So give me a break here I am not savvy with boat motors, I just like to fish. So for my first boat I just recently got a 2006 tracker V16 laker. It has a 2014, 20hp mercury tiller motor with a 10P prop. I know 20hp isn’t much but I feel like it should be faster than the speed of smell. It’s very sluggish and doesn’t plane out very well, also it leaves a big wake behind it.  It’s transom is about 20” and the cavitation plate sits about 2” below the bottom of the boat. Is it pretty much a general rule of thumb that it should sit even with the boat? Want to get some insight before I spend money on a jack plate that I may or may not need. Any info is greatly appreciated. 

Sounds about normal for 20 horse on a 16 footer . I wouldn’t get a jack plate  

I would start by buying either a Tiny Tach for $49 or a no-name tach just like it on line for $20 and get a reading of your rpms with the throttle wide open and the boat normally loaded. It sounds like you're plowing water and might only be getting rpms up into the - here comes a guess - 4,000 to 5,000 range when you should be running 5k to 6k. 

 

It's like driving weak 4-cylinder car uphill in 5th gear. 

 

I'm basing this on my experience with a 2014 Grizzly 1648 loaded with carpet, 3 big batteries, 2 big guys, a deck extension and a Merc 9.9 long shaft 4-stroke. Brand new, straight from the dealer with the factory 8.5P prop I was only getting 4550 rpms and 5.5 mph. I bought a Merc replacement prop that was 6.5P" and the revs went up to 5800 and the speed went from 5.5 mph to 12ish. And the engine was still buried too low in the water.     Bass Pro measures the length of the transom sheet metal and not the actual vertical drop from the top of the transom to the water when they advertise transom height.

 

YMMV, but you won't know if your engine is running at the top of recommended range where the power is unless you measure it. The Tiny Tach has a little battery in it and all you do is wrap a single wire around one spark plug wire and adjust the setting for number of cylinders, etc.

One thing to note is when your talking props you have to specify the diameter and the pitch.

As noted by @Johnbt you really need a prop that gets you close the the mfg spec RPM.

You run outboards wide open throttle.

 

On 11/4/2021 at 10:06 AM, Chris Catignani said:

You run outboards wide open throttle.

Only only boats that are way under powered or slower boats would you want to do that.  In the boats I had, you would be running 70 - 80 MPH, if you ran WOT, and burn a lot of gas doing it.  Backed of 45 - 50 MPH or 4,000 - 4,500 RPM, the ride was a lot more relaxing and burnt half the fuel. 

On some boats, like my pontoon, its a total waste to run WOT.  I can run at WOT (5,500rpm) and run 19 MPH or run at 4,200 and 17 mph and burn a whole lot less gas.

59 minutes ago, Way2slow said:

Only only boats that are way under powered or slower boats would you want to do that.  In the boats I had, you would be running 70 - 80 MPH, if you ran WOT, and burn a lot of gas doing it.  Backed of 45 - 50 MPH or 4,000 - 4,500 RPM, the ride was a lot more relaxing and burnt half the fuel. 

On some boats, like my pontoon, its a total waste to run WOT.  I can run at WOT (5,500rpm) and run 19 MPH or run at 4,200 and 17 mph and burn a whole lot less gas.

All this is true...but essentially what your doing is called "over-propping"...and I'll leave it at that.

Every outboard I've ever owned (and its only 5) recommend WOT 5000 - 6000 RPM.

Wonder if a whale tail would help with plane ?

 

Said nothing about the prop, I was responding the WOT comment.  All motors should be prop'ed so they will turn the manufactures  recommend RPM with the boats normal operating load.  The motor should be able to turn that recommended RPM, that does not mean it should always be operated at that RPM, as the comment I quoted suggested.

A hydrofoil (whale tail) will improve any boats ability to get on plane.  Unless the motor is too small to plane the boat to start with.