5.0L OMC Cobra WOT RPM and prop questions

[Purduebarry]

Over the weekend I got a chance to open up the boat with just a small chop from some wind. I had two others in the boat for a total of three (myself included). I had about a half tank of gas. The hull has not been buffed in years so I’m sure there is some drag, which I’ll addess in the offseason, it’s not full of crust just needs a buff and I’ve read this can help quite a bit.<br /><br />At WOT I only reached 4,000 RPM’s, my speed was right around 40 to 42. Both measurements came from OEM speedo and tach. Motor sounded nice and smooth but I’ll also go for a tune up in the off-season. The boat has been well maintained but is new to me. Assuming eveything is running well and the other conditions mentioned above these are my questions;<br /> <br /><br />1988 Bayliner 1950BR (19.5’)<br />5.0L (230 HP) with OMC Cobra Stern Drive<br /><br />WOT 4,000 RPM’s seemed low. I should be getting 4400-4800?<br />14.5 x 19 Michigan Wheel prop, is this OEM?<br />Michigan Wheel suggests this prop for 36 to 44 MPH?<br />Should my speed be higher or did I get everthing I can expect from this prop?<br />MW suggests increasing pitch to 21 for 40-49 MPH and increasing pitch to 23 for 45-56 MPH. <br />Assuming the incresed pitch helps raise the RPM’s what will happen to hole shot and getting on plane?<br />With the prop I have on now is the boat lugging, by not reaching recommended WOT RPM’s? <br /><br />Also, how much trim should I be using to achive full performance at WOT? This thing does not have an indicator so I’m pretty much guessing. <br />Is it okay to adjust trim when at WOT? I could hear the motor cut out for a split second when I adjust trim under full throttle and I don’t want to do this if it’s harmful.<br />Any tips, do I want to see a rooster?<br /><br />The boat will be for general overall boating, tubing, speeding around but I don’t want to have to change props everytime I hit the lake. I want the best of all worlds (who doesn’t) good hole shot and better WOT, RPM and speed. And lastly, OEM version vs. performance prop? I’ve read that performance props, cupping, finish, etc. provide optimum combination of acceleration, bow lift and flat out speed…is this just hype?<br /><br />I am considering the Michigan Wheel Rapture for Stainless? <br />Or 3-blade version of the Vortex if I stick with aluminum?<br />What do you think and / or recommend?

 

[walleyehed]

Re: 5.0L OMC Cobra WOT RPM and prop questions

First, RPM will not increase with more pitch...it goes the other direction.<br />MW is aftermarket, and not my first choice, but not the last either. I'd be running a BRP/OMC OEM prop if running aluminum.<br />Performance designed props are far from hype, but you have to watch out for props that "claim" to be high-performance.<br />Trim range is pre-set and should not affect engine operation during the use of trim.<br />Trim out after plane-out is established, trim out until RPM raises without a speed increase, or until the boat may want to "porpoise". The prop may "vent" at max trim and rev a tad higher, just bump it back down until it hooks up-watch your speed, you'll see when you've trimmed too far.<br />Sounds like the hull needs a good cleaning...I think I'd do that before I changed props..parasitic drag can slow things down a bunch.

 

[Purduebarry]

Re: 5.0L OMC Cobra WOT RPM and prop questions

I went to the BRP site, you suggesting and OMC OEM prop? I think the MW is the origional prop but it could be dealer installed. Which BRP prop do you recommend, same size and pitch as current 14.5" x 19? What will this prop do that mine won't. What performance props do you recommend? I know it's not a race boat and a buff will help but I still don't think the WOT RPMs were high enough, can a non-buffed hull keep RPM's down too?

 

[Silvertip]

Re: 5.0L OMC Cobra WOT RPM and prop questions

A dirty hull causes drag. Sort like driving your car with the parking brake half set. The engine can't reach peak RPM with a great deal of drag. Also, as Walleyhed says, increasing pitch slows the engine down. Another way to look at that is that IF the engine has enough power, a 21 pitch prop THEORETICALLY will move the boat faster at any given rpm than the 19 pitch. Or - think of the 21 pitch as a higher gear than a 19 pitch.

 

[Boatin Bob]

Re: 5.0L OMC Cobra WOT RPM and prop questions

If it was me I would follow the advise, clean the hull, tune up the engine and if you are still not reaching WOT RPM's (assuming tach is good) then I would go down a size in pitch to a 17 in. This will probably bring you into the low end of the RPM range you want to be in (general rule of thumb is 200 rpm per inch of pitch change), couple of things should happen besides the RPM's going up, should notice a better hole shot but will lose a little MPH from the top end. Props are all about compromise and you will not find one that will give you perfect in all areas.

 

[Fastjeff]

Re: 5.0L OMC Cobra WOT RPM and prop questions

My old Capri 19 footer did 45 mph with 2 adults aboard, and that was with the 3.0 liter 4 cylinder. Your hull bottom must be dragging.<br /><br />Jeff

 

[bdirect]

Re: 5.0L OMC Cobra WOT RPM and prop questions

Okay I read his original post and maybe I am missing something here. He stated that the engine did something not right when he was running WOT and adjusted trim. I think he needs to address what happened based on his description. Dirty hull didn't cause it, prop didn't cause it but something wasn't right and to do anything else before finding out what is wrong with the engine(if anything is wrong) is putting the cart before the horse and may cause more trouble. Sounds to me like he should tune up that engine first and make sure he eliminates whatever that engine cutting out thing is before spending money on props etc.