Help selecting a prop

[Frosstyx]

I have an issue with my outboard not reaching wot rpm range. I recently completely rebuilt the motor had it bored 60 over, put forged pistons in it. I replaced all electrical parts including power packs, coils and all. The current prop has some dings and bends in the leading edges of the blade and appears to be the original installed prop.


The motor is a 1987 OMC Seadrift Drive 1.6l (Johnson 115 motor). 2.0 gear ratio.

WOT range is 4500 to 5500. From what I can find, 5100 is perfect.

Boat is a 1987 Glass master 18' CC. (No idea on weight.)

Current prop is a Johnson 13.75x15 Stainless Steel prop.

I have a hydrofoil installed.


I checked the prop shafts and it's straight as it gets. I'm only getting about 4000 to 4200 rpms out of the motor and topping out at about 28mph according to my GPS and the boat is slow to plane. . Could the bent blades and such be causing my issue or do I just need a 13 pitch prop? Im going to order a new prop but need to know if you guys think 13 pitch is too low for this boat? Also what should my max speed be on this rig? Bought the boat used and don't have any manuals or anything for it.

 

[flyingscott]

How did it run before you rebuilt it

 

[Sea Rider]

Could internal hull be waterlodgged, if so, that adds tons of extra weight to combo. Did you tached the OB before it was rebuilt ? Sitting any given engine on any given boat is no indication it will work top, you need to perform a water test lightly loaded on flat calm water cond and check if OB is inside factory wot rpm range. Running towards the max rpm range is mostly preffered, then can go for a prop maximization to suit your boating needs.

Like testing factory delivered props on new or old OB's with a tach, that way will know if in the sweet wot range, go for a prop maximization, in case prop is broken will know if best to buy same one or go for +,- in pitch. 2 less pitch should put your OB at the right wot range, but that will need to be water tested...

Happy Boating

 

[Frosstyx]

Guess I should have mentioned the hull condition. The floor, stringers, and transom have all been replaced (marine grade plywood and glassed in). I bought the boat as a restoration project. I added a casting deck to the back of the boat but it was only about 50 extra lbs. Everything was glassed in.

When I bought the boat the wood in the transom was completely saturated and the stringers in the back were dry rotted(they had leached water from the transom). It has foam boards for floatation in the hull between the stringers and they are dry as a bone. No sprayed in foam. The fuel tank was removed and leak tested before being reinstalled. There is not a water problem with the hull.

I did run the boat prior to rebuilding the motor but it had sat so long that the thermostats had corroded shut and within 2 trips it threw a piston ring from overheating (warning horn was bad). The boat was running closer to 4700rpm but it was waterlogged and only getting a little more speed. This is the original motor for the boat. It's a full transom with the seadrive bracket.

The prop was pretty dinged up when I got the boat but I have since found a rock bed while trolling at about 2 mph. I'd like to get it to plane quicker and I'm thinking this boat should go close to 40mph if I'm getting 28 to 30 at 4000 rpm.

 

[Frosstyx]

Here are some pics of the current prop and shaft.

 

[steelespike]

It is based on the 115 so within reason it should respond like a 115. Usually a 17 or 19" prop would produce about 5000- 5500 rpm and speeds approaching 40.
Boat is only about 1500 lbs including the motor and drive.The 15" prop I think indicates there was a performance problem.
If we shoot for 5500 rpm You need 1500 rpm and you can't do it with a prop.Rule of thumb 150 to 200 rpm per inch.A 5" pitch reduction only makes up about 1000 rpm. You would likely lose speed and gain rpm. with a 10" pitch.
Long story short something is wrong with the setup.Don't assume all is right just because everything is new.
Start with the simple stuff. Be sure it fires on all cylinders. Do a compression check and note the location of the plugs and the condition.
Check that the throttle opens all the way. Does the boat respond to up trim? Be sure timing is set right and advancing as it should.

 

[Sea Rider]

Being a second hand boat, probably previous owner was satisfied with its water performance, some don't like to go fast from A to B and probably didn't know a thing about OB's wot range when at full throttle. In moden boating times with more info available, boaters getting more skilled and the help of boating forums as this nice one is a must go for a prop maximization if needed if plan running wot and get the max out of his horse.

It it possible to contact previous owner and ask if prop was a original factory delivered one with that OB or he changed it afterwords, sort of playing with props, per recommendation, word of mouth whatever. If OB runs well throughtout the whole throttle range you have nothing to worry about OB health, go for a prop maximization, at least 2 less in pitch to current 15 prop.

