Cavitation/Slippage???

[BigTerp]

Have just recently been having issues with what I'm guessing is cavitation/slippage as my boat starts to get on plane. Never had this issue before so trying to figure out what the problem is. The best way for me to describe it is the motor "vibrates" as it is trying to get up on plane and feels like it losses torque. Similar feeling as if your truck tires slip on a wet road. Definitely don't think its a motor issue as the motor itself is running fine. If I let off the throttle a bit it goes away and I can slowly open her up and all is good. My buddy hit a rock a few weeks ago while we were duck hunting smashing up the drain hole. We opened the drain hole back up and had to do some welding to the outside of the drain hole. I also threw some 5200 around the welds. So the area around the drain hole is definitely not in the best shape. I'm thinking that is letting bad/aerated water back to the pump? Don't have a picture right now of the area, but everything was fine with the boat/motor until after we fixed the drain hole. I have a video from yesterday of it doing this that I'll try to upload, if I can figure out my computers editing software.

This is a '94 Johnson 50/35 with about 35 hours on it after a complete rebuild last summer. It is on a Tracker Sportsman 1648. Has a new foot and 3-blade stainless impeller that have been working great up until now. Actually got up on plane much better than my old aluminum impeller did. Motor is also running close to 5500 RPM's at WOT. Was always a little over 5300 before. Have never had issues with cavitation/slippage until after we fixed the drain hole. The drain hole sits dead center of the jet foot, so if it's messed up I guess that makes sense that it would send bad water back to the pump. If that is the case, whats the best fix? Thinking of smashing the drain hole closed, welding it up and smoothing the bottom of the hull right in front of the jet foot. Been wanting to put a drain plug on either side of the transom anyway for easier access.

 

[RiverBottomOutdoors]

Smashed the drain hole right in front of the foot but didn't touch the foot? No bent grate bars?

 

[BigTerp]

Nope, foot is fine. It was hit but only got a little scratch on the toe of the foot, purely cosmetic. We weren't moving very fast. I'm actually surprised how much it smashed up the drain hole. Pretty soft, thin aluminum though. Grates are in great shape, perfectly straight. This is also a new foot, I destroyed my old one earlier this summer. The new foot is in 1,000 x better shape than my old. My old had a chunk missing from the left side toe end ever since I got it.

Isn't the increased RPM at WOT also an indication of cavitation?

 

[Lil' Blue Rude]

Yeah I'd say the dents causing it to cavitate. You can move the drain If you want. Just getting it straight again is the big part. I've heard of people using short strand fiberglass reinforced body filler with good luck on aluminum boats. just have to use a good epoxy paint over it to seal it up good. Might have to lower your motor some if you can't get it straightened out.

 

[BigTerp]

Thanks Blue!!

There isn't a dent though. We straightened out the drain hole and had to do some welding around the hole where it was leaking. The problem I'm guessing is from the extra material around the plug hole and hull bottom and uneveness in the same area from welding. I'm guessing that is sending bad water back to the foot? If I do end up welding the drain hole shut what should I be trying to do to the hull bottom there. Just smooth it up as best I can? What about smashing up that rib/chine to make the hull bottom even in front of the foot?

Here is a video of what is happening. Not the best in the world, I didn't know I was even recording, but I think you can get an idea of the issue. Starts at :40.

[youtube]uvNEiX3cq20[/youtube]

 

[RiverBottomOutdoors]

Yep....it's cavitation. I agree with BlueRude.

 

[BigTerp]

Thanks, that's what I figured. I'll try to get a picture tonight of how the bottom of my hull looks and get some opinions on what I should do. Need to figure out how to tackle straighting out and leveling off the drain hole. That will probably be easier than lowering my motor with the custom transom riser I have installed.

 

[Lil' Blue Rude]

Yeah if it's not smooth you can get some cavitation. Doesn't take much to disturb water flow. You might lay a straight edge the length of the boat to see if it's got any hook or rocker to it. That could also cause some cavitation.

 

[Ranchero50]

Hard to tell for the video but it doesn't look like there's very much back splash either.

As said above, rework the bottom or lower the engine or maybe even add a rubber gasket between the back of the hull and the foot.

