cavitation ???

[chipben]

hi this probably sounds a stupid question to most of you but how do you know when your prop is cavitating/slipping?

im running a 15hp s/s mariner on a 3.1m (10ft) inflatable the transome is 16inches deep.
the cavitation plate is aprox 1-2 inches below the bottom of the transom, when the boats on a plane the cavitation plate is only just below the water is this right?

last question for now- what is ment by the term hole shot?
thanks in advance a newbie wanting to learn

 

[steelespike]

Re: cavitation ???

What you are refering to is ventilation.I called it cavitation for over 50 years but I guess it is wrong.
Anyway Ventilation is easy to tell.Your motor will race but the boat will probably actually lose speed.
Your motor should operate fine.You may have to adjust the motor trim for best performance.
Hole shot is the ability of a boat to get on plane.
It is usually a comprimose based on weight,hull design
horsepower,propeller selection and your performance needs.Your little boat should pop up quickly.

 

[chipben]

Re: cavitation ???

ok thanks from what you said my motor is not slipping.
I have no idea how longit should take to get on plane?
I opened the throttle slowly an gradually and it took about 10 seconds at a guess to plane. Is this good?
the prop is 9.25 x 7p.
the engine was previously used as an auxillary on a life boat.

 

[Texasmark]

Re: cavitation ???

Agree with Silver on the 50 years of being told the wrong thing about the name of the plate.

Here is what ventilation is to me (on the current play toy).

I have an alum boat with a pad (flat surface at center of boat for boat to ride on at high speed). Problem is, being alum, there is a piece of angle alum (1x1) welded in the center of the hull....like a keel.....makes a V. Purpose is to protect and reinforce the center weld but on with the story.

I run my engine up where the vent plate is above the hull. In a turn, this strip of alum introduces bubbles right in front of my engine.

If I goose it in a hard turn the rpm's go up about 200 rpms and stay there until I straighten out and run for 50 yards or so and all the air bubbles blow out. Then it settles back down to the initial rpm's.

Some props would completely blow out in this situation where the boat speed would drop significantly while the engine rpm's went out of sight but I am running a pretty HP prop and it hangs in there.
-----------------------
The good aspect of ventilation is deliberately introduced by props that are ported (holes drilled in the sides of the hubs near the front). The ports allow exhaust gasses to flow over the blades in the hole shot. This "air" allows the engine to temporarily rev up to higher rpm's which allow the engine to develop it's hp much more quickly. The time in the hole is significantly reduced. Once on plane, the bypassing water closes off the holes and the engine rpms become normal like if no holes were present.

Great for heavy arsed boats, and water skiing.

Mark

 

[steelespike]

Re: cavitation ???

To get a true indication of your hole shot you really need to open the throttle full quickly.Try it gradually until you get to know how your boat will react as you increase the throttle guicker.
Hole shot on your boat doesn't mean much because it should get up pretty quickly and your not likely to need a quick hole shot like when pulling a skier. Inflatables can be a little tail heavy with one aboard and may have a lot of bow rise out of the hole but still manage to get up quickly.