WOT RPM's and cruising RPM's

[tony_cliffy]

I am still working withm y dealer for final prop selction for my new 115HP 4-stroke Johnson. With my current prop the WOT RPM's are 5300. Johnson says this is "lugging" the engine and I want my WOT RPM's closer to 6000. I plan to re-prop to achieve this. But my manual says I shouldn't always run at WOT, instead run at less than WOT to achieve better fuel economy and engine longevity. This would have me running my engine probably 4500-5000...which has been described as the detrimental lugging!<br /><br />Am I harming my engine by running at less than WOT? I feel like I am getting conflicting advice.<br /><br />Thanks!

 

[Duck Loads]

Re: WOT RPM's and cruising RPM's

I'll take a stab at this. I'm sure others will join in.<br /><br />There is a difference between an engine that has a prop to run at 6000 rpm (wot) and you back it off to 5000 and an engine that has a prop to only turn 5000 rpm (wot). The engine at 5000 rpm @ wot is under a greater load that the engine that is backed off to cruise at 5000 rpm. Make any sense? I'm sure you'll get a better explanation soon.<br /><br />Be safe,<br />Duck L.

 

[Spidybot]

Re: WOT RPM's and cruising RPM's

Right. If your engine reaches or comes close to the upper range @ wot limit, it is a SIGN of 'correct prop'. Means that the prop is good for all rpm as it puts only the expected load on the system while running.<br /><br />So, a prop that - with your normal load and running conditions - reaches the desired rpm without any other performance problems (grip/ventilation etc.) is right even for cruising at lower rpm.

 

[rickdb1boat]

Re: WOT RPM's and cruising RPM's

tonycliffy <br /><br />If you prop for 5800-6000. At 4500-5000, you will not be lugging your engine. You will be allowing it to run at its best, within it's powerband and with greater fuel econemy and longer engine life. Running at 5800 will cause no damage to the engine either, but it will cut down on fuel economy. I have a pontoon with a 91' 70 HP Evinrude that sees most of it's life at 5500-5800 RPM's and has had no problems in all the years I've had it. Runs like the day I got it. I have it propped for 6000 RPM's and yes, I do run it at that RPM sometimes, but mostly a bit below. Once you get used to it, you can almost tell when the engine is running in that "Sweet" spot...

 

[walleyehed]

Re: WOT RPM's and cruising RPM's

Tony, Is the engine mounted directly on the transom, or is it up slightly-which hole are the bolts in at the top????<br />Not to complicate anything, but this info is very important in recommending the next step for your set-up..<br />Please advise.

 

[rickdb1boat]

Re: WOT RPM's and cruising RPM's

As Kenny says, be sure you are set up for optimum performance before you change props. If you are, then by all means, prop it to get the RPM's up to at least 5800-6000. Good Advice, Ken. Also, what type and brand of boat are you running this motor on? Is this a new boat? More info will help us help you!!!!

 

[LubeDude]

Re: WOT RPM's and cruising RPM's

Everything mentioned is true, these guys know what they are telling you! WalleyeHed has spent countless hours with props and set-ups!<br /><br />Look at it this way!<br /><br />Your automobile is geared properly from the factory as to normal driving! It has a transmision, (Lets use a 5sp standard trans), Your doing 55MPH, Now lets put the car in 3rd gear and it takes 4800 RPM to achive 55MPH, if you came up to a hill that was just steep enough that when you stepped on the gas you could not accelerate but hold the 55MPH, thats what your boat is doing all the time at WOT! Figuring this hill goes on forever, you would get better milage in your car if you backed off say to 3800 maybe to 45MPH but you would still be able to accelerate back to 55 but no farther! Granted there is a difference here, but I think you get the Idea!

 

[tony_cliffy]

Re: WOT RPM's and cruising RPM's

The boat is a 2004 Crestliner 2070VDC, which is 20' long, has a 96" beam and and is a center-console aluminum semi-V johnboat. The engine is mounted directly to the transom and, I believe, uses the middle bolt holes of the engine. The boat is fairly light (Crestliner says 848 lbs. hull weight) and has a 20" transom. I will verify the exact mounting configuration when I go home this evening and re-post if anything is different).

 

[Cricket Too]

Re: WOT RPM's and cruising RPM's

tony...this is all great and true advise and I have only one thing to add about WOT. You said that your motor is a 4 stroke, and even though you will be propping(gearing in effect) your motor to reach 6000 rpm, I don't think that it is reccommended to run it that high very often like a 2 stroke. A 2 stroke can run all day at 6000 rpm like RB stated but a 4 stroke has alot more moving parts and does not have a constantly fresh charge of oil every stroke. Deffinetely get it to be able to hit 6000 but I would usually run it in the 4500 range, that's not to say you can't run it WOT sometimes, I just wouldn't do it continually like alot of people do with 2 strokes. Good luck with the new motor, should be fun.

