Ventilation cavertation

stul · 19 June 2007, 07:02

Hi
Not sure of difference ,what are the best ways to get over engine race, with little drive to push Rib forward.
Going to put on smart tabs on 4metre Narwhal rib with 40 hp mercury outboard
will get some pics to show set up
Stuart
ps my last engine did this but not so bad

Simon B · 19 June 2007, 07:53

VENTILATION: Gas either exhaust or atmospheric air cAuses prop to lose grip usually only happens at surface or if engine is trimmed out to far.

CAVITATION: Prop "over revs" causes a vacuum on one side of the blade, vapour bubbles form then collapse. Effectively you are bopiling the water at sea temperature. Can occur at any depth i.e. a submarine at 200m.

Check your pitch is about right then look at the trim set up and engine height.

PeteSmith · 19 June 2007, 08:53

Sorry this is my bug bear

, Ive heard so many people bank sharply in a rib to the point that the engine ventilates but peole say "Oh the engines cavitating"

Ive never seen a RIB Propeller cavitate (if you did you would see cavitation burn on the blades - pitting)

Ventilation is where air is mainly drawn in from the surface this is mainly due to suppliers putting short shaft engines on a rib that requires a long shaft, hence you often see that the transom (the strongest bit) has been cut down to take the engine. Its how to bodge!

Directly ablove the propeller is a horizontal flat plate this is the AVP plate "Anti Ventilation Plate" designed to try and prevent air been sucked directly down into the rotating prop. However is doesnt work very well when the rib is banking. How to solve it - Make sure the supplier has sold you the correct long shaft engine!!!

The following is taken from the Power Boat Training website
This article is from ABOUT.com

Propellers - The Basics: Part 2: Ventilation, Cavitation, and more...

Propeller Ventilation

Propeller ventilation is caused by air from the surface or exhaust gases being drawn into the rotating propeller blades. This results in the propeller slipping more than usual due to the reduced water load on the blades. The obvious symptoms of this are a sudden increase in engine RPM as well as a possible loss of speed. This commonly occurs when trying to turn the boat at high planing speeds or if the outboard or sterndrive is trimmed out too high.

In racing conditions this can also occur when following another boat too closely. The small bubbles in the water created by the leading boat can cause ventilation of the propeller of the following boat, with a subsequent loss of speed. This is why you rarely see high speed race boats following directly in the line of the leading boats, even if they are far enough back to be out of the spray and wake.

Propeller Cavitation

Propeller cavitation is less obvious than ventilation, but can be far more damaging. When the propeller blade passes through the water at an increasing speed, the pressure that holds the water to the sides of the blades is lowered. If the water is sufficiently warm, and depending on the speed of the boat, formation of water vapour (boiling) can occur. These bubbles that are produced then immediately collapse, releasing energy that can cause a cavitation burn on the propeller blades. This is one of the great advantages of stainless steel propellers. Due to their superior strength they can withstand cavitation damage better than aluminium and can also be produced with thinner blades to reduce the occurrence of cavitation.

Pete
><(((º>

brucehawsker · 19 June 2007, 12:28

Great reply, Pete. Makes a huge difference to have an expert explain this stuff

Simon B · 19 June 2007, 12:57

Well at least I had a go!

codprawn · 19 June 2007, 14:06

Quote:

Originally Posted by PeteSmith

Sorry this is my bug bear

, Ive heard so many people bank sharply in a rib to the point that the engine ventilates but peole say "Oh the engines cavitating"

Ive never seen a RIB Propeller cavitate (if you did you would see cavitation burn on the blades - pitting)

><(((º>

What about RIBs with surface drives or supercavitating props - there are a few about!!!

prairie tuber · 19 June 2007, 14:16

Quote:

Originally Posted by PeteSmith

Ventilation is where air is mainly drawn in from the surface this is mainly due to suppliers putting short shaft engines on a rib that requires a long shaft, hence you often see that the transom (the strongest bit) has been cut down to take the engine. Its how to bodge!

><(((º>

Thanks for the info Pete. I have a question, hopefully it doesn't sound too dumb.

I've got a 4.7m SIB on order with a very thick (2") marine ply transom that the factory usually sets up for a long (20") shaft, although they offer the option of having a V notch in the center of the transom so that a short shaft can be used (which is what I opted for). My rationale is that the V notch should not have any apppreciable effect on the transom's overall strength, but with the the outboard sitting a little lower it should greatly reduce the amount of torque on the transom-sponson interface (ie. less mechanical advantage applied by the engine's torque). Does that make any sense?

jyasaki · 19 June 2007, 14:43

Pete:

If you want to see cavitation, try running a too-small pitched prop and loosening your prop nut. One of these factors did it to me.

The burns are on an aluminum prop, about an inch from the blade tips, and are about an inch and a half long or so, on the back of the blade (as it sits on the boat.) They are reasonably deep when you consider they were caused by bubbles (haven't measured, but they are definitely into the metal.)

The loose prop nut thing came about from the lack of a spacer washer. The too-small pitch was from trying to select the correct prop (that one wasn't it.)

jky

nikster · 20 June 2007, 11:31

[QUOTE=prairie tuber;205894] Thanks for the info Pete. I have a question, hopefully it doesn't sound too dumb.

I've got a 4.7m SIB on order with a very thick (2") marine ply transom that the factory usually sets up for a long (20") shaft, although they offer the option of having a V notch in the center of the transom so that a short shaft can be used (which is what I opted for).

Me thinks your gonna regret that decision....!!!

prairie tuber · 20 June 2007, 13:36

[QUOTE=nikster;206014]

Quote:

Originally Posted by prairie tuber

Thanks for the info Pete. I have a question, hopefully it doesn't sound too dumb.

I've got a 4.7m SIB on order with a very thick (2") marine ply transom that the factory usually sets up for a long (20") shaft, although they offer the option of having a V notch in the center of the transom so that a short shaft can be used (which is what I opted for).

Me thinks your gonna regret that decision....!!!

Please explain.