Picking the Perfect Prop by Laurie Cork

[MotherNature]

^{First Posted on UB August 21, 2004
Article Published in BassWest USA January – February Issue 2007}

Picking the perfect prop for your boat can be a challenge in today's world of ever changing hull design. Thankfully, most manufacturers have a standard prop that they suggest or that is included with their boat packages. This standard prop will be a balance of performance, stability, and top end speed, giving you a comfortable easy to drive ride. Most boat manufactures have already put quite a bit of time into testing their hull design with several different motor, jack-plate, and prop combinations; obviously they want to give you the best bang for your buck so that you will come back for more. However, each boat will ride and drive a little different based on weight of equipment and passengers that each individual owner will carry. The placement of that equipment can also affect the performance of a boat. If you just quite can't satisfy your "Need for Speed" or just feel your boat could handle rough water or cornering better, a change in props could help.

Characteristics of increased speed, handing, hole shot, and to reaching optimum engine RPMs can all be changed by changing the pitch and/or blade configuration of your prop. This is a good place to note that by improving one characteristic you might sacrifice another. Commonly when you increase speed you will lose some handling capabilities; the boat may not corner as well or you find that you have to trim way down in rough water. The goal is to find the best prop for your needs, boat configuration, and comfort zone.

Okay some basic information. Typically, a stock 3 blade prop will generate more top end revolutions per minute than a stock 4 blade of the same pitch. This is simply because there is another blade to spin. It's not that the prop is heavier but simply because there is more prop surface to propel the boat forward. When changing from a 3 blade to a 4 blade prop with the same pitch you will see a decrease of 100 to 200 revolutions per minute. Visa-versa, when changing from a 4 blade prop to a 3 blade you will see an increase in RPM. Simply increasing and decreasing RPM doesn't in itself change your boat speed. What these props do to the lift of your boat, balanced against RPM's will determine the increase or decrease in speed.

A 4 blade prop is going to "bite" the water stronger because there are 4 blades gripping instead of three; this gives your total boat more lift. Not just bow lift but you will notice that your stern has also lifted out of the water. With more boat out of the water, a person might think instant speed! However, don't forget you have lost some top end RPM because you went to the 4 blade prop. This gripping action of the extra blade also gives a boat better handling characteristics in cornering, rough water drivability, and most times steering torque will be reduced.

To get the lost RPM back you can raise the motor with a jack-plate. When you start changing motor height it is very important to constantly monitor your motors water pressure, if water pressure drops below manufacture recommendations you run the risk of overheating your motor and causing serious damage. By raising the motor, you will increase your RPM's but you will be sacrificing hole shot. This is where a balancing act comes into play. You must decide what amount of increased planning time is worth the top end speed and RPMs. If you raise the motor too high it will hit the RPM limiter during takeoff before your boat gets on plane; too low of a motor position and your planning time will be minimal but your RPM and top end speed will suffer. Motor manufacturers recommend that when running your boat at full throttle, your RPMs should be within 200 of the maximum limit. Running your motor at full throttle with too low of an RPM is putting excess load on your motor and can shorten its life.

Where to start? Before we get into talking about which prop to start with, we need to identify what exactly is "Pitch". Pitch is the theoretical distance in inches that a prop would travel if there were no slip. If you have a 25 pitch prop, for each revolution of the prop it would move forward 25 inches. A simple way to think about it is the higher the pitch the more water it grabs in each revolution, therefore it's harder for your motor to turn it and your top end revolutions per minute will be reduced.

If your current set up has your motor running at or just below the recommended RPM range, when you change from a 3 blade prop to a 4 blade prop you will want to drop the pitch of the prop to allow you to maintain recommended revolutions per minute of the motor. A good place to start is to balance the increase in prop surface with a decrease in pitch to allow you to maintain the same revolutions per minute at full throttle. Depending on hull configuration and weight distribution, expect 100 to 200 revolutions per minute increase as you decrease pitch size in the same prop. If you are using a 27 pitch 3 blade and your current RPM is acceptable, you will want to drop 1 to 2 pitch sizes when going to a 4 blade propeller so that you can maintain the RPMs. There is no stead-fast rule for this, and it becomes a trial and error process but you can get close to start.

In order for your boat to run at its optimum performance level with different props you may have to try several different set-ups to reach your goal. Each prop may perform better at different motor heights. Typically, a 4 blade prop will run at a higher elevation than a three blade. Now it starts getting really confusing! The higher you raise your motor the more RPM's you can achieve. Usually you can run a 4 blade prop at a higher motor position and this will increase the RPMs that were achieved from the original motor setting. In theory you should be able to stay with the same pitch prop when changing from a 3 blade to a 4 blade prop, if you can raise the motor to compensate for the lost RPM. However theoretically true it's not necessarily better to raise your motor. How high you raise your motor can affect many things from top end speed, to RPM, to handling and your hole shot time. It becomes a delicate balance between all the characteristics and what you as a boat owner are looking for.

Bottom line, a 4 blade prop will increase your boats stability! In experimenting with different pitch and motor height combinations you may be able to improve your boats driving characteristics without sacrificing your top end speed. Fortunately, many dealers and propeller shops will allow you to use a couple different demo props to help you in your decision before you put out the big dollars.

Laurie Cork

[merc1997]

some good info here, but there is a lot more to it.  your blade length and rake are very big factors on both the 3 and 4 blades props.  how much cupping is in either one and where that cupping is placed.

typically with the correct 4 blade you will run slower on a straight line compared to the correct 3 blade, but when running boat traffic and such and actually be faster over a 4 or 5 mile distance.  this is because the 4 blade will not drop the nose in corners and lose speed and most 3 blades will do.

some hulls will do better with a 3 blade while others seem to do their best with a 4.

having the right prop and motor height combination and make all the difference in the world how a hull will ride and drive.  it can turn a rough riding hull into a smooth dry ride.

hydraulic jack plates are worth the money if you can afford one.  you can dial it in to the just right spot with changing loads and lake conditions.

thanks for bringing the subject back up laurie.

bo