AAGGGFFFF Slow Boat!!!

[KnotConnected]

Something of an old issue that I'm bringing back up, but I can _NOT_ get more than 25 MPH while WOT out of my '94 Cabrio 280 with single 454( Bravo-3). Emptied the on-board water tank, trimmed every which way (tabs and engine) and kicked all the fat people off, but 25 is the top she'll do) It's been something I've put up with while I've fixed and worked on bigger issues on the boat for the first years I've owned it, but the difference between a 60 mile trip at a 18 MPH cruise speed and 32 MPH cruise is starting to be a bigger deal to me now.

I know the bottom at this point in the year has some garbage growing on it, and that this can have a significant performance issue, but could it account for a 10 MPH loss?

If I have the bottom professionally sandblasted and re-painted this winter, what else should I check? The engine starts right up and runs as expected (800RPM idle cold, 600RPM idle warm, ~4200 at WOT).

 

[Susquehanna Squid]

Can I assume you have tried other props/pitches?

 

[alldodge]

Sounds like your motor might be just tired. Have you done a compression test?

The boat dry weight is around 7200 lbs, so after gear fuel and such you should be in the 8 to 8500 range. The 7.4 of those years (like the one mine came with) were only 300 hp, which is not much. If you replace with a 502 or punch up the 7.4 you could get here running.

I'm assuming you have a 2.0 drive so your slip is worst then mine.

What pitch props are you turning?

 

[SeaDooSam]

I have definitely heard of garbage on the hull slowing a boat 5-10 mph, but as others said it may be a mechanical issue as well

 

[RGrew176]

It seems to me that 4200 RPM at WOT is way low. I had a 1996 Bayliner 2855 Ciera with that same 7.4 Bravo III combo turning 28 pitch props. My Bayliner according to the GPS topped out at 51 MPH at 4900 RPM. Growth on the bottom can and will really knock down your top end. I rack stored so I never had any bottom issues.

 

[shrew]

The Mercruiser 7.4, like most Merc. engines has a Mac RPM range of 4400- 4800. 4200 would be low. Growth will also impact performance. Even too many layers of bottom paint can build up and add a ton of weight over time.

What is the pitch of the BIII props?

 

[wahlejim]

I will just echo the sentiments here about WOT and prop pitch. That seems low and you may have to explore different prop options. With that being said, you have a monster of a boat with, imho, an undersized engine. With that combo, any growth a the bottom will severely impact performance. A combination of prop, WOT, and growth will definitely hinder your performance by at lest that much.

 

[redneck joe]

what they all said.

 

[Old Ironmaker]

The same issue on a old Bayliner Capri with an old 305 I/O the boys had in the shop this year. 1 cylinder compression was way down and after piece mealing the thing it was a rebuilt crate engine that was NFG to start out with. Never thought to check compression on a crate engine all shinny and fresh.

Check compression and spark while you're at it.

 

[hogbare35]

A partially stop fuel filter could contribute to this problem .

 

[Captain Caveman]

Start with a compression test as it's cheap, easy, and doesn't require a mechanic. If that checks out and you've got access to a different prop, try swapping them. Otherwise my big suspicion is that the bottom of your boat is dragging down your speed.

I know the bottom at this point in the year has some garbage growing on it, and that this can have a significant performance issue, but could it account for a 10 MPH loss?

I believe that the real culprit is the boat bottom. However, you stated that "at this point in the year" it has some garbage growing on it. That seems to imply that it was cleaner earlier in the season. If that's the case you would see a change in speed. However, if you've had stuff growing on the bottom of the boat for years (my suspicion) then you need to get that cleaned.

You can scrape it yourself and then get it painted if money is an issue.

 

[Old Ironmaker]

My boat is docked on a grimy creek and we have Zebra Mussels. My homemade cleaner for hulls is 1 part Lysol Toilet Bowl cleaner and 1 part Hydrogen Peroxide. Wet hull and apply with a chemical sprayer. Use a deck brush right away and rinse. It's amazing. It doesn't work if you let it sit. I've tried every commercial hull cleaner and this works best.

 

[wahlejim]

As a cleaning professional, I highly recommend against that combo. toilet bowl cleaners are acidic, Hydrogen peroxide is basic. They will balance each other out, which is why it doesn't work if you let it sit. My feeling is that the toilet bowl cleaner is doing the work before the peroxide balances. Toilet bowl cleaners are excellent boat hull cleaners, if you can maintain the finish on the boat because it will dull over time. Use a 2 step process instead and you will be better off. Toilet bowl cleaner first, then a peroxide mix to neutralize the acid left on the hull.