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How important is top rpm in prop selection?

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  • How important is top rpm in prop selection?

    I am gettng 5400 rpm out of 6000 redlline on a yamaha 100 hp 4s, (2.31 gear ratio) on a 21 foot, heavy, flat bottom, fiberglass skiff. The performance of a borrowed 4 blade prop (13.5 x 15 TP express) is acceptable, actually a huge improvement over the old yammie 3 blade13.25 x 19k. The 4 blade allows me to run my jack plate all the way up v. the three blade venting by raising it just an inch or so. Top speed nearly the same at 30 to 33mph.

    I appear to be in the range but how important is it to get toward the top end of rpm range? I am pretty sure going down in pitch one or two degrees might dial it in better? Top speed is not a huge factor within reason...I'm presently running mid to low 20's, 90 percent of the time due to conditions. Running shallow, good holeshot, pulling a tube on occasion, lower planing speed and carrying heavier loads is my main objective

    I am considering both turning point and powertech at this stage. When I go to order one should I go 13, 14 or just stay with the 15? Both companies have a exchange program but id like to get it right the first time to avoid boating delays during exchanges.

    I'm thinking a 14 to be safe but would like to hear other opinions.

  • #2
    I forgot to add my avg estimated load is 750lbs, fuel, gear, passengers. My test results however, were done alone with about half that weight. Also the lowest position of jack plate has the AV plate parallel with bottom of hull. With the 4b on the motor it appeared that the best speed was achieved near the upper, middle end of jack plate travel.

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    • #3
      Running 90% of WOT is costing you 10% (3.x mph) off your top end.

      You didn’t say what rpm the 19, 3 blade gave you, but I would stick with the 3 blade and drop down to 14-15 pitch depending on the 19 results.

      Ran a 4 blade for a number of years. Get better all around performance with a 3
      ....

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      • #4
        The 3 blade gave me 4900 to 5000 rpm. My only issue with the 3 was that it would ventilate unless jackplate was buried,. I dont know how much of that was due to being over propped or if the proper 3 blade would run on the surface as well as the 4 blade appears to. This motor/prop was a takeoff from a 24 foot aluminum boat so that may eplain why it didnt work so well on my heavy rig?

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        • #5
          the motor manufacturer publishes a window. both an upper limit and lower limit where the motor should be on RPM at WOT

          you have to be between those two points

          do not lug your motor below the lower limit, and if needed, back off the throttle to not exceed the upper limit.

          lugging a motor leads to replacing the motor
          Cheesehead boating the Gulf Coast of FLA 27.51° N, 82.53° W

          1988 Cruisers Rogue 2420 -VP 290 DP now powered by custom 468 - https://forums.iboats.com/forum/owner...988-rogue-2420

          Past Boats
          1970 Wooster Hellion - Merc 9.8
          2002 SeaRay 190BR - 5.0 - A1G2 - "Cheeseheads in Paradise"
          1984 Avanti 170DLI -3.0 stringer- "Ship Happens"

          What’s behind you doesn’t matter.Enzo Ferrari

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          • #6
            Each boat/engine combo it's a unique installation. Variable deck losd is a serial wot rpm killer, prop the motor not lightly loaded but with the usual load you'll be having on board as solo boating is very boaring. If a 3 blade prop ventiles, motor sits too high on transom or it's a combo of bad trim angle and motor/transom height match. Sit motor at a given height for prop not to ventilate when on flat and rough water cond as well and at tight close turns at speed.

            OEM props performs very well, just a matter of selecting the right pitch. There's 2 possible top options, running middle wot rpm achieves slight more top end speed with slight less top hole shot, running max wot revs achieves superb fast hole shot with slight less top end speed. Both scenarios dials spot on water performances if having a fixed load on deck, say a family oriented combo. Which one do you prefer ?

            To play with props pitches safely need to have the motor running in top shape along a tach doing its homework right.

            Happy Boating
            Last edited by Sea Rider; July 5th, 2019, 08:28 AM.


            Sea Rider 320, 380 Sibs, 450 Rib, 2 Strokes Tohatsu 5,18 & 30 HP Proud Smokers

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            • #7
              What is your engines WOT range? If it is 5500 - 6000, then you are definitely over propped, if it was 5000 - 6000, then you are in the range, though in the bottom half, Either way, provided the tach is accurate, you need either less pitch or less blade area. I know you said the Yammie 3 blade blew out when raised(probably a prop with little Cup or Rake), and the 4 blade TP is better. I would find a more suitable 3 blade(with cupping and lots of Rake) to get the rpms up and increase the boat speed.

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              • #8
                Originally posted by cr295 View Post
                The 3 blade gave me 4900 to 5000 rpm. My only issue with the 3 was that it would ventilate unless jackplate was buried,. I dont know how much of that was due to being over propped or if the proper 3 blade would run on the surface as well as the 4 blade appears to. This motor/prop was a takeoff from a 24 foot aluminum boat so that may eplain why it didnt work so well on my heavy rig?
                Heavy is relative...my 22’ is over #5k. A 100 hp motor would have a tough time even getting it on plane
                Let’s start with the basics:
                What boat is this on?
                Motor leg length? 20 or 25”?
                Why the jack plate? Not typically used on “heavy” boats
                ....

