planing out and rpm's

[djarrett]

I recently got a new boat and i have a few questions.

This is my second one but my first is only a 14' aluminum with a 7.5 merc. The boat i just got is a 1984 procraft 1650v bass boat with a 1984 yamaha 150 ETLN. This boat has power trim and a jack plate. When i take off i trim my motor all the way down and go. The boat comes up on plane real quick and seems to level off fine but my tach is showing 4200 rpm's. The owners manual says to run it at 4500-5500 rpm's. Now when the tach is showing 4200 my speedo is showing around 50 mph. As i start to trim the motor up the engine increases in rpm's and i'm showing 4400 on the tach and about 55 mph. Am i supposed to keep trimming it up until i reach the correct rpm's or should they already be in the correct range after i been going wot for a little bit before i start trimming? I am kinda nervous about going any faster because this a new territory for me, i got the boat to fish out of. I'm really a newbie at this and have read all these post about how bad it is for the motor to run out of its range, this boat will have to last me for at least a couple of years. It also has a hydro foil on it and i am thinking of taking it off and trying it without it, would that help bring the rpm's up also? Also did a compression check and had 105-110 on all the cylinders with a cold motor because i thought maybe the engine was a little weak because of the low rpm's.

Anyway sorry for this being so long but i tried to put all the info i have on here.

 

[QC]

Re: planing out and rpm's

1) Get rid of the hydrofoil and destroy it so no one else wil use it

2) Recheck WOT RPM after that, you will probably get into your range and, yes, it is supposed to be all she has, trimmed out, loaded typically.

3) Verify your tach.

4) Just to be clear, although I am sure you understand this, the WOT RPM is not where you should necessarily run the boat, but it is an indicator that she is propped correctly . . .

P.S. I really liked you in the #88, sorry that you're having to limp along with Michael . . .

 

[djarrett]

Re: planing out and rpm's

Ya, i get ribbed alot about my name but its cool; i enjoy it and i am a big nascar fan.

After doing a search on here i am ashamed of having the hyrdofoil on it and will pull it off before this weekend. Maybe some kid will want it to put on the trunk of his car like the fast and furious movie.

I was really confused by some of the post i've read. I understood that its easier on the engine to run at wot if the rpm's are in the correct range. I even read one about someones' engine actually running cooler at the faster rpm's. I don't care a thing about running around dale hollow at 60+mph, i enjoy cruising around about 45mph.

Not sure how to verify the tach but i'll do a search...

And by trimmed out do you mean all the way "up" on the trim gauge?

 

[QC]

Re: planing out and rpm's

i am ashamed of having the hyrdofoil on it

Well then we are being successful with our little campaign here . . .

I understood that its easier on the engine to run at wot if the rpm's are in the correct range.

I've heard something like that too, but I am not an OB guy. Test data shows fuel economy is better lower, so I would question that though.

Not sure how to verify the tach but i'll do a search...

You need a shop tach or a multi-meter type deal. Poke around here like you said.

And by trimmed out do you mean all the way "up" on the trim gauge?

Yes, but she might "porpoise" there and may be why the guy put the hydrofoil on. Do a search for trim tabs and/or Smart Tabs. Lots of threads. Most boats are fastest trimmed all of the way up and just starting to porpoise (rhythmic up and down motion of the bow). Then you control the porpoise with trim tabs . . . If she won't porpoise, then OK, but you could probably shift some weight aft and get the bow a little higher then . . .

Good luck in Chicago DJ

 

[Silvertip]

Re: planing out and rpm's

Outboard manufacturers recommend wide open throttle ranges to enable you and the boat dealer to provide a prop that "allows" the engine to reach those RPM at wide open throttle. The prop is what determines whether or not the engine can reach those rpm on that particular boat. If the engine can't reach that rpm band, it is being "lugged" and engines don't like that. That is also inefficient because the throttle must open too wide for any given speed. On the other hand, a prop with too little pitch allows the engine to run outside the high end of the band. This is also bad in that it means the engine is "over reving". Over reving and lugging will both destroy an engine. That said, once the proper prop is installed, you don't have to run wide wide open. After all, that's what the throttle is for. The secret to trim is to keep trimming up until speed starts to decrease when engine rpm is increasing or staying the same. You should also have a water pressure gauge to ensure the engine is getting enough cooling water at high trim and high jacking levels.

 

[djarrett]

Re: planing out and rpm's

Alright i understand now.

Pulled the hydrofoil off yesterday evening. I am looking forward to this weekend to see how it feels without the big wing on there.

I do have a water pressure gauge and i watch it close.

Thanks guys for the time and your help, i think i was worried over nothing.