93 Johnson Pro115 prop help

[Mark-RI]

Thanks for reading my post. I have a 1993 Nitro 170 TF Bass Boat, a 1993 Johnson Pro 115hp the Prop in the exhaust area says 14x17 3 blade and its aluminum. At WOT its is at 5000 rpms and top speed is 40 trimmed . This seems quite slow.

My old boat a 98 Nitro 700lx (same size boat) with a 90hp Force did 55. I bought that boat new and never checked the prop size I was quite satisfied.
These speeds were checked with a gps.

Any thoughts would help Thank You
The prop may have been changed who knows?
I would like more top speed as I only fish in this boat. And I would like to change to a SS Prop too, and a 3 or 4 blade ??? I hear different stories on both?

Thanks again Mark

 

[junior1113]

Re: 93 Johnson Pro115 prop help

if all you want is speed a high performance 3blade ss will do it. first thing i would do as with every boat is maximize setup. where is captivation plate in relation to bottom of boat. you could easily pick up rpms and speed if this is not right. picture would help also how sure are you of the running condition of this motor. i had an 88 spl on a 16.8 skeeter that did over 50 and pushed a ss 3 x 19" pitch at like 5400

 

[captkevin]

Re: 93 Johnson Pro115 prop help

What is the maximum rpm for your motor?
Something doesn't seem right - would expect faster
If you go up in pitch you will go down in rpm's - if you are only getting 5k with the current prop that won't work.

 

[crb478]

Re: 93 Johnson Pro115 prop help

With your motor I would be shooting for about 57-5800 rpm's and I would think that if everytihng is set up right and it is running right you would be able to reach that with the 17 pitch prop that you have. How much does your boat weigh? Is there any floatation that has gotten water logged? Is your AV plate even with or above the bottom of the boat? Do you have a jack plate? I can not see dropping to a 15 or 13 pitch on that boat so I believe that there must be a setup problem, mechanical problem, or boat problem.

 

[Mark-RI]

Re: 93 Johnson Pro115 prop help

Thanks guys for your input.
Ok here is what I have for info for you. I cant find the actual weight, but the tag inside the boat says 1090 lbs loaded with fuel and passengers with a 120hp motor. There is no water leaks its dry as a bone. Runs great, starts ok cold, warm no problem. I have included a picture of the back of the motor, looks like the av plate is about 2 inches above the bottom of the boat, its about even with the flat part you can see in the picture. No Jackplate. and from what i can find and spoke to a local boat dealer that sells Johnsons he said 5800 rpm. If you need anymore info please feel free to ask. The engine runs smooth, no popping coughing or anything unusual. This thing should run atleast 50 if not better. I know I am comparing my 98 Nitro to it but it only had a 90 on it.

Thanks again in advance
Hope it stops raining sometime soon here in RI its getting sickning.

http://markrenaud.webs.com/apps/photos
/photo?photoid=41656322

 

[crb478]

Re: 93 Johnson Pro115 prop help

Picture did not come through. You do not by chance have a stepped transom do you? If so you may need to raise the motor more.

 

[junior1113]

Re: 93 Johnson Pro115 prop help

go to photo bucket for a free sample and down load from there to here. recommended wot rpms for that motor is 5250-5750 so 5500-5700 is perfect for optimum effiecency and performance

 

[Mark-RI]

Re: 93 Johnson Pro115 prop help

Thats what I have guys, I appreciate your efforts in helping me. I am sure this prop has been changed, it looks way to good to be 15 plus yrs old. I just hate to have to bring this boat in for service on something I can easily do my self. My local dealer said about $600 to put a ss prop and make sure its in the rpm range. I can prob do it myself for half. Thank You Mark

I was just looking in the compartments and found a used prop, it is a 13x19 now who knows. What would I expect if I installed that prop? Thanks again.

