I am posting this to help a buddy of mine(he is not a member here) please help since he can not get more speed or RPM from his current setup,
all suggestions are welcome and thanks in advance.

here are the specs
2000 Lund 1650 Explorer SS (Side console)

Specifications:
Weight 850 lbs (not as heavy as I thought)

Capacity 1350 lbs

Max HP 75 HP

Max Passengers 5 Adults

Centre Line 16’ 8”

Transom 20”

Amidship Depth 34.5”

Bow Depth 36.5”

Stern Width 79”

Beam 81.5”

Thickness (Freeboard/Bottom) .063/.080

Fuel Tank 19 Gallon


Motor:
2003 Honda 75hp (BF75LRTA) Long shaft

Specifications:
4 Stroke SOHC

4 Cylinder, 12 Valves

1590 CC (97 Cubic Inches)

75 HP at 5500 RPM

Gear Ratio 2.3:1

L Type - Dry Weight 373 lbs

He currently has a four (4) blade propeller that is 13”x15 Pitch Right Rotation and the boat will max out at 35MPH and 4000 RPM – the motor is suppose to hit 5500-6000 RPM at Wide Open Throttle.
makes me think he needs a 3 blade 13"x19 pitch or a 13"x21 pitch prop, not sure though, what do you guys think?

First things first, how deep in the water is the centerline of the shaft (center of the prop) he will need to measure the distance on level ground (with the boat level) to the bottom of the boat at centerline, then measure from the ground to the middle of the prop, subtract the numbers and it will give him a distance the prop is below the bottom of the boat.

If the distance is more than 4-4.5 inches he needs to raise the motor. If the motor is running too deep in the water he is fighting drag with the motor. Once he gets to 3.5-4.5 inches, he will need to run the boat WOT again and see what he gets. The best you can expect is to gain about 200 rpm for each DROP in pitch. if he is getting 4000rpm now with a 15 pitch prop he would need to drop down 5 to a 10 pitch to gain 1000 rpm. Thats seems a little excessive and I think he needs to raise his motor. If you can post more of the information it would be much easier to give an answer once you get the measurements.

Wow just re-read your post, he has a 25" long shaft motor on a 20" transom, that will pose problems as I don't think you could ever raise that motor high enough to get the cavitation plate out of the water when on plane. The only other way to get the motor high enough would be a jackplate. Take the measurements and go from there. I know I have a 25" transom with a 25" motor and I cannot get my motor more than 4.5" below, so it will definately be a tough one. He definately needs to get a smaller pitch prop to get that motor in the rpm's it needs.

....assuming that he's got the tach hooked up right and is really only pulling the revs you list. Something like 11 inches is indicated, but you're so far under the rated rpm that this is only a guess. I'd get the tach checked first though...
Doug

Hmmmm... something does not seem right, first have the tach checked for accuracy, the rpms really seem low.

Yes double check the shaft length vs what the boats needs.

If everything is correct you dont want to go to higher pitch.

My first question is how much boating experience does your buddy have? Does he know how tilt and trim works on a boat to maximize boat RPM and speed?

Is the MPH speed you give based on GPS or the boats speedometer? Boat speedo's can be off by 5 MPH and even more in some cases so should not be trusted for accuracy.

Is the prop stainless steel or aluminum? I would definitely NOT suggest a 19 or 21 pitch prop for that setup. Maybe 17 pitch at the most but lets address the RPM issues before your buddy invests in a prop.

I have 2003 Lund 1650 Explorer with Suzuki 70 HP four stroke. I typically use either 15 or 17 pitch stainless steel 3 blade props to get into the 5400 to 5800 WOT RPM range of the motor. My motor is raised two mounting holes which puts the upper bolts in the third hole from the top. That puts my cavitation plate 2.5 inches above the bottom of the boat.

The fact you are achieving 35 MPH at only 4000 RPM with a 15 pitch prop tells me something is wrong with the tachometer or speedo or both. Or your buddy is not trimming the motor correctly. A 4 blade prop is going to turn approximately 200 RPM less than a 3 blade and is typically not as fast because of increased resistance of the additional blade. But hole shot tends to be better with four blades. If the motor is mounted all the way down on the transome versus raised a hole or two, that is costing you another 100 to 300 RPM. So in the worst case you should be turning around 5000 RPM at 30 MPH. Depending on what prop you have, optimum performance should be more like 5800 to 6000 RPM and 35 to 37 MPH.

Is there any chance of trying another prop just for testing the accuracy of the tach? It'd be best if your buddy holds off on any prop purchase until the RPM issue is resolved. It could the guage is off or your buddy is not trimming the motor high enough to get a combination of max RPM and MPH.

The model BF75LRTA in the post signafies the motor is a 20" shaft, not a 25". "L" means 20".He has the correct motor on the boat. A 17 pitch prop should be what he needs. Make sure the cavitation plate is level with the bottom of the hull to maybe an inch above. Any deeper in the water and you are dragging. Did he mount the motor himself or did a dealer do it. Any respectable dealer would not let the boat go out of the shop like that.

thats good

At 4000 engine rpm, a 15" prop on that Honda (2.33:1 gear) would push the boat at about 20 mph. As others have said, something is wrong with the numbers here- tach, speed, prop pitch- something isn't right.