Logo
FULL SITE MENU

Under Achieving rpmbs wrong prop

M

markchiz

Contributing Member
"I drive a 1991 four winns fre

"I drive a 1991 four winns freedom 170, with an OMC 3.0 cobra.
In neutral, i can rev it to 6000+ rmp, (only revved it there for a split second to test), but then when i'm in drive on the lake, (by myself, empty boat, no weight) i can onyl reach 3400 rmp tops throttle wide open, and about 26 mph. I normally cruise at just under 3000 rpm, an 21mph.
I was just readign an article about props, and it said if your engine is nto reaching higher RMPs at full throttle, then you have the wrong prop size or pitch?? I'm not sure what pitch my prop is, but does this sound like my problem?
This is my first boat, and i am nto sure what to expect from a 3.0 140HP I/O on a 17 foot boat, but i thought i might get a little more speed and rpm range, dont you think?
Can anybody else with a similar set up (3.0 on a 17ish foot boat) attain higher rmps in drive, and a higher top speed? or is my inexperience in boating coming in to play now?? help!"
 
could be the wrong prop or cou

could be the wrong prop or could be the engine is weak (for any one of 200 or 300 reasons). I would guess that your 17' boat lightly loaded should do around 36 to 40mph with a 3.0. And a 17 pitch or 19 pitch prop would be appropriate to get the engine into the 4200-4600 rpm recommended range at wide open throttle assuming everyhting is working correctly and you have the correct outdrive and your boat isn't water logged and yada yada yada.
 
"engine is rebuilt just last m

"engine is rebuilt just last month. but then again that doesnt mean its nto weak. everything works great on it. I dont think its water logged, before i dropped it in, it was sitting in a dry garage for 2 full seasons. I plan on pulling my boat out of the marina in a few days, is there any way I can check to see what pitch the prop is? should it be labeled?
any way to tell if my engine is "weak"?
starts first crank, revs up quick in neutral, idles soundly, sound great. That extra 15 MPH would be great to figure out!"
 
There are places that can do a

There are places that can do a dyno test right off the prop shaft.
The prop should say it's diameter and pitch on it or at least have a part number that you can google
 
"what about prop diameter?
17

"what about prop diameter?
17 or 19 pitch prop of what dia?
also, are they OMC specific, or are there props that fit alot of aplications?"
 
"what about trim position??
i

"what about trim position??
i'm a new boater (this is the first season i drove a boat...ever) and i have been driving it with the trim all the way down...and someone asked me this morning what position i drive it in...i was kinda puzzled, cuz i thought it was to be driven all the way down. apparently i'm way wrong?? could this contribute to my low rpms and speed at WOT? i sure hope so...cuz adjusting the trim will be alot easier than re-propping."
 
"I can't see the trim maki

"I can't see the trim making that much difference but it will make some. Most folks will start off with the drive all the way down (or trimmed in) this brings the bow down quicker and gets you up on plane quicker. Once up on plane you start to raise or trim out the drive, this raises the bow which gets a little more hull out of the water, less hull less drag more rpm more speed. You will have to play with the trim to find your sweet spot which of course will change depending on how you load the boat, people, gear, fuel, etc. If changing props you need to know what you have as a starting point and as a rule of thumb each inch of pitch can change your RPM's by close to 200. So for example if you are running a 19 pitch and go for a 17 you should pick up close to 400 RPM."
 
"thanks for the input everyone

"thanks for the input everyone.
I took it out tonight, played with the trim. Gained about 500 rpm and about 5mph. still just barely over 4000 rpm and 30 mph. checking my prop tomorrow when i pull the boat out, and see what i'm working with. i'd like to get about 35-40. we'll see."
 
"These are recommendations fro

"These are recommendations from the original omc manual prop chart- all props listed are three blade aluminium. first # is prop diameter, second # is pitch. Listed for a 17'-20'runabout with a 4 cyl. 3.0 omc engine- For 24-32 mph use a 13 3/4 X 15 prop. For 30-40mph use a 13 1/4 x 17 prop. For 34-42 mph use a 13 X 19 prop. For 38-46 mph usa a 12 3/4 X 21 prop. Just remember- usually the higher the speed the less power you will have and the lower the rpm's will be (the longer it will take you to get up on a plane or harder to pull out a skier.) The slower "power" props will get you up on a plane faster, with higher engine rpm's but a slower top end speed. Props are always a compromise. Depends on what you want to do. Good luck."
 
How can I find out the correct

How can I find out the correct prop to run on a 35 hp Evinrude serial #E35ELCTS. It on a 16 ft. V hull Sea Nymph fishing machine with a transom higth of 21 in.
 
Back
Top