RPMs @ WOT

[johnmark]

I have a 1983 Johnson 50 hp (J50BELCT) on a 15 ft 1965 wizard boat. this motor has a SS prop 12.25 x 15 and @ WOT she was running around 4000 RPM what RPMs should this motor run at dont seem to plane well and small wake seems to knock her off plane too . Thanks a ton you guys. Bear with me as I'm kind of new to boating.

 

[hwsiii]

Re: RPMs @ WOT

You should be turning 5,500 RPM with that motor as it has a 2.42:1 gear ratio, please verify this. You do need a tachometer and a GPS, even a hand held one or even some phones have a GPS built in to them. You an buy a digital tach and hour meter here for $ 35.
http://cgi.ebay.com/ebaymotors/Tachometer-Hour-Meter-Outboard-Jet-Ski-Quad-Go-Kart_W0QQcmdZViewItemQQ_trksidZp4634Q2ec0Q2em14Q2e l1262QQhashZitem27ab0dee04QQitemZ170373541380QQptZ MotorsQ5fCarQ5fTruckQ5fPartsQ5fAccessories

H

 

[johnmark]

Re: RPMs @ WOT

I have a tiny tach. it reads 4200 rpms @ wot. I only have a seloc manual it states 4500 rpm @ wot looking for a speed o.

TN-25 posted this for me "According to the 1983 brochure, the gear ratio is 12:29 (.414). It was rated at 50 h.p. @ 5000 rpm. Full throttle operating range is 4500 - 5500 rpm.

Curiously they also offered a 60 horse version of that very engine (44.99 c.i. or 737 cc; same as the 50) the same year (1983) that was apporoved for operating from 5000 - 6000 rpm at full throttle (it made 60 horses @ 5500 rpm.)"

 

[Silvertip]

Re: RPMs @ WOT

Not so curious - Johnmark! If you look at the displacement (cc) of the engines from any manufacturer you will generally find at least two of them have the same displacement. There are even three and four engines of different HP ratings, all with the same displacement. HP is not gained by displacement alone. In a four stroke engine it camshaft design, compression ratio, and carburetion/fuel injection calibration. In a two stroke the same principles apply but it involves porting, intake and exhaust configurations.

 

[hwsiii]

Re: RPMs @ WOT

John, a 12:29 gear ratio is actually a 2.42:1 gear ratio. 28/12 = 2.42:1 and the manufacturers WOT RPM is 5,500 RPM. You are only turning 4,200 RPM at this time, to reach 5,500 RPM you need to buy a 11" pitch prop in the exact same one you have now on the boat, and that will raise your RPM to about 5,700 RPM,which is what Dhadley recommends for the new fuels used on older motors.


H

 

[johnmark]

Re: RPMs @ WOT

Thanks guys this helps alot. I looking at getting a new prop. I'm going to fool with trying without the hydrofoil and diff. trim holes (Settings) to see if the rpms change any. maybe more weight to the bow.