OMC 3.0 Liter Prop Pitch Suggestions

[Old Yeller Boater]

I'm very new to boating, so please don't flame me. I'm having some difficulty finding good basic information on prop sizes and pitches for my application. Hopefully you guys can give me some suggestions of props or perhaps locations where I can find some good information for my situation.

I have a sunbird 17 foot open bow running a 3.0 liter OMC with an OMC Cobra outdrive. It's a 1987 model. My speedo does not work, so I can't tell you max speed, but my RPMs max at about 4400 with two people, or 4200 with 4 people. I'm currently running a 13 1/4 by 17 pitch prop.

I notice with more people in the boat it takes a lot longer to get on plane and I usually have to have NO people in the back two seats in order to not be driving along with the bow in the air for about 10 seconds. I need to get another prop so that I have a spare, and I'd like to find another size that will allow me to get on plane better. I'm not concerned about top speed. the boat is genarally used for pleasure riding or pulling a tube. I usually will have 4 adults in the boat but would like to get reasonable performance if I happen to have 6 people in the boat too.

Should I try to go up to a 19 pitch, or perhaps higher?

thanks!!!!

 

[steelespike]

Re: OMC 3.0 Liter Prop Pitch Suggestions

We 're certainly not going to flame you we were in your position once too.
If you read the stickies at the top of the page and the helps on the prop store page you will better understand about prop selection.Be sure to ask for any clarifications.
My feeling is that normally that setup would run with a 19 or 21" pitch prop.
at about 4600 rpm and approach 40 mph.Unfortunately without your speed the rpm
doesn't mean much.But to me your rpm with a 17" prop should be higher perhaps even over rev with one or 2 aboard. Higher number props are like high gear in your car more speed less power, lower number props are like 2nd or 3rd gear more power less speed these are generalizations and there are exceptions.Be sure the motor is in excellent tune,throttle plates in carb opening all the way.Prop excellent,bottom clean.Let us know what you find or any questions.

 

[fishrdan]

Re: OMC 3.0 Liter Prop Pitch Suggestions

If your 17P prop has marginal>poor performance you want to go down in pitch, like a 16P or 15P prop. Though, I question why you would need to do this unless there are other considerations, high altitude, heavy boat (soaked foam), poor engine performance and needs a tune-up, other things SS mentioned.


I have a Mercruiser 140HP (basically the same as your 3.0 OMC) and agree with what SS said. I run a 15x19P for all around good performance, but could probably run a 21P for light loads. Under a moderate load (2 adults, 2 kids, camping gear, 1000#+) I'm turning around 4200rpm and running in the mid 30's with the 19P prop. If I'm going to have the boat loaded to max capacity I drop to a 17P prop. If I boat at high altitude I also have to go to a lower pitch prop, 17P or 15P depending on altitude.

 

[180shabah]

Re: OMC 3.0 Liter Prop Pitch Suggestions

OYB - nobody gets flamed here just for being new, just don't ask about oil before doing a search

I agree with the above, something just doesn't seem right with your setup. Need to look at the:
engine - compression test and FULL tune up
Boat - dirty hull, extra water weight
drive - verify gear ratio, should be 2.0:1
opperator - are you trimming properly, all the way down/in for take off then up until it ventilates then back down enough to stop ventilation.

 

[Old Yeller Boater]

Re: OMC 3.0 Liter Prop Pitch Suggestions

thank you for the helpful input. I don't know for sure what my speed is as the speedo is not functional on the boat. I'll take my GPS along for a ride sometime and see what it shows. I'm guessing that I'm running in the mid 30s at top speed, but that's just a guess.

I had the motor worked on over the winter. Not the motor itself, but replacing exhaust gasket and valve cover. Otherwise, the motor starts and runs well, with no excessive noises going on. The mechanic didn't report any additional needs to me, and I do know that it's not buring oil based on starting it in my driveway and noting the exhaust.

Boat is generally clean. Not sparkling, but just shadow of dirt from the lake. Elevation is about 600 above sea level, so it shouldn't be starving for air.

Is there any way to find out what my final ratio is? My boat was most likely pieced together and re-sold, so I don't know how much is original or not.

