1984 20 HP cavitates badly on turns

[numbersguy]

I have a 1984 20 hp mercury on my 16 ft starcraft fishing boat.

Motor starts every time and runs great. I know the boat is rated up to a 40 hp but I'm not on a huge lake and it works great for me.

I bought it used 4-5 years ago and other than tune ups and winterization I really havn't done much to the boat or motor. it does have a set of dolfin stablizers mounted on it. Do those help?

When running at 3/4 to full throttle, the slightest turn will cause cavitation. Is the motor mounted too high? My fishfinder sensor sticks down about 2 inches below the transom about 14-18 inches from the motor.

Is there a general rule for how low the motor should be mounted?

Thanks

numbersguy

 

[sschefer]

Re: 1984 20 HP cavitates badly on turns

I have a 1984 20 hp mercury on my 16 ft starcraft fishing boat.

Motor starts every time and runs great. I know the boat is rated up to a 40 hp but I'm not on a huge lake and it works great for me.

I bought it used 4-5 years ago and other than tune ups and winterization I really havn't done much to the boat or motor. it does have a set of dolfin stablizers mounted on it. Do those help?

When running at 3/4 to full throttle, the slightest turn will cause cavitation. Is the motor mounted too high? My fishfinder sensor sticks down about 2 inches below the transom about 14-18 inches from the motor.

Is there a general rule for how low the motor should be mounted?

Thanks
numbersguy

It's probably the fishfinder transducer thats fouling the prop water. Mount it even with the bottom of the hull so that when you're on plane it's skimming the water.

The motors main cavitation plate (first one above the prop) should be even with the bottom of the hull. If the engine is setback from the transom then it's roughly 1" above the bottom for every 6" of setback.

 

[numbersguy]

Re: 1984 20 HP cavitates badly on turns

Thanks for the information,

I'll make the change in my transducer this weekend and report back.

I acquired an old pontoon boat that with the motor between the pontoons too.

How low should that be?

Its become a project fixing things one after another, but I finally think I have them all licked. is that more of a water level issue?

Thanks again.

numbersguy

 

[jtsailjt]

Re: 1984 20 HP cavitates badly on turns

If you can't stop the cavitation by adjusting motor height, try a stainless prop. That can make quite a difference.

 

[sschefer]

Re: 1984 20 HP cavitates badly on turns

Thanks for the information,

I'll make the change in my transducer this weekend and report back.

I acquired an old pontoon boat that with the motor between the pontoons too.

How low should that be?

Its become a project fixing things one after another, but I finally think I have them all licked. is that more of a water level issue?

Thanks again.

numbersguy

Pontoons can be tricky. Setbacks can be designed to get the motor into clean water but that can be costly. That said, the trick is to try and find clean water (or as clean as possible) for the prop to run in. Setting it further in, if possible, may help. Until recent design changes occured, toons just weren't design to do much other then get from point a to b at a nicy leisurely, slow pace.