suzuki 140 cavitation problem

[smithcover]

recently purchased 140 suzuki to repower my 1990 grady white tournament 19.
runs great but cavititates a bit just before full plane. It loses bite - over revs for a second or two- then regains bite and all is well.
I discussed this which the dealer and he thinks that it is not the prop. he thinks that it may be mounted too high.
I dont agree. it is mounted about 3/4" higher than the old yamaha engine but i noticed that the old engine had a doelfin mounted on the cavitation plate. can anyone shed some light :
1- Should i experiment with a new propeller? speed is not an issue it goes 38 but i rarely exceed 30. would a 4 blade be better?
2- Should I install a doelfin?
thanks in advance for any insights.
smithcover

[Harper2]

Are you sure that what you experience is cavitation, and not ventilation? Your dealer may be right if the problem is ventilation and not cavitation. Are you full down on the motor trim when it occurs, or have you at that point already trimmed up a bit? Mounting too high can cause ventilation of the prop, as can trimming too high, or not trimming down in tight turns. With the motor mounted at the proper height, the cavitation plate should be pretty much even with the keel with the trim in the full down position.

Ventilation is caused primarily by the prop aerating due to turbulence in the water column (water aready partially aerated before passing through the prop plane) or actually coming partially out of the water and picking up air. Cavitation is caused by the boiling of water caused by an extreme pressure drop as the water flows over the blades, and is affected by prop design and application, e.g. cup, pitch, RPMs, diameter. In instances of extreme cavitation, you can witness burning and pitting of the blade, generally around the trailing edge.

[artdf175]

I have always heard this as cavitation as well but Mike.. technically correct again
I have owned a 21 ft North River for about 6 years
I had it into the dealer about two years ago and the salesman took one look at the boat (with the motor still tilted up in the travel position, mind you) and said your motor is mounted too high..
At his suggestion lowered the motor ..
as it turns out the top prop was catching air.. boat ran fine.. now it runs great.. huge difference in performance..
should not be a big problem to lower the unit one set of bolt holes
I think it would be a lot cheaper to do that than expend $500 for a new prop
my dealer told me he would remount it if I didn't like the way it ran.
if the dealer is telling you it is mounted too high.. a pretty cheap fix
Art

[keakar]

you are creating a problem where there is none, if you buy a new prop you cant set the correct pitch size if the motor is running too high out of the water anyway so lower the motor so the ventilation/cavitation plate is even with or slightly below the bottom of the hull (not above it) and you will find it runs great with no issues. then and only then should you think about going buy a new prop for it.

on a side note, that harper2 guy is smart for a new guy, almost as smart as that other guy named harper that used to post on here

drop your engine lower to where it needs to be and then retest, if the dealer mounted your motor for you and they stand behind their work, then he should offer to adjust the height for you for free since they mounted it wrong in the first place.

[Moonlighter]

Setting correct engine height is important for lots of reasons. Before you play with changing props and all the expense that involves, you MUST get the engine height set correctly first. Even the best prop will not perform to its optimum if the engine height is wrong. Cant stress enough that you get the height sorted as the first priority. Once that is done you can play "swap the prop".

The most common problem is engines set too deep in the water, but occasionally you get one that is too high. From what you said, your new engine is set 3/4" higher than the one it replaced, so there is a possibility that it is too high.

Whilst an experienced eye will look at motor height and make a educated assessment, really, there is only one really reliable and accurate way of checking engine height. It involves an on-water test.

Take the boat out on a good smooth piece of water. Trim in and accelerate onto the plane. Take the boat up to a good "fast cruise" speed - On a DF140 I would expect this to be about 4800- 5000rpm and trim out to the optimum trim setting that you usually use. Let it settle.

Then get someone to hold the wheel while you go down to the transom and look over the stern to see the leg of the engine. The correct height is when the anti-ventilation plate, the large plate immediately above the prop, is just skimming the surface, or getting splashed.

If its invisible due to being underwater, the engine is too deep. If its all the way out and the prop loses grip, it's too high.

Adjust height accordingly until you get it right. It is quite easy and inexpensive to do this.

This is how guys such as Ken at the propgods set engine height. I have used this method to help set up my boats and my friends boats on many occasions and it always works.

Here is a link to a thread on the Verado Club website that is regarded as a very authoritative thread that sets this out and shows photos of engines set and the right and wrong heights.

Propping correctly & how to test props.....

One other point: all props handle differently even though their pitch and diam might be the same.

You havent said whether you are using an alloy or stainless prop. Stainless props generally grip the water better than alloy. 4 bladers generally, tough not always, provide more stern lift. Some 3 bladers with more cup or with larger blade areas give more lift.

But the starting point is getting the engine height into the right zone.

[Harper]

on a side note, that harper2 guy is smart for a new guy, almost as smart as that other guy named harper that used to post on here

Harper2 ain't as smart as he thinks he is.

[WEDOCQ]

I agree with them, too high. I experimented with raising my motor on a 21' Arima, DF140 and got the same symptoms as you are reporting.
-Shawn

[noelm007]

You need to get the setup 100% before you start putting band aids on gaping wounds.....then start to look at max RPM and props and so on. It is not rocket science, the basics have to be right first though.