Am I getting full RPMS

[bsh21wash]

19ft Seaswirl 4.3 liter OMC Cobra with 3 people in the boat I was getting about 4300 RPMS at about 43 mph does this sound about right? I also messed around with the trim and didnt really notice a difference. Thanks

 

[Haut Medoc]

Re: Am I getting full RPMS

I'm surprised that you are getting that much.....
I wouldn't do a thing different......​

 

[Don S]

Re: Am I getting full RPMS

The specs for the 4.3 (not the 4.3L HO) is 4200-4600 rpm. at NORMAL load.

 

[bsh21wash]

Re: Am I getting full RPMS

So 4300 would be on the lower side? I could still potentially have more power?

 

[Haut Medoc]

Re: Am I getting full RPMS

Potentially, yes.....
You will have to get a different prop that will get you to 4600 rpms....​

 

[ziggy]

Re: Am I getting full RPMS

to get the extra rpms, yer gonna need to go down in pitch i'd think. this will get you more power. out of the hole. but may also make it not go as fast. i don't know why yer trim aint making any difference. i don't know omc's, the mercruisers i've delt with ya trim the drive up far enough ya can start ventalateing. that'll bring the rpms up.....though not desireable at all.

 

[bsh21wash]

Re: Am I getting full RPMS

Ya it has a 19 pitch on her right now. Maybe I wasnt tilting it up far enough?

 

[QC]

Re: Am I getting full RPMS

More RPM is not necessarily more power . . . Almost all test data I have seen shows that boats are fastest when propped toward the bottom of the RPM range, of course hole shot suffers, but it depends on your priority. If the load you were carrying is typical or heavy I wouldn't touch anything unless you are dissatisfied with the hole shot

Also, If up trim is not making the boat faster I would shift some weight aft and see if that makes a difference. I would keep shifting weight aft until she porpoises at full up trim and full speed, and then put a little back up front or add trim tabs to control that porpoise. You should trim up for speed until either: a) your speed starts decreasing, b) you hear the prop start to ventilate or c) you reach the max trim limit.

 

[bsh21wash]

Re: Am I getting full RPMS

Trim all the way up?? Wouldnt the drive be out of the water then?? What did you mean porpoise? Also what will it sound like and do when it ventilates? Thanks

 

[jedimaster]

Re: Am I getting full RPMS

Porpuising means the bow will rise and fall repeatedly.
To find the fastest trim angle I just trim it all the way(so the bow is up high and the drive is at its lowest) until it starts to porpuise a bit then I bck it off slightly until it levels out. Usually one or two hops is all it takes. It could be that your trim is not adjusted properly and your not able to trim it enough?
Not sure about Ventilate but it sound slike he means your jumping the prop right out of the water and your engine revs like mad, basically your pulling a dukes of hazard in a boat. This usually happens when you jump off a big wave or wake from another boat.

 

[Haut Medoc]

Re: Am I getting full RPMS

Trim all the way up?? Wouldnt the drive be out of the water then?? What did you mean porpoise? Also what will it sound like and do when it ventilates? Thanks

He doesn't mean all the way up.....
He means up as far as you can go with out cavetation/ventilation...
Porpoise is when the boat bobs up & down, like what porpoise do when they follow alongside a boat......
With ventilating you will experience an increase in RPM & a decrease in foreward motion, like the prop is not 'biting' any more.....​