cavitation plate height

[lewski]

I have a 14ft aluminum and need to buy an outboard. The height of the transom measures 18.5 in. What length of shaft do I need. Should the cavitation plate be equal to the bottom, can it be a littlle below, cAN IT BE A LITTLE ABOVE...THANKS

 

[Georgesalmon]

Re: cavitation plate height

motors are usually 15, 20 and 25. you need a 20" shaft motor. guessing your cav plate should be about an inch above the transom bottom, but every boat hull is a little different.

 

[jigngrub]

Re: cavitation plate height

Optimum is plus or minus 1" below or above the bottom of the hull/transom.

You can install them lower or higher with advantages and disadvantages for both.

http://media.channelblade.com/boat_...s/dts/dealerlibraries/8441/copetipstricks.pdf

Scroll down to the outboard motors.

 

[jestor68]

Re: cavitation plate height

As stated, the short answer is a 20 inch shaft motor. Ideal height is with the cavitation plate about even with the bottom.

That is easy to accomplish with the 20" by mounting it with a 1" thick spacer on top of the transom, so your transom height is now 19.5 "......perfect.

 

[H20Rat]

Re: cavitation plate height

anti-ventilation plate, not cavitation plate...

 

[Bob_VT]

Re: cavitation plate height

To properly measure your transom it is a vertical measurement and NOT along the transom itself.

Once you mount an outboard there is a critical X measurement that places the lowest plate about 1" (some higher-some lower) above the lowest line of the keel. This is where mounting and adjusting the outboard becomes important for optimum performance.

Allot of the mounting is also related to what kind of outboard and steering (tiller vs console). Do you have your eyes on a specific outboard? What is the hull rated for?

 

[jestor68]

Re: cavitation plate height

anti-ventilation plate, not cavitation plate...

A rose by any other name is still a...... Down here we call it a cavitation plate.

 

[H20Rat]

Re: cavitation plate height

to those that know what cavitation is, the anti-cavitation plate makes zero sense. Cavitation is caused by a vacuum forming over the edge of a prop, and then imploding back upon itself. Its very, very rare in an outboard prop, especially not at this horsepower level. Its also very destructive, the implosion takes a little chunk of metal with it. (cavitation happens on high horsepower, high pitch, small props. IE, PWC's have cavitation problems.) Cavitation has nothing to do with air, its more like a lack of it!

Ventilation is just that, you are getting air from the surface to the prop, hence, anti-vent plate...

Here is what cavitation damage looks like... Lots of micro pitting like the metal itself is turning to powder.

 

[jestor68]

Re: cavitation plate height

Tell it to the boys in Destin.

I see the two terms as being interchangeable; the term "cavitation plate" being the colloquial equivalent that I've seen used more often in my 55 years of involvement in boating.

In fact, it's been only recently that I've noticed the term "anti-ventilation" being used by the folks who insist on being correct.

 

[Texasmark]

Re: cavitation plate height

Problem with all the good info is that you have an inherent problem with your "tinny".....as I do with mine.

The two sheets of alum that make up the hull are joined at the center of the bottom of the boat. An escutcheon or aluminum cap is placed over the seam to protect it which also provides some lateral stability in turns ( this "keel" helps to prevent side slipping). In tight turns at high speeds, bubbles are released from the movement of this "keel" which can wind up in your "slipstream" (water going over the blades of the prop) and cause some ventilation (engine revs, speed drops) if the engine is too high.

So follow the other's comments and if you have a problem in turns that you can't tolerate, just drop the engine one notch from the height you selected, based upon their comments. Or you can leave it up (mine is 1.5" above measured vertically) and put on a SS prop that is cupped and be on your way.

HTH,
Mark

 

[26aftcab454]

Re: cavitation plate height

My 18 ft Lonestar El Dorado had a standard 20" 115hp mounted 1" high. the boat would fly out of the hole onto plane in 30 to 40 feet. the trim was only good to about 25%up. at 43mph with a 19" prop

just bought a 14 ft semi V aluminum and run a 35lbs trolling motor on it.

 

[64osby]

Re: cavitation plate height

You don't mention what you're looking for. Max HP, 2 stroke, 4 stroke. Assume it is a tiller.

If your looking at an older 2 stroke the late 50's up to the early 70's motors have a shaft length of about 18" (this is considered short)(thinking I have the years pretty close).

Sounds like one of those would fit perfectly on your 14' tinny. I have a 10hp and an 18 for my 14'er and both work fantastic and run as good as any newer motor.

 

[Texasmark]

Re: cavitation plate height

A rose by any other name is still a......View attachment 178343 Down here we call it a cavitation plate.

I posted somewhere on this site that all my life here in TX. (Gulf coast to Okie border) it was also knows as an anti-cavitation plate and wasn't until I got on here that I was corrected. Thinking about it, they were correct and I had been using the wrong terminology my entire life. Oh well, "life's a beach and then you die". Grin.

Mark

 

[Outsider]

Re: cavitation plate height

Here is what cavitation damage looks like..

Pray tell, what motor is that 'prop' from. Looks strangely like an 'impeller' ... :faint2: