Is my merc running at full power???

[matt88]

Been working on this boat all summer and have it mechanically dependable and structurally sound but I expected more performance from the old girl. It is a 15ft imperial speedboat with a 1973 85hp merc. So far I have replaced the transom,floor,carpet,etc,etc,etc...The engine has had both carbs rebuilt,new water pump impeller and a new prop. Top speed is a mediocre 38mph. I have found the limit of which I can raise the engine on the transom so the motor is not too low. It runs and handle great but I have talked to people with similar style boats and power train and have been told on more than one occasion that my boat should run between 50-60mph. I have also checked and set the timing on the engine. It was running 5600 rpm with a 21p prop so I now have a 23p on it. Havent rechecked the rpms yet. (Only have a timing light/tach to read the revs) Only thing I find odd is that it will run 32-35mph at about half throttle and will only go 3mph faster at full throttle.(Is this normal??) The data plate on this boat reads 750lb dry weight,85 max hp,1240lb gross weight. My gut tells me this boat weighs more but that is what the plate says. I also found a small bulge in the bottom of the hull in the rear of the boat (just noticed this last weekend). It appears to be from a repair made by the previous owner. Will fix that before hitting the water again but I still dont think this would be a 10-15mph burden??

 

[jerryjerry05]

Re: Is my merc running at full power???

38mph from an 85hp is not bad.If you have good compression and spark and the tach is right.Then your about where your gonna be.
Check the cavitation plate and see where it is against the transom.
Too high or too low can make a difference.
You have trim and tilt? That can make all the difference.J

 

[captkevin]

Re: Is my merc running at full power???

With a 85 hp motor you are doing pretty good. You need to check your maximum rpms with the new prop to make sure you are not over proped. If the motor doesn't have power trim top end will suffer.

 

[Faztbullet]

Re: Is my merc running at full power???

Thats about right with that engine/hull and you can bet the ole hull has gained weight over the years from being water logged..

 

[Chris1956]

Re: Is my merc running at full power???

Do you have PTT on that motor? If not, you need to try to tilt is out more, until it wont plane, and then back off a hole.

If you have PTT trim it out, further as you go faster until the boat porpoises.

A 23 Prop will drop your RPMs to 5200 or less, which is too low. Stick with the 21"pitch


BTW - drop the max spark advance to 21*, or the motor may blow up with the lousy fuel we have today.

 

[matt88]

Re: Is my merc running at full power???

Thats about right with that engine/hull and you can bet the ole hull has gained weight over the years from being water logged..

Never thought about that but it does make sense. I dont have power tilt and trim only manual and its trimmed all the way back now. I had the engine mounted too high the first time out and when I dropped the motor about 1 inch my speed picked up from 34mph to 38mph. Thinking about dropping it a bit more just out of curiosity. Its set about 1in above the keel now but I think I could still use some more bow lift. I also think a good stainless prop might do wonders. What pitch should I look for in a stainless prop. I like where I am now with the 23p aluminum because I am in the factory specs. Someone mentioned my rpms were too low? Do these motors run better above the factory rev line? For my engine the recommended rpm range according to the manual is 4800-5200. If they make more power at 5500-6000 somebody let me know!!

I think I set my spark advance at 27. Thats what is on the engine cover anyways.