Prop Shaft Splines '97 Johnson 115

[Cricket Too]

Looking for a new prop for the above engine, swinging a 14X17 right now, in decent shape, some very minor dings and scrapes on it and getting 5200 RPM at WOT with normal load. Looking to drop down to a 13.75 X 15 to bring it up to around 56 or 5700.

I have been shopping around and have seen that on some later model V4's, OMC started using the 15 spline V6 lower unit. My boat is in the water and I can't pull the prop to count splines all that easily, so I was wondering if anybody knew if my '97 had the 13 spline shaft or the 15.

If I had to guess I'd say it has the 13, just by looking at it and the fact that it's got a 14" diameter prop on it now, I would assume the bigger gear case would have like a 15" diameter on it.

Don't have the model # on me now but can post it back later if it's needed.

Was just about to order a prop, when I figured I'd check that first.

Thanks, Mike.

 

[Dhadley]

Re: Prop Shaft Splines '97 Johnson 115

If it has a prop nut with coarse threads, it'll be a 13 spline. If it has a fine thread nut it'll be 15 spline. If it's a 20" motor most likely it'll have a 13 spline shaft. I'd agrre with you on the prop diameter. If it were a 15 spline the prop diameter would most likely be (minimum) 14 1/4". 3 blade prop, not 4.

Now, dropping pitch to gain rpm with an outboard is the very last thing you want to do. Unless you're convinced the oil companies are going broke and you want to send more cash their way. Dropping pitch to gain rpm will, most likely, decrease efficiency. Play with the set up to get the most out of what ya got before you spend $$ on a new prop. Yes, I sell props and yes, I'm telling you not to buy one. Yet.

I can't tell you how many times we talk to someone wanting to buy a lower pitch prop and after working the set up they end up getting a bigger one. More pitch and more rpm = increased efficiency.

 

[Cricket Too]

Re: Prop Shaft Splines '97 Johnson 115

Hey thanks Dhadley. I did consider playing with the set up, but then I realized that the engine is already on the highest set of holes on the transom bracket.

This is the first season I have owned this boat, and it is the original engine on the boat...1997. I can see some washer marks on the holes one notch down, as if it had been sitting on these holes at some point.

The reason I didn't consider dropping it was really because of the WOT RPM and that fact that my hole shot is pretty good, it could be better, but I just assumed that a lower pitch prop would help out both issues.

I don't consider myself a propping expert by any means, but I do try to read as much as I can on this site about it.....I have been trying to find that explanation of X dimension you put in a post a while back, but can't seem to find what I thought I saw you post.

Anyway, I would consider dropping the engine down one notch, but the boat performs so nice the way it is, that I figured why mess with a good thing. I have a pretty decent hole shot for a 115 pushing a 19" WA, and when I do get on plane I have absolutely no ventilation at all, I can leave it at WOT once I get on plane and it will just take right off and I can even trim up a fair amount and bring my RPM up, and even trimmed I get no ventilation whatsoever and if I slow down, I can take right back off again without even trimming back down.....no problem.

Of course if I add a couple more people in the back of the boat, it struggles a bit out of the hole, but not too bad.

My main concern is running 5200 WOT, I do not want to lug this engine, and figured dropping my engine down a notch might help my hole shot all around, but would also create more drag, there by reducing my already low WOT.

I am willing to try anything you think will help this rig out, and if there is any way to get a little more hole shot and also bring my WOT RPM up by about 500 RPM, I am happy to do it, before buying a new prop and decreasing my engine's efficiency.

So any advise you can give is much appreciated.

Sorry for the long post, but figured all the info I could give you would help.

 

[Dhadley]

Re: Prop Shaft Splines '97 Johnson 115

If the bolts are in the highest holes in the stern bracket, the motor is as low as it can go. If the bolts are in the lowest holes, it's as high as it can go.

However, the way you describe the trim & holeshot etc, the performance indicates it can go higher. By any chance do we have a 25" motor on a 20" transom?

 

[Cricket Too]

Re: Prop Shaft Splines '97 Johnson 115

Sorry Dhadley, I should have explained myself better. The bolts are in the lowest holes meaning it is as high as it can go. This is a 25" shaft motor, I assume since this is a 19' boat that I do have a 20" transom, although I haven't measured it, but I can't imagine it would be a 25" transom, and it's probably why the engine is mounted so high.

The anti-ventilation plate is just above the bottom of the keel, maybe an inch above, which is why I never bothered to move it at all, because it was in the correct spot from everything I have ever read, not to mention how well it performed.

So the hole shot and trim I mentioned before are with the engine as high as it can go on the transom. And I do trim a fair amount once I get on plane, it lifts the bow out of the water nice and cruises right along at about 22kt at 4200 RPM.

