question about max rpm (WOT)

[rsfeller]

i'm got a V/P 4 cyc (AQ125A) and the book recommends the max RPM is 4400-4800 (WOT). although early in the season i only ever could get it to 4400 RPM.<br /><br />now it's late in the season (freshwater bottom growth?) and the max RPM are about 4200 and my respected mechanic says i have the correct/recommend prop for my application.<br /><br />should i be running the recommend 400RPM under listed MAX or 400 RPM under the ACTUAL max...thus i should be cruising at 3800RPM?<br /><br />the only issue with this theory is that i can barely sustain plain at RPM below 3900.

 

[Bondo]

Re: question about max rpm (WOT)

Just Clean your boat bottom,+ You'll be right back in the Sweet Zone.....<br />Or, You Could always try the next Smaller Prop.....

 

[magster65]

Re: question about max rpm (WOT)

Exactly, all boats slow down during the season. If you're at the minimum recommended rpm at the end of the season you're tuned/propped well. A lot of people run their boats outside the guidelines.

 

[airman]

Re: question about max rpm (WOT)

It's almost always better on the engine to get on a clean plane, even if you have to spin it a little faster. It sounds like your boat needs just about full power to do that even when everythings clean. I've done mid season cleanings with a scrub brush on a long pole at the dock or with a snorkel and mask. It doesn't take that long.

 

[Boatist]

Re: question about max rpm (WOT)

I agree with everybody, clean the hull. You will get better speed, better fuel mileage, better performance. Also check and see how much extra Gear (weight) you have added. Running a boat with a dirty bottom is like running a dragster with a parachute.

 

[rsfeller]

Re: question about max rpm (WOT)

thanks for you input.<br /><br />what about this question i posted:<br /><br />should i be running the recommend 400RPM under listed MAX or 400 RPM under the ACTUAL max...thus i should be cruising at 3800RPM?<br /><br />i'm just curious to what the recommendation in the book is refering to...running a few rpm under book max or actual max...

 

[Doug Durako]

Re: question about max rpm (WOT)

The three I/Os I have owned since 1978 have all been propped so the motor can hit the maximum recommeded rpm at wide open throttle, with a clean hull.<br /><br />Not that I use it that much, but I like to have the max if I need it.<br /><br />Plus, I can always tell immdiately if the motor is acting up.<br /><br />90 percent of the time, we are running at the 3000 to 3500 cruising rpm range.<br /><br />Also, I have not left a boat sitting in the water since the first one back in the 70s. Seems like all we did was clean the bottom of that darn thing!<br /><br />Back in those days, we easily lost 400 rpm due to creeping crud on the bottom through the summer season.

 

[magster65]

Re: question about max rpm (WOT)

You never want to run at WOT for any length of time. It wears the engine quickly and if there's anything amiss it'll hurt the engine. 400 rpm below maximum rpm as a cruise is spinning/working your engine quite hard, too hard in my opinion (if that's WOT).

 

[Bondo]

Re: question about max rpm (WOT)

" the only issue with this theory is that i can barely sustain plain at RPM below 3900." <br /><br />That's a Real Problem.......<br /><br />Most boats "Cruise" at 2800 to 3500rpms......

 

[rsfeller]

Re: question about max rpm (WOT)

it's not a real problem, i think you as many are over generalizing a boats varibles! <br /><br />the rpm needed to sustain my plane is a function of engine power vs weight of the boat. considering my boat is rather large for the engine it's no surprise. <br /><br />i'm still waiting for someone to explain to me if the manuals recommendation of cruising 200-400 RPM under WOT is 200-400 under the actual max RPM i can obtain, or the recommend RPM i was getting earlier in the season...

 

[Doug Durako]

Re: question about max rpm (WOT)

Cruising at 400 rpm under the max recommended is to hot for extended periods. Can't imagine a manual that would recommend that.<br /><br />You should be able to cruise all day at 75 percent of the lower side of your max recommended range, which I think is what Magster and Bondo are trying to tell you.<br /><br />Is your tach accurate? If it is, borrow a prop of the next smaller pitch size and try it out.

 

[rsfeller]

Re: question about max rpm (WOT)

cannot confirm the tach's accuracy...but i know the running under 400RPM is correct.<br /><br />i think most here are forgeting that im' running a 115HP 4cyl VP on a 22' cuiser that must weigh a good 4K lbs...<br /><br />i will try to find a smaller prop next season.