Understanding Your Prop
One of the most important parts for proper engine performance in
any boat is the prop. Like tires on a car it is the only thing that connects
you to the road, or water in this case.
Three main factors determine how a prop
will perform are: #1 the number of blades, #2 the diameter of the prop, #3
the pitch of the blades. The number and diameter of the blades determine
how much contact you have to the water. Like wider tires wider blades firmer grip.
How ever unlike a car there is no transmission to change gears with after you
get moving. Unlike tires the prop will always be slipping against the water.
Think of the pitch of a prop like the gears in a car. First gear, or a low pitched prop,
or a higher gear ratio. At 12 to 1 for instance the engine would rotate 12 time to
every 1 turn of the transmission’s output shaft. This gives you the power to get
moving before you shift to a higher gear. A prop pitch works in the same way.
a 12:1 pitch means that for one revolution of the prop it will screw itself ahead
12 inches. Trouble is you can’t shift gears once you get moving.
If your boat is over propped it will feel like taking off in third gear in your car and put continuous strain on your drive train wearing the upper half of the engines connecting rod bearings . You need a prop that will slip enough in the water at first to allow the engine RPM’s to get up into the power range but grabbing more and more as the boat moves faster through the water. With a properly sized prop you should feel the same smooth pull from low to top end and should just be able to reach the engines RPM red line with full throttle at the boats top speed. A higher pitched prop will give you more resistance at take off than a smaller pitched blade the same diameter because more surface of the blade is being pushed against the water. Speed props of higher pitch will be 2 bladed allowing a lot more slippage and a slower start but a higher top end speed. Like taking off in third gear, it’s tough and you have to ride the clutch to get started but your top speed is better than first gear. 
If your pulling skiers you would use a lower pitched three blade prop, better
grip and pulling power out of the hole but a slower top speed, like starting out in 1st gear
of your car. However if you undersize your prop, to small a diameter and or pitch, your engine may
be running to freely at top end causing the lower half of the engines connecting rod bearings
to wear excessively.
It all gets a bit more complicated than that but for our purpose we will keep it simple. A good example is the way I would prop my 16 foot fishing boat with a 40 horse outboard. For me a normal load would be a full live well, 2 gas tanks, 2 large batteries, two electric motors, side planning gear, and a hundred pounds or so of fishing tackle and gear. With a three bladed 10:1 pitch prop she would pull up on plane fairly fast and cruise about 38 MPH just under full throttle. Some days we would be stopping by the beach and have coolers, an extra person and would like to pull the knee-board around. I used a four bladed carbon composed 9:1 pitch blade for this. The top end was cut back to 34 miles per hour but it would pull the load up on plane with out any trouble. 
Without the extra weight the boat would jump out of the water and reach top end
fast but could over rev the engine at full throttle.
The four composite blades did not flex or slip much at full load and kept a firm grip on the water.
This gave me faster acceleration and better control when pulling skiers or boarders.
For those times when just me and the dog would go for a cruise and play knock me off the bow I had a 11:1 pitch prop with cupped edges. It took a little longer to get there but would top out at 42 miles per hour just below full throttle and the red line. I hope this helps you understand a bit more about your boats performance and prop selection, till next time The Castmaster. |