Right prop for 24' pontoon

Steveatpa

Seaman Apprentice
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Apr 3, 2017
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31
Hello all, first post.
We recently acquired a Sun Tracker 24' pontoon boat. Very nice condition as the PO really cared for it.....or seemed to.

The engine is a Mercury efi 90 hp which replaced the 40 hp it was originally equipped with. The boat scoots along nicely at (I'm guessing) a moderate 20-25 mph. My only concern is this is my first outboard (besides an old rowboat), and to see the engine cruising at 5000-5100 rpm seems excessive. WOT is about 6100 and seems to do this effortlessly too.

The OEM 13 pitch prop is in decent condition over all. Not beat from continually hitting objects and dirt, but does have a small ding and scrape on one of the blade edges from obviously hitting something. I thought that maybe I'd replace the OEM aluminum prop with a stainless 15 pitch to see if I can bring the cruise rpm down around 1000 rpm and possibly achieve WOT at around 5800-6000.

Again, this is all kinda new to me so I'm here to learn if anyone has any experience and suggestions in doing such prop swaps on a pontoon boat of similar size and caliber. Also worth noting is I'm not trying to turn this pontoon boat into a "performance" boat either but merely just looking for maximum efficiency from the motor and cruise speeds. Thank you and look forward to reading threads looking for information.
 

HotTommy

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Mar 15, 2013
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In my limited experience, your boat is getting excellent performance. So I wouldn't be in a hurry to change anything. That said, Mercury has a website with a prop selector to help you choose the best one. There is also a forum here at iboats dedicated to prop questions. In addition to any answers you get here, you may want to try those sources as well.
 

ahicks

Captain
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Sep 16, 2013
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Going to agree with Tommy. What you have going on now sounds pretty good to me.

Regarding dropping a few hundred rpm, I wouldn't. What little you'll pick up in gas mileage and reduced wear (through reduced rpm) is likely going to cost you dearly when it comes to accelerating - like when trying to pull the kids on a tube. A prop with more pitch could turn the boat into a real dog in that respect.

Regarding stainless steel, I'm not big on them at all for a pontoon boat. There's very little to be gained, not enough up side to make it worth it. The downside is they are SO much heavier than aluminum, there is increased wear on the shifting mechanism inside the lower unit when maneuvering (forward, backward, forward, repeat as long as necessary) there is MUCH more prop momentum involved every time you shift. The other thing is if you hit something solid with a stainless prop, because of it's extra strength, you are much more likely to break lower unit components (vs. breaking a blade off an alum. prop). Those are my thoughts anyway. -Al
 

Steveatpa

Seaman Apprentice
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Apr 3, 2017
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Great points on the added mass of stainless steel..... Didnt think of that.
What about motor rpm's? Is cruising all day at 5100+ OK for these motors? I'm a maintenance nut so making sure fluids are in tip top condition is important to me, but still rpm's are rpm's and accelerated wear is not what I'm after. Thus what I was wondering if I could help the motor work not so hard.
Thanks for the input too.
 

HotTommy

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I have a 2016 Mercury 115 HP four stroke on my old pontoon boat. Maximum sustained RPM is rated as 5000-6000. I seldom find the need to operate at full throttle for extended periods unless pulling a tube or skier. In those cases my RPM tops out around 5400. ... I guess my point is that if you plan to spend a lot of time at full throttle near 6000 RPM, then maybe a different prop is a reasonable idea. But if you don't actually spend much time at 6000 RPM, why bother? .... As for the cost of cruising all day at 5100, I think your major expense will be fuel. For example, a thousand hours at 3000 RPM in my boat will use about 2500 gallons of fuel. A thousand hours at 5000 RPM will use about 6000 gallons of fuel. If I were worried about the cost of wear and tear on my engine, the cost of that extra 3500 gallons of fuel would likely be enough to convince me to operate at a lower RPM.
 

ahicks

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Cruising all day at 5100 won't hurt a thing - but - my bet is you'll find there isn't enough difference in cruising at 4k or so, and 5100 to pay the gas bill for the extra 1100 rpm.

