What pitch? (I know... its a loaded question)

[my05hammer]

OK, I got the johnson 115ETL77 on my 16ft Invader Intruder tri-hull working like a top. Then for my second time in the water I took someone who was supposed to know the Rock River and we destroyed the prop on some rocks. Before it was ruined, we were getting up to 36mph according to GPS. But the motor seemed to be low in rpm's, like around 3800-4200. I am going to check it with a strobe tach the next time I take it out after a new prop.

The question is, the factory prop was a 13X19. I want to get two props and one hub. I want a 13X19 and I would like to try a 13X17 to up the rpm a little. Will the 13X17 increase the top speed a little by not lugging the motor? Or is the 36mph all I am going to get and the 13X17 will just help get on a plane better and tow tubes better?

I would love to see 40 mph with this boat (its the gearhead in me).

 

[steelespike]

Re: What pitch? (I know... its a loaded question)

First check your setup.Raise the motor on the transom to a point just before venting(cavitation) becomes a problem.Usually the antivent plate(just above the prop)ends up about an inch above the bottom.Be sure the carb(s) are opening all the way.You need to be at at least 5,500.
If your guessing just put a gun to your motors head.Your strobe tach is a good way out but may be difficult to aim on the water at speed.But it can be done.
You may want to start with a 17" If it ends up with a slight overev you can still use it to help pick the right pitch.If your rpm guess turns out to be right your going to need 1300 rpm that could
put you at about 13" pitch.Thats way out, the 115 typically runs 17 or 19.

 

[SparkieBoat]

Re: What pitch? (I know... its a loaded question)

your tach is off/broke..or regulator/rectifier is bad.

 

[my05hammer]

Re: What pitch? (I know... its a loaded question)

My boat doesnt have a tach. Thats why I will be using the strobe. I plan on getting a 19 and a 17 and using the 19 for a back up.

 

[SparkieBoat]

Re: What pitch? (I know... its a loaded question)

a tach is a worthy investment, you can pick a good used one up for not much.

 

[my05hammer]

Re: What pitch? (I know... its a loaded question)

I would like to have a tach. I'll have to figure out how to hook one up to the motor and get one for it.

 

[Texasmark]

Re: What pitch? (I know... its a loaded question)

I used to run a 19P on an 18' Caravelle tri-hull (1972) with a 125 Johnson with no problems....and I didn't have PTT. WOT was 39 (I remember because I could never get that extra mph and hit 40) with the normal load of 2 adults and gear.

With the prop rating they use now, your 115 is probably at least as good on hp and the lower unit gears are probably the same ratio. Bet your Invader was built with the same shaped hull, maybe even within 10 years of mine (They were popular around here), so one would expect increased performance with the smaller boat with essentially the same engine.....I was running the OMC SST when it first came out and that could be a consideration.

So, not knowing anything about your boat, I would get the 19 in SS and figure out why you can't get more top end out of it....a 115 on that boat ought to be up in the mid to high 40's with the 19 and pop up a heavy slalom too; obviously not at the same speed.

On the tach, most remote control wiring harnesses have a loose wire for that purpose and it is already wired to the engine. The tach supplier will include instructions on hookup. Someone on here can supply you with a wiring diagram also and the color of the tach input wire from the engine. Other inputs are 12v and gnd.

Good Luck,

Mark

 

[my05hammer]

Re: What pitch? (I know... its a loaded question)

The boat is a 1974 16 foot Invader Intruder Tri-hull with a 115ETL77 Johnson outboard. With the original 65hp Merc on it my brother was able to get 25-28 mph out of it. He was claiming that with the 115 I should see mid 40?s to 50 mph out of it, but with the original 13X19 prop (before I damaged it) I got a max of 36 mph according to GPS. The engine was not up to rpm, and once in a while the prop would spin on the hub and I would have to practically bring it down to an idle to get it to engage again. I had a prop repair guy look at it and he verified that the rubber hub was spinning.

So hopefully you are right and when I get my new props, the boat will see the low 40?s, but up near 50 mph would be awesome for sure. (I am ordering one hub and two props; a 13X19 and a 13X17)

Thanks for the info on the Tach!!

 

[Beefer]

Re: What pitch? (I know... its a loaded question)

I doubt the 'factory prop' is the one that's one there 37 years later, and I also doubt that a 19p was the original setup for that boat. If you're the original owner, you can confirm or dispute that.

When I got my boat, there was a 19p prop on it, and would max out at 4000 rpm (rec WOT is 44-4800), and 27mph. I dropped down to a 17p, and the WOT is now at 4400rpm, and my max speed (GPS) is 33mph (wind and current help). It also sounds a ton better with the 17p. I'll guess that the 17p is going to bump your speed up to close to 40 if not more, because now the motor will be more efficient.

What you need to do is NOT buy both props, buy one prop and a proper tachometer. A tach is probably the most important gauge on a boat. It can tell you tons about how your motor is doing. Once you have the tach and the new prop, you can then determine how far off you are from your recommended numbers, and then adjust as necessary. You may be fortunate enough to have a prop shop that will let you try out different pitched props to determine which is best for your particular setup.

For a tach, you can get a 'Tiny Tach' (do a google search) for less then $50, and it's so easy to hookup, even a caveman can do it.

