• If this is your first visit to the iboats.com Boating Forums, be sure to check out the FAQ. To post a question or comment, begin by signing up. To start viewing messages, select the forum that you want to visit from the selection below.


Announcement Module
No announcement yet.

Mph, Rpm, Prop Pitch Question

Page Title Module
Move Remove Collapse
Conversation Detail Module
  • Filter
  • Time
  • Show
Clear All
new posts

  • Mph, Rpm, Prop Pitch Question

    HI guys. 1998 chapparal 200LE 20 ft. 4.3 mercruiser 190hp
    how fast should this boat go? wot range is 4400-4800. currently with 21 pitch prop 4800 rpm, 42 mph half to full load. I switched to a 23 pitch prop to get more speed. with 23 pitch prop 4400 rpm, 42 mph. what gives? the previous owner said he had it up to 50 mph with just him on board. what can I adjust? thanks for any help. I want more speeeeed. I think the 4.3 is almost undersized for a 20 footer.

  • #2
    Re: Mph, Rpm, Prop Pitch Question

    Mercury Marine posts numerous boat/motor test combinations. This one should be very close to your boat's. http://www.mercurymarine.com/look_de...ecks&mercV6=14

    Notice the prop and speed is very close. Sounds like if you want to go faster you need a faster boat


    • #3
      Re: Mph, Rpm, Prop Pitch Question


      A LOT of people exaggerate how fast their boats go!

      There's a few things you need to know.

      You need GPS (Pitot speedometers are not accurate)

      Verify your tach and determine your drive ratio.....when you have that info:

      Then you can use a Prop-Slip calculator to determine what your boat should do (approx)

      Use 15% slip. (common for single prop planing type boats)


      I use xxx oil and have never had a problem........
      This post was created with OpenSuSE x64 Linux!


      • #4
        Re: Mph, Rpm, Prop Pitch Question

        thank you both I was just kinda curious. the mph is not gps. the mercury motor test with the chapparal is close. the 200le is quite narrow beam is 7'4" and lighter than the test boat 2600lbs. so i just figured this boat should go faster than 42. well maybe if the speedometer is not accurate maybe it is going faster than 42 lol. why does the speed stay at 42 when i switch prop pitches and the rpm goes from 4800 to 4400. pitch 21 to 23. thanks.


        • #5
          Re: Mph, Rpm, Prop Pitch Question

          why does the speed stay at 42 when i switch prop pitches and the rpm goes from 4800 to 4400. pitch 21 to 23.
          Ayuh,... Because you ran outa Horsepower....
          Any Grease is Better,..... Than No Grease at All.......


          • #6
            Re: Mph, Rpm, Prop Pitch Question

            Have you verified the weight of YOUR boat on a scale. On my boat the difference between normal load (400lbs passengers and full fuel tank) and myself running with an almost empty tank is almost 3mph(but it will over-rev to get there). That and the previous owner probably lied, or just had no idea how fast it really went. That is pretty common.


            • #7
              Re: Mph, Rpm, Prop Pitch Question

              As Bond-O says, you ran out of horsepower.

              This is a dyno chart of a 220 HP MEFI Vortec marine V-6. Yours will have the same basic curve but will peak at 190 HP instead of 220 HP.

              See how this curve shows the motor putting out 226 HP at 4800 RPM? And it's putting out about 215 HP at 4400 RPM? A 10 HP difference.

              Basically, your boat requires 190 HP to go 42 MPH. The 21 pitch is allowing it to deliver 190 HP at 4800 RPM. The 23 pitch is allowing it to deliver probably 185 HP at 4400 RPM. A good rule of thumb for this size boat is that it requires 10 HP to make a boat go 1 MPH faster. You are easily within a plus/minus 1 MPH speed range with the 21 and 23 pitch props, so you can't see any difference in speed on your speedometer. No matter what you do with the prop, you aren't going to go faster than 42 MPH unless you find some more horsepower (or less weight). With an almost empty fuel tank, 1 skinny person in the boat and a nice tailwind, you could probably get another 5 or 6 MPH, which would get you into the high 40's, probably close enough to call it 50 MPH by the previous owner.
              Any opinions expressed above are worth exactly what you paid for them!


              • #8
                Re: Mph, Rpm, Prop Pitch Question

                Wild, I had a 19 SeaRay with the same powerplant. I got about the same MPH and RPM as you using a 21" pitch Rapture SS Prop. I would be surprized if you could get that boat to 50MPH. The power needed to increase speed on a boat is nearly exponential.


                • #9
                  Re: Mph, Rpm, Prop Pitch Question

                  Wild, I have a Chap and a 4.3 carb as well. That may be where the similarities end as our hulls are likely very different. But I will chime in anyway for a fellow chappy. I get exactly 49 mph at 4750 rpms with my Chap. The boat weighs 2900 dry plus 33 gallons (never been out with less than 3/4 tank) plus four of us and gear, two are little kids and my wife and I are slim. My prop is the original slightly disfigured, alumimum 21 pitch. With the above load, no matter the wind or bow cover or whatever, the RPMs always top out the same. Chaparrals are not known for speed. I do not know about your year (since the hull was patented in 2001) but Chaps have an Extended Plane Hull. That means the surface touching the water goes all the way back greatly improving the ride relatively speaking. Look at most boat brands and you will see at least a foot of length not touching the water. My 18.5 footer has the hull size of a 20 or so footer touching the water. The ride is probably great in your boat but all that hull and accompanying weight makes it slow. If you want speed, switch boat brands since Chap is a luxury boat.
                  2003 Chaparral 183SS


                  • #10
                    Re: Mph, Rpm, Prop Pitch Question

                    Look at it this way. The prop is the transmission for your boat and you only have ONE gear. The prop needs to get you out of the hole yet prevent the engine from over reving at the high end and also to prevent lugging at that speed. What you did by installing a 23P prop is to shift up a gear which means, as the others said, you ran out of power. Just like in a car or truck there is only so much speed any engine in any car can deliver with the gear ratios it has. You may think your basic family sedan should easily do 140 MPH at 4000 RPM because you are only turning 2000 at 70. Unfortunately it takes very little HP to cruise at 70 but as wind resistance comes into play (water resistance in the case of a boat), the amount of HP it takes to increase speed goes up tremendously.