Wot

[Andrew Leigh]

Hi,

if a manufacturer states that the rpm range is 5000 - 6000rpm what then is the WOT. Is it 6000rpm or should one be conservative and keep it down to say 5500rpm to save the motor? Why the range? Why not state a fixed rpm with a tolerance?

Thanks
Andrew

 

[hkeiner]

Re: Wot

A range is specified because it is a range of WOT RPMs that is acceptable, rather than only one specific acceptable RPM. Mathematically, I guess one could represent this same range as "5500 RPMs plus or minus 500 RPMs" and mean the same thing. I assume it has just been traditional (and perhaps clearer) to just state it as a range rather than have everyone do the math using a tolerance.

Having said this, I also understand it is better to prop a motor to run at the higher end of a specified acceptable WOT RPM range. The motor runs cleaner if propped at the higher RPMs and it also helps avoid lugging the motor when extra weight is carried in the boat.

 

[tashasdaddy]

Re: Wot

it is impossible to get a hull, and motor set up to run a specific rpm. the proper set up involves the hull design, and weight distribution, motor height on the transom, and trim of motor to the boat, and most important is the correct prop to good with all the rest. most motors do not reach the optimum horsepower until the hit 5000rpms, above 6000 they start to loose efficiency.

 

[Texasmark]

Re: Wot

On the initial question, 5000 to 6000 possibly is used for the non-technical person to easily understand.

Engines are rated to deliver minimum rated hp at a given rpm. How much you really get from your "production" engine depends upon how all the ducks lined up when it was built. If everything was perfect, you might get 10% more than the engine that had everything wrong....but still came off the line meeting minimums.

Then you get a suggested range of operation at WOT. This is an assumption that in operating your specific rig, if you are within that range at WOT, then you can expect normal service out of it and that it will perform without damage at the lower throttle settings. Outside that range, you could be lugging it or over-revving it and the corresponding engine damage resulting from that could be expected (but in the case of over-revving, not necessarily occur).

We have learned on here from the guys that know, that OB engines like to run at the higher end for best performance and long life. So, if your engine is rated in the center of this range, then I guess one would assume that you will be getting more out of your engine than what it's rated for....as long as the hp torque curve holds up for you if you are operating it at the high end.

HTH

Mark

 

[gss036]

Re: Wot

I don't believe in running at WOT on any engine for a long period of time. I think you will eventually seize your engine. I say this because I have several friends over the years that have tried that and end up buying a new engine. If a new engine is in your dreams, then by all means run WOT until you blow then engine.
Most engines will have a "sweet spot" for running and you can listen to the engine runnig and determine that. Usually for a big V-6 Mercury it is around the 4000-4400 rpm range. You will also get better fuel economy in the range. Something like 10-12 gph vs 18-20 gph at WOT. Again if speed is your thing, go for it.

 

[drewmitch44]

Re: Wot

I used to think the same as "gss" as far as running the engine at wot will hurt it. Now that ive been around a little bit and talked to some people i am starting to think otherwise. I used to think like this, "you dont run your car at WOT so why would you put your outboard through that?". Well like i said ive talked to a few people and even mecanics and yammi dealers and they all have told me that the motor actually runs better and will last longer if you do this. I used to run my motor at about 3600 and that seems to be the sweet spot like you had said but thats just for fuel economy. Ive been told that these motors are designed to run at the top of their range more so than middle like that. It keeps stuff cleaned out and like i said the motor is designed to be run like that. I now dis-agree with "gss" in his statement that its going to blow up the motor runnig at WOT. I do agree that the fuel consumption will be greater at WOT but it will last running at WOT. Thats my 2 cents on the topic and like i said my opinion has changed in the last couple of years on this. To me it did not seem right to keep a motor pinged like that but like has been said its made to do that!

 

[rodbolt]

Re: Wot

beats me why the large spread.
but like its been said most manufactures want the enine capable of the high end of the range.
most outboards that "Blow" during high speed runs usually blew due to lack of maint or overpropping.
its funny watching some of my nascar bubba customers. always yakking about this car or that driver yet think nothing of asking their high performance bass rig to run 5500 RPM on year old gas,3 year old impellers and have never changed a fuel filter cause it never needed it.
usually they are the ones that decry the RPM range.
but if properly set up and maintained a modern outboard can turn 5000 all day without an issue.
only reason I dont run my F150 that hard is because of the fuel consumption.
my E-TEC is another story

 

[HighTrim]

Re: Wot

"if a manufacturer states that the rpm range is 5000 - 6000rpm what then is the WOT"

Andrew, Ive read your post a couple times now, and it seems what you are misunderstanding is the acronym WOT.

The best propeller size for your boat and engine combination is based on the recommended operating range at wide open throttle (WOT) for your engine, which you will find in your operator's manual. This will be expressed in terms of a certain horsepower at a certain RPM (revolutions per minute). WOT does not stand for the numerical RPM value per se.

The goal in prop selection is to determine what propeller style and size will maximize performance for your boat, while allowing your engine to operate in the recommended RPM range. The correct propeller will prevent the engine from over-revving, yet allow it to reach the minimum RPM where maximum horsepower is produced.

As far as the contradictory ideas about running at WOT or not, the main thing to remember is that it is imperative that the WOT RPM fall within the range specified by your engine manufacturer. That is not to say that you must drive around all day at WOT, that is just silly, and uneconomical.