 

[Mark-RI]

Re: 93 Johnson Pro115 prop help

Is everyone stumped?? I guess my best bet is not to purchase a new stainless prop for $400 but to purchase 4 aluminum props and try them all till I get the best performance. My calculations tell me if I need to gain 800 rpm but yet go faster I need to loose 4 inches in pitch and go to a 12-13 inch pitch if available to get me where I need to be. I was hoping to get this done by today as I have a Tournament tomorrow morning. Thanks in advance for your help. Mark

 

[Mark-RI]

Re: 93 Johnson Pro115 prop help

UPDATE!

I went out over the weekend, I installed that extra prop I found. Its a 13 x 19 well here is what happened. I could not go over 2200 rpm and the boat was up in the air, 7.4 mph was all it would do. never got on plane. Had a passenger with me who is my normal fishing partener.

Went back out Sunday I had changed the plugs for the heck of it. I had a friend with me, 100 lbs lighter than the first passenger. Same problem. Then after about 5 min with the boat in the air then it came down and got on plane and the boat went 48 mph and the rpms were 4800, speed checked by gps. faster but unusable. Back to the ramp put the original 14 x 17 back on and tested again. got 42.4 mph @ 5200 rpms I never trimmed this motor that far before to get the rpms that high. So I learned a little about my trim that day.

Maybe this info may help, thank you Mark

 

[hwsiii]

Re: 93 Johnson Pro115 prop help

Mark, I need a model number off of that 19" prop and the 17" prop you used so I can figure out which prop would be the best for your boat and setup. I do believe that you should try moving the motor up another notch though, bass boats tend to run on a pad and they can be very finicky about having the prop high enough in the water column and which prop they like. And are they SS or aluminum props. Dhadley is much better at recommending a particular model and make of prop than I am and maybe he will chime in on this. Get this information for us and lets see what we can figure out. If you could take a couple of pictures of each prop that would help also.
I found that your motor has a WOT RPM of 4,500 to 5,500 RPM and a gear ratio of 2:1, please check this for me and verify that information.
And when you were trying to get on plane did you trim the motor all the way in to the transom instead of out like you would for high speed, because that is the way it should be done for take off.


H

 

[Dhadley]

Re: 93 Johnson Pro115 prop help

Although the pic doesn't show a straight on shot of the AV plate in relation to the pad I'd agree that the motor is too low. If you can use all of the trim range without venting then it is too low, even for an aluminum prop. Before buying another prop, try raising the motor and maxing out the set up with what you have. Your target rpm with todays fuel is 5800. If you're using ethanol fuel you may not be able to reach that but you should get close.

 

[hwsiii]

Re: 93 Johnson Pro115 prop help

Dhadley, I have seen you mention many times about running the motor over the manufacturers recommended RPM, would you please explain that in layman's term so I can understand. I don't have anywhere near the knowledge you have in that area and I want to be able to understand the theory behind it. I am not a very mechanical person, so I don't understand the reasoning.

H

 

[Mark-RI]

Re: 93 Johnson Pro115 prop help

Thanks for all your replies!
I have been away for the 4th of July
DHadley, to answer one of your questions, I have not got the motor to over rev by trimming, my old Nitro would I guess you call it captivate?? when I trimmed to much, I watch the trim gauge on the dash for reference and listen, I also know by the gauge how high it takes to actually lift the motor for safe hauling and it seems I may be into the second stage of the lift pistons there are 2, looks like one for trim and the other for lifting the motor. I have had that gauge basically pinned while in motion. when this rain ever stops here in Rhode Island I hope to get out to see if I can raise that motor?? I will see if I can get a btter picture for you in relation to the av plate and the bottom of the boat. I went out over the weekend with it, had a guy with a 90hp go right by me, i wasnt happy. Same type of boat.

Thanks again
Mark

 

[junior1113]

Re: 93 Johnson Pro115 prop help

do compression test, and get that motor up. come back with results. if you can trim all the way up to second stage without blowout. you need to go up at least an inch. would almost bet you could go all the way up to bottom holes and still need to go up.

