1984 Merc 75hp low top speed

[thx997303]

I've got an 84 75 up Mercury outboard on a 16 ft mirrocraft from the 60s to 70s, closed bow with a console.

The outboard seems to run alright, but this combo is going a top speed of 31 mph on flat water, with just me and fuel onboard.

I'm suspicious of a few things with this boat. Just bought it and it seems somebody did a repower to this outboard from something else.

The propellor is one of the things I'm wondering about. It's marked 25x17p

The outboard has one of those big fins that looks like a whale tail attached to it.

The outboard smokes a bit at idle, but it clears up.

What should I be looking at here? I can't believe 31 mph is all this boat and engine combo will give me.

Any help is appreciated.

 

[racerone]

Maybe it is running on 3 of 4 cylinders.----Compression test and spark test to be done first.

 

[thx997303]

Compression is 120 or so on each cylinder. I'll have to test spark tomorrow

 

[Faztbullet]

You got a tach?? figures out to about 5Krpm

 

[thx997303]

I do not have a working tach at the moment.

 

[roscoe]

The propellor is one of the things I'm wondering about. It's marked 25x17p

Double check the numbers on that prop.
There is no way a 25" diameter prop will fit that motor.

Take prop off and look on the end for prop model number and post it here.

 

[thx997303]

I'd have to take it off tomorrow, but this is a picture of the the end of the prop

 

[thx997303]

Not sure it will help, but here are some numbers from the side of the prop.

 

[thx997303]

A Google search suggests is is actually a 13 1/4 x 17 pitch prop, and it seems by some calculations, it's probably going exactly the top speed it should.

Would it make sense to go with a steeper pitch and test rpm? I really feel like this 75 hp outboard should be able to push this boat faster. I've seen similar boats with 50 hp outboards exceed 40 mph.

 

[roscoe]

13.25 x 17 makes more sense.

First thing first.
A tach, even a cheap inductive tach, something like a Tinytach will do.

Do you have a spare prop? If so, try it after you rig up a tach.

That will tell you if it is worth trying a different pitch prop, and give you an idea of what to buy.

 

[thx997303]

I do not have a spare prop. I've got an inductive tach on the way.

 

[Faztbullet]

Need to make sure all things are set up correctly on boat...also do you have trim?

 

[racerone]

Is there foam floatation in this boat ?----If so is all confirmed dry ?----Or is it possibly water logged.----Do not forget to check out the boat itself.

 

[thx997303]

The boat has electric trim. There appears to be foam flotation, at least in the bow, I'll have to check it's condition. Appears dry. The boat doesn't leak, and is stored covered, but don't know till you look I guess.

I'm going to include a pic of the mounting here. Im not sure if the height is correct or not.

Not my first boat, but it is the first one I've expected more than 15 mph out of lol

 

[jimmbo]

The Trailer needs the bunks replaced(broken) and extended so the Transom is supported. Check the hull for straightness, as I suspect there is a Hook

 

[Buckischloo]

I grew up with a 16' Caravelle with a '72 Merc 65 hp. 32 mph was about all she would do. Maybe get the boat weighed and see if it is waterlogged. The wierd prop spec is strange.

 

[thx997303]

The bunks aren't broken, but I'll be replacing them soon. I'm absolutely sure there is a hook.

 

[jimmbo]

Looking at the pic, the Bunk looks bent and broke. You will need to reengineer the Bunk Support to prevent the Newer Longer Bunk from bending.

A Hook will kill the Top Speed, makes for a Boat that stays on Plane at lower speed, but...

 

[thx997303]

It's bent because the previous owner used Trex or something like that. Lacks rigidity.

 

[thx997303]

I did notice when I had it on the water yesterday that on acceleration, the port side would drop. It sat level when not moving, and moving a bit of weight to starboard fixed it. I'm guessing that's from any hook that may be there.