 

[steelespike]

Re: Wot

5,000 to 6,000 rpm. It is suggested to prop near the max rpm.This allows max performance best economy.And if you happen to load it down with a boat load of relatives it will still be able to operate within its rpm band.
You can select any reasonable cruising speed as long as its not against the
crest of the bow wave not allowing it to plane.
Its my opinion that a motor run all the tiime at max rpm will fail before one run at the lower rpm band. Will it be a measurable difference probably not.

 

[DJ]

Re: Wot

rodbolt makes a very good point.

Two stroke engines do not like to be lugged (overpropped). Neither do high performance (all 4S outboards) four strokes. Lugging creates huge loads on pistons and bearings. Not to mention carbon build up.

If your engine is rated at 5000 to 6000, prop it to run 6000 with a light load at WOT. When you load it up with people and stuff, it will probably run around 5500. It will be happy.

Just because your engine can run 6000 doesn't mean you have to. Your car can run 100+ MPH but you do not do it. Your car is geared to deliver maximum performance and economy at average speeds. The prop., on your boat, is your final drive "gear".

 

[roscoe]

Re: Wot

Hi,

Why not state a fixed rpm with a tolerance?

Here it is : 5,500 , +/- 500

 

[Texasmark]

Re: Wot

On WOT and overloading. I had a boat once that didn't have a tach so I don't know what the real rpm numbers were. It was a 125 V4 Johnny on a 17' Caravelle Tri-hull. Prop was 13+ x 17P. The setup was adequate to pop 2 slaloms up with 4 people (some little) in the boat.

I sold the boat to a buddy that wanted to fish one of the "line shacks" located on the Intercoastal Canal in So. Texas. Popular thing down there to fish under the lights at night for Speckled Trout (Spotted Weakfish).

The distance from the launch to the shack was 40 miles. The guy had the boat loaded to the gunwales with all his camping and fishing stuff, and lots of gas for the boat and generator, generator, and who knows what else. He ran the whole distance at WOT.....(he told me) he figured he could make it in about an hour.

I forgot the details of when it crashed, but upon arriving back home, he took the rig to the dealer to have one piston (with a hole burned in it) changed out......so he told me later at work. Wanted to know why I sold him a piece of crap boat......he was kidding; I thought. Maybe not.

The culprit here probably was the fact that he had the boat overloaded, engine couldn't turn up adequate WOT rpms, and if that wasn't bad enough, he ran it for an hour that way. I used the boat for over 5 years, taking the family out skiing and all the trimmings, and never had a problem; but I didn't do what he did.

Mark

 

[Andrew Leigh]

Re: Wot

Hi all,

thanks for the responses it is pretty much like I thought it was. A tolerance is always there to compensate for other factors that are not always under control.

My experience is that of the two boats I have owned the first was set up fairly well and "correctly" propped. My second boat I have had out once and have yet to closely check the speed, rpm's and prop but frankly I hope I don't find anything untoward. It pops out the hole well and goes well, in fact when compared to my other boat she slips onto the plane effortlessly. On the 20th I am taking her out again and will suss her out.

On the tolerance issue my first 17' bowrider which had a 115 V4 Rude on the back with a 17" ally prop. Johannesburg is high, about 6000ft. I could never get my prop to spin above 5200rpm with a reasonable load. Down at 1000ft I got an extra 400rpm easily. I suppose in hindsight I was really happy with the boat and never really questioned the prop selection. She was a little lax on holeshot though ...ah so we learn.

Cheers
Andrew

 

[DJ]

Re: Wot

Hi all,

thanks for the responses it is pretty much like I thought it was. A tolerance is always there to compensate for other factors that are not always under control.

My experience is that of the two boats I have owned the first was set up fairly well and "correctly" propped. My second boat I have had out once and have yet to closely check the speed, rpm's and prop but frankly I hope I don't find anything untoward. It pops out the hole well and goes well, in fact when compared to my other boat she slips onto the plane effortlessly. On the 20th I am taking her out again and will suss her out.

On the tolerance issue my first 17' bowrider which had a 115 V4 Rude on the back with a 17" ally prop. Johannesburg is high, about 6000ft. I could never get my prop to spin above 5200rpm with a reasonable load. Down at 1000ft I got an extra 400rpm easily. I suppose in hindsight I was really happy with the boat and never really questioned the prop selection. She was a little lax on holeshot though ...ah so we learn.

Cheers
Andrew

At 6000 ft. that older Rude needed to be rejetted.

 

[Andrew Leigh]

Re: Wot

Hi DJ,

being my first boat I assumed that as it was bought at 6000ft that it would have been jetted to 6000ft. As they say to assume is to make an "***" of you and "me". In hindsight you are probably right.

That does raise my curiosity, with a new boat (new to me) one obviously would want to ensure that all is well and set correctly before doing to much boating and potential damage. A couple of questions then.

Where does one find a reference for altitude vs. jet size? (1990 Yamaha 130 hp V4)

To what altitude do you think they would jet a motor when it's new.

It was also practice to have "running in" jets which were changed on the first service, these generally ran the motor rich. Is this the practice in OB's?

Years ago, in this country, carburated motor car engines jets specs were to sea level, I suppose that is the only constant. The dealer would apparently change these jets based on where the car was. Also all the after market repair manuals also came from the UK which also tended to be mainly sea level operation.

Thanks
Andrew

 

[rodbolt]

Re: Wot

all yamahas I deal with in the states are jetted at sealevel. never saw a chart for re-jetting a ymaha for altitude.
used to be OMC and merc not only ofered a rejetting chart they also offered different gear ratio sets for high altitude running.
main and pilot air jets for the US market are rather limited in selection for the yamaha.
thats another nice thing about EFI, it compensates for altidude