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Going from a Honda 50 to a Yamaha 70 four stroke

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  • Going from a Honda 50 to a Yamaha 70 four stroke

    Hello all,

    I have a Honda 50 four-stroke on a Stinger 160 tunnel hull, which weighs about 1500-1800 lbs, depending on loading. It currently generates 5500 rpm's at WOT and a top GPS speed of 30.2 lightly loaded in perfect conditions, with an 11.1x14 three-blade aluminum prop. With 2 people and a full load, I can hit 27mph.

    I'm repowering to a Yamaha 70 four stroke. I'm hoping to get better top-end and cruising speed; I don't care too much about quick hole shot. Any thoughts on initial prop composition and pitch to try? I thought I'd try a 17 or 19 pitch aluminum 3-blade then maybe switch to a SS 3-blade after determining the best pitch. Seems like the 19" pitch would work OK with the bigger motor?

    Any input from the prop wizards here would be appreciated.


  • #2
    By the way, the Honda has a 2.08 gear ratio and the Yamaha has a 2.33.

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    • #3
      The the yamaha prop selctor.
      http://www.yamahapropselector.com/propSelector/web/

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      • #4
        Hi Wes, thanks for the reply. I did the yamaha prop selector before I posted. Unless I'm using it wrong, that tool isn't applicable when repowering a boat because it asks for the current pitch and rpm of the Yamaha F70, which I don't have yet. The tool would be good after I repower, but I'm looking for a pitch to begin with on the F70.

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        • #5
          This is generally a best guess scenario. I like the 17" alum. plan. Done early enough in season, if it were too far off you could have it re-pitched to bring it in line with your target rpm. much cheaper than buying another prop.

          As far as target rpm, I've messed with quite a few mid range Honda's (40-90hp) and found that although they are fine at max rpm, backing off 4-500 leaves you with no loss of speed, and an engine that doesn't sound near as "busy" as an engine run at max rated. FWIW, -Al

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          • #6
            Try the mercury prop selector. Use the 60hp 4s with the 2.33gr. It's close to the same engine as the 70hp yamaha. Merc being 995cc and yam 1000cc. It will give you a decent starting place. If I had to guess, I;d start with a 19p prop because of the different gr and the increased power of the yamaha.
            https://www.mercurymarine.com/en/us/...or/#!/step-one

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            • #7
              For reference only
              • Boat type: Bass
              • Usage: Top Speed/0 ft
              • Engine Manufacturer:
                Mercury
              • Engine Type: Outboard
              • Engine Year: 2018
              • Number of Engines: One
              • Engine Family: FourStroke
              • Engine: 60 EFI 4cyl (995cc)
              • Drive / Gear Case: 40-60 HP FourStroke Command Thrust, 2.33 Ratio

              Details

              • Calculated Pitch: 19.12
              • Calculated Weight with engine(s): 1,566.40 lbs
              • Calculated Engine RPM: 5,875
              • Calculated Boat Speed: 41.23 mph





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              • #8
                Does it have the big or small gearcase???? Prop diameter will make a difference.
                When in doubt,Trim it out!!!
                Business Owner & Certifed Technician
                Boats I own & use
                20ft Javlin w/250 Junkrude
                17ft Bullet w/200 Merc (SOLD!!)
                13ft Allison w/150 I-6
                17ft Hydra-Sport 175GT
                15ft Allison w/75 Stinger(project)
                14 x 48 Drifter with 1100SS Merc

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                • #9
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                  Thanks everyone for your help. The Mercury prop selector tool was helpful.

                  Faztbullet, I think the F70 only has one size for gearcase? The prop diameter generally ranges from 13" to 14", depending on pitch.

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