Underpropping is good!!

Discussion in 'Sport Boats' started by Arminius, Jun 30, 2022.

  1. Arminius

    Arminius Active Member

    756
    Oct 30, 2019
    Seattle
    Bowrider 200 Select, 2003
    5.0L MPI, 260 hp w/Alpha 1 Drive
    Underprop your boat for great performance! Modern engines have electronic protection against excess rpm so there is no reason to worry.

    I was going to a 17" on a 140hp Suzuki DF140 on a 18' hull that was a dog. Not a 2-stroke definitely. Anyway, the box mistakenly contained a 15" so I gave it a try. Dynamite! Might get up on a slalom ski behind this boat. All these years I've been propping for red line when trimmed out at WOT. Why? Guests generally hate a fast, bumpy ride; especially in a light OB.
    It was to prevent the engine from overrevving and blowing up, I guess. Now engines have electronic rev limiters so, why worry? Trimmed out, Mr. Suzuki says "No more! at 6500 rpm."
    4" of pitch reduction made a sports boat out of this old fisher. The Suzuki can protect itself and doesn't need the old incremental approach in prop selection.
     
    dtfeld likes this.
  2. blaster

    blaster Well-Known Member

    May 23, 2011
    potomac river
    2010 sea ray 205 sport
    F-250 CCLB 6.2 4x4 3.73 rear.
    Mercruiser 4.3 mpi alpha 1.62
    ?????????????????WTF????????????????
     
  3. techmitch

    techmitch Well-Known Member PLATINUM Sponsor

    May 1, 2008
    Chesapeake Bay, Middle River/Frog Mortar Creek MD.
    1999 270 DA
    Twin 4.3s W/Alpa I Gen II's
    [​IMG]
    I'll say it again. Lost, very lost.
     
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  4. Stee6043

    Stee6043 Well-Known Member GOLD Sponsor

    Jun 1, 2015
    West Michigan
    1997 Sundancer 400
    7.4L Gassers
    Lower top end MPH will reduce the chance of the plywood bow cover coming off. From a passenger safety standpoint you're making a wise decision.
     
  5. Arminius

    Arminius Active Member

    756
    Oct 30, 2019
    Seattle
    Bowrider 200 Select, 2003
    5.0L MPI, 260 hp w/Alpha 1 Drive
    Archimedes, an old Greek guy, invented these screws we use to motivate our boats. They are gears bearing against an incompressible liquid, water. The angle of their tines is measured as pitch, the distance the screw would travel through water (with no slippage) in one complete revolution. The 19" pitch would travel that distance while the 15" does not go as far but is easier to turn. So the shorter pitch has a mechanical advantage and also increases engine rpm at a given speed moving it further up the power curve on a gas engine. Greeks were really into screwing and were famous experimenters.
     
  6. Shenanigans77

    Shenanigans77 Well-Known Member PLATINUM Sponsor

    590
    Feb 17, 2020
    Lk St. Clair
    2003 320 Sundancer
    Twin 6.2 Mercruiser Bravo III
    Bouncing any motor off the rev limiter repeatedly is bad juju. But i digress
     
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  7. techmitch

    techmitch Well-Known Member PLATINUM Sponsor

    May 1, 2008
    Chesapeake Bay, Middle River/Frog Mortar Creek MD.
    1999 270 DA
    Twin 4.3s W/Alpa I Gen II's
    Let me guess. Your one of those guys that drives his 6 speed car on the highway at 65 mph in 5th gear because you feel it has more power that way?
     
    Chris-380 and Stingrayaxe like this.
  8. OllieC

    OllieC Well-Known Member SILVER Sponsor

    Mar 11, 2013
    MinniSoCold
    Sara Belle
    2005 Weekender 215
    Mercruiser 5.0 mpi, Bravo III
    Remind me to never buy a power boat from the Seattle area.
     
    Stingrayaxe and blaster like this.
  9. mdolesh

    mdolesh Active Member

    285
    Dec 18, 2019
    Edgewater, MD
    2013 370 Venture
    Twin 300 Verado
    So the plane could take off...if it only had a smaller prop! Problem solved!
     
    MattB92 likes this.
  10. b_arrington

    b_arrington Well-Known Member GOLD Sponsor

    Feb 21, 2007
    Setauket, NY
    Back Cove 37
    AB Ventus 9VL
    Cummins QSC 8.3 600
    upload_2022-6-30_16-25-42.gif
     
  11. Arminius

    Arminius Active Member

    756
    Oct 30, 2019
    Seattle
    Bowrider 200 Select, 2003
    5.0L MPI, 260 hp w/Alpha 1 Drive
    Airplanes have variable pitch propellers allowing the engines to reach higher revs during take off when heavily loaded. I live by Lake Washington where Kenmore Air sends floatplanes full of fat fisherman to Canada on weekend mornings. The 9 cyl radials rev high as the pilots flatten the props, reducing the pitch, so they will have power enough to break loose and climb loudly.

    The important point I make is that traditionally boat props are over pitched to prevent the user from blowing the engine up. The future is here and we now have effective and reliable overrev preventers built into our engines and we no longer need to be concerned about rare abuse when picking a prop. I was amazed at the amount of performance available by going down a couple inches of pitch. I wouldn't have tried but for the accident of getting a 15" in a 17" box.
     