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                • #9
                  My experience to date has been that the 4 strokes sound awful busy at max rated rpm. At something like 500 rpm lower than that, they sound MUCH better and might be even faster - but just a hair. Bigger point is you don't loose much by not running them right at max. The exception being if you're setting the engine up on a boat that's going to be used for skiing. Those need to be propped for max performance right out of the hole, and the only way you can do that is with a prop that lets the engine turn up max rpm. Suit yourself. It's a compromise - or run a second (extra) prop that allows you to have a good cruise, and the other a good ski.

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                  • #10
                    Oh it’s such a variable and complicated thing reallt. I could literally write war and peace on my experience and experimenting.
                    Keeping it short though it’s no doubt about not only the right pitch but right prop too. Some props will just not perform close to the surface. Then if you get one that does...you’ll find it may allow the engine to raise the rpm and even exceed the rpm range...where the same pitch but lesser quality prop was getting much lower rpm because it wouldn’t allow lift.

                    Also really worth remembering that a common miss conception is that the higher the pitch, the more speed you’ll see....even in some cases if a bigger pitch prop is in the lower end of the rpm range...it won’t always see more speed than a lower pitch prop which is at the higher end of the rpm range. This is because with the best of the props...the more rpm they operate at...the better they are at lifting the hull out the water. There is also the fact that all engines have a peak power figure where they make the most power according to the power curve graph. This is usually near the top end of the rpm range stated by manufacturer.
                    An example with my boat is that a 21” laser 2 which will occasionally allow my boat to even hit its limiter...is quicker than a 23” laser 2 that will only achieve only just into the lower end of the 4600-5000 range. I’m pretty sure my engine makes more power at around 4800 upwards than it does at 4600. Again, the prop being more effective at lifting the hull at higher rpm.

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                    • #11
                      Excellent info gentlemen. Thank you for your imput and experience.

                      WOT throttle range is 5000 to 6000 for this engine.

                      Boat is the old, long defunct predecessor to the southern skimmer. It has lower gunnel height than a carolina skiff but very similar otherwise, flat bottom, heavy glass layup, 20 inch shaft. The boat is heavy compared to similar carolina skiff type hulls.

                      I think I'm pretty sold on the 4 blade prop. It has better bite, stern lift, hole shot, slightly lower planing speed. The difference between the old prop and this are drastic: shockingly better in all respects.

                      I just think a 14 pitch would give me a couple hundred rpm increase, slightly better load carrying ability without going over the redline of 6k? I could easily make due with this prop, im just thinking a 14 or even 13 would be better for my purposes?

                      This 15 pitch, 4 blade ran great with the jack plate all the way up with the engine much higher than i ever thought possible.. That means the AV plate was 5" above the bottom and it still fed water to WP. At full throttle rpms increased close to redline and it started slipping ever so slightly but it didnt blow out. I dont plan to ever run it at the full up position at full throttle especially in the shallows but i wanted to test it none the less. At lower rpms, say 4000 and 20 something mph it didnt appear to slip at all in the full up position.

                      A few mph lower top speed IS absolutely not an issue with me. 90 percent of the time im running in low 20's or high teens due to water conditions due mostly to not wanting cracked teeth or compressed vertebrae.

                      I realize my unique requirements are somewhat of a unicorn in the boating world but i would much rather rather be slightly under propped where i can control the rpm, even if i had to dial back the throttle travel than end up as others alluded to, the motor is lugging along over propped. I think the safe bet is to order the 14 then I have the option to dial it one step up or one step down. Feedback?

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                      • #12
                        I'm with you here on the 4 blade prop decision. And you aren't the Lone Ranger here. There are a LOT of guys running similar boats in the Gulf waters

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                        • #13
                          Originally posted by cr295 View Post
                          The 3 blade gave me 4900 to 5000 rpm. My only issue with the 3 was that it would ventilate unless jackplate was buried,. I dont know how much of that was due to being over propped or if the proper 3 blade would run on the surface as well as the 4 blade appears to. This motor/prop was a takeoff from a 24 foot aluminum boat so that may eplain why it didnt work so well on my heavy rig?
                          If you are into aluminum props getting the Turning Point Hustler you'll get rake and cupping, both help to control shallow (near surface) running....plus some ports for a dazzling hole shot. Since HP is torque and rpms and torque usually comes from cubes and long strokes, both of which 1 hp/cu in 2 strokers lack, they need to get their HP via rpms........besides mine never liked to bog down...they liked the top and usually a bit past for best running.

                          Cup on the prop may have the effect of another inch of pitch up at the higher rpms....so if you are running uncupped now you need to consider that. Also rake should grab better.....inch of pitch change is like 150-200 rpm change, inverse to the change in pitch
                          If you are new to boating or have a new boat, a knowledgeable friend could show you how to operate your boat and save you a lot of grief, maybe some money, and maybe your life.

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                          • #14
                            jimmbo It sounds as if you are really doing well now. Great stuff....but...the prop geek in me can’t help thinking that if you are doing so well with that 4 blade...close to the surface with your jack plate....ohh...mmm...I’d really want to be trying a real 3 blade specialist and talented prop that is well known for these conditions. Jimbo and I know exactly what prop could do just that.

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                            • #15
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                              Originally posted by QBhoy View Post
                              jimmbo It sounds as if you are really doing well now. Great stuff....but...the prop geek in me can’t help thinking that if you are doing so well with that 4 blade...close to the surface with your jack plate....ohh...mmm...I’d really want to be trying a real 3 blade specialist and talented prop that is well known for these conditions. Jimbo and I know exactly what prop could do just that.
                              I think you put my name by mistake at the front of your post

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