In any case, I'd hold off an making any changes until you've had it out a few times, so YOU can judge for yourself after getting a feel for what's going on..

BTW, Congrats on the new boat!
 

Silver Eagle

Master Chief Petty Officer
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Mar 16, 2010
Messages
852
We had a 24 foot pontoon boat with a Mercury 90 hp two stroke. I never ran it above 5,000 ever .Most of thee time I'd run it at 3600 rpms.16 mph .I used the boat on the Chesapecke Bay. How come you run the motor so hard.,Are you a speed nut. If you are get a bowrider. Sometimes just for fun I'd kick it up to 4800 rpms for short time 22mph. Your set up seems perfect to me ,leave it alone.
 

Steveatpa

Seaman Apprentice
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Apr 3, 2017
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31
Thanks again for the advice everyone. I've talked to Mercury and even they feel that the prop may be a little to light for the application. I'm looking into just getting another less expensive (softer and lighter too) aluminum prop of either a 14 or 15. Havent decided yet and will probably take it out some more and getting a better feel for the current 13 pitch before coming to a decision.

And Silver Eagle, no I'm not a "speed nut" and maybe you missed that part found in my original post where I said I wasnt after making this a performance boat. Rather I've been around boats long enough in my life that its obvious when a hull is dragging along verses cruising along. I dont see the point in running an engine at 3500 rpm if the hull is shoving though the water like a barge at 10 mph. At that point idling along at 5 mph would be worthwhile... But instead this boat wants to run cruising planed out at around 5000 rpm. Again.....which is why I asked the question as to whats the "normal" rpm range for these larger OB's. And after a good discussion with Mercury, running at or close to the low side of its upper rpm range apparently is normal. Not sure I'm still OK with that which is why I'm after a possibly steeper pitch prop. But nonetheless.
 

Silvertip

Supreme Mariner
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Sep 22, 2003
Messages
28,758
Increasing prop pitch will lower the WOT rpm with whatever load you've been doing your testing. Keep in mind that if you are doing these test with a light load, once you increase pitch and then get heavily loaded (weight wise) you may actually be lugging the engine and hurting fuel economy. In my view you have the ideal setup. The boat has a throttle lever for a reason. Pull it back a tad if you don't like 6000 rpm lightly loaded. The benefit comes with heavy loads or towing water toys.
 

WaterDR

Senior Chief Petty Officer
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May 8, 2012
Messages
730
You can cruise at any rpm and not hurt anything so long as it's not bouncing off the limiter.

Also....the rpm gauges are not always perfect either.

For setting a prop on a toon, take it out with the lightest load you would typically have. The rpms should be close to the max range without hitting the limiter.

At heavy load, you want to be in mid range or so.
 

darrklim2

Petty Officer 1st Class
Joined
Jul 5, 2007
Messages
224
Whether it's a pontoon or flat bottom boat, 6hp or 300hp. It makes sense to get the right prop for you application. This forum where I learned that 10 years ago when I joined. I don't know the max recommended RPM of your engine but you want to test with your common load. If it's recommend to be 6000 RPM then you want to be as close to that without going over as possible, at WOT and normal load. Especially since a pontoon is generally a non-planning hull(arguments can be made for lifting strakes etc.), "hole shot" and "getting on plane" are not factors.

​Speed at any RPM is irrelevant to getting the best performance, fuel economy, efficiency and " wear and tear" of the engine.

​Just my $.02.
 

ahicks

Captain
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If you talk with a shop doing prop repair "in house" tell them what you have going on and see what they say. Ask them about the potential for repitching the prop you have enough to drop 500 or so rpm.
 

Steveatpa

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Apr 3, 2017
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Updating the thread.....

I had the 14x13 alum prop fixed and had the shop put a nice cup on it while they were at it. The prop guy said the aggressive cup he added would almost be the same as re-pitching it up to a 14.

Well boating season started and we just had the chance to take it out and see how it drives. Surprisingly its quite different than before too. Now it will cruise much faster at a lower rpm, and wont rev as high as it used to. We cruised around the lake at about 4000 rpm instead of 5000 as before. And instead of hitting 6100 on tap, it will get to about 5800 at WOT. Nice..... So I'm assuming that the cup was significant or the dented blade knocked the whole thing out of wack more than I thought, or a combination of both.

