29 mph from OMC 3.0: Help wanted!

[gillis]

Hi, <br /><br />I thought that I would post a couple of questions to you experts out there as I am a "newbie" to boating. I acquired a 1990 Sea Swirl 17' boat equipped with an OMC 3.0 135hp engine. I got it from a good friend, who is basically an auto mechanic specializing in electrical work. He just got finished rebuilding the engine, running new lines, replacing the fuel system, alternator, you name he did it and it sure looks nice! What prompted him to perform this work was the fact that he complained of speed and performance issues. If I remember correctly, his two complaints were top end speed and difficulty pulling "heavy" skiers out of the water.<br /><br />I took the boat out today with a friend and we were only able to achieve a top speed of 31 mph, but for the most part averaged a top speed of 29 mph. To me, this just seems overwhelmingly off the mark. At a minimum, should I not be able to hit at least the upper 30's? I have yet to identify what prop is installed. I don't even know what is the ideal prop that I should be using with my boat. However, I am sure that if I did have the right combination of size and pitch it would not increase my speed 10+ mph! <br /><br />In addition, the engine has been tested twice for proper compression and it doesn't seem to be having any issues. I am also not certain as to what RPM I obtain when running WOT. Perhaps a boat mechanic could measure this for me? <br /><br />Second, the throttle is very, very difficult to shift from neutral-to drive or reverse. In fact, I almost have to "force" it back and forth repeatedly before I can shift it in gear. Shouldn't the throttle shift rather smoothly or effortlessly? I had a very difficult time today trying to dock because I didn't seem to have very good control or response. This was very aggravating and seemed very dangerous.<br /><br />Last, when I run the boat at my WOT speed of 29mph, (laugh) I can let go of the wheel and the boat instantly pulls hard to the left. However, when I run at half speed I experience no issues. Surely, this is not common? <br /><br />Seeing as I am a newbie to boating, I would appreciate any help or advice regarding the issues that I have mentioned above.<br /><br />Sincerely,<br />Scott

 

[plasticfailure]

Re: 29 mph from OMC 3.0: Help wanted!

yeah, i think you should get a tachometer if you don't already have one. For the throttle being hard to push, there should be a tension adjustment somewhere on it; I don't know where that would be though. <br /><br />I would run too long at WOT without knowing where you're taching at. Your prop could be too small and you could be over reving it. <br /><br />Also how are you measuring the speed? using gps would be the best.

 

[trentmh]

Re: 29 mph from OMC 3.0: Help wanted!

I was the friend on the boat today with Scott and I can answer the speed question. We used a GPS to measure the speed primarily because we did not believe the speedometer in the dash although it turned out to be correct. I'm planning on putting a tach on real soon....maybe tomorrow. There is a good possibility that the prop on there now is a 15x17. I say that because the previous owner gave Scott an almost new 15x17 prop that he bent upon launching.<br /><br />Trent

 

[cobra 3.0]

Re: 29 mph from OMC 3.0: Help wanted!

I have a similar set up. 16 ft bowrider 1986 3.0 Cobra and get 38 mph(GPS) with one person on board with a 19" pitch stainless OMC prop. The conditions have to be just right to hit that speed. Usually with 3 people on board and a 14" x 17" prop reving 4500-4600 rpm, I get somewhere around 34-35 mph. With 6 people 30 mph.<br /><br />You need a tach that's for sure. Your prop may be the problem)...or a water logged boat. Are you trimmed out correctly(just before it starts porpoising)? Is the engine timed correctly and using at least mid grade fuel? Mine just lugs when the timing is off. I had to time mine by ear while running to get it just right. <br /><br />My engine has never been rebuilt and I know the compression readings could be higher on mine.

 

[gillis]

Re: 29 mph from OMC 3.0: Help wanted!

From a "trim out" perspective, yes. The only way that I am able to hit 31mph is when I "trim up" until I porpoise, which I can then only hold it for a few seconds. If I then "trim down" just under porpoise I can max out at 28-29 mph. Regarding the fuel, I am not sure what kind was previously used but decided that it would probably be best to run 93 octane pump fuel.

 

[Elk Chaser]

Re: 29 mph from OMC 3.0: Help wanted!

You are more than likely real close to the top end speed. You might achieve another 5 to 10 by changing the prop but your hole shot will suffer some. Get that tachometer installed before you start messing around changing props.<br /><br />My old 19' Wellcraft 165HP 3.7L did about 35 mph with two people and a 21p aluminum prop. I was able to get it up to near 40 with a 21p stainless prop. The 21p prop was not real good at the hole shot for skiiers and I would change to a 19p prop for that which reduced the top end some.<br /><br />The boat did tend to want to pull on the steering at full throttle, even more so with the stainless prop and much less when I changed to 19p prop. Could not really ever take my hands off the wheel though. <br /><br />Try adjusting the trim tab (hopefully you have one) to compensate for the left pull, that's about all you can do about that.<br /><br />I really don't think you have to run a high octane fuel, the performance change is not even noticable at the HP your running (if your looking for more performance from fuel that is). IMO it's not worth the extra money these days unless your running a high compression motor that really needs that fuel.

 

[gillis]

Re: 29 mph from OMC 3.0: Help wanted!

Thanks for all of your assistance. What about my throttle issue?

 

[cobra 3.0]

Re: 29 mph from OMC 3.0: Help wanted!

My 1986 3.0 engine does stipulate at least 89 octane fuel (mid grade in Canada).

 

[marunr]

Re: 29 mph from OMC 3.0: Help wanted!

For your shifting problem read:<br /> http://www.funkopolis.com:8080/~stuart/cobra/

 

[trentmh]

Re: 29 mph from OMC 3.0: Help wanted!

We took this boat to a reputable mechanic yesterday for a thorough inspection. First thing he noticed was that three sensors were inoperable which was causing the shift issue. <br /><br />He also did a compression test and 3 of the 4 cylinders test at or above 145. One however tested at 110. He said he wouldn't be able to say why until after he got it on the water. I don't know whether we should be concerned about this or not at this point? What do you think could be a likely culprit for this newly rebuilt engine?

 

[KaGee]

Re: 29 mph from OMC 3.0: Help wanted!

Originally posted by trentmh:<br /> I don't know whether we should be concerned about this or not at this point? What do you think could be a likely culprit for this newly rebuilt engine?

Could be several things.... but I'd take it up with the rebuilder, that is if there is a warranty.

 

[trentmh]

Re: 29 mph from OMC 3.0: Help wanted!

Gillis and I just got our boat back from the shop after they repaired the shifting issue on this boat. The mechanic took the boat out for a test spin and said that at WOT it was turning 4200 RPM's, but he was still only able to achieve 32MPH measured with a GPS. The current prop is 15x17. I realize that if we went to a 19 pitch prop we would decrease the RPM range and possibly lug the engine. My question is how would reducing the diameter effect RPMs? The manufacturer said that the stock prop that originally came on this particular boat was a 13-1/4"x17. Our objective at the moment is top end speed. Would a stainless prop possibly help and if so, how much speed would you estimate the gain would be?

 

[cobra 3.0]

Re: 29 mph from OMC 3.0: Help wanted!

The 15" diameter seems very large and yes, a smaller diameter will spin faster. You sure it isn't a 14" diameter? 13 1/4" x 17" sounds like the right one to me too. A stainless would give you 1-2 mph more over a good aluminum of the same correct pitch and diameter.