WOT RPM

R

Roy

Member
Joined
Jan 1, 2000
RO Number
114
Messages
186
How bad would it be to have a prop that limits WOT rpm on Mercrusier 5.0 MPI/Alpha 1 to only 4400 rpm? Spec says 4600 - 5000. Thanks...Roy
 
Has it ever turned rated WOT RPM?
Do you know the engine is producing rated power? Like, is it in tune?
If it turned rated WOT RPM before, what changed?
Fouled bottom?
Damaged propeller?
Overloaded?

IF IF IF it's running right, clean bottom, not overloaded, then I'd say you need less propeller pitch. That 5.0 is producing 260 Prop Shaft HP (PSHP) and that makes it pretty high strung. Probably a HP per cubic inch measured at the crankshaft. For that reason, I don't think it's a good idea to "lug" it by running full load at less than full RPM.
 
Yes, JD hit on something. If you're at the end of the season and you have growth on your boat, the 4400 you see now may bery well hit 4600 with a clean bottom and running gear. You should only consider your RPM accurate if your hull is clean.
 
let me rephrase that. Your RPM is probably accurate (as much as the gage is accurate). What isn't accurate is your WOT. With a clean bottom your boat will run faster allowing the shaft RPM to increase slightly.
 
I'd rather see much closer to 5000. If it were mid season I might not immediately pull the boat, so long as I was not running a heavy boat. But, if it were me I'd do something by next season at the latest. What I would not do is only try to get to 4600. I'd shoot for 4800 min.

But that's me and I've never been a fan of lugging engines. Lugging equals heat and heat equals wear and broken things at unpredictable times.
 
As long as you don't run for extended periods at WOT your OK. If your cruising in the 3200 to 3600 range slight overproping helps economy and helps keep the noise down. Many commercial boaters seriously overprop for economy. Especially the displacement hull crowd.
 
The main thing when over propped or low WOT is not hammering it to get on plane. Take it up easy. Keep your cruise down to 3200 RPM or so.

Do complete tune-up and clean bottom, then do another WOT check. Re pitch prop as needed to where the prop guy says.
 
Thanks for the feedback. Everything is brand with the engine/drive and the bottom is also clean. I never operate at WOT. I basically have 2 speeds - 3200 for cruising and idle speed. My objective is economy so maybe I should keep what I've got. It's a 4 blade S/S prop.

Thanks....Roy
 
A good prop shop could also tweak your prop to pick up a couple of 100rpm.
 
Going back to what JD asked, has that engine ever reached the recommended WOT?
 
Yes. It did reach 4800 with a 17 pitch, 4 blade. Current prop is a 19 pitch, 4 blade.

Roy
 
Wait. You made 4800 with a 17 pitch (which was right in the sweet spot) and moved to a 19 pitch? Why did you make that move? Each inch of pitch or size is usually at a cost of 100 RPM. The next time you get it serviced, have it changed back.
 
Roger That, Stephen.
My prop is a MerC "Vengeance" stainless series, three blade and sized as in Signature. With that, we can reach WOT RPM which is only 4400 for our series engine. That's probably deceptive, though, since for some reason our Alpha outdrive is 1.62:1 ratio. That's unusual for a V8, they're usually 1.4-something. The 17" pitch would be more typical with the 1.4-something gearcase. Never had a 4-blade but I'd guess it's a little more efficient, meaning less slippage and a little more limitation on RPM...
I wonder what the boat is, what it weighs, what the load is.
I'm in the group that wants to be able to achieve WOT. For one thing, I'd rather have my boat that little more responsive that less pitch affords, even if it takes a pinch off the top end speed.
 
It's interesting that years ago a 350 block carried a recommended WOT of around 4000 RPM. After the advent of roller cams this was upped to 5000 and beyond. So from a structial stand point you are not going to hurt an engine if you can only get 4000 RPM. what you lose is some of the HP gain that the extra RPM gives you. If your an all out performance addict, by all means prop to the higher limit.

If your like the most of us and cruise 400 or more RPM below your highest capability you are not hurting anythig and getting better economy with less wear and noise.
 
It is generally a myth that more pitch improves economy. From an engine standpoint economy is usually achieved at a mid range RPM but every engine is different. None of that means much in a boat where prop slip overwhelms everything else.
Even if you knew the peak efficiency point of your engine what is more important is where the prop and hull do best.

Getting the weight out and going slow are key economy booster things to do.
 
and Fords were 3800-4200.

Best economy is acheived when running with the highest vacuum reading for your desired RPM. That means running with the throttle open as little as possible to maintain your RPM/speed.

So going back to some of the basics, the faster you go(with a deep V planing hull), the higher the boat will run out of the water. That gives you the smallest wetted surface and the least drag. To get your best economy run up to your target RPM(or speed) and a little faster. After you get there, incrementally back off the throttle and you can maintain your target RPM by running the boat slightly higher out of the water which means less drag and less throttle needed to maintain that speed.

...of course, rough water knocks that ability out the door.
 
Keep in mind , with higher prop pitch keeping the WOT below spec RPM, the TPS on your MPI will normally need to be at a higher % of full to maintain any given cruise RPM, and with the increased props slip you are unlikely to gain enough add'l speed to overcome the higher fuel at that RPM.

Between the greater load placed on the engine, likely poorer economy, and noticeable decrease in ability to maintain RPMs at one throttle setting running into or down heavier seas, using a prop that is not just at the bottom of the RPM specs but 200 RPMs below the engine mfgr's recommended minimum , the 19" pitch prop seems like a bad choice. If you add gear or more POB it will be worse.

I would pitch to attain at least mid to higher end of the mfgr's spec, and they provide that recommendation for good reason.
 
quote:
Originally posted by jtybt15

and Fords were 3800-4200.

Best economy is acheived when running with the highest vacuum reading for your desired RPM. That means running with the throttle open as little as possible to maintain your RPM/speed.

So going back to some of the basics, the faster you go(with a deep V planing hull), the higher the boat will run out of the water. That gives you the smallest wetted surface and the least drag. To get your best economy run up to your target RPM(or speed) and a little faster. After you get there, incrementally back off the throttle and you can maintain your target RPM by running the boat slightly higher out of the water which means less drag and less throttle needed to maintain that speed.

...of course, rough water knocks that ability out the door.

Click to expand...


Every model of boat is going to be different. In my situation I have a 350 Merc 2 barrel in a 25ft inboard and the boat is fairly light (4600 dry I think). I run slightly over-propped and choose to run at about 3200 RPM. That puts my planing hull on top of the water with still plenty of RPM before I top out at 4000+ RPM. For my situation, this gets me about 8GPH at about 22 knots. If I increase RPM, I only go marginally faster and sacrifice a lot of fuel. Working with the prop shop he was able to dial in my needs with just one adjustment.

Roy, you need to go back to your prop shop and work with him. You will need your current performance values and weight of your boat. He can then help guide you to an appropriate conclusion. Keep in mind that the recommended RPM is not the speed you should cruise at. You should cruise at no higher than 90% of the WOT specs. Some don't agree with that thinking but in my opinion running at WOT constantly is beating the engine.
 
put what ever prop that does what you want at the speed you want to go. i way over prop so that i have a great quiet cruising speed i can stay on plane at 2000 if i want to fly i change prop to wind her up to 4800 props are like women they all will get the job done but a few are much better at the task than others :) as a note we go the other way with fishing boats putting a low pitch on to get rpm up and idle hull speed down to 1.3mph how a prop carries a boat and trimability also comes into play
 
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