Remember; 1" of propeller pitch is equal to approx. 200rpms. So by going from a 19p to a 17p, you will gain roughly 400rpms, and vice verse.

 

[my05hammer]

Re: What pitch? (I know... its a loaded question)

I doubt the 'factory prop' is the one that's one there 37 years later, and I also doubt that a 19p was the original setup for that boat. If you're the original owner, you can confirm or dispute that.

What I meant by factory prop is that it is a Johnson prop with a Johnson PN and it is the correct dia and pitch for this engine according to the Johnson manual that I have for my engine. Thanks for your reply. Your experience with the change in pitch to a 17 with increased top speed has confirmed my suspicions that I too can benefit from a 17 pitch prop. I think I may just buy one prop and then look into the tach. Thanks for the hint on the Tiny Tach.

 

[Texasmark]

Re: What pitch? (I know... its a loaded question)

I totally agree with your brother but while running a cupped 19P SS. You are dealing with a situation where the boat performance has a benchmark, his benchmark....it's not some estimate with nothing to support your numbers for your "specific" situation. If you do the 115 installation correctly you should get numbers proportional to his estimates. If you don't "fix what's broke."

My neighbor across the street at the time and my bro in law both ran 60 Johnsons on 15' Invader tri hulls and could easily get in the 35 mph range if alone or load up the family and go water skiing too. They would run 15-17P alum props (gear ratios were different for the 60 and 125) I remember the numbers as I was running a 125 J, omc SST prop, on a heavy 18' Caravelle tri hull running 39 max and I could outrun them but not by all that much. And these engines were built before BIA changed to prop ratings which your 115 has. So theoretically, you have the same engine I had but on a 16' boat with a modified V tri hull rather than my deep V trihull which rides smoother, but takes more whoopie to make the numbers.

Good Luck,

Mark

 

[my05hammer]

Re: What pitch? (I know... its a loaded question)

I just realized there was a major typo in one of my posts earlier that stated the max speed I was getting on my boat with the damaged prop was 6 mph according to GPS. It was not 6 but 36 mph. I think that 36 is good but I think that the motor has more in it than that. Its just going to be a matter of getting it all set right.

 

[SparkieBoat]

Re: What pitch? (I know... its a loaded question)

6MPH at 3600 RPM????? you have a spun hub?? I am confused you may want to start a new thread with a detailed and precise explanation of your problem.

 

[ntreder]

Re: What pitch? (I know... its a loaded question)

Steelespike, or anyone.....

I just repowered from a 2005 115 Merc carb to a 2010 150 Etec.
pre-repower
Merc
RPM 5000 max mph 42.7 GPS with avg load
14x21 aluminum prop

Etec
prop 1 - 15x17 alum max rpm 6000, max mph 47
prop 2 - 14.75x19 ss viper max rpm 5000, max mph 49.5

question is I don't think the dealer mounted the motor exactly the same as the 115 as far as where the cavitation plate sits. Is there a general rule as to where that should be.

Oh yea boat is a POLARKRAFT 187 WT kodiak.

that 115 screamed, and I'd expect more out of the 150 etec. There also seems to be more spray in general coming from the back of the boat motor and sides (and I do understand that there is 80-90 more lbs in that new motor....

thanks!
RPM with

 

[ntreder]

Re: What pitch? (I know... its a loaded question)

one more thing, the dealer is getting me a 17 pitch stainless viper to try instead of the 19 pitch. But everything I've looked up online recommends for that boat and load calc...a 14.25x19....maybe this is all because the motor isn't mounted perfect...but you'd think dealer knows what they are doing

 

[SparkieBoat]

Re: What pitch? (I know... its a loaded question)

ntreder, you should start your own thread, hijacking is frowned upon here.

 

[Texasmark]

Re: What pitch? (I know... its a loaded question)

ntreder, you should start your own thread, hijacking is frowned upon here.

Not only is it frowned upon, it is illegal on the site.....get a grip sucker.

Mark

 

[my05hammer]

Re: What pitch? (I know... its a loaded question)

6MPH at 3600 RPM????? you have a spun hub?? I am confused you may want to start a new thread with a detailed and precise explanation of your problem.

No... I reread an earlier post of mine and saw that I had typed 6 mph. I forgot the 3. So I wanted to clarify that the boat was NOT going 6 mph but was actually 36 mph. And yes... it was doing 36 mph with a spun hub.

I just bought a Solas Rubex 13X17 prop (and a Garmin Echo 100) from the iboats store.

 

[Beefer]

Re: What pitch? (I know... its a loaded question)

No... I reread an earlier post of mine and saw that I had typed 6 mph. I forgot the 3. So I wanted to clarify that the boat was NOT going 6 mph but was actually 36 mph. And yes... it was doing 36 mph with a spun hub.

I just bought a Solas Rubex 13X17 prop (and a Garmin Echo 100) from the iboats store.

Be sure and post your results. I'm interested, and it will help others in the future.

 

[my05hammer]

Re: What pitch? (I know... its a loaded question)

Will post results for sure.

My prop comes in on Monday and my brother and I are going to try the boat out on the Mississippi on Wednesday.