TOPIC: Your Thoughts on Cavitation Plates?

Or Anti-cavitation plates. My '66 Starflite 100 came with one but after reading some scary stories about them I took it off.

I have to admit it seems like a case by case basis, some people seem to have good luck with them. You guys have any experience with them?

Cavitation plates are supposed to be a permanent part of the lower unit.Hydro-Foil stabilizers or whatever generic name are bolted on to the stern of cavitation plate as a aftermarket part.As you noted it is a case by case scenario. I personaly would try to stay away from them and figure out what the problem is without using them.Some guys have them to get on a plane quicker or to allow for a slower on plane speed,depending on boat size ,weight, use and engine HP.Many variables as to why or why not, which would be drag and a miniscule slower top end in most cases.What "scary storys" have you heard?

Found lots of stories of wallowing once the plates were added and of snapping the factory plate off the lower unit, a few stories of rolling the boat to the point where it took on water and a one story of flipping a boat at speed resulting in a fatality,

My logic was the Evinrude should know what they're doing and it's probably hard to improve on what they designed.

Stories or facts ? Hard to believe a high speed fatality,anyone that has a clue would not use one for the speed needed to flip a boat.I would imagine you would have to be hitting well over 75mph on a choppy waterway to flip a small boat.Once you hit 40 mph under those conditions your backing off the throttle.The "fin" would be way too much drag to let the boat hit that speed. Ever see a racing lower unit,aerodynamic and as small as possible $$$.
Rolling a boat to the point it took on water ? You would have to be in over 5'-6' seas in whitecaps and running on a beam reach with the boat improperly balanced to have that happen. Along with being a total newbie and never taken a boating saftey course comes to mind.Anyone with a pee for a brain hits large waves straight on and the same goes with following waves.
As far as ripping off the actuall cavitation plate.Improperly installed comes to mind as well as hitting a object. Maybe a bad casting or weld job and the resulting pressure might snap off the cavitation plate.

Yes the motor Mfgs know what there are doing,they are assuming/hoping the right size engine is put on the buyers boat.I am sure the foils do there jobs in certain situations or they would not still be selling them,although marketing is a huge factor.Just like deer whistles that every other car had on there bumper years ago, did they work or were they heavily marketed ?
IF you already know or lead to believe they are 'killers' and ruin your cavitation plate , then why ask ?

They are often used on underpowered boats, or on boats owned by people that don't know they have the wrong pitch propeller, or on boats loaded down with junk that isn't needed and just thrown in any cubbyhole to get it out of sight- usually in back, so the motor has to work harder to plane. And, yes, they weaken the factory plate by exerting many times more leverage (pressure) against it. Drilling mounting holes doesn't help strength any, either. If you think you need one, something is wrong with your setup.

Kerry wrote:

They are often used on underpowered boats, or on boats owned by people that don't know they have the wrong pitch propeller...

Thanks. My little 17' Fabuglas weighs in at about 750 pounds dry, 100 hp is largest motor they recommend for it so it's hard to imagine it would be inadequate. I'm still working out the finer points of trim but I've gotten it a lot better than when I started. I'll be moving the battery and seating forward a bit. I also need to learn more a prop pitch.

Friends used to have a 17' deep-V boat with a 66 100 HP Johnson.

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4 people in the boat and it would pull up 2 slalom skiers.

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Never needed anything like an added fin on it.

Before moving items towards the bow make sure you have the correct prop.Sounds like your way over pitched,some people do that thinking it will improve gas milage,wrong. You need to know what the maximum RPMs are for the motor and a tach, even if hooked up temporarily.This reading should be taken with a normal payload or as close as you can.You should be slightly under not over the Max RPM that it calls for.100-200 RPMs would be a acceptable number. After you do that report back with info.Many FGers will be glad to inform you of what pitch prop will optimize your ride.

Mr. 88 wrote:

... After you do that report back with info.Many FGers will be glad to inform you of what pitch prop will optimize your ride.

I have to admit having only a basic knowledge of pitch. All I can say is that it's the prop that came on the boat. Still have a layer of ice on everything here so it will be a while before I can do anything beyond hooking up the hose.

I've thought about wiring in a tach, I'm doing little repair work right now anyway so I might see if I can find a vintage looking tach/hour meter to pop in.

The CG changes are minor, mainly I'm returning the seating to its factory configuration and moving the battery up about 2' to make room for a second gas tank, I've only got 12 gallons right now.

You may have to put a post in the JEGO forum to get the RPM rating if you do not have a manual or it is not marked somewhere.The prop being part of the pkg does not equate it to being the correct one.21" prop turned through a piece of wood in theory moves forward 21", slippage and load affect that number in water.If your motor is rated for 4500RPM and you can only attain 3800 RPM with a 21" PProp then you are overworking that motor.You would go down to a 19" or 17" to get better performance out of motor and allow it to run at a efficient load range.I forget if every 1" is equal to 100 rpm, someone will chime in and give the correct number for that formula.3800 rpm x 21" = X~12= amount of feet traveled. 4500rpm x 19 [or 17] = X~12=[more] amount of feet traveled... This will show you what you gain by not overpitching. It will get you out of the hole quicker and give you better response at all speeds.

Manual calls for full throttle operating range of 4500-5500 rpm and lists several acceptable prop sizes. The current prop simply says P R 109, don't know if that refers to anything relevant or not. I'm looking for fuel economy over speed, but that sounds like a new thread.

And until I get a tach on it while under load it's pretty much speculation, sounds like.

Step # 1 is to determine the condition of the motor.

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In the past I have looked at boats and said " there is something wrong with the motor or the boat " based on the propeller that was installed.

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No point in bringing up the revs by reducing pitch if the motor is running on 3 of 4 cylinders

professor wrote:

Step # 1 is to determine the condition of the motor.

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In the past I have looked at boats and said " there is something wrong with the motor or the boat " based on the propeller that was installed.

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No point in bringing up the revs by reducing pitch if the motor is running on 3 of 4 cylinders

Very true slipped my ol mind...Compression test would be the first and easiest step before spending any money on a prop.

They eliminate porpoising, but slow you down on the top end. My local Zodiac dealer puts them on every boat he sells. Try it with and without and see what you think.

In my experience, those fins are almost always a band-aid solution to another problem. As noted above, those problems can include being underpowered, improperly propped, poor weight distribution, waterlogged hull, poor hull condition, motor too deep/shallow, motor angle incorrect, etc. But, hey, for $40.00, I can "fix" my boat, without fixing the problem!

And, yes, they can, and do break the anti-ventilation plates. Cast aluminum does not like to flex and will crack when stressed over time.

- Scott