OUTBOARD PROPELLERS FOR GENERAL BOATING USE
The majority of you readers who are currently reading this probably have absolutely none to very little knowledge about outboard propellers and their functions.
Common questions that you might have would be like;
1. What propeller is best for my boat?
2. What’s the difference between 3 and 4 blade propellers?
3. Are there specific propellers for different outboard motors?
4. Can I run two different propeller pitches on my motors?
5. What’s propeller pitch?
6. What are people talking about when they mention counter – rotation propellers?
7. What’s the difference between stainless – steel propellers and aluminum propellers?
8. Would a stainless steel propeller work with my boat and motor?
9. If my propeller is broken, can I get it repaired?
10. And, what on earth is an outboard propeller?
Well, I can assure you that by the time you have finished reading this article, you WILL have a firm understanding what an outboard propeller is and what its functions are. You will have a good understanding as to what the best propeller for your boat setup would be and you will be able to make decisions based upon your own knowledge and not some sales consultant that is only intent upon selling you any old propeller regardless whether it is correct or not.
So, let’s get to answering those questions that I’m sure you have…
1. It is always difficult to establish which propeller would be best for a boat setup as everyone has different setups. There are certain guidelines for establishing the correct propeller application. 1. We look at what the RPM of the motor at Wide Open Throttle (WOT), this rpm should be between 5200 – 5500 for a 2stroke motor and between 5200 – 6000 for a 4stroke motor. If the rpm is higher than 5500 then you have a too lower pitch prop on and if it is below 5200 then you have a too higher pitch prop on. 2. We look at the amount of people that are normally on the boat. More weight means you would need a lower pitched propeller. 3. We look at the size of the boat and the size of the motor on the boat. In general, a bigger boat would result in a lower pitched prop and a bigger motor with more Horse Power (HP) would result in a higher pitched prop. 4. Most importantly, you have to see the characteristics of the boat on the water. If the boat battles to get on the plane then you would need a lower pitched prop and if the boat revs very high and doesn’t have much speed, then you would need a higher pitched prop.
2. The difference between a 3 and 4 blade propeller is; a 4 blade propeller gives you much better out the hole performance, a better more comfortable cruising speed, better handling characteristics from the boat and a better fuel efficiency as you can have the boat at a lower cruising speed. The 3 blade is a bit better in top end speed though.
3. Some aftermarket propellers come with a multi-fit kit thus enabling the propeller to fit on most makes of outboard motors.
4. On a twin motor installation you could run two different propeller pitches but there would be a distinct performance disadvantage and it is highly recommendable to get two propellers that are of the same pitch otherwise trying to balance equal thrust becomes almost impossible to do so.
5. Propeller pitch is the angle of the blades of the propeller to the hub of the propeller. Propeller pitch can also be described as the theoretical distance a propeller will move through the water for each revolution (i.e. a 21 pitch propeller should move a boat forward 21 inches for each revolution, assuming there is no slippage).
6. A counter rotation (or left hand rotation) propeller is a propeller that has its blades fixed in the opposite direction to that of an ordinary right hand rotation propeller. These two propellers are used in unison for larger boat setups with two high horsepower (normally above a 60HP motor) motors because due to the two propellers rotating in an inwards direction, it cancels out the torque affect that would normally be caused by the bigger motors. If counter rotation propellers are not used on bigger setups, the torque affect would make the boat very difficult to steer.
7. Aluminum propellers are a lot cheaper than stainless steel and normally come standard with outboard motors. Aluminum propellers flex when they are operated at higher speeds thus causing a general loss in performance. The outboard motor must also be at a fairly low setting on the boat transom as too little prop in the water with an aluminum would cause cavitation (when the prop slips in the water because of air bubbles in the water around the prop) problems. A stainless steel propeller remains rigid in the water thus eliminating flex thus giving much better performance. A stainless steel propeller also delivers outstanding acceleration and bow lift and delivers a much better handling performance. The stainless steel also lasts a lot longer in salt or polluted water than an aluminum propeller. The downsides of a stainless steel propeller are its cost.
8. Yes, a stainless steel will work with any boat setup; a standard aluminum setup is when the cavitation plate (that fin thing that is just above the propeller) is in line with the bottom of the boat. A stainless steel setup is when you raise that cavitation plate about 4 cm above the bottom of the boat (you normally raise the motor by about 2 holes on the transom but on certain setups it can vary). The reason that a stainless steel propeller can be higher out the water is because of its non – flex ability in the water. On smaller motors (less than 50HP) a stainless steel propeller doesn’t provide as bigger performance gain as on a bigger motor. THIS excludes racing where a stainless steel prop is definitely a better option.
9. Yes, providing the damage isn’t too serious, you can get the propeller professionally repaired at certain places.
10. A propeller is the funny looking thing that spins in the water when you move the throttle leaver backwards and forwards. It is not advisable to put your hand near it when it’s spinning!
Well, that’s it. I hope you found this article interesting and that you come away with a much better understanding of outboard propellers than when you started reading.
