slow OMC

[l1lmonsta23]

I have a 20ft '85 renken cuddy with a 2.5L OMC sterndrive, with the original OMC outdrive. The boat runs great with just me in it, top speed of 38mph at around 3700rpms (off my depth finder), " I'm a little ****" as soon as I get some people onboard, (3-4 more riders), I can't get the boat to go above 22mph, or get it to trim off and level out. Do I need to re-work my outdrive for a different gear ratio? How can I get some more pep? I would like to do some skiing and tubing, are these just pipe dreams, due to the 120hp motor? Any advice would be greatly appreciated.

 

[Helstrm]

Re: slow OMC

Is 3700 rpms at Full throttle? Is so You are over propped or you have engine issues. What size and pitch prop dop you have? I would thing that engine should be running about 4800 rpm at WOT if you are setup right.

 

[l1lmonsta23]

Re: slow OMC

I do not know how to determine the specs. on my prop. Yes WOT is around 3700-4000rpms. I tried adjusting the ignition timing but there are no lines tho let me know where tdc is. I have the mark on the pulley. How would i determine the prop. specs. and what should the correct ones be? Thank you very much for your help.

 

[imported_scott_m]

Re: slow OMC

Look for numbers stamped onto the hub of the prop. You might see something like 19P or 21P. That indicates the pitch. I think for every inch of pitch you give up, you'd pick up 200RPMs, so to get your motor up to say 4800, you need to drop about 5 inches of pitch from where you're at. <br /><br />Also, is this boat stored in the water? If so, bottom growth may be holding you back.

 

[l1lmonsta23]

Re: slow OMC

No, I trailer the boat. Could the previous owner have possibly made that much of an oops in his prop. selection? Also, does the pitch on the prop. determine whether I get more of a "hole shot" as opposed to top end speed?

 

[Helstrm]

Re: slow OMC

Yes. a lower pitched prop will both increase hole shot and decrease top speed. The PO may have been after top speed but over propped so much that your boat can't get out of the hole when you are loaded down. Running your rpms that low is putting a hell of a load on the engine at WOT. I would think with your boat you would want to run no more than a 19p. Maybe a 17P but without knowing what is on the boat now it is only a guess. With a 17P I would think you would get on plane a lot faster and top out around 30mph but like I said all we can do is shoot in the dark without more info.

 

[l1lmonsta23]

Re: slow OMC

ok, well i appresiate the input. I will check the pitch when I get home this afternoon, and repost tomorrow with my findings. Thank you very much for your timely replies.

 

[l1lmonsta23]

Re: slow OMC

i meant quick replies.... sorry about that

 

[whywhyzed]

Re: slow OMC

You know what...That's a huge boat for a 2.5<br />I wonder if the tach is correct.<br />I think I would be ok with 38 mph with 1 person<br /><br />I had a 21 cuddy with the 3.0 and it would only do 35 mph with 1 person<br /><br />The tach may have settings on the back for 4/6/8 cyl, check that you are on 4<br /><br />also ignition problems can cause the tach to read funny

 

[Helstrm]

Re: slow OMC

I have an 84 Sunbird Cuddy 19' with a 7'10" Beam running on a Volvo AQ125a . With a 19P prop 2 adults and 2 kids small cooler. I run 29mph WOT.<br /><br />I should pick up 2 or 3mph when I change out my foil for some smart tabs

 

[scottkanderson]

Re: slow OMC

I have a 86 Renken with a 4.3L and original OMC drive. I run a 15 pitch prop for wakeboarding and I can only get a top speed of 33mph with the RPMs at 4400 WOT (This is running it at 9,000 ft, so if your at sea level you can do a lot better). Of course you have different gear ratio's. Here is a chart http://www.iboats.com/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=1;t=013313 This is all based on wether or not your tach is acurate. You need to have that checked first so you don't blow your motor by dropping to much pitch. My Tach was off by over 400 RPMS and I moved that switch around on the back and made sure it was on 6 cyl, and now it's within 50 RPMS

 

[whywhyzed]

Re: slow OMC

Originally posted by telluride:<br /> My Tach was off by over 400 RPMS and I moved that switch around on the back and made sure it was on 6 cyl, and now it's within 50 RPMS

isn't that something - my 1970 slick had been set on 2 cyl (orig OMC tach)instead of 4 cyl for 34 years until i bought it

 

[l1lmonsta23]

Re: slow OMC

ok well it turns out i have a 13 3/4 x 15p so i have been thinking of going to a 13p but that just seesms crazy. any thoughts?

 

[cobra 3.0]

Re: slow OMC

As mentionned above , check your tach for accuracy. <br /><br />It's possible that you need a 13 pitch. That's a quite a small motor for a 20 ft boat. How are your people distributed in the boat? Too many at the back will slow your boat down to a crawl. Is your trim angle correct? I imagine you have a trim button to adjust trim?

 

[l1lmonsta23]

Re: slow OMC

yeah i have a trim switch, but that doesn't really help when the boat is loaded down. I have also been thinking about getting some trim tabs. I will check the tach setting as soon as i get home this afternoon. I took the dolphin things off the outdrive, and that actually helped alot, me and a buddy went fishing yesterday and i was cruising at 37mph at 4200rpms WOT

 

[stomper]

Re: slow OMC

I have a 77 17' glaston with a 140 omc, was doing the same thing so I thought it had the wrong prop. Got to thinking and I run a compression test, I had 2 weak cyl. Luckily it was just a head gasket, $48 and about two hours the boat runs like a new one. This weekend I had four people over 200 including me and a couple kids. It gets out of the hole fast like it should now so I'd check compression and stuff like that before you go buy a different prop.

 

[l1lmonsta23]

Re: slow OMC

Thanks stomper, I'll check that, especially since the cheapest I've seen the 13p prop. locally was about $175.00. $175.00 for something that may not fix the problem? Seems a little silly.