Given how well it drove, at this stage I'm going to enjoy it for awhile before I start looking for a SS prop. :)
 

clemsonfor

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Sep 19, 2005
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I guess you missed the whole part about SS being much more wear on the drive components ? These guys know what there talking about. I would just use the aluminum prop. Performance wise you will gain nothing and just increase wear as you shift with SS.
 

Steveatpa

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Apr 3, 2017
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So then I guess engine manufactures using SS props is just all marketing hype and bling...? Really, this is worth knowing.....
 

ahicks

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Stainless is the stronger material obviously, letting the designers work with thinner blades with less flex and blade designs that strength allows. NONE of the resulting benefits will be noticed in the relatively low performance envelope the small to mid range horsepower pontoons work in. There's nothing a 300+ dollar stainless prop will do that a 125 dollar alum. prop won't do - other than maybe look pretty when it's on a trailer.

If you were talking about an engine with 2-3 times the power of a 90, mounted on a boat that will be used frequently at speeds twice what the one we're talking about might run at - THEN you might start seeing reasons to justify the price of stainless. The only other reason? 'Cuz you want one. You don't need to justify that.....
 

Steveatpa

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Apr 3, 2017
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Interesting..... Thank you for the input. I'm taking it all in for sure.

As many have expressed in various threads and write ups', the opinions on SS props is vast and wildly all over the map, which certainly makes understanding whether or not getting one for the average person is worth the money.

For instance, I've heard from reliable sources many things like:
You'll save money over time as SS props are stronger and will blow through small debris, whereby aluminum props will need constant repairing. NOT total destruction either but merely fixing small damages throughout usage which cost every time the prop is in the shop.
You'll gain prop efficiency as anything over 4000 rpm with aluminum will cause blade deflection and effectively reduce the props ability to do its job.
You'll gain money over time simply because of the fuel savings due to the increase in the props efficiency...
There's no difference in shifting performance or wear on the lower gears as the water drag placed on all props in water will cause the same force during shifting. (makes sense considering physics)

So for some people this is a straight forward choice, but for others there's many debatable variables which need to be weeded through..... Thanks again though. I appreciate the feedback.
 

ahicks

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I'm faced with the same decision. I need a new prop for a Honda 90 on a 24' toon. The engine came with a prop that's too big. After a lot of research, I've decided I can't justify the price of a stainless prop.

I don't buy into long term efficiency benefits, as they sound much like an insurance salesman telling me about something I don't need.

Not buying into the blade flex story either - not at the performance levels we're discussing. Rpm's have little to do with it. If something is going to flex a prop, it's going to be horsepower and mph loading a prop to silly limits. 90hp and 25mph is child's play.....

I'm also not buying into the constant maintenance story, as many MANY years of experience on all sorts of boats has taught me otherwise. Yes, you'll bend one once in a while, but I'll take that damage any day over potential bent or broken drive components. And don't let anyone tell you that a stainless prop won't nick or bend either....

Those, and the extra inertia loads while shifting (sorry, nobody is going to convince me there isn't a difference) are what formed my opinion. Use it for what it's worth.
 

bruceb58

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Mar 5, 2006
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First off, I am a huge believer in using SS props. Have had them on almost every boat I have owned if I could. I do have the exact same engine as you on a pontoon and I realize that the performance increase of using a SS prop on my pontoon is not worth the extra price in my estimation. It's possible that you might make up some of the price in fuel but depending on how many hours you put on your boat yearly, but that break even point may be many years away.

As far as more damage being done while shifting, as long as you pause between shifting between reverse/forward so that the prop slows down and/or stops, there will be no extra wear using a SS prop. Way more load due to the water than the spinning mass.
 
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ahicks

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Right, I don't mind spending the money - IF - I can justify it somehow. I hate to remember what I had tied up in a chrome-moly prop on a big block drag boat I had. THAT was justifiable!
 
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