RPMs and speed

[Triple502]

So over the winter I replaced my starboard engine in my 95 Maxum 3200 SCR. The motor is a Mercruiser 5.7 270 hp mated to a Bravo II drive with the Thunderbolt V ignition. I replaced the cap, rotor, wires and plugs. Also put new manifolds and risers as well as a new fuel pump. I went ahead and had both carbs rebuilt and I have been chasing bugs all spring and summer. I worked out a few minor issues I was having except one. It seems the motors are working harder than they did last year to keep the boat on plane. Last year I could run at about 3250 rpms and it would go about 25 mph (on the GPS). This year, the motors are turning at 3500 rpms just to keep her on plane and she is barely topping 20 mph. The bottom has a fresh coat of paint and the drives do not appear to have any growth on them. The fuel is new and I replaced all of the fuel separaters at the beginning of the year. This is a heavy boat and is probably underpowered anyway, but can't figure out the difference from last year. I did service the drives with new oil, gaskets and lubed everything. The motors seem to run fine just can't figure out why they have to work harder than they did last year. The fuel cell is 180 gallons and I have not had her over a half tank all year. Help!

 

[Scott Danforth]

more RPM and a slower speed indicates that you have either more drag, or the trim is not right, or your trim tabs are down.

 

[Grub54891]

Could the fresh bottom paint have affected the top end? was the old paint wore smooth before?

 

[Triple502]

Funny thing is, seems the boat stays on plane with the tabs down (applying about 25% on gauges). When i raise them all the way up the boat slows and struggles to stay on plane. The paint actually looked pretty good at the end of last season. I powerwashed the bottom and put a single coat of ablative paint on the bottom. Going to take it over to a beach this weekend and check the bottom with my mask and snorkel.

 

[alldodge]

seems the boat stays on plane with the tabs down (applying about 25% on gauges). When i raise them all the way up the boat slows and struggles to stay on plane.

Boat should not need tabs to stay on plane, coming up first time maybe, a little one side to the other to level, sure. You picked up some weight or your not trimming the same way

 

[Triple502]

"Boat should not need tabs to stay on plane, coming up first time maybe, a little one side to the other to level, sure. You picked up some weight or your not trimming the same way"

Exactly what I thought about the tabs, I really do not know what I am doing different from last year. I think I may have the marina haul it out and see what the bottom looks like. I know I have not added any extra weight to the boat, in fact I have been keeping the fuel level lower than normal to try and keep weight down while the new motor was getting broken in.

 

[gtfireftr]

Are you slipped in water, or do you trailer?
If in the water and you did not use her very much, I bet you have growth on the bottom....
Scrape clean and you will see a significant difference..... I just did the exact same thing..

 

[wahlejim]

Have you verified both Tachs with a shop tach?

If so, I think the next step is getting the boat weighed to check for waterlogging

 

[Triple502]

I am slipped and it's in the water. The marina cannot pull it this week so on Sunday a diver buddy of mine is going under to take a look. I have verified with a shop tach once motor was in and used same to set timing. The boat was hauled out in early November and was put up while I made the motor swap. My wife reminded me this morning that after the boat was launched (early April) it did not move from the slip. We had a crappy spring so she didn't leave the slip until June. I do remember there being barnacles all over the drives and have since been cleaned. I am thinking the bottom has a forest growing on it. Thanks for the input guys I'll let you know what I come up with. For the record, this is my last engine swap and my last boat with outdrives (too many moving parts). :mad-new:

 

[gtfireftr]

I am willing to bet you have discovered the majority of your problem.... good luck....