Although a 13 pitch is a wild guess, it's a good starting point, unless wanting to go straight for a 11 pitch, assuming those props are available in 2 pitch increments. Anyway will need to test them at wot. Personally would buy 2, a 13 & 11 pitch and stay with the one that achieves mear or max wot range lightly loaded. Use the other as spare or sell it inmediately after wot test.

PD : Discard with a hub slip test if current prop is in 100% working order prior passing to less pitch props, you never know what you might find hidden..

Happy Boating

 

[steelespike]

Keep in mind this is a form of I/O. Dry weight is only 1500 including the drive. Hole shot should be strong his is slow even with a foil installed.
His max rpm is about 4200 at about 28 mph.The motor is struggling for some reason.
Maybe he could get the prop tuned up it is stainless. Though it may load the motor more and it's already struggling.
You can't make up 1300 rpm with a prop Once the motor is right a ss 15 might be the one.
If you review 115 performance reports at the Etec site you'll see 17" and 19" are pretty common props.
His motor is fresh It should compare well.

 

[Frosstyx]

I am going to order a 13 pitch and see what happens. Worst case is I have an extra aluminum prop. If it works then I'll have the stainless prop gone over by a local prop shop and keep hold of it. I am going to recheck timing and my throttle links and such as well. I do know that regardless of of the prop is causing all the issue, it's been dinged up to bad to to have caused some of it

 

[steelespike]

I guess a new 13 should help out the motor.It should feel peppier but top end may be a wash or even less.
You might try a little choke at speed. Also there is the chance your on 3 cylinders.You wouldn't be the first to not realize
it but for the lack of power.

 

[Sea Rider]

Can't have best of both worlds as hole shot & top speed with a 13 at same time, top speed will be slight less, nothing to worry for not sleeping well. Preffer a faster hole hot and loosing some top end speed, besides very few boaters runs hammer down al day, just hammer down for a faster hole shot and then back throttle to maintain combo on a nice plane along excellent fuel consumption.

A dinged prop as yours should not have any or very slight effect on performance. Improper blade balance, bent prop shaft or a "on it's way" to have a complete spun prop would. Before ordering a 13 pitch prop, make a prop hub slip wot test and confirm 15 prop is 100% operative, Probably a 15 is a good water performer for your combo which you don't know for sure, that's why is a good staring point to check current prop cond in use. If found kaput, your new dilema will be : get same new 15 pitch or move to a 13 ?

Happy Boating

 

[fhhuber]

Well... if the engine is lugging due to load I wouldn't expect the prop to be slipping on the rubber hub insert. The prop slipping would usually lead to over-rev of the motor but the boat still not getting on plane.

 

[steelespike]

Well... if the engine is lugging due to load I wouldn't expect the prop to be slipping on the rubber hub insert. The prop slipping would usually lead to over-rev of the motor but the boat still not getting on plane.

Agreed.

 

[Sea Rider]

Well... if the engine is lugging due to load I wouldn't expect the prop to be slipping on the rubber hub insert. The prop slipping would usually lead to over-rev of the motor but the boat still not getting on plane.

A prop has 3 performance conditions : works spot on, on its way to become spun, kaput. Is it so tiresome to remove prop, mark it, put it back, go for a wot spin lightly loaded, remove, check ? this procedure will avoid lots of wildguessing, assumptions, unnecessary extended OB troubleshooting, could be in front of your eyes and don't see...

If prop is found to be 100% ok, work from there...

Happy Boating

 

[Frosstyx]

Guys, I just checked my spark on cylinders one and three (port side). Spark won't jump over 1/4 inch. I think I've fried a powerpack. This happened last year. I will post in the Johnson forums to get some advice. All the electrical components were switched put except the stator and timing base. Imma order a new pack but really need to figure out what's causing this to happen. The starboard side gap jumps strong at 3/8ths. Port won't make it over 1/4 inch and it's weak.

 

[steelespike]

As you may know In the cylinder under compression it is even harder for the spark to jump.

 

[Sea Rider]

A new power pack seems is on its way...

Happy Boating

 

[Frosstyx]

Actually a stator on the way, did some checking and the stator seems to have gone bad. It is, after all, factory original on a 30 yr old outboard.

 

[steelespike]

Now we should see numbers closer to normal.

 

[b.gagnon]

A 115hp on a 18' lightweight boat running a 15p prop should redline!
High speed side of the stator could cause the problem you are seeing or a powerpack. Did you check it with a DVA? Ohms do not tell the whole story.. You should remove the kill wire from the powerpack to make sure it's not bleeding to ground, and causing a weak spark.....Also check the engine mounting height(could have been mounted higher before the boat was taken apart), if it is sitting too low it will rob a 3-500 hundred RPM! The hydrafoil is prob causing you to loose 100+rpm if it stays in the water while on plane!
Ger Trim Tabs.....