Engine damage if running outside of recommended rpms @ wot

[mralda]

I am trying to find out what damage is done to an outboard if you run a propeller pitch that does not allow it to develop the recommended rpms @ wot. In a lot of posts I've read that you can (or should) have different prop pitches for different tasks... pulling skiers, cruising, or top speed. It is obvious that only one of these propellers will let the motor rev up to recommended range. I think that when over propped, the engine lugs upon initial acceleration (much like a car that is started on second gear), but once on plane I don't feel there is much strain on the engine (same as an automobile). Please, can somebody tell me what damage is actually done to the engine? Thx.

[powerabout]

depending on the model and the year and your fuel.....
overproping can cause it to detonate and burn a hole through a piston faster then you can say shuzbutt

[mralda]

Evinrude Etec 115 hp 2007, running regular gas in Guatemala and regular TC-W3 oil. When I bought the boat, the engine had about 40 hours (previous owner indications) and a 14.5 x 17 inch aluminum 3 blader, which only let the engine rev to 4400 rpm, but the boat was wonderful! 4 to 5 seconds to plane... see post on v4 Evinrude. This time I just really, really want to know what damage is done and if someone has personally seen the damage (or the testimony of an outboard engineer would also do). Thx.

[pistnbroke]

correct revs for your motor are 4500 -5500 ....a 17 pitch sounds fine ..I had a 20 p on my yamaha 115 and have a 19p on my 90 hp johnson..anything from 15 -23 pitch seems normal ..but it depends on the the boat ...how long is it ??? what is the make of the prop ?
has the prop been hammered to give a higher pitch or cup ?
Is the prop ventlated ..ie got hole in the hub ..
Is the rev counter accurate ?? how do you know?
4-5 seconds to get onto the plane is a long time ...again how big and heavy is the boat ?
let me know
Noosa prop shop ...Noosa Australia

[ondarvr]

Out boards are designed to run within a certain range at WOT, when it can't reach this RPM range at WOT, it also means its lugging at lower RPMS. The problem with lugging is that carbon will build up very fast, which can lead to very high combustion temps and sticking rings, this can bring your once good running motor to a quick and pricy stop.

Boats aren't like cars that when they get up to speed it requires very little HP to keep them there, they use all of the HP at each RPM to stay on plane. Imagine driving up a steep hill with your foot to the floor in high gear going 35MPH, there's lots of strain on the engine.

Targeting the higher end of the RPM range will let the motor live a long and happy life.

[pistnbroke]

still need to know if rev counter accurate as that is the only reading that is making you think there is a problem...
If you drop the pitch to 15 you will raise the revs 400 ..but the lowest pitch in the book is 17 for this motor


But it would need to be a big heavy boat say 24 ft to go to a 15 pitch

[mralda]

Thanks for all your posts. Boat is a Zodiac ProOpen 650 RIB (21'4" and weighs arond 1700#). According to the manufacturer wot is 5500 to 6000 rpms. Just checked my tachometer against Tiny Tach commercial unit and it reads OK. I'm changing the motor height this weekend and see what happens. RPM's with the 14.5" x 15 (ProPulse composite porp) are around 5400 but speed is only about 28 mph.

[pistnbroke]

you got your revs wrong the book says 4500-5500 sound like the boat is a bit big particularly if its a RIB

Cavitation plate level with bottom of transome unless its on a pod when it shold be 2 in up for every 6 in of pod length .....

[Frosty]

You are worrying too much, You have a big boat and 28MPH sounds about right for your 115HP.

It does'nt HAVE to run 5000 RPM all the time.

If it is at 4500 thats ok as long as your not wide open at that.

You are allowed to cruise!!

I take it this boat is not even loaded yet but will be in the future for diving etc, people and diving gear?

In that case you need to go down to 15 if you can get one.

Or put another 115Hp on it.

[mralda]

Hello pistnbroke, according to Evinrude's page, wot is 5500 to 6000. Boat came with an aluminum 14.5 x 21 and I used to run the boat 24mph @ 3000 rpm. Problem was it only saw 4400 rpm at WOT. I put the ProPulse (adjustable pitch) at 15 inches and got the WOT to 5400, meaning I still need to go down to 14'. But at 15, top speed is only 4 mph above what I used to cruise at! Cruise now is only 18 mph. Usual load is my wife and 2 kids, half a tank of gas (35 gal), some food & drinks.

[pistnbroke]

I agree with Frosty AGAIN ! you are only 100 rpm out from what my book says ...if you came in my shop and asked for a prop I woud say 17 or 19 to start with .....can you get a 17x 13.5 or similar to try ?? If you run it WOT it will drink the gas cruise at 4000 rpm is fine .What Hp is the boat rated for ? maybe the motor is really a bit small for it ...

E tech 115 quoted 4500-5500 ........... 115 Fitch 5250-5750

No evinrude or johnson is rated to 6000 until you get to 200hp

3000 rpm is a bit low for cruise ..I get 20 kts at 3000 and usually run at about 4000

[mwatts]

if you came in my shop and asked for a prop I woud say 17 or 19 to start with .....can you get a 17x 13.5 or similar to try ??
I think you mean 13.5 x 17 ? ;)

mralda: I think the 14.5 x 17 was a bit oversized for your engine. The 14.5 x 15 prop you tried came close, but apparently it's pitch is to low (low top speed). Try a higher pitch with smaller diameter, like a 13 x 17 or 13.5 x 17. Anything between 15 and 21 pitch is used on your engine, but diameter is always in the 13 - 13.5 range.

[Frosty]

Well I always call pitch before dia

a 13.5x17 is a 13.5 pitch but Ive not seen pitch in halfs.

[mwatts]

Well I always call pitch before dia

a 13.5x17 is a 13.5 pitch but Ive not seen pitch in halfs.

Ehm, I'm quite certain that "officially" it's the other way around. Dia before pitch. So a 13.5 x 17 is a 17 pitch.

mralda said his engine originaly had a 14.5 x 17 prop. I don't think you could get a 17" diameter prop on a 115 HP without cutting of the cavitation plate. ;)

[Frosty]

Yes your right im sorry --getting old.

[pistnbroke]

I am going to print and frame that Frosty !!! and yes I did mean 13 x17p

Mind you we have had much success in cutting down props ..oops ..its not really the diameter I find that makes much difference but the blade area/blade shape .... given that the central 1/3 of the blades does very little

I would still like to know what the HP rating plate on the hull says as I am not convinced the motor is big enough...

[powerabout]

pistnbroke
When you say central 1/3 is that to the hub or the leading edge?
Just interested that all
Regards
Powerabout
PS yes I agree boat sounds underpowered

[pistnbroke]

inner third of blades

however I do think that if you used the fitted prop at wot for long periods you might have trouble if you are running at cruise say 4000 then its not so much a problem ...unlike quoted earlier the motor will not be running at max output ( at 4000 ) because the throttle is not fully open ..say 75% which means the gearbox is not over stressed ...now we know why the previous owner sold it ...the motor is too small ..but having said that not unreliable and probably more economical .....