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Load and the affects of WOT rpm
skennelly
Member Posts: 2,196 ✭✭✭✭
I kind of know the answer to this already...but hoping some others can confirm.
Last weekend I had the most people I ever had on the boat...9 people, 2 coolers misc gear.
Boat is a 270 FV 350 mag mpi...I know it's underpowered for that type of load.
Question is I had the throttle pegged and I want to say rpm's were at maybe 3400 rpm and tough to plane at all. Eventually did in calmer water and I could back off the throttle.
Under such load the engine would never reach the spec'd 4600 range correct?
When it's 2 to 4 people on the boat it acts much better. I just want to make sure there isn't something I should be checking on the engine and that's it's normal that rpm's would be so low.
Thanks
Last weekend I had the most people I ever had on the boat...9 people, 2 coolers misc gear.
Boat is a 270 FV 350 mag mpi...I know it's underpowered for that type of load.
Question is I had the throttle pegged and I want to say rpm's were at maybe 3400 rpm and tough to plane at all. Eventually did in calmer water and I could back off the throttle.
Under such load the engine would never reach the spec'd 4600 range correct?
When it's 2 to 4 people on the boat it acts much better. I just want to make sure there isn't something I should be checking on the engine and that's it's normal that rpm's would be so low.
Thanks
2002 - 270FV Mag 350 B3
Comments
This is just my opinion but if you have a boat loaded and it's wide open at 3400 RPM you are lugging your engine. If you do that much you will kill it.
I respectfully suggest that you find out what your normal load is and prop the boat to that situation. 1"prop (rake, cup etc. equal) raises or drops the RPM by 200.
Al often has props for sale at great prices @Alswagg.
What is your WOT with 2-4 people and the gear you normally carry? prop for that.
BTW with the exception of 3 very highly modified blueprinted/balanced engines I have owned I have NEVER run my engines at WOT for any length of time. That's just my opinion.
My 4.3LX's (3 of them), my 350 Magnums (5 of them) my twin 502 mags - I only ran them at WOT for brief periods during break-in and at periodic intervals after that for maintenance. By brief I mean run it up to WOT count to 5 and run it back down to 3400 +/- RPM.
I have never liked keeping a "stock" engine at WOT for any length of time, no matter what the manufacturer or the techs say. That opinion was reinforced by two of the best engine builders I have ever met. One an American at Hawk Racing Engines in Ft. Lauderdale and the other a Canadian who owned his own computerized shop.
That's my opinion, but as I have stated a few times in the past I have sold a ton of boats. I never needed a broker as friends and dealers wanted my boats. Every one of the I/Os I have owned is still pulling without a re-build.
Set your prop right for your load, use a quality oil and filter, change them according to your use and the engines will out live you!
Probably nothing wrong with your engine. Next time out with a light load, run it up to WOT to make sure everything is normal. Heavy load prevented boat from planing out, which in turn prevented the prop from turning at it's most efficient attitude, which prevented engine from getting into WOT rpm range. I have had a large load like that before and could only get on plane by putting the trim tabs down all the way and having a couple heavy people move to the dinette all the forward in the cabin. Once I got up on plane, I was able to back off on the tabs, decrease throttle a little bit and move the people back to where they were. But in the future I decided if I had a bunch of people, I would just put-put along at 1200 rpms and enjoy the company, it's a cruiser, the trip is more fun than the destination anyway!
if you run your rig like that often, you're gonna want at least an extra propset- and one that is way less steep than the one you now have, and you'll want to take the few minutes it takes to change them before using them...
your engine isn't lugging if it can hit 4800RPM @ WOT with a lower to moderate load... it's safer to turn higher RPM's/less load than it is to be WOT, washing the cylinders (running silly rich) and less RPM's from the engines vantage... the governor will limit RPM's to protect the engine @ around 5400 RPM... the dizzy won't start to spray spark until 6k rpm's or so, and valves won't float until around 6500RPM's... so... WOT between 4600 and 5200 is safe for the engine (and where the cam profile is designed to provide max torque) .... so... find a prop that makes best use of that range and allows the engine to find it without exceeding it and without hesitation...
Normally me and my wife and perhaps 2 other friends are out and I never have to go much past half throttle to be happy...I've ran WOT for short periods and it runs within specs so I don't believe anything is wrong just needed some of that necessary feedback.
@skennely, No disagreement nor disrespect.
I'm glad that you don't plan on repeating that scenario (and it's just my opinion) - if I understood it correctly.
That is, if you had your throttles "pinned" or nearly "pinned" as in full forward position and you were only turning 3400 RPM you were, IMO, severly lugging your engine and that will IMO kill your 350 real fast.
Having your throttles "pinned" and running at WOT within your normal RPM band is a completely different scenario and NOT the same as what I thought described.
To be honest - I wouldn't run my engines at more than 75% of their WOT capability for any length of time anyway...BUT ....having my throttles very near or at their full WOT position ("pinned') and only turning 3400 rpm......never, never, never - for me.
@Liberty44140 I agree with your first sentence! My opinion is that it is probably good to run up to WOT and down a few times a summer. I think it's good for the engine's rings, burning off crud etc.
In my opinion, though - EXACTLY as you said. I took the engines up for a 5 count and then back down.
IMO Not only no flag on that play and fine for the engines but - probably good for them!
I have run my race engines at WOT for 20 minutes but I had special fuel pumps (Gaffrigs) with special fuel gauges with safety set points and alarms as lean fuel blows engines faster than anything. Lean fuel = soaring heat, super fast then BAM. I also had windage oil pans with special purpose oil pumps and filters that kept a constant oil pressure along with the aforementioned blueprinted/balanced and racing prepared engines.