 

[Dhadley]

Re: 93 Johnson Pro115 prop help

The deal about the rpm has to do with combustion temps (not water temp). We have very different fuel today than we had when that motor was manufactured (or even last year). You don't want the motor to run so that the compbustion temps are beyond manufacturers design parameters. Lots of things can cause the combustion temps to go above what the motor will stand and low top rpm is one of those things. Low top rpm is a major cause of coking which can lead to broken rings. With todays fuel 5500 is border line lugging for the OMC style pistons.

 

[hwsiii]

Re: 93 Johnson Pro115 prop help

Thank you very much for explaining that, I never understood why you made those statements and I didn't know how to discern which motors it applied to. So on older OMC outboards I will start propping them out for 5,800 RPM instead of 5,500 RPM. I will start doing that today, I have a 1995 Force 40 that that factory specifications call for WOT RPM to be 5,000, should I be running that at higher RPM as well.

Thanks,
H

 

[TDmanager]

Re: 93 Johnson Pro115 prop help

The deal about the rpm has to do with combustion temps (not water temp). We have very different fuel today than we had when that motor was manufactured (or even last year). You don't want the motor to run so that the compbustion temps are beyond manufacturers design parameters. Lots of things can cause the combustion temps to go above what the motor will stand and low top rpm is one of those things. Low top rpm is a major cause of coking which can lead to broken rings. With todays fuel 5500 is border line lugging for the OMC style pistons.

I want to be sure I understand what you are saying. Are you saying we should always run out boats at full throttle to avoid problems? How does what you are saying affect cruising speeds. In my case, I currently have a 17 OMC stainless prop and a 20 Raker. The 17 will run 34 at 5600 rpm and the 20 will run 35 at 4900.

We use the boat for just cruising at about 25 mph. My wife does not like to go any faster. At 25 mph the 17 turns the motor 4150 and the 20 turns the engine 3600. We really like the 20 Raker the best as it lifts the bow of the boat higher and the motor sounds so much smoother and quieter. at the same throttle setting the 17 goes 25 mph and the 20 goes 27.5 which makes it seem the 20 Raker is more efficient. My questions are, am I hurting my motor using the 20 Raker on my 1979, 70 hp Evinrude? Will running a premium grade of gasoline help if I continue to run the 20 Raker?

Thanks

 

[Dhadley]

Re: 93 Johnson Pro115 prop help

No, you do not have to run wide open all the time although if the set up is correct and the motor is running correctly you can, no problem. I know lots of folks who do and I must admit that years ago apparently I was in much more of a hurry than I am these days.

The theory is that if the motor is lugging at wide open, it's lugging throughout the entire rpm range. If it's running correctly at top rpm, it'll be fine at cruising rpm - whatever that rpm is.

Then you get into ring design. Some designs are more affected by coking than other designs. Then there's fuel calibration vs ignition timing and plug heat range.

Then you get into why the manufacturer quotes a given rpm. That has a lot to do with how they submit a motor's designated horsepower to NMMA. A motor may be "rated" at 5500 but will run better and last longer at a different rpm.

You also have motor design. An old OMC crossflow, small bore looper and the big bore loopers will last at a much higher rpm than where their rev limiters are. Some of the other designs, like the 3.3 liter Evin, won't last very long above the stated top rpm range. Just a design issue.

 

[TDmanager]

Re: 93 Johnson Pro115 prop help

We use the boat for just cruising at about 25 mph. My wife does not like to go any faster. At 25 mph the 17 turns the motor 4150 and the 20 turns the engine 3600. We really like the 20 Raker the best as it lifts the bow of the boat higher and the motor sounds so much smoother and quieter. at the same throttle setting the 17 goes 25 mph and the 20 goes 27.5 which makes it seem the 20 Raker is more efficient.

Considering the info above, with my specific engine and prop, am am I hurting my motor using the 20 Raker on my 1979, 70 hp Evinrude? Will running a premium grade of gasoline help if I continue to run the 20 Raker?

Thanks for the good explination for the reasons for having the correct prop.