What's my WOT RPM? and Trim Question?

[pvisser]

Hello.<br /><br />I have a 1983 19' Doral with a 3.8L V6 OMC inboard engine. This engine really moves the boat well. My questions are as follows:<br /><br />1. What is the WOT for this engine? I have been running it at 5000, pretty comfortably, which gets me about 37 MPH. I still have more room to push on the throttle. This is the RPM's as shown on the dash gage.<br /><br />2. One the throttle there is a trim lever, with a matching gage on the dash. Ranging from Up to Down. Here come the "dumb" question. How do I use the trim properly?<br /><br />Thanks

 

[sculbert]

Re: What's my WOT RPM? and Trim Question?

To be honest I don't know the exact WOT but it sounds to me like you are running around 500 rpm to high. I looked in my Mercruiser book and all the engines are designed to run between 4100 to 4800 which is for a 4 cylinder. The rest are 6 and 8's The 6 and 8's run bertween 4100 to 4600.<br /><br />It sounds to me like you could use the next size higher prop and get alittle more speed. without much drop in bottom end. Are you running a Stainless steel prop or aluminium? If aluminium you can got to the next size prop in stainless very easily. <br /><br />Now for the trim. That is a very impotant item. I usually test mine by the seat of my pants and speed and RPM As your boats starts to plane out you want to raise your trim a little. You want to watch the rpm's and speed. find a comfortable balance for the too and make sure you aren't propoising the boat. IE.. jumping in the water up and down. This will plane your boat out for peak performance and speed.

 

[Northern Eclipse]

Re: What's my WOT RPM? and Trim Question?

From reading your post it seems that your boats prop is to low a pitch, your rpms are to high and speed seems a bit low, My guess would be your current prop is a 15p or 17p, that boat should be able to turn a 19p or 21p prop and be aleast running low to mid forties, that 3.8 is a strong motor and produces good torque so it will turn a bigger prop, see if you can try a couple of props from a Marina or prop shop, most good outfits will let you try a prop, with the rule of you break it you own it.

 

[pvisser]

Re: What's my WOT RPM? and Trim Question?

Thanks for the help thus far..<br /><br />I changed the Prop. It was a 17p, now I have a 21p on it. The WOT is now about 4800, and the boat cruised along nicely in fairly choppy water at 40 MPH.<br /><br />BUT..<br /><br />When pulling back from WOT the throtle sticks for a bit.. Any idea how to resolve this? I was hoping to just WD-40 it...

 

[sculbert]

Re: What's my WOT RPM? and Trim Question?

The sticking might be from you not normally running at full throttle. therefore you might have some buildup in the throttleplate casuing it to stick. You can use some carb cleaner to try to clean it out.

 

[Rickles]

Re: What's my WOT RPM? and Trim Question?

Don't trust the dash tach. for proper RPM.I see many boats that are off by 3-500 rpm regularly.Use a good "shop" tach to verify RPM.

 

[pvisser]

Re: What's my WOT RPM? and Trim Question?

I took it out again this time with a buddie's shop tach, and indeed the RPM's were out just as Rickles said they mey be. The actual reading at WOT with a 21p prop was 5010. I have ordered a 23p prop. The marina also figures this should bring us to where the RPM's should be.<br /><br />Does this sound ri8ght to ya'll?<br /><br />The 3.8l engine does put out alot. I mean alot of torque.

 

[Ralph 123]

Re: What's my WOT RPM? and Trim Question?

Once on plane, slowly adjust the drive up until the boat begins to porpoise (bounce) then trim back down slowly until it stops bouncing. It is a constant process as you change speed, conditions, etc. It becomes second nature after a while.<br /><br />PS- Drive should be all the way down when getting on plane. Lots of great books and videos on the subject.

 

[Jdeagro]

Re: What's my WOT RPM? and Trim Question?

Getting on plane with a 23 may be a bit difficult.<br />However, if the engine RPMs are correct then you should stay with the 23 pitch and look elsewhere to fix the planing issue. Otherwise the engine will continuously run faster than necessary at all speeds (more gas ans engine wear).