 

[pflathead]

Re: WOT RPM's and cruising RPM's

Tony<br /><br />Dont forget that rpm on an empty boat is going to be different than with it fully loaded. We have to find the middle of the road to get it just right. I have had two dozen boats in my life and none have been just right, so who am I to advise. I'll shut up!!!!<br /><br />Pflathead

 

[walleyehed]

Re: WOT RPM's and cruising RPM's

I have had two dozen boats in my life and none have been just right, so who am I to advise. I'll shut up!!!!<br />

Pflathead, it takes someone with a fair knowledge base to even understand when "none have been just right", so you're many steps ahead of most in recognizing that simple fact, so don't cut yourself down......Good work!<br />Now, lets see what he finds on the mounting bolt location when he returns....

 

[tony_cliffy]

Re: WOT RPM's and cruising RPM's

Kenny (or others),<br /><br />I was wrong about my engine mounting. My engine is mounted using the top most bolt holes. Do you think this looks OK? I hope an image is appearing below (first time posting an image here...).<br /><br />

<br /><br />

 

[angus63]

Re: WOT RPM's and cruising RPM's

Tony<br />Looks like your cav plate is about 2" below the bottom of the boat. I would bump it up to even or even slightly (1") above if you want to optimize.<br />Good luck

 

[Hooty]

Re: WOT RPM's and cruising RPM's

Raise the engine until it ventilates when turning, then drop down one hole.<br /><br />c/6<br />Hooty

 

[walleyehed]

Re: WOT RPM's and cruising RPM's

Ditto on Hooty's re.......do it ONE HOLE at a time....<br />Trim the engine out about 2", put a 2X4 on a floor jack...slide jack under skeg with handle extending to the rear, pump jack to take some weight off of bolts, loosen bottom 2, remove ONE top bolt then loosen and removed other top bolt while you steady the engine. Have a buddy slowly pump the jack until you can slide a top bolt back thru #2 hole and tighten bolt/nut. install #2 bolt, tighten it, then secure both bottom bolts before removing the jack....should be set for test #1.....Good luck.

 

[Spidybot]

Re: WOT RPM's and cruising RPM's

Just to clarify, Tony:<br /><br />Moving the motor up on the transom takes the lower unit out of the water thus making less drag, improving rpm and speed.<br /><br />As Hooty points out the limit is when the prop drags air down (ventilates) when cornering at speed. Therefore one step down when that happens. Once the height (also referred to as X-dimension) is right, you have the proper running conditions for picking the right prop (or keep the one you have)

 

[tony_cliffy]

Re: WOT RPM's and cruising RPM's

Kenny,<br /><br />Are you saying you too think the motor is mounted too high?<br /><br />For this style boat (see pics above) do you think raising the engine a couple inches will increase my RPM's? My problem is that the current prop (a 14x23) will only turn 5300 RPM's at WOT with two people and standard gear. I was considering dropping to a 14x21 prop. But if I reduce drag by raising the engine will this possibly achieve the desired RPM's (5800-6000) in a more efficient manner?<br /><br />Finally, given this is a new boat, shoul dmy dealer known he was mounting it low such that I can ask him to adjust it? He tends to argue against any change I ask him to make. Is there a resource I can use that says an engine should be mounted with the antiventilation plate even with the bottom of the boat?<br /><br />Thanks to all!

 

[walleyehed]

Re: WOT RPM's and cruising RPM's

The "standard" for position of the AV plate is "even with" the bottom of the boat, again, this is just the starting point as a general rule.<br />You should gain about 75-125RPM per hole up, and as far as max performance, my answer is "Yes" the engine should be raised....how much is dependant on factors that include prop style, weight of boat, design of hull and center of gravity.<br />Raise the engine one hole at a time and test....you may notice ventilation when turning at top-speed, which can be fixed by 2 methods, possibly 3....1 would be to simply lower engine back down 1 hole, or#2, Have your prop "worked" for more cup (bite), and #3, possibly consider some set-back of about 6".<br />You would be surprised at what you can do with this set-up if you want to take the time to play with it, and yes, most dealers don't want to mess with the set-up procedure mentioned, as it "will work" as a factory set-up, just not as efficiently as what we are talking about here.

 

[Hooty]

Re: WOT RPM's and cruising RPM's

Kenny iz saying the engine is mounted to low on the transom. Raise it up (^).<br /><br />c/6<br />Hooty

 

[Dhadley]

Re: WOT RPM's and cruising RPM's

Raise it way up. Once you get it out of the snow it'll be better.<br /><br />Seriously, go up.<br /><br />Good luck!