Prop review fo rmy pontoon

[silver007]

Hi again.

1995 Sun Tracker Party Barge 24', 1985 lbs + 305lbs engine + 700 lbs people + 500 lbs gas and other stuff = 3500 lbs or so total
Aluminum 13.75 x 15 prop
Tracker Mercury 90hp ELPTO

Mostly cruising around 2600 rpm, no skiing. My goal is engine efficiency and longevity.

WOT is right at 5000 rpm, speed around 18 mph trimmed up a fair amount.

Max rpm on the engine says 5500. Should I change props to raise rpm closer to that?

Engine runs good as far as I can tell except for low idle vibration and shake, which may be from what seems to be a slightly off center hub (and apparent nature of the engine). Had a tune up by previous owner about 6 months ago.

 

[Sea Rider]

If the rule of thumb applies (+ 200 rpm per one less pitch) go for a 13 pitch same diam prop to dial 5500 wot revs as usually loaded. On a toon distribute deck weight, passengers evenly and trim OB to ride at 90? (neutral trim).

Happy Boating

 

[silver007]

Thanks, so I should run at the stated max rpm at wot?

 

[Sea Rider]

When OB dials max wot rpm range as loaded, can throttle at the rpm/speed you like most. With a less pitch prop will have better hole shot if toon is possible to plane or better fast displacement speed, better fuel consumption throughout the whole throttle range. Could send current 15 pitch prop to a prop shop and have it re pitched to a 13 pitch..

To check if OB will rev towards max 5500 wot rpm range will only be possible during a water test as loaded on flat calm water cond. OB will achieve better prop trust when sitting at 90?.

Happy Boating

 

[ahicks]

With no intentions of skiing, I see NO GOOD REASON to turn the engine to max rated rpm. Keeping it down 500 rpm will let it run quieter, and likely use less fuel.

 

[Sea Rider]

Insane, out of this world to pretend go skiing with a 90HP-OB powering a toon, with a 300 HP installed will do...

With a OB running max wot range will asure going for bigger loads & passengers and not just the 700 lbs people and the 500 lbs of stuff currently stated. Don't want to lug engine if going that route, right ?

Once at max 5500 wot rpm range as loaded, just thottle less to 1/2 to 3/4 throttle for less fuel comsumption if that's an issue. It's not how fast you reach your boating spot, enjoying the cruise at less speed is the way to go for most boaters.

A toon is not a speed demon so useless going full throttle, has more hull drag than any other boat.

Happy Boating

 

[ahicks]

Sea Rider, as English is not your first language, I'm going to ignore your "insane" comment. Further, I'm not going to waste my time on a rebuttal. Clearly you have no idea of what you're talking about here.

 

[silver007]

I think Sea just meant it was a crazy notion, not that you were crazy ahicks . It sounds like even though running around wot is not ideal, I should lower pitch to get motor at ideal rpm at any given speed.

 

[ondarvr]

Yes, ideally for longevity the motor should be able to reach the upper end of the RPM range. If it doesn't it means its lugging at all RPMs, this results excess carbon buildup, which leads to a shortened life expectancy.

 

[silver007]

Thanks ondarvr.

 

[ahicks]

Yes, ideally for longevity the motor should be able to reach the upper end of the RPM range. If it doesn't it means its lugging at all RPMs, this results excess carbon buildup, which leads to a shortened life expectancy.

We're generally on the same page most of the time, but, with all due respect, I have to disagree here. Our OP has a prop on a 90hp powered 24' pontoon boat that's turning within 500 rpm of max rated at WOT (5000). He has no intention of pulling anything, and my suggestion, because he doesn't need 100%, is there is no reason to spend the money for a prop that would allow max rpm.

There's a reason engine manf's give a range of rpm's for a target vs. a specific "designed for for use at" rpm. Regarding "lugging", I doubt seriously this engine will be lugging at any rpm running the prop he has. He said 5000rpm, not 3000. This is not like operating a car or a bike accelerating from a stop in top gear with the engine bucking in protest. THAT'S lugging! Regarding carbon build up, that's generally due to junk oil. Modern 2 strokes run on name brand oil generally don't have an issue here, especially liquid cooled engines.Putting around is how the majority of outboards are used the majority of the time. Even a dedicated ski boat isn't "always" pulling a skier!

Make no mistake here. I generally always prop for max rpm on my own rigs - but - I'm a performance nut. Our 90hp 24' 'toon IS set up to pull, and when loaded really light, is actually over max rated by a couple hundred rpm! That does not mean that's the only "right" way. The existence of more than one "right way" is something I learned a long time ago. -Al

 

[Sea Rider]

Gee, why boaters are alluded to ? Google translator is the culprit...

Going back to the topic ; OP posted...

"WOT is right at 5000 rpm" "Max rpm "on the engine" says 5500"

Should I change props to raise rpm closer to that? Yes, you should.

If OP will carry more load or passengers while OB was previously loaded revving at 5 K, with that extra new load OB will rev towards min or lugging side which is not optimun. Ideal is for OB to rev towards max wot rpm range as loaded, better is if with a fixed load all the time.

