Help with speed 90hp mariner 16ft boat

[If you ain't first you're last]

I have an 80's 90 hp mariner straight 6. Its mounted on a 16ft starcraft obo (deep vee) with a jack plate and tilt and trim. It has a laser 22pitch prop. I'm only getting 37-38 mph. I feel like I should be getting more. The boat is pretty light. I built it from scratch and it's all aluminum floors and framing. I can get that speed with 1 person or loaded down with 4. I just installed a rmp gauge but it's not working correctly so I'm ordering a new one. What speed should I be getting? Any help or advice would be great!

 

[Chris1956]

Top speed is hard to predict, as it is based upon power, boat weight, hull design and trim. Are you trimming it until she starts to porpoise? Is the hull true, with no hooks or rockers?

Is there spray coming off the nose at cruise speed? If so, you are plowing water, which robs speed. A 22 pitch prop may be too much for that motor to spin. The motor is running on all 6? The IL6s can allow water into cylinder 6, via crank seals, putting out the fire...

Just some ideas to investigate.

 

[DeepCMark58A]

Use the gps on your phone to get an accurate speed over ground more accurate that the speedos in a boat.

 

[If you ain't first you're last]

I've used my helix 9 and gps on my phone both ready 37-38 at wot

 

[If you ain't first you're last]

I believe it's running on all 6. I'll check. What's the best way to check look at the plugs and then spark check? Not plowing water nose picks up really quick. I trim it until I start to bounce and then I trim back again until it stops. I get some spray off the back but not a lot. Seems normal.

 

[QBhoy]

If you are trimming her up and with jack plate...man you should be pulling way more than that.
I had a fletcher 16 ft (probably heavier) and a 90hp merc (3 cylinder 2 stroke) with a black max 19”. She would pull 45/46 gps.
I would do a compression test.

 

[Chris1956]

If a spark plug is real clean, like brand new, it is likely that you have been getting water in that cylinder, steam cleaning it.

If she has good acceleration, the prop is probably a reasonable pitch. Your carbs are opening all the way? The timing is set correctly? Your compression is good on all 6?

 

[jimmbo]

A 90 HP inline6 has a WOT range of 4500 - 5000 rpm.
Why do you say your tachometer is not working correctly? Did you set the pointer on rear of it to the correct setting for your engine? If it is not working at all, the problem is not likely the tach, but the rectifier is fried.

 

[Faztbullet]

That about all you will get on a deep vee hull....to much drag