What to look for in a new prop?

Question

I would like to increase the top-end speed of my boat. I've heard that replacing the prop can help.

What should I look for in a new prop? What does the pitch do? Is stainless steel better than aluminum? If so, why?

Answer 1

Alot of your question depends on the hp and torque rating of your motor.  You might consider moving from a 3-blade prop to a 4-blade or a slight increase in the prop diameter or pitch.  A dealer would be the best place to take a look at your setup and give you the pros and cons of the different options.

Blade pitch is how much the blades are tilted.  Typically, more pitch "should" increase top end speed.  However the increase in pitch makes your engine work harder to get the prop to spin since it also increases drag forces on the blades.  Also high pitches can create cavitation which can lead to prop failure and engine/drivetrain damage.

Aluminum - lighter and spins up faster, but more prone to damage

Stainless Steel - more durable, but heavier

Answer 2

Props… Ok

Here are some basics; but before you read this keep this in mind. You need to talk to someone who is familiar with your engine/boat package to maximize your prop selection. Each boat performs best with it’s own setup and what works for a 18’ bass boat wont for a 24’ bow rider (as an example)

Most props are described by two numbers. An example would be a 14X19. The first number is the diameter of the prop (measurement from the center of the prop to the blade tip multiplied by two… usually in inches) The second number is the pitch of the prop. Prop pitch is the theoretical distance a prop will move froward in one revolution with zero slip. The best way to explain this is: if you placed the 14X19 prop in a tub of sand and turned it one full rotation it would move forward 19inches. Pitch is a theoretical number simply because a prop MUST have slip in order to function (Way too much physics involved to go into now). So the prop I listed above is a 14” diameter X 19 pitch

Larger + heavier boats tend to want more diameter and less pitch where lighter and faster boats are the opposite.

As far as number of blades is concerned, if all other design aspects are the same (and they never are), fewer blades =faster. More blades = more hole shot thrust and greater stability at speed( This gives you: shorter hole shot time, more stability, gets on plane +stays there at a lower throttle setting, Improved mid range speed, better low speed handling, less likely to ventilate (break loose and “spin” out) in a turn.) This may or may not be the case with your boat depending on what engine you are running and your hull's characteristics.

Material. There are two common materials aluminum and stainless steel. First things first: If you tend to hit your prop on the bottom of the lake or another hard surface, stick to aluminum. It transfers way less damage to the gears and shafts than stainless. If you want performance stainless' rigidity and It’s ability to be molded into thinner blades make it the obvious performance winner. I have seen boats gain 7 MPH by changing from and aluminum prop to a stainless of the same pitch. I have also seen them gain nothing! So once again it depends on your boat package!

When choosing a prop the # 1 goal is to make the engine run in its operating range (ask your dealer or check owners manual). When changing prop pitch adding one inch of pitch will reduce Wide-open throttle RPM by approx. 150-200. Subtracting one inch will do the opposite.

Changing from a 3 blade to a 4 will generally drop RPM 50-100 at W.O.T.

A few other terms you may want to know when you talk to your dealer:

Rake= the angle of attack the blades of the prop are mounted on the hub (center of the prop). It’s similar to pitch but not the same thing.

Cup= a rearward curling bend at the tip of the props blade. Greater cup helps with surface piercing applications.

That’s prop 101. But to get the right one for your boat YOU NEED TO TALK TO A DEALER! Otherwise you could throw a lot of money down a hole with no return.

Hope this helps! And if you have any other questions on this stuff (I know it’s a lot of info) contact me through this site.