 

[BigTerp]

Thanks Jaime!! Yeah, I get zero splash back into the boat. Never have.

Have been thinking of getting rid of that drain hole anyway. It's in an awkward spot (underneath my gas tank). Thinking of moving it to either side of the transom. How would you suggest I go about closing up that hole and getting the bottom in that area back in shape? I'm thinking that would be easier and more beneficial than lowering my motor.

Here are a few pictures of the back/bottom of my boat. The white stuff is 5200 I put on where it was welded.

Here is my foot. It is in excellent shape.

 

[BigTerp]

Oh, and here is what the drain hole looked like before. Not sure if I should try to get it back to that shape, or smash it up even with the rest of the hull?

This was after I imploded my foot earlier this year. You can see the drain hole in the picture though.

 

[BigTerp]

Was looking around trying to figure out how I'm going to tackle fixing this and came across some posts about intake fins. What are the pros and cons of intake fins? Would they solve my issue?

 

[Ranchero50]

That drain hole is ugly... Grind the crap off the side of the lip and redo it. Or just remove it and put one on each side of the motor.

Wings...

 

[chevyrulz]

i'm no expert but i assume the fins aid grip in turns & help holeshot?

i posted my thoughts on the cavitation issue in your other thread

 

[JoshKeller]

Before going and doing more welding on it, try sanding down that 5200/welds down smooth. Lots of jagged edges it looks like.

 

[BigTerp]

Thanks!! I plan to take a flap wheel to it to try to smooth things out as best i can. Didn't realize how ugly those welds were until they started causing me problems.

Ultimate goal is to delete that drain hole completely and rework/smooth out that part of the hull. I was getting almost 30mph and hole shot was amazing before all this drain hole business happened. So I'm hoping that will solve my problem.

 

[dhoganjr]

Was looking around trying to figure out how I'm going to tackle fixing this and came across some posts about intake fins. What are the pros and cons of intake fins? Would they solve my issue?

The intake fins would help, but you would be better off trying to fix the issue first.

If you decide to get them have them welded on. I had them on the 115, they help a little with steering providing a slight rudder effect. Also allow you to run the motor a bit higher, especially finicky ones if you don't have a good or straight bottom. And they funnel water to the intake reducing cavitation in turns and chop.

The cons were it seemed to pick up more debris; leaves, gravel, twigs, or anything close to the intake. The same funnel effect to eliminate the cavitation funneled more debris to the intake. My first set was the poly ones uhmw, destroyed those on the first trip, went over a stump and twisted them into a spiral shape and pulled through the washer and nut that held them on. The next set was the ones from Outboard Jets, the aluminum ones, they are .100 Guage maybe .125. They bend when you hit something but you can straighten them with a hammer and block while on the water. I also kept a spare set in the boat just in case. So if you use them get them welded on and made with thicker material.

Good luck, hope you get it fixed.

 

[BigTerp]

Thanks man!! Good info. on the intake fins. Ranchero50 is getting me lined up to repair the mess on the bottom of my hull. He's done some other work on my boat and is AWESOME at what he does. Plan for now is to delete the current drain hole, fix up and smooth out/level the bottom in front of the jet foot and install a new drain plug on the starboard side. Hoping that fixes my issues [-o< I might take another look at intake fins down the road, but if I can get things running like before I'll be a happy man!!!

 

[97Aero]

Sounds like you did the same thing I did over the summer. Ranchero did a fine job of beating that drain hole into submission on my boat. Lucky for me it was to the left of my tunnel. How's the water level right now? I'm in Afghanistan but will be back in 10 days to do a little fall smallie fishing.

 

[BigTerp]

Took Josh's advice and took a grinder with flap wheel to the welds around the drain hole. Got them all knocked down pretty good. I then took some 220 grit sandpaper I had lying around and hand sanded everything around the drain hole and hull bottom in front of the jet foot until it was as smooth as a babies butt. Took the boat out Saturday to do some fishing and everything was back to normal Zero cavitation and my WOT rpm's dropped back down to 5400. Going to call it good for now and see how it does this fall/winter during hunting season. Still want to eventually delete that drain hole and level out that center chine. Will probably wait until I smack up the drain hole again though.