WOT RPM for 1995 Johnson 50 Hp.

[vissertrades]

Hi all,<br /><br />I just got a 1995 Johnson 50 Hp put on my 15' Smokercraft "Big fisherman" boat. It had a 12 1/4 x 15 prop on it when I used it last weekend. At WOT and trimmed it would touch 5900 rpm with me and a few kids on board. I *think* it would have hit 6000 rpm easily with just me. I talked with the place that rigged the boat and they gave me a 11 3/4 x 17 prop to put on it. My question is, could I have caused engine damage by pushing this motor over 5500 rpm? I ran WOT maybe 6 times, each time less than 1 minute. Also it seems to run fine. Is there anything I should have checked out?<br /><br />Thanks

 

[DJ]

Re: WOT RPM for 1995 Johnson 50 Hp.

Piet,<br /><br />You're fine.<br /><br />WOT range is 4500 to 5500. The 2 & 3-cylinder loopers like rpms. 6K won't hurt it. They cannot stand to be lugged.<br /><br />If you are really concerned, put the 17" on when the boat will be lightly loaded.<br /><br />Remember, you don't have to run it wide open all the time.

 

[Dhadley]

Re: WOT RPM for 1995 Johnson 50 Hp.

5800 rpm is your target rpm with an average load. Over 5500 will not hurt it at all. If its propped out at less than 5500, that will start to damage it.<br /><br />Good luck!

 

[vissertrades]

Re: WOT RPM for 1995 Johnson 50 Hp.

Thanks for the reply...My kids made me do it I'm going with the 17 pitch just to be safe!

 

[Dhadley]

Re: WOT RPM for 1995 Johnson 50 Hp.

Safe is the 15........<br /><br />Or put the 17 on, youll probably lose about 3-400 rpm and then adjust the X dimension to gain back 2-300.

 

[vissertrades]

Re: WOT RPM for 1995 Johnson 50 Hp.

Alright, now I'm confused . Why would Johnson put 5500 rpm as the maximum? I *think* I could have trimmed up some more and hit 6K with 3 kid passengers so I'm thinking the 17" would put me slightly over 5500 rpm loaded. Am I missing something? Also the 17 is slightly smaller in diameter.

 

[Dhadley]

Re: WOT RPM for 1995 Johnson 50 Hp.

Yeah, I dont like it either when they go down in diameter as pitch goes up.<br /><br />As Djohns said, 2 strokes dont like to be lugged. It builds combustion temps and thats the mortal enemy. Its like pulling a trailer uphill and stuck in high gear.<br /><br />I would try the 17 and then like I said, I would work with the set up to improve performance even further.<br /><br />Let us know what happens with the 17.<br /><br />Good luck!

 

[vissertrades]

Re: WOT RPM for 1995 Johnson 50 Hp.

Follow up to my rpm situation:<br /><br />Yhe 11-3/4 x 17 made little to no difference in WOT rpm's. Boat seemed faster but I can still hit 6000 rpm easily. What to do, cupped 17 prop, go to 19 pitch? Stainless 17 or 19 pitch? Dealer will work with me to prop this correctly so what's my next move?<br /><br />Thanks!

 

[Dhadley]

Re: WOT RPM for 1995 Johnson 50 Hp.

Thats good! Yes, try a 19 if youre still getting 6K easily. And it should be faster yet. These props are all the same brand, right?<br /><br />Thanks for the update!

 

[vissertrades]

Re: WOT RPM for 1995 Johnson 50 Hp.

No, I think the first one was an OMC and the second was a Michigan Wheel. Another thing I just thought of was that I filled up with Amoco Premium this time (93 Octane) Last time I had Speedway regular (87 Octane). Maybe that was the additional gusto! Also, the 17 pitch was 1/2" smaller in dia. Rule of thumb, 1/4" dia = 150 rpm?<br />1" pitch = 200 rpm? My net effect on paper would have been only 100 rpm lower!

 

[vissertrades]

Re: WOT RPM for 1995 Johnson 50 Hp.

BUMP!

 

[DJ]

Re: WOT RPM for 1995 Johnson 50 Hp.

Piet,<br /><br />You need to stay with one brand of prop. to get consistant readings.<br /><br />I am no fan of Michigan Wheel props. I've never had one that worked right.<br /><br />A stainless "cupped" prop will bite the water better. I have found that they run a few rpm's less than aluminum. My experience anyway. <br /><br />I would put on the 17" OMC and just be aware that you don't need to run it WOT all the time. Remember, boats tend to gain weight, gear stashed here and there.<br /><br />By the way, your engine does NOT need premium fuel. Premium is actually detrimental to the engines health. Premium tends to run cooler which builds up combustion chamber deposits.

 

[vissertrades]

Re: WOT RPM for 1995 Johnson 50 Hp.

I exchanged the Michigan Wheel prop for the OMC eqivalent size (11-3/4x17). Blades on the OMC prop are double the thickness and seem bigger. Also, the OMC prop seems to be cupped! I know the first prop was OMC also, it had thicker/bigger blades but not sure about the cupping. I'll let you know how it turns out. To make things even more complicated I just picked up the owners manual for my '95 J50TL and the specs in the back say rated HP is at 5500 rpm with a WOT range of 5000-6000 rpm. I also have the '95 OMC service manual but the specs are 50 hp @ 5k rpm with WOT 4500-5500 rpm. Go figure, I'm calling Johnson in the morning!

 

[vissertrades]

Re: WOT RPM for 1995 Johnson 50 Hp.

Owners manual was for 3 cyl. looper! Back to 4500-5500 WOT for 2 cyl.

 

[vissertrades]

Re: WOT RPM for 1995 Johnson 50 Hp.

UPDATE! Got to try the OMC 11-3/4 x 17 and can wind the motor up to 5,800 rpm at max trim with me and two kids aboard (360 Lbs). With 3 kids, my wife and me it will hit 5,600-5,700 rpm (550 Lbs). Not bad? I think I'll stay with this prop! Also, had a high idle so I adjusted the linkage/levers per the OMC service manual and it's perfect. Funny thing was this engine was never touched as the factory sealant was still on the adjustments! Must have been a Monday or Friday built unit!

 

[vissertrades]

Re: WOT RPM for 1995 Johnson 50 Hp.

After thinking about it and considering my tach reads 100 rpm low I think I'm going to try a OMC 11-1/2 x 19 prop. Sound like a good idea? Owners and service manual both say target rpm should be at peak HP which is 5,000 rpm. I'll still be beyound that with the 19 pitch, should I jump right to the OMC 11-1/4 x 21? Comments please!

 

[vissertrades]

Re: WOT RPM for 1995 Johnson 50 Hp.

bump!

 

[vissertrades]

Re: WOT RPM for 1995 Johnson 50 Hp.

Ok, now what? I got the OMC 11-1/2 x 19 out in the water and rpm's were 5,500 at WOT (new prop just out of the box), but boat seemed SLOWER??? Also, prop seemed (felt) like it was slipping and I could not trim out as much as with the 17. What gives? In the corners the prop would ventilate (slip) and I had to slow way down. Hole shot suffered and it felt like top end did as well. I'm stumped! Should I try a OMC SS 17? New 19 was cupped but blades seemed thinner than the OMC cupped 17. BTW, I have a mark on the motor that references having the anti-vent plate parralel with the boat bottom (set up with a level). Any advice?<br /><br />Thanks!