And after that type of run I used my automatic oil changers to change all of the oil and filters. IMO that's how to run engines at extended WOT. That's just my opinion - others may disagree. How many engines have I hurt in 40 years? None.
Could you run a normal 350 at WOT for longer than a 5 count. Sure you could. How long without stressing it? I don't know.... 5 minutes? 10 minutes? ...but why? IMO you're just burning a lot of gas and risking a glitch in fuel or oil delivery.
I have a 260 EC, 350 MAG B3. I've had 11 people onboard, 6 adults, average 200 lbs each, and 5 kids average 40 lbs each. So 1400 lbs of humans onboard (it was a big family). Full water tank, full gas tank. Full cooler under wet bar, full fridge in cockpit. All my fishing gear loaded up. The ONLY thing that was empty was my waste tank.
It obviously took longer to plane (about 17 or 18 seconds), but with trim tabs down, it got onto plane at around 22 to 23 mph, and then went to normal cruising RPM at 80% throttle, hitting 4000 to 4100 rpm, travelling around 29 mph.
There were +1 ft waves, which does make it more difficult to plane, but nothing serious. If I had 3 ft waves, probably much more difficult to plane.
Anyhow, to answer your question, once I was on plane, the engine worked the same as if I was not loaded (ie. 2 to 3 people on board). So, I can't explain why you were limited to 3600 RPM, unless you weren't able to achieve plane.
Boat Name: King Kong
"Boat + Water = Fun"
Boat Name: King Kong
"Boat + Water = Fun"
Dont worry about it, you have a fluid (lake water) damping the engine. Vs with cars a clutch and tires, and with an auto the trans fluid and tires.
You can run the engine at WOT as long as the revs stay in check. the danger to engines is not lugging them, its the jerk at TDC under high RPM loads.
the stress on a engine is exponential with RPM,
so LUG away at low RPM if you want.
as for running the engine at WOT, these engines are so under stressed and such a low RPM, its not an issue. The valve trains are good for 5500, and the rest is fine at those low RPMs as well.
these are literally the most under stressed simple V8s that exist.
In the absence of other problems, engines can handle WOT.
all it sees is WOT and high RPM
drive the boat how you want... enjoy it..
I have run with 8-9 people aboard . Boat will plane but have to push it hard UNLESS you use a little finesse and your trim tabs and becomes much easier. I will put trim tabs all the way down before i start, trim motor down a bit more then normal and then slowly work up the throttle. Put it up like a 1/4 wait for boat to catch up, then half wait , and then like 3/4 and give it a few seconds and it will pop right up on plane. Bring tabs back up ( i usually leave them like 1 led bar down helps porpoising) then trim up motor a bit and back off throttle and usually cruise like 3400-3700 rpms which is around 25 mph-26 mph. Once boat is up on plane weight in the boat doesn't have a effect on mine and probably yours as well. Cruises the same. At WOT I can see 35-38 mph. but takes some time to build up to the final rpms . More people I talk to with cruisers the cruising speed of 22-25mph is pretty much average among all the big boats including sea rays, top speed usually 32-38 mph in most boats too. Of course lighter Sea RAY SS etc will be quite a bit faster.
Loads against marine engines are based on hydrodynamic loads. These are calculated geometrically. Loads on automotive engines are based on aerodynamic loads. These are calculated arithmetically. Completely different.
Think pushing a big hull through water or an aerodynamic automobile on bearings through air.
A marine engine can be run at WOT as long as its WOT falls within its proper operating WOT rpm band.....but, I repeat, why do that - other than periodically for maintenance (as I stated above)
However, if a marine engine is run at WOT and this falls well outside its proper WOT operating band you will harm it.
If you have a newer engine and it logs your "driving" habits you will probably void your warranty as the situation will be considered abusive. Don't listen to anyone, including me. Consult your manual or call Mercury.
Why do you think there is such a defined process for propping your boat? Why have so many guys on this forum asked propping questions regarding the performance of their boat?
I don't want to argue or preach - I have no right to. I just don't want anyone to hurt his engine.
Each of us run our boats a little differantly, When I take off, tabs down, drive tucked in, and hammer it, WOT, until its on plane, trim up, tabs up, set throttle at desired speed to conditions. tabs down just a little. once every other outing its Wot for a couple miles checking temp, rpm, speed, listen, oh it sounds so good, then back down to desired speed. longevity in mind trying to take care of my cruiser.
@randy 56.... Agree that what you are doing, IMO, is absolutely fine.
I have been spoiled with all of my Rinkers as I always bought the biggest engine that could be put in the boat. So, I always had power to spare.
My EC 310 would often do 34mph at 3400RPM The EC 360 would often do 35 mph at 3500 rpm because their engines were sized appropriately for the boats and the drives were trimmed right. It's not rocket science.....get the correct size engines for your load!
I didn't have to go full throttle with the EC310 because it had twin 350 Mag Bllls and the EC 360 with the 502 Mags Blll X drives was such a beast that 3/4 throttles was fine.
Once in the EC 360 with 11 on board and headwinds with chop I pinned it. They roared.
Then as usual tabs off and touch the drives up until some porpoise then touch down. Of course with Vessel View I have optimum cruise indication, so that helps too.
When I didn't have V V I just watched MPH and tweaked the drives accordingly.
Like you @randy56, I adjusted the shims on my drive trim rams for the total negative trim possible as I knew I would never be able to totally trim them up anyway. In fact I trimmed them up very little.
A few times a summer, I'd run the RPMs up to wot for a 4 or 5 count but wasn't really worried about that because they got a plenty good work out putting the boat onto plane.
It does sound fine hearing those engines roar at WOT.
with 7 or 8 people she needed everything to align just right lol
people in the cabin
tabs in and throttle all the way down
and going only 18mph