Happy Boating

 

[ondarvr]

Sea Rider

We don't always agree, but I respect your communication skills. On this subject we do agree.

 

[silver007]

My main goals again are efficiency and engine longevity. I'm thinking a lower pitch would reduce even the slightest chance of lugging, whatever amount, versus the 15 that's on it. Also I seem to have gathered that these engines prefer not to "putt" around at low rpm, which the lower pitch would also help relieve to an extent when I'm 'putting' along. The more I think about it, it seems if the engine can't reach max rpms then something needs changed. Barring any throttle linkage problems I think I'm better off with a 13 pitch. I hope everyone's advice has helped me reach sound logic!

 

[ahicks]

If you are nervous about the prop you have, go for it. Put the next size smaller one on it to make you sleep better.

However, if you are wondering if the prop you have is hurting anything, you'd need to answer the question regarding Mercury's suggested max rpm range, and how you might be hurting an engines somehow while operating within it. The fact you are currently solidly within it, with the rather substantial load you have on board (1200lbs), tells me there is no issue. In fact, you could easily leave at least half of your load on shore, which would likely result in a tach reading much closer to max rated.

This brings up the question of whether to seek the correct prop when lightly loaded, or with expected/average load. It's commonly done both ways. You can make a good case where doing it either way is correct. So your call there!

Assuming for just a minute that you did operate your 'toon with half your indicated load left on shore (or most of it, just for giggles), and that did bring your tach reading up close to max, is it your feeling the engine would be lugging less or turning a little higher rpm at your normal cruise speed? That's something you can try, but I'm pretty sure I can predict how that's going to go....

There's a Mercury WOT RPM chart here:

https://www.rubexprops.com/boat-prop.../?make=Mercury

90

1989-2004

5000-5500

As far as putting around hurting anything, skip the "expert opinions". Check out the people using pontoon boats around you to see how they are using them 99% of the time. If you see what I see, thinking if there were an issue, it would be more apparent.......

 

[silver007]

Thanks ahicks. This is what I'm thinking. I would think that the logical direction to go would be to make decisions with it loaded as it will be 90 to 100% of the time, which in my case would be the 1200 lbs. Sort of like if I have a vehicle towing then I'd want lower gears to prevent any engine lug no matter how small, and to keep the tranny from being overworked. It "may not hurt" it, but I'd have to say the safest route would be lower gears.

Your question about wot rpm's lightly loaded now has me curious, but even if I hit 5500 unloaded I still think a lower pitch would be ideal for the engine.

Thanks for that chart. It gives me a range instead of just max, and tells me my gear ratio. So that sort of opens up a gray area. Maybe a 14 pitch instead of a 13. Sheesh. It makes me wonder, when Mercury gave the max rpm, were they insinuating that you should reach that loaded or unloaded, or that's just the max, nothing insinuated?

As for the locals, honestly I wouldn't trust any of them to know what they're doing. I'm in a metro area. Take that however you want

If any of this sounds off, let me know. I'm new to the sport and you won't hurt my feelings!

 

[ahicks]

Re: Merc's suggested operating range, the higher rpm should be viewed as a red line/never exceed. The lower is open to interpretation I guess. For the last 40 years or so, I've been looking at it as the lowest rpm to keep you within design parameters, and solidly within the engine's power curve. You can get further into this if you like, if you can find a copy of the engine's power graph. On that you can see the rpm the engine makes the most HP as well as the most torque. Max torque nearly always lower than max HP. It would be interesting to see how those numbers line up with Merc's suggested operating range. I have a sneaking suspicion, but don't know for sure.

Maybe worth mentioning, is the fact the kids next door have the same engine you have on a 25' Crest III, but they spend a LOT of time pulling tubes and skiers. We set that engine up with a prop that turns up max rpm for that reason. They run a 13" prop on that.....

Last, boats are like people. You always find more of them in metro areas. Yes, you get the clueless newbies that haven't figured out there really are rules of the road that must be observed - like the guy going the wrong way around a lake or buzzing markers outlining a swim area. For the most part though, most actually know what they're doing, and have been doing that for quite a while. MANY with pontoon boats have been boating for many years.... having converted from other type boats as they discover 'toons generally make for a more "universal" boat.

 

[Sea Rider]

If OB revs at 5000 wot revs as "usually" loaded with a 15 pitch prop, go for a 14 pitch and check max achieved wot revs, impossible to know if OB will rev + 200 or 500 more revs. If too low, send prop to a prop shop and have it re pitched to a 13 pitch. Anyway going for a 14 or 13 pitch prop will achieve better hole shot is toon is capable of doing so, if not, better initial fast displacement speed than with a 15.

Forget all about what others do or go boating with, each combo installation is unique, besides others don't cut their veins going for a prop maximization to run max wot revs as loaded, very few boaters here likes doing so, will include myself.

Happy Boating

 

[ahicks]

Finally, we agree on something... If that's what you've been saying from the beginning, pardon my inability to interpret what you're saying properly.

"others don't cut their veins going for a prop maximization to run max wot revs as loaded, very few boaters here likes doing so, will include myself."

-Al