Cavitation By Design - Gimmick or Not

[rcarduino]

Hi,

I have a stainless steel cavitating prop on my 19ft bowrider. The prop has holes ahead of each of the three blades to allow exhaust gas to escape the hub onto the blade surface where I am told it lubricates the blade allowing it to cavitate while the engine spins up into its power band giving a skier a better start.

The engine hits is max RPM when the boat is lightly loaded and gets close enough with an increased load, hole shot is good and top speed is where it should be, however, on the few occasions that I used the boat offshore in a swell, the lack of grip in the swell has been really apparent, I can't see the grip issue being any different when towing.a skier, so is it a gimmick, could I simply get a prop shop to weld the holes closed or would I loose something by doing so ?

Thanks

Duane

 

[Frank Acampora]

Re: Cavitation By Design - Gimmick or Not

You do not need to weld the holes closed to test the prop. Three simple plugs will do. If the holes are stock--that is, drilled by the manufacturer, they will offer various sized plugs to fit in and either close them completely or reduce the size.

You have been described almost the correct operation of the vents: A vented prop will allow exhaust gas to escape the holes during low speed operation. This aerates the water, allowing controlled slip to the prop due to VENTILATION, not cavitation. The engine is lightly loaded and can spin up to its maximum torque RPM, allowing faster acceleration. At higher speeds, water flowing over the gearcase and prop hub effectively closes these holes, allowing normal prop grip.

Cavitation is harmful to the prop. Ventilation CAN lead to cavitation (which is where the confusion is) but these vented props are designed to avoid this.

 

[steelespike]

Re: Cavitation By Design - Gimmick or Not

When operating off shore you may get better results if you trim down.
It would seem that off shore operation may put you right in the rpm range that vents operate in.

 

[rcarduino]

Re: Cavitation By Design - Gimmick or Not

Hi,
Thanks both for the replies. The prop is horrible offshore, I have only taken the boat out in a significant swell once and the lack of grip was alarming. Holding the back of or pushing through a swell seems to be exactly in the sweet spot for slip - not very enjoyable to pilot and not too safe either.

Great to know that I have the option of plugging the vents for offshore trips.

Thanks

Duane

 

[Outsider]

Re: Cavitation By Design - Gimmick or Not

The holes allow exhaust gas to flow out and over the blades, there is more 'slip', and the RPM increases quicker. As the RPM rises, less and less exhaust escapes until the prop effect returns to normal. Cavitation is a boiling of water on the low pressure side of the blades, not exactly the same thing ...

 

[jestor68]

Re: Cavitation By Design - Gimmick or Not

Mercury's PVS system offers 4 different size plugs(small, medium, large and solid) for their SS props that use the system. As far as I know, their's is the only such system, allowing the user to "tune" the amount of slip produced.

Most come with the medium plug installed. If that one gives you too much ventilation, try the small plug; or better yet the solid plug to eliminate the system that's not designed to be used on that type of boat.

 

[Fun Times]

Re: Cavitation By Design - Gimmick or Not

What model propeller do you have?

 

[jimbo_jwc]

Re: Cavitation By Design - Gimmick or Not

I bought one that has square holes and wondered about if they sold different plugs for it . I was able to find the hub kit . There is no branding on the one that I found . These are still available so must work on some applications . Prop Size;14" X 19" RH, 3 Blade Stainless Steel Lightspeed QS5070X

Their description Quick Silver Lightspeed

3-blade stainless propeller with high-rake design gives lots of bow lift
Vented for positive acceleration
Great for lighter weight boats with lots of power


Found this topic on this site for further info http://forums.iboats.com/prop-quest...aser-ii-blocked-ventilation-holes-512648.html

 

[rcarduino]

Re: Cavitation By Design - Gimmick or Not

What model propeller do you have?

Its a Stilleto that the previous owner put on the boat, its doing everything its designed to, the hole shot is great, WOT is right where it should be 45Mph+- on a 19ft bowrider with 1999 135 Johnson.

As the consensus is the vents are effectively closed at sports speeds, I will leave as is - the boat is low to the water so the prop isn't the only reason not to go offshore regularly and under inshore use the prop is perfectly well behaved..

Thanks all for the informative replies.

Duane.

 

[jestor68]

Re: Cavitation By Design - Gimmick or Not

Plugs for the older style square holes are part # 19220A4.

Google Quicksilver Vent Plugs and find venders that offer them.

 

[jimbo_jwc]

Re: Cavitation By Design - Gimmick or Not

Thanks bought 4 blade Solara's ? 17p for grip and big load old nicked 3 blade for spare and 3blade14x19 stainless vented to play with because I exceed WOT on 130hp 3.0 L with 14.25 x17 original .

 

[Texasmark]

Re: Cavitation By Design - Gimmick or Not

My experience with deliberately porting my non ported SS prop to get hole shot and fast acceleration plus max out mph with high pitch, is that porting is not made for rough water. Sucking the throttle in somewhat and trimming down will help, but if you are in really big water you want more blades and no holes.

The idea of plugging and not to suit the occasion sounds great. Only caveat I see is that you may want more thrust and less pitch to handle the loads placed on the engine in high seas.....aka a different prop as mentioned.

Personal preference is a prop for the condition whereby you set the throttle and the boat adjusts accordingly. In my opinion, when you are constantly on the throttle to take up the slack and then cut her back, you are under thrusted and over pitched.

I have 4 props for my boat!

Mark

 

[oldman570]

Re: Cavitation By Design - Gimmick or Not

I have had the same trouble as Rcarduino and went thru several test of vented and standard props with different pitch's, before I found a Piranha composite prop that fit the bill of easy change of pitch with a lifetime hub center warranty. It dose not have any way for the hub to slip/spin on the prop shaft, and I can replace any one blade or more at any time. The blades will shear of if they hit something of substantial resistance, and not damage the motor. I carry extra blades for it, of different pitch, on the boat and change them as needed when I do my boat check before leaving the house. The cost of replacing blades is a lot cheaper that rebuilding a prop. Changing blades can be done while on the water as soon as I have hit something. I have yet to have to replace one, and have hit my share of wing dams, logs, stumps, and rock bottom over the last 10 years. You can do a search on the web, and see the test of the props against steel and aluminum props, and all the info on them if you wish. JMO
Oldman570