  12. MattB92

    MattB92 Active Member

    230
    Nov 9, 2019
    Barrie, Ontario
    1994 Sundancer 270
    7.4 w/Bravo II
    I love this guys threads, always entertaining
     
  13. Thornton69

    Thornton69 Well-Known Member

    977
    Sep 15, 2014
    Northern BC
    1981 245 SDC
    2021 Solar 420
    '97 5.7 Bravo 2
    '20 Merc 25 EFI Jet
    Wonder if in the long run, under propping a marine engine would have the similar effect it does with under loading a generator. Continued high rpm's with low combustion pressures can lead to cylinder glazing? Maybe even to the point of excessive blow-by and even fuel diluted crankcase oil? I would stick to the manufacturer's recommendations.
     
    techmitch likes this.
  14. Arminius

    Arminius Active Member

    756
    Oct 30, 2019
    Seattle
    Bowrider 200 Select, 2003
    5.0L MPI, 260 hp w/Alpha 1 Drive
    Extended high speed runs are seldom made. Someone complains or uses diplomacy. The gas gauge goes down too quickly, whatever. Glazing is not a problem on auto engines while carbonization from running too slow may be. Top speed sells boats to novices so the manufacturers' recommendations may tend to over propping. Fill your new boat with your fat relatives and try to plane with the new engines and you may wish for a lower pitch. The new engines tend to be gutless because they are expensive and the manufacturers are trying to keep the price of the package down. You really don't want a V-6 IO but that's what they are touting. Your 2.0 L outboard is now a 4-stroke with half as many power pulses as the old 2-strokes. Sellers tweak the specs to sell product. Go with what works for you.
     
  15. Thornton69

    Thornton69 Well-Known Member

    977
    Sep 15, 2014
    Northern BC
    1981 245 SDC
    2021 Solar 420
    '97 5.7 Bravo 2
    '20 Merc 25 EFI Jet
    If at given cruise speed, do you think you will be getting better fuel economy with an engine running at 3700 rpm opposed to 3400? Just curious. Fuel/air ratios being the same, I would suspect the one moving more air would also be moving more fuel?
     
    Nater Potater likes this.
  16. bobeast

    bobeast Dance the Tide SILVER Sponsor

    Oct 22, 2017
    Isleton, CA
    2002 310DA
    350 MPI w/V-drives
    Relying on the rev limiter seems ill-advised. When it kicks in, you'll drop off-plane like a stone. Once you figure out the threshold you'll likely never rev it that high which just makes YOU the rev limiter. I'm not seeing the up-side.

    All that said, I'm sure a plywood bow cover would lower the pitch.
     
    Last edited: Jul 1, 2022
  17. scoflaw

    scoflaw Well-Known Member

    Aug 10, 2011
    cape cod mass, cape coral fl
    1999 Powerquest legend 260 sx 502 mag
    2005 Baja 242 islander 496
    502 mpi Bravo 1
    496 mag B1
    Rev limiters don't drop you off plane. When you bump it you know it, that's about it.
     
  18. blaster

    blaster Well-Known Member

    May 23, 2011
    potomac river
    2010 sea ray 205 sport
    F-250 CCLB 6.2 4x4 3.73 rear.
    Mercruiser 4.3 mpi alpha 1.62
    Also, when you bump the rev limiter there is a record in the ECU that can be used as a possible excuse to void a warranty. My friend has a Suzuki 20 outboard that had an issue and he had to fight with the dealer after they said he hit the rev limiter x number of times. They were trying to get out of a completely unrelated repair to the tilt mechanism and something with the throttle cable. He was able to convince Suzuki after showing his motor was under propped and that he had asked for a replacement propeller but was denied a trade by the dealer.

    It is always best to operate equipment as intended by the manufacturer, especially during the warranty period.
     
  19. ttmott

    ttmott PhD in OCD TECHNICAL Contributor GOLD Sponsor

    Apr 3, 2012
    Space Coast Florida
    2006 52 Sedan Bridge
    Cummins QSM11
    O come on boys - easy..... some good points are being made.
    I had several different props for my Gambler 206SS with Merc 225.
    17 pitch was for skiing and tubing; it would pull your arm sockets out coming out of the hole. But if you didn't watch it the rev limiter would hit at 50 MPH.
    26 pitch was a dog out of the hole but could propel that boat into the triple digits. It would not allow the motor to hit the limiter. At least not that I could hear.
    26 Pitch in action -
     
    Nater Potater and copb8tx like this.
  20. Arminius

    Arminius Active Member

    756
    Oct 30, 2019
    Seattle
    Bowrider 200 Select, 2003
    5.0L MPI, 260 hp w/Alpha 1 Drive
    The 5.0 MPI in my 2003 Bowrider turns a 19" High Five. We do get a kick out of running at WOT in a flat water lake. GPS speedo shows under 50 mph when I begin to trim. I turn 5150 when the fault horn starts honking to let me know 5200 is the limit. I just trim down a bit and the horn quits and we are stable at 52 mph.

    But as to the subject Suzuki 140 DF w/ 15" Suzuki prop on Campion EX18, now it pivots on to plane and slams up to 33 mph @ 6300 rpm. I start to trim right up to 36 mph and go no further than 6500 rpm. The rev limiter is gentle but the tach goes up, a red light flashes and I lose 50 rpm and then bounces back up. No sudden moves, it just won't go any further.

    I read that this rev limiter tech is common place where the BMWs and killer crotch rockets are limited to 160 mph or whatever the Euro cops demand. Actually, I had my rear wheel seize on a 74 I'd built myself at 80 mph. It was unreal. The rear tire screamed and the bike started coming around under me. Events unfolded in slow motion but I realized i better pull the clutch but the lever refused to move. I had the leverage of the old mousetrap and the strength of panic. Got it in, the wheel started turning and I pulled over. The primary chain had broken and piled up under the clutch pack distorting the shaft and partially seizing the clutch push rod. Whew!

    Boating is OK. I decided this after soloing and realizing you can't just stop and think it over while aloft as the gas gauge keeps dropping and you really do need a field. Anyway, hitting the rev limiter on a boat is civilized entertainment.
     

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