Cavitation trouble

[Asaltweapon1]

I have a aluminum 18 ft. Crestliner with a 50 hp Honda 4 stroke. My problem is that the motor gives a cavitation noise at certain rpms, turns and rough water. The cavitation plate on motor is good with the transom. Maybe a inch high. I've changed prop size, shifting weight. Nothing!! My thoughts are jack plate or setback plate..Any suggestions!! Thanks

 

[jestor68]

Re: Cavitation trouble

Generally, ventilation is caused by the motor being mounted too high for the prop being used.

A 20" motor on a 20" transom mounted all the way down on the transom should have the AV plate about even with bottom. What hole is the motor mounted in?

What kind of prop are you running? If it's aluminum, a better SS prop should allow it to grip better.

A better description of exactly what model 18 ft Crestliner you have might help also.

 

[steelespike]

Re: Cavitation trouble

It's normal to trim down for turns and rough water.
Is it revving or just making a noise. If it growls a little occasionally That's an indication the height is right on the maximum edge.
Your boat style,prop size and the wot rpm and gps speed will help evaluate your setup.

 

[Asaltweapon1]

Re: Cavitation trouble

Ill have to check of the model of the boat. Since I bought the boat used. The noise it makes sounds like air getting sucked in and I lose some power. I cant imagine the set up is wrong since it was done by a marine place. I was told not to do ss prop because it could rip out my gears if I were to hit something. I use this boat for fishing, crabbing, and duck hunting. So it does go in shallow water. All suggestions are helpful.
Thanks Guys

 

[steelespike]

Re: Cavitation trouble

Are you sure it is a 18 ft?Is it a bow rider,or center console,side console?front deck?Utility?
All props have a cushion hub that should protect the gear case in the event of a strike.
In the event of a soft strike (mud or sand) a ss prop will hold up better.
In a hard strike the hub should work but the ss prop will need repair and would cost more than al to repair.
If you hit anything with an al prop it will in al likelihood need repair.
The initial cost of ss is about 3 times that of aluminum.
A jack plate allows fine tuning the height and does get the prop in cleaner (less disturbed)water.
It's possible you could lower it one hole and solve the problem without losing performance or
needing a jack plate.
Do you know your prop size and wot rpm and gps speed?
Is there anything on the transom or bottom of the boat that might disturb the water near the prop?

 

[jestor68]

Re: Cavitation trouble

I noticed on the Honda web site that they offer aluminum 4 blade Solas Honda props in pitches ranging from 9 to 14 inches.

Since 4 blade props tend to resist ventilation better than 3 blade props, one of those might help your situation.

By the by; since you bought the boat used, it is possible that the previous owner altered the motor height. You can usually look at the mounting holes on the motor bracket and see if more than one hole has been used.

The thing about SS props damaging lower units is pretty much an urban legend; spread rapidly thru internet forums.

 

[Texasmark]

Re: Cavitation trouble

Cupped props are really designed to eliminate ventilation, not cavitation which is the sucking in of surface air just above the propeller and the plate above the prop is aptly named "anti ventilation plate" for that reason. Cavitation is another problem of a different nature.

If you want to go fast you need to have your engine high on the transom and a prop designed to run that way. You also want to trim your lower unit out away from the boat as far as you can to help to get the bow up, reduce wetted area thus reducing drag and improving top end performance. Sometimes a jack plate is necessary to get the prop high enough and far enough behind the boat to accomplish the goal. A prop designed for such service is a must and many are out there. While you are at it, if running a 2 cycle engine, they love the rpms so run it at or above (I do, have been for 20 or so years) the recommended full throttle rpms.

The "big guns" on the site, who have apparently retired and were heavy in racing and running fast and all, said toget your "setup fixed" then get the right prop.

Mark

 

[Asaltweapon1]

Re: Cavitation trouble

STEELESPIKE- The boat is a 18ft side console. the motor is mounted at its lowest hole. Ill have to check the prop size. I run the motor around 5,800 rpms which is recommended which gives me 28-30 mph depending. There is a 1' flange across the bottom of the boat. That's the design. I bended that flange up to try an help water flow. Did not work.

JESTOR68- I already tried the 4 blade prop approach with no different results.

TEXASMARK- I the motor may be just a hair high. Marine place said it looked good to them. My concern is not with the speed. Its directed towards the noise and losing some power briefly. I'm anal when it comes to my things. I like everything to be perfect. lol

Thanks Guys I really appreciate all the help!!!
When I get home from work ill take a pic and try an post it to show you guys what im working with.
Thanks Again

 

[steelespike]

Re: Cavitation trouble

Is the flange completely clear of the water? Is the flange adjustable other than bending? Removable?
Does the motor actually gain or lose rpm with the noise?
Is the keel large and does it run all the way to the stern of the boat?
You may have to use a service to deal with the pictures.

 

[Asaltweapon1]

Re: Cavitation trouble

The flange is welded across the bottom of the boat. Honestly, I'm not sure with what the rpms do when i have the noise. What is the Keel? Im trying to load a pic.

 

[steelespike]

Re: Cavitation trouble

delete

 

[steelespike]

Re: Cavitation trouble

You will need a picture site to post a picture I've seen youtube pictures on here.
Technically a keel is the backbone of the boat but it is also the fin that runs down the center of the bottom.
It helps the boat track correctly some times a boat will have multiple keels A jon boat might have 4 or 5.
Some times as many as 3 keels may be close enough together to interfere with the prop Usually the keel will end 6 to 12" from the transom so as not to disturb the water in front of the prop.
Maybe the flange is aftermarket and may be disturbing the water.
Perhaps just the welds need to be smoothed.
Does the motor have a foil/whale tail on it? Attached to the antivent plate.
I think your probably running a 13" pitch.