Here's my stats... Is it right? 155 commercial rpms

cdoliver · Sep 11, 2008

Thanks for the help. I have a 18ft Ebbtide dynatrac. It weighs 1150lbs. Powered by a 1990 155hp omc commercial on it. It runs great, but I'm only getting 50mph @ 5000 RPMs. Using a 14.5X 21P omc prop. The Motor is all the way up (top hole) no jack plate. I have a 14X19 prop that gets me 49MPH @ 5400RPMs. I'd like to get the rpms where they should be, but I'm thinking that a 17p is just the wrong direction to take. Thanks again.

rndn · Sep 11, 2008

Re: Here's my stats... Is it right? 155 commercial rpms

The 19 inch added 400 rpm's which follows the guide of 200 rpm's for each 1 inch of pitch. A 17 inch prop should get you to 5800 rpm's.
Have you weighed the boat to make sure it's not water logged as I tend to agree with you that an 18 footer with 155hp should be able to handle more than 17 inches of pitch.

krakatoa · Sep 12, 2008

Re: Here's my stats... Is it right? 155 commercial rpms

Guys
Actually I have a 1993 Mariner 115 HP, my prop have a 19P, it goes up to 5,200 RPM and reach a top speed of 40 MPH. My boat weight is over 1250 punds, this tell you something?

parrilladk · Sep 12, 2008

Re: Here's my stats... Is it right? 155 commercial rpms

what is the operating rpm range of that engine? 40mph at 5,200 is not bad for a 115hp

parrilladk · Sep 12, 2008

Re: Here's my stats... Is it right? 155 commercial rpms

Here's what I found for your motor:
Manufacturer: Mariner
Model: "ONE FIFTEEN"
Year: 1993
Horsepower: 115.0
Cylinders: 4
Type: 2 stroke
Gear Ratio: 2.07:1
Min WOT RPM: 4750
Max WOT RPM: 5250
Spline Count: 15

You are very close to the upper end in rpm. The calculated prop slip is 11% (theoretical top speed at 47 mph). Is your prop banged up? Or is your boat waterlogged? 11% seems too much for slip at wot.

cdoliver · Sep 12, 2008

Re: Here's my stats... Is it right? 155 commercial rpms

I think my thread got hijacked... Back to the 155 commercial,

I understand I have a v8 gearcase with a different gear ratio. What is the standard fix to use this motor for recreational use? Do I just use a 17p?

Randybeall · Sep 13, 2008

Re: Here's my stats... Is it right? 155 commercial rpms

What is the rpm rating on the 155 commercial. The gear ratio will make a difference in what prop pitch is required. That said, you can ignore the ratio and simply match prop to engine rpm rating and go on, that is why there is no prop recommended for an engine by the maker. It must match the engine,gear ratio, hull drag, and a little black magic. Happy boating.

cdoliver · Sep 17, 2008

Re: Here's my stats... Is it right? 155 commercial rpms

Ok, well with a 14X19 I can get 5500RPM's @ 50MPH (GPS). I checked the boat, theres no water, everythings dry. I just don't understand why it's slow. I thought I would at least get 55-60mph. It's in it's top hole and no blowout when you trim it up all the way. Any thoughts? Thanks

Dhadley · Sep 17, 2008

Re: Here's my stats... Is it right? 155 commercial rpms

The clue is in your last statement. "No blowout when you trim it up all the way". If you can do that it's probably still too low. Do you have a 25" motor on a 20" transom?

BTW - good little boat, neat ride.

Randybeall · Sep 17, 2008

Re: Here's my stats... Is it right? 155 commercial rpms

Dhadley makes an execllent point. What is your engine number? Might tell us something. Then again since you mention that you have a different gear ratio maybe someone changed the lower unit.....Where is the anti vent plate when you are on plane. If it is below the water it is too low and will slow you down.

Dhadley · Sep 17, 2008

Re: Here's my stats... Is it right? 155 commercial rpms

It could be on top of the water when on plane and still be too low.

krakatoa · Oct 1, 2008

Re: Here's my stats... Is it right? 155 commercial rpms

cdoliver said:
I think my thread got hijacked... Back to the 155 commercial,

I understand I have a v8 gearcase with a different gear ratio. What is the standard fix to use this motor for recreational use? Do I just use a 17p?

:redface::redface: Im sorry.....

steelespike · Oct 1, 2008

Re: Here's my stats... Is it right? 155 commercial rpms

I was looking at the Etec 150 HO on 5 ranger boats and a Stratos I think actual hp is in the range of 175 Fastest speed was 62 from the stratos,
I think the Rangers ran from 52 to 58 mph. The HO is designed for speed and the boats were all intended to go fast though heavier than yours at about 1500.You may have to get the vertical height up a little as suggested.
Is the V8 gearcase phyically bigger than the 6?

Dhadley · Oct 1, 2008

Re: Here's my stats... Is it right? 155 commercial rpms

The V8 case is indeed larger. And is not nearly as hydrodynamic as the Lightning gearcase on the E-Tec HO's. I still agree with his statement in his first post that going to a 17 is going in an undesirable direction. Everything he's mentioned indicates the motor (prop) should be higher. The Ebbtide Dynatrac, in my opinion, is a pretty good responding hull design. The larger V8 gearcase will always be more drag than the standatd V6 but I think the hull will respond to height.

Here's my stats... Is it right? 155 commercial rpms

Petty Officer 1st Class

Commander

Senior Chief Petty Officer

Seaman Apprentice

Seaman Apprentice

Petty Officer 1st Class

Petty Officer 1st Class

Petty Officer 1st Class

Supreme Mariner

Petty Officer 1st Class

Supreme Mariner

Senior Chief Petty Officer

Supreme Mariner

Supreme Mariner