Cavitation/Ventilation

I own a 1989 Sun Tracker 18' Bass Buggy with a 70 HP Evinrude, Love the boat. However when cruising at about 18 mph I get this loss of power I can hear cavitation/ ventilation. I don't loose speed just stops gaining speed. Sometimes this happens at 19 mph or as slow as 14 mph depending on how choppy the water is. Then when I slow down a little I can feel the outboard grab water. My max speed has been 19.8 mph then cavitation/ Ventilation happens no matter the water condition. I have tried moving all the weight to the back, all tilt positions etc.. Also this happens when I make a turn at slower speeds. You can see pictures at billbburns.com as to what I think the problem might be. There is this thingy welded to the transom assembly, looks like a piece of angle welded to the edge. Considering cutting this off to see if that helps. Should I or is there a good reason for this thingy? Any Ideas or suggestions would be greatly helpful?
OkieLakeLady

Re: Cavitation/Ventilation

Yes, I would personally cut that off, IT looks like someone added it in an attempt to "fix" the problem.

There are several reasons that cause it to cavitate at higher speeds.
#1 At that speed there is alot more turbulance caused by the wakes of the pontoons hitting each other and adding alot of air bubbles to the water that your prop is trying to grab.

#2 The pontoon is "attempting" to plane and is probibly forcing the nose down while lifting the rear which also lifts the prop higher up into the "turbulant" water

#3 your hitting the peak torque of the engine which is putting maximum "twist" on the prop so its trying to push TONS of water and the smallest amount of air in the water will cause it to aerate a.k.a. Blowout.

POSSIBLE fixes (easy to most difficult)
#1 LOWER your engine as low as it will go on the transom

#2 Dedicated pontoon prop like a "elephant ear" 14x13 or 14x11 for your engine, Possibly 4 blade if the 3 blade doesn't get the job done

#3 Underskinning will reduce "drag" caused by the crossmember thus reducing the introduction of air to the water.

#4 Add 3rd pontoon with the motor close to the back of the middle toon, this allows the engine to run in good clean water AND provides more flotation and load capacity.

Or you could just live with the cavitation as its dang near impossible to eliminate it on some pontoons.

Re: Cavitation/Ventilation

Have you considerd getting a Sting ray brand stablizer and mounting it up ? I put one on my 90 hp e tec and it took just about all the cavitation away .

Re: Cavitation/Ventilation

Thanks for information and tips, I like the idea of a third pontoon. Matter of fact I know where there is a bass buggy with careless weeds growing through it. I have a certified welder friend that willing to help. I will contact the owners and see if I can acquire the boat. For cheap I hope. In the mean time I'm going to try the Stingray hydrofoil to see if that helps. Would like to put off the add-a-toon project till winter so we can take our time. Not sure what under-skinning is could you clarify Rancherlee.
Thanks a Million
OkieLakeLady

Re: Cavitation/Ventilation

Rancherlee said:

POSSIBLE fixes (easy to most difficult)
#1 LOWER your engine as low as it will go on the transom

#2 Dedicated pontoon prop like a "elephant ear" 14x13 or 14x11 for your engine, Possibly 4 blade if the 3 blade doesn't get the job done

#3 Underskinning will reduce "drag" caused by the crossmember thus reducing the introduction of air to the water.

#4 Add 3rd pontoon with the motor close to the back of the middle toon, this allows the engine to run in good clean water AND provides more flotation and load capacity.

Or you could just live with the cavitation as its dang near impossible to eliminate it on some pontoons.

Click to expand...

Looking at your pictures before I purchased anything, I would lower that motor one bolt hole (FREEBIE) and see how it works there.

Re: Cavitation/Ventilation

Lowering the motor the last hole will give it a try. Can a outboard be too low in the water at rest and how would I know?