My only real issue is the 5200 WOT RPM, I would really like to get it higher as I can't stand the thought of lugging the motor. But a little stronger hole shot would be nice too.

I tried to attach some pics I have of it on the trailer, but it seems the files are too big for this site, I may have to just post a link and attach them somewhere else.

Thanks for your Dhadley,

Mike

 

[Dhadley]

Re: Prop Shaft Splines '97 Johnson 115

If you have a 25" 60 degree 115 you may indeed have a V6 style gearcase (same size but different than a true V6 unit). If you have a 25" motor on a 20" transom I'd suggest spending a little on a manual jackplate. That will get you all the adjustability back. If it were me I'd use a 6" Bob's convertable and use it in the 3" offset to start with.

 

[Cricket Too]

Re: Prop Shaft Splines '97 Johnson 115

This is the 90 degree block, it's a bubble back, not sure if they had the bubble back on the 60 degree, seems like it would be a little tight.

I didn't even know they ever made a 60 degree V4 until I went to get some parts for this engine and they asked me if it was the 60 or 90, took me by surprise, but I knew it was the 90.

It is definitely the 25" shaft though, it shows it in the model #, J115TSXEU.

I also assumed this was a cross flow when I got it, then I pulled the plugs and took a look and it's a looper, which somebody else afterwards showed me in the model # as well. I guess the "S" in the model # means "special styling", or SPL which I knew it was because it says it right on the side of the hood. I always knew this meant no TNT or VRO/OMS, but apparently is means no cross flow also, at least that is what I was told. This engine does have factory TNT on it though, which is the "T" in the model #.

I was a little bummed to see it didn't have OMS on it when I went to look at it the first time, but I have to say that it doesn't really bother me as much as I thought it would. I fill up 30 gallons at a time and that lasts me for 3 weeks or so, this engine is great on gas, so it's not a big deal to mix 30 gallons, like say 200 like on my friend's boat.

So you think a jackplate may be the way to go here? How much economy/efficiency do you think I will lose if I do drop down to a 15" prop on this current set up?

Thanks, Mike.

 

[Dhadley]

Re: Prop Shaft Splines '97 Johnson 115

Yep, thats a 25" crossflow. The problem is that without the ability to raise the motor any higher you'll not be able to take advantage of the technology built into the prop. With a jackplate you'll gain all that back plus you may gain from the setback too.

 

[Cricket Too]

Re: Prop Shaft Splines '97 Johnson 115

Hmmm, I could have swore that when I looked down the plug holes, I didn't see the "bump" on top of the piston that would indicate a X flow, so I assumed it was a Looper, then somebody told me the "S" for SPL in the model # meant that it wasn't a cross flow either.

I could very well be wrong as I was changing plugs kind of quick and didn't look all that hard down the hole, I could have just missed the "bump" on top of the piston. If you say it's a X flow than I take your word for it, it's fine with me as I know the X flow system is a very efficient and time tested way of scavenging exhaust.

I'm pretty sure I'm dealing with the 13 spline prop shaft on here though, I seem to remember the shaft having pretty aggressive threads when I pulled the prop to grease the shaft and check for fishing line in the spring. I will check again though.

So maybe I should ride this prop out for the rest of the season and look into a jack plate then. Although the way the anti-ventilation plate sits now I'm not sure how much higher it can go before the prop would start to ventilate. It is about an inch or 1 1/2" above the bottom of the keel, so any higher might be pushing it.

I will try to get the pics here to show you, but it looks like it shouldn't go much higher.

I think I have a friend with a 15" prop that will fit this motor, just to try it. Do you think with this current set up I will get at least 400 more WOT RPM out of a good 15" pitch prop? Even if I do lose some efficiency?

 

[Cricket Too]

Re: Prop Shaft Splines '97 Johnson 115

^^^^

 

[Dhadley]

Re: Prop Shaft Splines '97 Johnson 115

If you have a chance to try a 15 without buying it, by all means do so. Try as many props as you can. The more comparison info you have the better.

 

[Cricket Too]

Re: Prop Shaft Splines '97 Johnson 115

Thanks Dhadley, I will try the 15" this weekend, I think it's a 13.75 X 15. I will let you know what I see out of it.

How will the drop in diameter by .25 of an inch affect performance? I have seen some 15 pitch props for sale in the 14" diameter, but most seem to be 13.75" diameter. Is there any advantage to having bigger diameter?

Also, I was thinking if I drop the engine on the transom one set of holes, I will probably get a little better hole shot, but my WOT RPM would suffer, is that correct?

Thanks for all your time Dhadley, I really appreciate your knowledge.

Mike

 

[Cricket Too]

Re: Prop Shaft Splines '97 Johnson 115

^^^^^