2002 200 max HPDI Cavitation?

[Wormdunker]

I have a 518vx with 200 HPDI Vmax. Yamaha 25M prop. A couple weeks ago I noticed my RPMs would "race " when getting on plane at about 3000 RPMs to 3500 RPMs. Additionally, the boat has a bit of a slippery feel once out of that range, like the boat isn't tracking right. Very strange feeling just feel like I have a micro chine walk but at the same time it feels as though my prop is slipping in that RPM range.

At idle and all other rpm level the motor sounds great, runs great.

I decided that perhaps my prop wore out, I had some nicks and signs etc.... too. So I replaced it with a Tempest 25 pitch at Mark Croxtons suggestion.

Took it out today and the boat performs the same way. It gets out of the hole great but then at 3000 the rpms seem to race or at very least the engine gets louder up to 3500. Then once past 3500 it runs on plane but still has that slippery feeling.

Any thoughts on what I am experiencing?

Thanks for the help.

[Texas Larry]

Have you changed anything else, or did it just start doing this? Change in load, engine height (prop to pad) new accessories, etc?

[Wormdunker]

No changes to anything. Checked hull bottom too. Wondering if slightly loosened power steering could cause it?

Today had the overheat alarm go off. Noticed low flow from pee hole. Gonna change out water pump instead of just impeller just due to age of motor.

[Texas Larry]

This is an interesting problem! Live wells full? Mine did something like this when I had a lot of water in the bilge after a rain and I forgot to pump out after it rained on me - but it took forever to get on plane. Sounds like a load issue with too much weight in the rear on one side. I cannot see where steering or a water pump would cause a blow out issue on take off.

Are you able to trim the engine all the way down on take off?

[Wormdunker]

Ok I am happy to say I figured this out.
Symptoms were:
1. Squirrely handling in mid range RPMs (3000-3500). When I would turn the wheel and boat felt like it hooked up but then back to over revving when the wheel was straight tor turning to the left. It drove like a knuckle ball across the water the bow would waver left and right like was sliding on ice.
2. At 3000 Rpms the prop would lose grip on the water and over rev through 3500 rpms.
3. Boat had no bow lift, trim didn't affect the lift on the hull.
4. Due to over reving at 3000 rpms, after about 1/4 mile motor over heat alarm sounded, this happened twice as I was trying to troubleshoot.

My actions to remedy:
1. New prop.
2. New water pump (due to over heating) Turns out I needed this anyway.
3. Because the boat was running like a knuckle ball it felt or sounded as though I could have had a miss. I changed spark plugs.
4. Also related to surging I decided to change fuel filters.
5. While I was there I figured I would change lower unit oil. Very few particles, relatively clean.
6. Why not re-paint the lower unit? Looks brand new.
7. While in lower unit I noticed why my speedo wasn't working, Plugged pick up. now unplugged and working.
8. Multiple adjustments to my Trim tab, no impact whatsoever to performance.
9. Give the hull a thorough once over while on the trailer, but what the heck better check where the bunks contact the hull. I went for a swim. NO problems.

Ok so after all that the boat was still screwier than a Phil Niekro Knuckle ball.....
I was confused. Turns out My transom transducer had been kicked up one day when my son was out. The Humminbird kick up brakes popped up when he pulled it up on shore. But the ducer also tipped downward 2 clicks in the bracket. At the beginning of this ordeal I noticed the kicked up bracket. I also noticed the ducer was tipped back further than it should have been. I popped it back in the bracket and adjusted it ONE click. I should have gone with 2 clicks making it more level.

Turns out the air bubbles coming off that transom ducer caused ALL MY Problems. I thought I would write this up just in case someone as dumb as me ran into this problem again one day. Hopefully I can help save them about $1000 trying to fix it.

Good news, My out board is purring like it is brand new. All in all I am glad I did all the maintenance.

[Wild Heethen]

Thanks a lot. Never dreamed of that. I have same boat and engine. First I've ever heard on here.

[Wormdunker]

Pretty crazy how a few air bubbles can alter the performance of a boat so much.