Cavatation

[Sea Wolf]

Bill- I've bought a stainless steel 19" pitch three blade PowerTech prop and a stainless 15" three blade prop which I will be testing between now at the Shasta gig, so should have some more real numbers by then.

I have the disc brakes installed, but still have to tie down the lines properly and bleed the system and connect the back up lock out solenoid to the back up light feed in the factory trailer tow package plug.

We're spending a lot of time with my Mom who's in a rehab/convalesent hospital right now, so progress is not at All Ahead Full power setting on the C-Dory project helm. Joe.

[C-Bill]

Joe,

I've also got some boat projects that should have been done some time ago. Being this far (Carson City) from the ocean makes it rather difficult to get what I need. I have some incentive with October 8 being just around the corner.

Bill

[Chica]

Chica here again. The 22' angler has had the caditation problem from the first time on the water which was a relatively calm lake here in Montana. Two people in the boat with no extra gear-two fishing poles, tackle box, and a small cooler. On Vancouver Island, three adult males, and a teenager, salmon rods, and a large cooler. The comment was made that we seem to have less problem when two people where sitting on the cooler in the back of the boat. My wife and I certainly had more problem than when there were four in the boat.
My cavitation plate is 1&1/2 inches below the bottom of the boat, and the the bar that tilts the motor is in the farest forward hole. I do not know the diameter or pitch of the prop, but will talk to the Honda people about this.
I can drop the motor down one more hole, which should put the motor down another inch. Any thoughts? Chica

[TyBoo]

[C-Bill]

Chica,

I just went outside to check my cavitation plate. The bottom of my cavitation plate is 3/8ths of an inch below the bottom of the boat (plus or minus 1/16ths). And I don't run with trim tabs. My motor is a Yamaha 90 2-stroke.

I know this sounds stupid, but here goes - Is your keel inline with your lower unit?

Bill

[TyBoo]

Nope on the keel - it is actually lower than the cav plate. The measurement is taken from the lowest point of the hull bottom, not the keel. And I wouldn't consider that question stupid at all. Asked pretty much the same thing myself a few years back.

I use a straight edge held against the bottom of the hull and tilt the motor so the cav plate is in line with it. Doing that, a measurement of 3/8" low is right on the money if you are using the plate closest to the prop to measure to.

[C-Bill]

Tyboo,

My question dealt with the lower unit being right or left of the keel, in which case there would be the possibility of a bubble trail hitting the prop.

Bill

[digger]

Hi Chica.......Ron here (aka digger on Snoopy-C). Born, raised, and schooled in Butte. Now living in Spokane. Also retired. I've used Snoopy-C with the Yamaha 100 4 stroke sitting down on the transom, which placed it exactly 1 1/2 inches low (cavitation plate below the boat bottom). Snoopy ran fine that way, although the fuel consumption has been around 3 nmpg. I raised the motor 2 holes, and voila, slight cavitation, with nmpg increased to about 4. Still trying to figure whether or not I need to change it back. I don't have trim tabs on Snoopy, but did on another boat. I'd try this ---- run the trim tabs all the way up to where they are not impacting the boat, and then bring the boat to about 4800 rpm and adjust the main motor trim until you get no porpoising -- hopefully, you'll get no cavitation at this time.........at that point slightly raise or lower the main until it steers easily, and feels stable (too much down trim makes the boat plow, hard to steer, and unstable). This should probably require the main trimmed down a bit more than parallel to the boat bottom. At that point, tweek the tabs down by bumping the controls. When you feel the boat start to be impacted slightly, make sure the tabs are even and bump the main motor trim up a bit. You should get an increase in speed and rpm without the slip. If you have a gps, you can watch the speed and rpm. I've actually had a boat gain speed and with a simultaneous rpm decrease when properly trimmed. The prop is biting the water and moving the boat with a decreased prop slip.

Once you find the optimum motor tilt for the weight distribution, you can usually leave the motor alone, and control the tilt and trim using the tabs from the beginning. Sort of like controlling an airplane at that time, because the tabs really provide for sensitive control both stem to stern and side to side.

