18 ft OMC Cobra 2.3L Prop Pitch?

bwpilot20

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Jul 20, 2009
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I've been reading through the forums and have done some research but... still need help. I've got a 1990 VIP Vision 1700 that is just under 18ft long, weighs about 2100lbs dry, with an additional average of about 500lbs of people and gear onboard. It has a 128hp OMC Cobra setup and currently has a prop that was blowing through the WOT even at 5000 feet in Colorado. I since have moved down to Missouri where the elevation is around 1000 ft or less... My max RPM range is 5200-5600 rpm and I'd be ok with sticking around 5200 rpm ideally. I've looked all over the prop that is on it right now and can't find any numbers, but that's basically worthless anyways because I can't ever run WOT without blowing way through redline, and so I have no ability to reference what pitch I need. So I'm wondering if anyone can give me some suggestions on prop sizes? I ran through the "Prop Wizard" online calculator and it was giving me 13 1/4 x 17" size props for waterskiing and etc... and 19" props for top speed. My boat is kinda rare, so what's a common size prop to run on an 18 ft bowrider around 1000 ft elevation with a four banger? :) I don't really have the money to buy multiple props and experiment, so I'm just hoping to get something close that will give me the ability to push that throttle all the way down and see where my WOT is without blowing my engine... Oh yeah, I've tried to find out the gear ratio but the manual is vague and just says 2:1, so I'm not sure if that's realistic or not, but if it helps, the gear ratio would be stock for the boat since we made the classic OMC outdrive mistake a few years back and filled the gearcase from the dipstick, leading to a nice $2100 surprise.
 
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hwsiii

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Jan 25, 2009
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2,639
Re: 18 ft OMC Cobra 2.3L Prop Pitch?

The first thing you need to do is check the tach with a known good tach. Then pull the propeller and look inside it where the nut holds it on and in the back of the prop for some numbers. With full gas and one person in the boat you want to turn full WOT of 5,600 RPM. The motor needs to be capable of turning WOT RPM when it is lightly loaded for the longevity if the engine. If you set it up for 5,200 RPM that is not good, it is putting a load on the engine at all RPM. kind of like running a car up a steep incline all the time.

H
 

bwpilot20

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Joined
Jul 20, 2009
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3
Re: 18 ft OMC Cobra 2.3L Prop Pitch?

Pulled the prop and it's a michigan prop, under where the prop washer sits it has the word "Michigan" and what appears to be the letters "P", "J", and a zero all spaced out about half an inch from each other. Just outside the washer area are the letters "JPS" and the only other numbers/letters are on the outside of the prop hub it has a little symbol that looks like a sideways "D" and the numbers 1052 immediately after that sideways "D" symbol. There is nothing else stamped on the prop anywhere. Any idea how to derive a part number from that information so I can get the pitch of this prop? I contacted the previous owner and he said that he thought the prop was a 15 pitch.

About the Tach, the tach was checked last year and found to be reading correct. I guess I could check it again, but over the years it seems like it's been reading consistent based off sound of the engine... plus it usually seems happy cruising at 5000 rpm, but of course that's with the throttle at roughly 75%. I have gone ahead and gone WOT before just to see what it would settle out at and I reached a steady 6200 rpm. I know it was over-revved when I did that because I definitely had valve tapping around 6000-6100 rpm indicated (and that's running Mobile 1). Before I switched to Mobile 1 I could get valve tapping at about 5400 rpm indicated.
 

hwsiii

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Joined
Jan 25, 2009
Messages
2,639
Re: 18 ft OMC Cobra 2.3L Prop Pitch?

Pilot, from everything I found that motor is supposed to run at WOT from 5,200 to 5,600 RPM. The valves shouldn't tap at 5,600 no matter what oil is used, when was the last time the valves were adjusted. I am just trying to figure out what condition the motor is in.



H
 

bwpilot20

Recruit
Joined
Jul 20, 2009
Messages
3
Re: 18 ft OMC Cobra 2.3L Prop Pitch?

