20:53
Faizan
When you start considering altering the performance on the MerCruiser TKS 3.0 L four-cylinder engine in your inboard-outboard boat, the simplest solutions are best. Only about 35 percent of your boat's power goes to propelling the boat. This means changing the propeller can change the way your boat's power is transferred to the water. Such a change can enhance its efficiency, change its acceleration or raise its top speed, but all are trade-offs.
Propeller Basics
The propeller that comes with the engine and outdrive when new has the largest diameter the propeller can have. The size is limited by the cavitation plate above the propeller. But the pitch of the propeller -- the distance forward that the prop pushes the boat in one full revolution -- can be changed. A pitch of "17" means the boat moves forward about 17 inches with each revolution of the prop.
Across-the-Board Performance
You know how fast you can push the MerCruiser; the maximum speed for the GMC four-cylinder lies between 4,400 and 4,800 rpm. This means when the boat's at full throttle, the engine's speed, indicated by the tachometer, should read at or near that speed. If the engine speed peaks before it reaches full throttle, you need to increase the pitch. If the engine hasn't peaked when you reach full throttle, you need to decrease the propeller's pitch. A 1-inch increase in pitch will slow the engine by about 200 rpm; a 1-inch decrease speeds up the engine by about the same amount. Choosing the propeller for the best all-around performance is a trial-and-error process. Most dealerships have propellers you can borrow to experiment with different pitch props.
A Better Hole Shot
"Hole shot" is a period of flat-out acceleration from a standing start. It requires you to reduce the pitch, though trial and error, until you arrive the rate of acceleration you want. Like all other propeller changes, there's a trade-off -- the boat will reach its maximum engine speed before you can push the throttle all the way forward or, if you push it all the way forward for the fastest acceleration, you'll need to watch the tachometer to ensure you don't exceed the maximum engine speed. It also means that, while you'll have a better hole shot, the boat's top speed and efficiency across its full operating range will drop.
Higher Top End
If you want a higher top speed, or "top end," change the propeller for one with a higher pitch. Again, this is a trade-off: You're exchanging acceleration and overall performance across the full range of throttle settings for the top speed of the boat. The greater pitch means that the motor will "lug" at low speeds, and when you reach maximum throttle, you won't reach the maximum engine speed.
What Happens to the Other 65 Percent
Only about 35 percent of the horsepower that makes it to the propeller drives the boat through the water. Of the other 65 percent, about 27 is used to fight choppy water or currents. About 18 goes to overcoming the friction between the boat and the water -- the friction component of the laminar flow. About 17 percent is used up in fighting the suction of the boat's wake, and about 3 percent overcomes wind resistance.
Propeller Basics
The propeller that comes with the engine and outdrive when new has the largest diameter the propeller can have. The size is limited by the cavitation plate above the propeller. But the pitch of the propeller -- the distance forward that the prop pushes the boat in one full revolution -- can be changed. A pitch of "17" means the boat moves forward about 17 inches with each revolution of the prop.
Across-the-Board Performance
You know how fast you can push the MerCruiser; the maximum speed for the GMC four-cylinder lies between 4,400 and 4,800 rpm. This means when the boat's at full throttle, the engine's speed, indicated by the tachometer, should read at or near that speed. If the engine speed peaks before it reaches full throttle, you need to increase the pitch. If the engine hasn't peaked when you reach full throttle, you need to decrease the propeller's pitch. A 1-inch increase in pitch will slow the engine by about 200 rpm; a 1-inch decrease speeds up the engine by about the same amount. Choosing the propeller for the best all-around performance is a trial-and-error process. Most dealerships have propellers you can borrow to experiment with different pitch props.
A Better Hole Shot
"Hole shot" is a period of flat-out acceleration from a standing start. It requires you to reduce the pitch, though trial and error, until you arrive the rate of acceleration you want. Like all other propeller changes, there's a trade-off -- the boat will reach its maximum engine speed before you can push the throttle all the way forward or, if you push it all the way forward for the fastest acceleration, you'll need to watch the tachometer to ensure you don't exceed the maximum engine speed. It also means that, while you'll have a better hole shot, the boat's top speed and efficiency across its full operating range will drop.
Higher Top End
If you want a higher top speed, or "top end," change the propeller for one with a higher pitch. Again, this is a trade-off: You're exchanging acceleration and overall performance across the full range of throttle settings for the top speed of the boat. The greater pitch means that the motor will "lug" at low speeds, and when you reach maximum throttle, you won't reach the maximum engine speed.
What Happens to the Other 65 Percent
Only about 35 percent of the horsepower that makes it to the propeller drives the boat through the water. Of the other 65 percent, about 27 is used to fight choppy water or currents. About 18 goes to overcoming the friction between the boat and the water -- the friction component of the laminar flow. About 17 percent is used up in fighting the suction of the boat's wake, and about 3 percent overcomes wind resistance.
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