What should I be expecting

[GiMLit]

Facts
1977 Cobia 18.6 ft Bowrider originally powered by a 120 hp with electric shift drive.
It now has a transplanted 3.8 liter (185 HP) OMC Stringer with 800 Series drive (has 1.77:1 on sticker) on the from a 17 ft Tempest with a 2 bbl carb. Model # 982566 Serial # J 269796 from what I have in my OMC manual that is a .57 ratio I assume the .57 is a final drive and the 1:77 is upper case ratio.

Prop is a aluminum 15x17 in good shape but not new.

Lastnight I was getting 4000 at WOT GPS has me at 38 MPH with 3 adults and 4 kids. Comes up on plane decent. I am not disappointed in these #'s but considering I am 400-500 away from ideal RPM I am out of the power band and could be better.

I have tons of automotive experience but very limited marine.
With the low RPM at WOT I would have to look at engine performance issue as a reason (I have gone over things and I believe the engine performance is good) or have to lower my prop pitch.

I have read some others with this same 3.8 liter and they are running much taller pitch props with proper RPM and higher Top speed.
There was a very knowledgeable member here until recently who goes by the nicks- Hy Stat, kjdkjd and also whywhyzed, who was running as tall as a 23 SS prop with much higher speed.

Question are - I know there are no sure set answers but base it on your experiance and thoughts.

1. What would you expect me to be running as far as speeds ?
2. What prop range would you expect? - 15x17 seems like it is reasonable to me. To drop to a 14.75 x 16 or something like that I am giving up Top Speed for hole shot and RPM - I think I am missing something and shouldn't be loosing top speed, or would I ?

Any ideas, thoughts etc are wanted and welcome.

Thanks

 

[GiMLit]

Re: What should I be expecting

Thanks for moving this. I actually meant to post it here in props.

 

[GiMLit]

Re: What should I be expecting

hehe posting to my own thread.
DOH
Reading other threads. Wondering my WOT RPM. I have yet to do any runs with a light load ie me and a couple tanks of fuel or even me and a light fuel load.
I might see another 200 RPM and obviously with it more Top Speed. Still I am off RPM by a good 300 RPM or so I would think.
Again any input would be great.

 

[codertimt]

Re: What should I be expecting

Just MHO, but I think you're in pretty good shape...and you simply have a tach issue. Assuming the drive ratio is actually 1.77:1(or .57 which is 1 divided by 1.77), then 38mph at 4000rpms would equal a negative prop slip...so basically theoretically impossible.

However since the speed is via gps and thus should be accurate, if you plug the numbers with an average 12% prop slip it comes up to about 4700rpms...

Here is the prop calc, I like to play with...

http://continuouswave.com/cgi-bin/propcalc.pl

Anyway, not sure of the hull design of that Cobia, but that seems like a decent speed for a loaded boat... Other folks numbers and prop choices aren't necessarily applicable unless they are in a similar hull and you know the gear ratio is the same...so make sure of that before worrying about their numbers too much...

So, I say verify the tach number and go from there...

 

[GiMLit]

Re: What should I be expecting

Thanks for the link and the input.
I am going to test my RPM readings out and see what I have. Like I said I am not unhappy with what I have so far but I think I can have more

 

[GiMLit]

Re: What should I be expecting

OK thought maybe I had wonky tach readings so I hooked up a shop tach and went out on a test run tonight before we pulled off the lake.

RPM 4165 @ WOT
GPS 38 MPH
Prop is a 15x17 Aluminum
Drive ratio is 1.77 (.57)

K if I run this through the prop calculator I am at -6% slip.
WTH is going on? LOL

Besides the negative slip issue, something I thought was odd, this run was the first time I have had less than 7 people aboard (had 2) and top speed was the same as with gear and 5 additional people.

Thoughts
If I was to drop to a 14, 14.25, 14.50 or 14.75 prop what if anything would each give me in RPM increase? If so what pitch would you recommend I go to?

Like any person tuning something I would like to have my cake and eat it to.
I would love to add Rpm and get in the prop range and also top speed but basic prop selection has me stepping down in prop pitch for RPM increase and ending up with less top speed.

Love to hear your thoughts.

 

[steelespike]

Re: What should I be expecting

I'm not a pro but unless you have some kind of crazy gear ratio I think yopu should be running a 20 or more pitch and signifigantly higher speed.
And with a 17 easily max or over rev the rpm.Something isn't right.

 

[GiMLit]

Re: What should I be expecting

I am just confused on how I am getting such screwed up numbers. Obviously something has to be off. Slip in the negative clearly can't happen.

My RPM being so low is also making me curious.
My compression is a little low @155 which if you were just looking for reasoning for RPM being low then you could swallow it.
The calculations on slip still being off means there is something I am missing.
I even went as far as looking at other gear ratios in my manual for other engine combinations which would explain the calculations being off but it still makes little sense.
I guess to confirm the gear ratio I will have to count it out and verify it is what I think it is. Then I can continue chasing this around LOL

 

[codertimt]

Re: What should I be expecting

Well, you've verified the rpms are reading correctly and we know you have a good gps speed reading(unless you're testing on a fast running river, in which case it could be off still...). So that leaves prop and gear ratio as the variables. One way you can sort of evaluate the gear ratio is turn the motor by hand 1 revolution and see if the prop turns about 1 3/4...

Other than that, the only thing is to verify the pitch of the prop.... Do you know the entire history of the prop? Perhaps it was repitched at some point? You're numbers work out pretty good if it was actually a 19" instead of 17"... I would guess a prop shop could do this...or if you could demo a different 17" and see how it performs?

That's the only ideas I can come up with... Good Luck..