[Billy]

Guys, I just changed over to a 90hp Yamaha from a 70 Johnson but I don't think the motor change had anything to do with the reduction in cavitation I used to get. I didn't have near the problems that Chica describes but following seas and "topping" out on some of the more pitched seas got the old motor to revving. I have both a hydro plane device on my motor and trim tabs. I think the difference has been that I mounted the new motor one hole lower than the one which would put the cavitation plate about even with the bottom of the boat. I've not had the motor on long enough to attest to the negatives of increased drag but it was sure nice recently when I didn't have the problem of "over" revving in a fairly large seas. If my experience is any indicator, perhaps Chica's motor was inadvertantly mounted too high?

[Chica]

Chica here again-Went to see the Honda dealer today(about an electrial problem in the trim switch). I mentioned the problem of cavitation to him and he went outside to view the C-Dory. He quickly noticed that the two transducers that I had mounted were possible too low in the water and suggested I raise them. While the boat was at the Honda dealer I went to see a fellow who repairs props and sought his thoughts re. the prop as being my problem. Without being aware of my previous advice he asked me about my fishfinder. He also suggested I raise them up and have a test run before I change props or raise or lower the motor. I hope it is that simple. Tomorrow I will give it a test run and report findings. Thanks for all the advice, it's great having such a good "sounding" board.
Snoopy-C, nice to hear from someone from Butte-America. Chica

[Sea Wolf]

Chica- Very interesting observation on the part of your prop/serviceman!
It also points up an oversight among the possibilities list that we didn't think of, but seem a very real possibility in hindsight.

Mounting transducers too low could well provide the disturbance necessary to allow air to be sucked down to the rotating prop.

Two days ago, when visiting our pontoon boat at Shasta Lake, I started the motor (40 HP Mercury 4-stroke) to charge the battery and drive any moisture out of the engine, etc. To aid in the warming up process, I put it in gear at about 1/3 throttle (It has four 1/2" dock lines on each side in it's covered slip.)

When I added a little bit more throttle, a whirlpool like air vortex formed on the starboard side of the cavitation (ant-ventilation) plate, and made incredible sucking noises as it pulled air down to the prop and jetted it out the back.

I believe this side of the motor is more vulnerable to this phenomenon, as the right-hand prop's blades are going down on that side in forward gear and the port side blades are coming up on the other side.

What this may mean, is that it would be better to mount transducers and any other turbulence inducing hardware on the port side of the engine instead of the starboard side where the induced turbulence could make an air pocket that the prop could turn into a downdraft vortex and ventilate.

This would only apply to devices mounted with a certain distance of the prop/anti-ventilation plate, of course. Keeping them away from the engine also makes sense to limit "noise" or interference from echoes, etc.

Problems arise, however, when some of us try to mount two motors, trim tabs, a couple of transducers, a swim step, and USC's Marching Band on the transom.

I can't recall reading any admonitions not to mount any transducers or other devices near this side of the engine shaft, but maybe some of the rest of you have.

This may also help explain why devices such as the Doel Fin, Sting Ray, and others are useful in helping prevent ventilation, particularly on turns, as they physically block the induction of air downward.

I hope that raising/repositioning your transducers solves your ventilation problem! Keep us posted. Joe.

[C-Bill]

Joe,

You are very correct in your analysis. And, I've read what you reference in the instructions relating to mounting transducers. But, its not as easy as you state, because the instructions make reference to the direction of prop rotation, which can be confusing to some people. And, to make things more confusing, there are counter-rotating props on the twin outboard setups.

Bill

[Chica]

Chica here, unable to test the boat due to heavy winds and worse, low water in the reservoir with no ramps open. Stopped at the Yamaha shop on the way home and the motor guy there said I needed to drop the motor down one hole. I have a lot of options to try after I see what raising the tranducers brings. Later, Chica

[Chica]

Finally got Chica in the water with both transducers raised to just level with the bottom. This solved 95% of the problem. Only with the most unrealistic combination of trim-tilt and trimtabs could I get any cavitation at high RPMs. I never would have believed that transducers would have caused such a problem. I do think mounting transducers on the port side of the main motor would be a positive. Thank you all for your imput.
Chica