1997 90hp Johnson can't hit Max rpm

[Joetomass]

I have a 90 hp Johnson on a 21ft pontoon with a 13 pitch prop. My max rpm is 5000 with 2 people and very little gear. With 4 people and same gear I drop to 4700-4800 with 8 people I drop to 4100. I was told 5500 was max rpm for this engine. But never can hit close to that. The engine seems to run fine. Just not shore if I should try to change prop or adjust something on engine. Also my prop now is beat up and needs to be replaced but not shore what size to change.
Thanks for any help

 

[sutor623]

Re: 1997 90hp Johnson can't hit Max rpm

What's the diameter on that prop? 4 blade or three? That's the beauty of a boat that seats lots of people ou almost need two or three props for different occasions. I try and hit max Rpms on a light load, that way if you are weighed down you can still fall into spec. 8people on a 21ft with 90hp isn't going to be cooking up river, but you are right for wanting to match your Rpms correctly. I'd try finding a smaller diameter prop or keep same diameter and run an 11 pitch. That should put you around 5500rpms on a light load. You could probably also get a 9pitch for days when ou have a heavy load or want to pull a tube. I think I order to cruise properly with 8people you will need to have two props, personally.

 

[sutor623]

Re: 1997 90hp Johnson can't hit Max rpm

Hey one more thing, do you have any speed readings on light an weighted down boat so we can calculate a prop slip?

 

[Joetomass]

Re: 1997 90hp Johnson can't hit Max rpm

I am at 23mph GPS at 5000 rpms with me and my wife with nothing onboard but life vests etc. just double checked my prop is 13.75x13. Also was thinking about timing? Is that possible

 

[Joetomass]

Re: 1997 90hp Johnson can't hit Max rpm

3 blade

 

[sutor623]

Re: 1997 90hp Johnson can't hit Max rpm

Hmmmm I think your gear ratio is 2.0:1 If that is the case you are getting 25% prop slip which is high. Not entirely uncommon for a pontoon though. A rule to follow with displacement hulls( like pontoons) is to run the largest diameter prop that you can (which I think 13 3/4" is it) that being said, I'd like to have one of the prop gurus take a look at this. I'm thinking you need o go down in pitch but I also want to see how we can minimize your prop slip numbers. I'm going to get the prop boys in on this convo.

 

[sutor623]

Re: 1997 90hp Johnson can't hit Max rpm

Hmmmm I think your gear ratio is 2.0:1 If that is the case you are getting 25% prop slip which is high. Not entirely uncommon for a pontoon though. A rule to follow with displacement hulls( like pontoons) is to run the largest diameter prop that you can (which I think 13 3/4" is it) that being said, I'd like to have one of the prop gurus take a look at this. I'm thinking you need o go down in pitch but I also want to see how we can minimize your prop slip numbers. I'm going to get the prop boys in on this convo.

 

[steelespike]

Re: 1997 90hp Johnson can't hit Max rpm

Keeping in mind that pontoon speeds are all over the place your prop size,rpm and speed is right on.
A 11" 4 blade prop will get you more rpm and may hold up better with added weight.But pontoons don't plane and as you add weight you drag more and more structure through the water. Slip numbers will be high 25% is within normal expectations.
Skinning thd bottom may help a little and adding a water glide may allow it to plane like a runabout but slower.
Weight distribution may help.

 

[Joetomass]

Re: 1997 90hp Johnson can't hit Max rpm

Sorry I prob. Sould have said that before I did skin the bottom an added a water glide this summer. My numbers are about the same with or without the glide I did however get much better weight handling and more stable ride with a fully loaded boat and in ruff waters with the glide. Spoke with guys at water glide about not gaining any speed and they seem to think motor is not hitting full throttle. We spoke about changing prop pitch but he is puzzled that there was no increase in rpm. So that's what making me thing I'm not hitting max rpm. They told me I should see about 100-200 rpm increase just from adding the glide. Could timing be a issue???

 

[Joetomass]

Re: 1997 90hp Johnson can't hit Max rpm

Any other input on this ????