Low top speed, 454 Merc

[Allout123]

Hey all! I bought a 25' 1991 Century 5500 CLX with a 454 7.4L Mercruiser and bravo 1 outdrive. I had it out for the first time this weekend but I'm having issues with top speed. The boat planes out and runs great, at WOT I'm around 4000 RPM's.

It has a 4 blade prop (not sure on the pitch, I can check if it's labeled) and I'm seeing around 38-40 MPH top speed.

I know this is an unaturally heavy boat, but with all that power shouldn't I be seeing some more speed? If anyone can point me in the direction of things to check (what should my compression be at, etc.) I would appreciate it. Thanks!

 

[tpenfield]

See if you can find some tests or specs on what the top speed is supposed to be. If the speed is in fact low (which it seems to be a tad low) then it could be the engine . . . it could be the 4-blade prop . . . or it could be the boat, given its age. . . . water retention and all the joy that comes with it.

 

[Stargazer1967]

The pitch of the prop is usually stamped on the outside of hub. Not sure of the rpm range of that motor but 4000 rpms seems a little low. If you are over-pitched, It could cause this condition. I also agree with tpenfield as to other causes.

 

[Bondo]

Ayuh,.... Welcome Aboard,.... You should be seein' 4400 to 4800 rpms, Wot, trimmed clean, 'n Runnin',...

Unfortunately, it Could be alota things, simple like water in the fuel filter, or bad things like a tired motor, or just too much prop,...
A diagnostic Tune up usually leads to the cause,....

 

[Scott Danforth]

Something is wrong. A big block in a 25' boat should be hitting 4800 rpm and over 50mph.

That hull is almost exactly the same as my cruisers hull which would hit 48mph with a small block. Although the centry is a couple hundred pounds heavier, you should still be seeing much better performance.

Do a compression check
Check the quality of fuel in the fuel filter
Make sure the exhaust flappers havent dropped and blocked your exhaust

 

[Allout123]

Sorry for the delay guys and thanks for the input. I made a mistake I didn't catch when I wrote this, I actually have a 3-blade prop, not a 4. It's a quicksilver mirage stamped 48-13702-A41-21P which I assume is a 21" pitch.

With some help and explaining from a friend about getting a smoother/cleaner trim the boat seems to like 4400 RPM's before I raise it farther and lose speed. Still, this only got me to about 43-44MPH top.

I swapped the fuel filter and the fuel is brand new high octane non ethanol with some sea foam in it, plugs are brand new and gapped correctly, and it runs clean and smooth so I didn't really think of carb issues.

I was going to test compression next and also was curious about the exhaust flappers Scott was talking about but wasn't sure how to check them.

Thanks guys!

 

[alldodge]

With 21 pitch, 1.5 ratio drive, 44 mph and 4400 rpm you have a slip of 24.5% which is bad.

Don't see your compression being the issue. If the compression was weak your slip would not be as bad, but speed and rpm would be lower. I would suggest weighting the boat and it should be around 4500 plus the trailer. Also look at the bottom of the boat to make sure its clean.

 

[Allout123]

With 21 pitch, 1.5 ratio drive, 44 mph and 4400 rpm you have a slip of 24.5% which is bad.

Don't see your compression being the issue. If the compression was weak your slip would not be as bad, but speed and rpm would be lower. I would suggest weighting the boat and it should be around 4500 plus the trailer. Also look at the bottom of the boat to make sure its clean.

Thanks for the info. I did some reading on prop slip to understand it some more, and I see the correlation where in a good setup it should go down as speed/throttle increase.

I will get it weighed this week because regardless I'm curious myself. As far as the hull goes, I sanded and painted the bottom with rustoleum marine paint this month, everything looked good and should be smooth.

 

[Hoosmatroos]

FWIW, I've had the same prop on my 7,4 ltr MPI in my Sea Ray 230 BR, heavy boat but runs great. Just swapped it out with a 19". A total different boat. RPM's are going up to 4600 instead of 4200 with the 21". Didn't test the max speed yet. But previously it was around 44 miles (70 KM'/h).

 

[Scott Danforth]

the flappers are in the vertical section of the y-pipe behind the risers and the down elbows. if you over heated or ran out of cooling water, the flappers get burned up and the parts drop into the exhaust cavity of the outdrive blocking exhaust flow.

have you verified the tach readings with a shop tach, and were the speeds by GPS or the pitot tube style dash mounted speedometer?