Firshrdan (or anyone else, for that matter), your comment about soaked foam has me curious. I've noted that my boat seems to sit heavy in the stern when in the water. The floor is supposed to be sealed, but there are a number of screw holes from previous owners and I've wondered if the inside of it was full of water or something. However, I've not noticed any soft wood in any locations around the floors. I have considered drilling some holes somewhere near the bottom of the boat just to see if anything drains. Perhaps somewhere in the bilge area? Is that a good idea? If so, what location would be good to do that?

Thanks to everyone. your input is greatly appreciated!!!

(by the way, I did note the stickies at the top and did some searches, but I wasn't finding the specifics about my boat/motor size, so that's why I asked.)

 

[180shabah]

Re: OMC 3.0 Liter Prop Pitch Suggestions

I'll take my GPS along for a ride sometime and see what it shows.

GPS is the only way, spedo is innaccurate even when it is working

Is there any way to find out what my final ratio is?

Spin the engine by hand, until the prop turns one time. Count engine revolutions. Could be 1.5, 1.6, 1.8 ro 2.0 2.0 is the correct ratio for a 4cyl.

 

[steelespike]

Re: OMC 3.0 Liter Prop Pitch Suggestions

The first thing I would check is that the throttle plates in the carb open all the way.Remove the flash supresser and with motor off open the throttle all the way
observe the plates as you move the throttle.A compression check is pretty easy.
Water in the foam would normally be in the rear and any place in the center I
would drill a hole big enough for a pinky just barely through the floor hopefully between the frames/stringers.You could also weigh the boat.If watert soaked the boat would have a really sluggish hole shot and struggle to stay on plane at reduced power.Unfortunately some I/Os can be sluggish out of the hole and struggle to stay on plane you have to decide if it seems extreme to you.With one or 2 in the boat
imagine you were pulling a skier you should feel he is getting up fairly easily especially with a 17" prop.

 

[Old Yeller Boater]

Re: OMC 3.0 Liter Prop Pitch Suggestions

Thanks. Throttle plates will be checked this evening for sure.

Sluggish, it certainly seems that way, but without experience for comparison, it's hard to tell. I may try to find another boat in my area that I could borrow or ask someone to test mine on the water to get a comparison.

I generally have to run 3400 to 3600 rpm to stay on plane. Does that seem right?

If the boat is water soaked in the foam, what's the solution? Should I cut the floor open, allow to dry, then seal up properly?

by the way, since I've owned the boat, I only have it in the water for day use and otherwise, it's stored, covered with a boat cover, and then plastic tarp over the boat cover just to keep it from weathering too much. It stays dry with the covers on it.

 

[90sunbird]

Re: OMC 3.0 Liter Prop Pitch Suggestions

i had the same boat only in a 90 stock the boat should do about 40. it nice on the water but i also had the soked foam issue i left the boat in the sun and then put in inside with a dehumidfier i never realy dried out all the way but it was better.. you better check the engine out cuse someting isnt correct, 3.0 in that boat is a 130 hp and it runs well, but like every one said your engine isnt right i used a 21p prop on it and i was up at 4600 at like 41 mph before i worked the engine

 

[Old Yeller Boater]

Re: OMC 3.0 Liter Prop Pitch Suggestions

Any other suggestions on drying the boat out?

 

[180shabah]

Re: OMC 3.0 Liter Prop Pitch Suggestions

Any other suggestions on drying the boat out?

Do you even know if it is wet yet? Saturated foam requirws floor removal, foam removal, probably stringer and/or transom repair, new foam applied, new floor installed.

Check the easy stuff first. Throttle, compression, gear ratio, etc...

 

[Old Yeller Boater]

Re: OMC 3.0 Liter Prop Pitch Suggestions

thanks, that's just what I wanted to find out, just how extensive it would be for the dry out process. I was thinking perhaps cutting a 1 foot square out of the floor in some unexposed area (like under the rear seats) would be enough to expose the problem and allow it to dry out.

I checked the butterflys last night. they are wide open at full throttle. Gear ratio and compression are on the list.