Cobra 3.0L I/O Prop/RPM question

Hello folks. I have no idea where to start. My engine at WOT runs at about 32-34MPH and is doing about 5500RPMs. According to what I've seen, this is not optimum. What I've heard many times is that my normal operating RPMs should be 45-4800, and I should be getting 35MPH easy at this RPM. My question is, what is a good starting range for pitch, type, and composition of propellor? My prop isn't marked in any way, and it's very damaged, so I'm sure changing it will help me, but I have no idea what to start with.

I have a 1986 Rinker V170 with a 3.0L Cobra 4cyl (140HP) I/O.

Does anybody have a similar rig and can you tell me what kind of prop you run and what types of numbers you get? I'm really most interested in getting the highest speed out of it.

Re: Cobra 3.0L I/O Prop/RPM question

*bump*

Re: Cobra 3.0L I/O Prop/RPM question

WOW! can't believe you have no response since 6/9.Prop numbers usually appear under the prop nut.If you read the stickies at the top of the page
it will help post your question better.Your prop may make it hard to get good numbers.You may be able to get it repaired and they can tell you the size.
Then you could use it to get good numbers.
Usually wouldn't suggest without more precise info but..a 19" pitch should be a good starting point.If its a little light you will still have a good hole shot and speed you may have to watch the rpm.With a healthy motor and the right prop you should see about 40.

Re: Cobra 3.0L I/O Prop/RPM question

I just got a 1992 Forester Diva GT 1800 with a 3.0 L Cobra OMC SX 4 cylinder. I don't think the gauges are reading out quite right b/c it says my RPMs are only about 3600 at full throttle. When the boat does plane out the speedometer says about 33 mph but I think we are going faster. I have no idea what kind of prop is on the boat, but it is really doggy when you kick it down. It would be a real chore to try get a wakeboarder or skier up.Where should I start with getting this stuff fixed??? How can I check to see what the RPMS are truely running at?
I am going to try get a GPS in the boat to check speed. Would smart tabs help? Any suggestions??

Re: Cobra 3.0L I/O Prop/RPM question

Welcome,You should really do your own post.It gets really confusing to all involved.Imagine what it would be like with 5 or six answers for each post and responses for each of those.
First thing please read the stickies at the top of the page.Prop size is usually
on the barrel or under the prop nut. Beg,borrow or take a friend with a GPS
for a ride. One or both of your gages could be wrong.But I think they are close.You may have the wrong prop or a engine problem With the prop size your max rpm and the GPS speed we should be able to figure something out.
Wouldn't hurt to have some details on the boat.When you get the details start your new post we'll catch on.

Re: Cobra 3.0L I/O Prop/RPM question

Thanks for the advice and the welcome. Unfortunately when my girlfriend purchased the boat it did not come with the owner's manual and she lives a state away with it. I will start my own thread when I get more info. I belong to a forum like this for my truck and they usually want you to add to existing threads but I will definately take your advice. Thanks again.

Re: Cobra 3.0L I/O Prop/RPM question

I have a 1989 16 foot Forester with the GM 3.0 and OMC drive. I just bought the boat so I don't know a lot about it but maybe my info will help you. I have read that the RPMs should be 4300-4500 max at wide open throttle. Mine with the trim gauge at level runs about 4250 and an indicated 44 mph. Adjustment of the trim up a tad and it will hit 4400 rpm and right about an indicated 47 mph. I am not sure what prop is on it, but the guy gave me 3 extras when I got it and I will see what they are and post them. This is not GPS verified speed, but I have had a lot of boats and it has to be close because it is really moving out. I think the engine is pretty darn healthy and I think I got a decent deal on it for only $500 with it only needing a shift cable.

LM

Re: Cobra 3.0L I/O Prop/RPM question

I took a look at the prop on my boat mentioned in the above post and it says 13.2x18. It seems to work well.

lm