Sorry, didn't mean to make you worry about my engine, the engine runs great and has for years. I was just saying that with the original oil that the dealer was using the engine would get valve tapping issues at WOT rpm, I could have been mistaken about that happening at 5400 rpm, but regardless, the problem was totally solved by using mobil 1 synthetic. I also upgraded the ignition system when I did the oil swap and that may have helped also. In fact I did a ton of work to the boat after we first bought it used back in 1996. :) It's a low time engine with only about 200 hrs total on it.

Any idea on my prop? I've looked all over Michigan wheel's website trying to find a part number that matches the "1052" that is on the hub, but no joy... By the way the prop is aluminum. I also have yet to figure out what the JPS means that is stamped on the prop... I've noticed that all the 17-18ft boats at bass pro that have the 3.0L mercruiser setup are running 21 pitch props, but that seems like a pretty steep pitch change if I am indeed running a 15 pitch right now...
 

hwsiii

Commander
Joined
Jan 25, 2009
Messages
2,639
Re: 18 ft OMC Cobra 2.3L Prop Pitch?

Pilot, welcome to Iboats prop forum and we will try to help you as much as we can, but there is certain information that is needed in order to help you. Working over the internet and not being able to see the boat or prop the question is about makes it much harder to propose a solution to the correct prop that you need, and most people don?t know the make and model of the prop . When we get information in bits and pieces we continually have to look back and forth to try and piece the information together. The following form has all of the information that is needed to help you with your problems in finding the right prop. If you do not know all of the information that is asked for on the form please give us the most information you have available as I have a program I designed to find the right prop for any planing hull boat that needs this information. It requires time to find all this information when we have to look for it and in my defense I expect you to be willing to expend as much effort in finding the right prop as I do. We hope to be able to help you get the best prop we can for your particular uses, and this information is required to do that.
It is not as simple to pick the right prop as most people coming to this forum think it is, that is why we ask so many questions, it is for your benefit we ask all these questions. And if you have a stainless steel prop or are changing to one the blade geometry of the prop can make a lot of difference in speed and RPM. Most of my calculations are done assuming an aluminum 3 bladed prop will be used, when changing to a SS prop or changing within SS props we need to discuss what you are trying to accomplish.

Iboats Boat and Motor Info

1. Year, make and model of boat
2. Length, width and base weight of boat, look for boat decal on back of boat
3. Number of people and gallons of gas normally on boat
4. What do you use the boat for
5. Is it a Deep Vee and if so how many degrees of deadrise
6. Year, make and model of motor
7. HP and gear ratio of motor IMPORTANT
7A If you have a mercruiser and don?t know the ratio get your serial off the outdrive and go here http://www.sterndrives.com/mercruisersterndrives.html
8. Manufacturer?s recommended Wide Open Throttle (WOT) range
9. Anti-ventilation Plate height above keel of boat if it is an outboard in inches
10. Is it a bass boat or does it have a pad bottom
11. Does it have a hydrafoil or trim tabs
12. Make, model, diameter, pitch and whether SS or aluminum prop
13. WOT RPM and speed from your current prop and how much gas and how many people were in the boat for the test data and is the speed by GPS. If you do not have a tach you can buy a Tiny Tach for $ 50
14. Are you at sea level, or a much higher elevation
15. Has your motor been tuned up lately and have you checked that the carburetor butterfly is opening all the way, checked compression, and looked at the plugs and checked spark, all of the forgoing could be the reason your prop is not attaining full RPM
16. How long has this prop been on the boat and why, at this time, do you think it is the wrong prop
17. Does the prop show any damage that you can see
18. What problems are you trying to cure or what are you looking for the boat to do that it is not doing the way you think it should or to your expectations

REMEMBER, The numbers I give you will be NO better than the information you give me

The only thing I ask of you is to come back and give me a report of WOT RPM and speed for my database.

H
 
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