Boat performance changes?!
 

The boat:
'99 Formula 330 SS
Dual Merc 454 L29 engines (SN: 0L34xxxx) w/ about 400hrs/ea (stock AFAIK)
Bravo 3 outdrives (2:1 ratio), rebuilt by the previous owner two years ago
26P props

So last year I remember being able to hit about 4600RPM and getting about 45-48MPH @WOT on a good day; other day I could only hit roughly 4200RPM and about 40-42MPH @WOT. This year I seem to be limited to 4000RPM and 38-40MPH even on a good day?!?! Speeds are measured with my Raymarine E7D (GPS). Can anyone explain to me what all can cause problems like this? In all cases both engines performed almost exactly the same, telling me it's very unlikely to be:

• Gauge misreadings for RPM
• Engine running problems (both engines are super smooth)
• Damaged props
• Damaged outdrives
• Anything else that the engines don't share.

Here are the things I checked/tried (with no change):

• Reduce weight on boat
• Change fuel/water separators/filters
• Change air filters
• Switched off of 10% ethanol fuel (boat runs MUCH more reliably)
• Checked throttle cables (WOT brings throttle to max point before it hits a hard stop)
• Tried running Engine without drive engaged (ran it up to about 4700RPM with the drives disengaged before quickly bringing it back to idle)
• Checked props for damage - there is a little bit but it doesn't seem any worse than last year
• Double checked to make sure my props are correct - from everything I found they are
• Checked to make sure my trim tabs were retracting and were fully retracted during testing
• Tried trimming the drives to see if it would affect the speed or not - it didn't seem to change more than maybe 100RPM

Here are some ideas I have for the culprit, although I'm still very much a noob at this so I could be wrong...

• Bottom paint - I know it really needs to have this done, I'm working on it...
• Barnacles - Kinda ties into the bottom paint, I know they're there but I doubt they could make this much of a difference.
• Fuel filter(s)
• Fuel pump

If you have any insight or experience with this kind of thing, PLEASE post.

Edit: One other thing I forgot to mention - Even though the throttle is moving to the max position when I move the throttle control levers, they hit this 4000RPM wall at about 3/4 throttle - from there nothing changes as I go from 75% to 100% throttle.

barnacels might be the problem,they can really cause a lot of drag,also btm paint will really slow you down.

Any growth on the bottom will slow you significantly. Before you do anything else haul the boat and clean the bottom.

Clean the bottom.

Clean bottom. I've seen this same question with answer on another boating site geared towards big fishing boats.

Clean the bottom. I had a Sea Ray 40 EC with twin 3208 400 horse diesels that with clean props and bottom ran 32 MPH. After 6 months in the water it wouldn't make 25...send a diver down and clean with a brush ($300) it ran 28~30.

The $hit that collects on the bottom kills your performance.

As a side note...don't leave your Bravo 3's in the water ...you will be re-building them every 2 years if you do and you will eventually replace them. My partner has Volvo Duo props on his Sea ray Amber Jack and he just spent $7500 re building them after 2: years in the water....and the Volvo's are far superior to the Mercs.

Yep, Sprink is giving you some good advice. Lots of posts out on the internet about Bravo III's succumbing to galvanic corrosion when left in the water. For some reason they are much more prone to this than their Bravo I/II cousins.

Also - the hull drag issue is a no brainer. There is a lot of surface area down there that you are trying to push through the water. Get it cleaned very thoroughly and you will regain much of your lost performance. Old gas from last year could be another issue. The Merc EFI systems have a knock sensor that will retard the ignition if it is detonating due to bad gas. Get some fresh gas in there and see if that helps.

Two stainless steel props sitting next to an aluminum case..

Merc added an extra zinc under the cavitation plate and a bullet zinc on the prop shaft in the early to mid 2000's to try to help with galvanic corrosion but they still do not fair well when left in the water.

Yep, and Merc also added an addional Mercathode puck to help combat this. I understand that they now use 2 pucks per drive and they are mounted on the hull directly next to the drives. (They are tied to the drives via bonding straps on the inside).

Either way, do not leave Bravo-3's in the water.


Oh, and your problem is almost 100% bottom growth, as everyone stated. The smallest amount of slime will low you down. Barnacles are bad. Check your tabs as well. A small build-up on those will really slow you down.

Yep, and my outdrives have all the protection above (dual pucks per drive and extra zincs). I would love to haul my boat out, but it would mean buying a new truck... Since I'm not in salt or even brackish water, I'm hoping this level of protection will buy me a few years.