I am looking at a nitro 640 lx with a 50 hp motor. My question is with that hp would I be able to get on plane with 3 people in it. 

There's more to it than just how much hp.  With the right prop, a 50 should get on plane just fine.  However, it may be way too small of a prop when you are running light, with just you in the boat.  A lot of times, when running smaller motors, you need to keep a couple different props handy.  When running by yourself or with a lite load, you may need to run a 17" (just using that as a reference) but when you throw three lard**ses in it, you will have to drop down to a 15" or maybe even a 13" to be able to get up on plane.  The top speed will also drop considerably.

 

Hull design plays a huge role in how well a boat will get on plane also.  A 52" wide flat bottom will get on plane with a heavy load a whole lot easier than a 42" V-bottom. 

 

The thing you do is always try to be able to buy the max hp motor the hull is rated for, with the most CC displacement.   Which in this case, I think that boat is only rated for a 50.  However, if that's the Force 50, don't look for much hole shot.  Those are antique technology motors and don't get on plane very well at all.

 

The reason for paying attention to the CC size, it takes torque to get on plane, and it takes CC's to make torque.   One brand motor might get a little more aggressive with their porting and sell a smaller 600 CC as a 50 hp, where another might detune one of their larger motors and sell a 800CC as a 50hp.  The 800 would get you up and going a whole lot easier than a 600.

 

One other thing, understand while stainless props cost more, they will get you up on plane a whole lot better than the same pitch aluminum.  So, if concerned about getting on plane, get stainless props.

I don't think you would ever get on plane with 3 people in that setup with any prop.

Right prop, proper tilt/trim setting and proper weight displacement will get you on plane. Been there done that. Put the fat guy up front. Lol.

If fishing three people is the norm, why not look at a bigger boat?

Right prop, proper tilt/trim setting and proper weight displacement will get you on plane. Been there done that. Put the fat guy up front. Lol.

If fishing three people is the norm, why not look at a bigger boat?

 

Thats what I thought is one person would have to sit towards the front to get on plane but it would be possible. I would say about 50% of the time we have 3 people so its not the norm but it still happens enough to where i wanted to make sure it could get on plane. I would like to get a bigger boat but was hoping a smaller boat would do until i can save up the money for a bigger boat. 

I had a 2001 640LX with a 75 HP motor on the back. I have had around 650 lbs. of people in the boat and it didn't seem terrible getting on plane. I'm not sure how a 50 HP would do, but the right prop and set up would be very important. Btw the max HP on my boat was 90 HP.

That might be struggling with a 50 on it. Make sure you try it out first before you buy and bring a friend with for your test drive. That way you'll see how it performs with 2/3's of what you're putting in it. If I remember right those can also be one of the funny hulls made by Nitro that will lean like crazy unless you trim right away.