rpm. A higher pitch will reduce WOT rpm as it places more load on the engine, a lower pitch will increase WOT rpm as it reduces load on the engine. A good approximation of actual pitch numbers is that an increase in pitch by 1" will reduce engine rpm by approximately 200. Conversely, a decrease in pitch of 1" will increase engine rpm by 200. As an example, if you were running a 10 x 11 prop and your WOT rpm was 5500 and you were aiming for 5900 rpm at WOT, you would need to increase your rpm by 400 rpm so you would decrease prop pitch by 2" and switch to a 10 x 9 prop.
From a practical point of view it can get very expensive to continue to buy props until you find exactly the correct one, so it pays to search for a dealer who will let you 'try out' props. Such dealers do exist. Mercury as such a program with some of its dealers (2). Other individual dealers may allow you to try out props if you plan to purchase from them. Of course you will need to return the prop in new condition - no grounding or stump-thumping during your experimentation. There are also Internet dealers who will let you exchange a prop for the same model with a different pitch, usually for a fee which is much less than the purchase of another new prop. They subsequently sell these props at reduced prices as 'single use' props.
If you are experiencing prop ventilation your motor may be mounted too high. If your motor is mounted at the correct height, a cupped prop may help. A cupped prop will also help in holding better in cornering. A 4 bladed prop will do the same thing, but from my own personal experience there was too much of a performance hit with a 4 bladed prop. Your experience may be entirely different, but in general a 4 bladed prop will provide more bite, better hole shot, and better midrange performance, with a slight decrease in top end speed over the same pitch 3 blade prop. Cupped props are generally only available in stainless, but I found a company named Turning Point Propellers who manufacture a series of aluminum props which are cupped. I am presently running a 10 3/8 x 13 Turning Point Rascal prop, and have been pleased with its perfomance and handling characteristics. I can get slightly higher top speed with a 10 3/8 x 11 (the only other one they make for my engine) but opted for the higher pitch as it provides better performance and efficiency in the midrange where I operate most of the time.
Many skiffs experience porpoising at higher speeds. Sometimes this can be corrected by redistribution of weight in the boat in combination with engine trim, but this is not always practical. I carry two 6 gallon gas tanks on my J16 under the rear deck and it was not practical to move these forward. I have one battery aft and one forward, and did not consider moving the aft battery forward although that certainly would have been an option.
There are a couple other solutions to the porpoising problem. One is a wing device that bolts to the anti-ventilation plate on the motor (Doel-Fin for example). These work by lifting the transom of the boat by the motor and effectively moving the center of gravity forward. This of course puts extra strain on both the engine anti-ventilation plate and the transom itself. Another choice is trim tabs. I personally evaluated both the Doel-Fin and Smart Tabs, which are self adjusting tabs attached to the lower transom. They consist of stainless steel or composote tabs attached to gas filled shocks similar to the kind on a rear lift gate of an SUV. Nautilus makes many different models for different weight boats. In my setup, the Doel-Fin reduced both the WOT rpms and top speed significantly, although they did get the hull on plane faster and allowed me to stay on plane at lower rpms. When I replaced them with Smart Tabs, both engine rpm at WOT and top speed returned to where they were without the tabs or Doel-Fin, and I got the same faster plane and lower planing speed benefits. I highly recommend the Smart Tabs. They also helped (in combination with a cupped prop) in reducing ventilation in cornering.
1) An excellent article about propellers - terms, definitions, calculations - http://sites.mercurymarine.com/porta..._schema=PORTAL
2) Mercury demo prop program - http://sites.mercurymarine.com/porta..._schema=PORTAL
Article submission for the first ever CSO contest.