28' Pontoon with 3.0 Mercruiser Prop

[tr@v1s]

Hey all,

I have a 1989 Harris 28' pontoon with 3.0 mercruiser. Current prop is 15 1/2 X 15 stainless. Seems like the recommendation is to stuck with aluminum props on mercruisers from what others have told me. The previous owners had switched to this prop because he damaged the aluminum one. Considering going back to aluminum, but not just because it is stainless.

There are performance questions that will help determine what I decide to do. So, here's what I've observed:

1. I never achieved over 4,000 rpm (read in mercruiser forum that I should be at 4400 at WOT)

2. Max speed is 23 mph (using gps)

3. Pulling a 3 person (Super Mable) tube with about 600 lbs on it (I know that's overloaded by about 90 pounds according to the tag) I still achieve 22-23 mph relatively easily (just no faster no matter how long we run).

So, do I need to go to another prop to get to 4400 at WOT? Should I be able to get any more top speed out of this barge with a 3.0 considering that it tows at the same speed it runs without anything behind it?

Educate this guy please.

Attaching pic of prop.

 

[Scott Danforth]

check your compression and the health of the motor as first order of business.

compression should be between 140 and 150 psi for a healthy motor

If your motor is healthy, you most likely need to drop in pitch for lightly loaded to get to 4400-4600 RPM.

you may need to drop in pitch for pulling the tubes. for your pontoon, I would consider dropping down to a 13p

I used to have 3 props on board with my 3.0 16' boat to cover the need between watersports and cruising

No, you probably wont get much more speed without a lot more motor, lifting strakes and a center toon log.

 

[tr@v1s]

check your compression and the health of the motor as first order of business.

compression should be between 140 and 150 psi for a healthy motor

If your motor is healthy, you most likely need to drop in pitch for lightly loaded to get to 4400-4600 RPM.

you may need to drop in pitch for pulling the tubes. for your pontoon, I would consider dropping down to a 13p

I used to have 3 props on board with my 3.0 16' boat to cover the need between watersports and cruising

No, you probably wont get much more speed without a lot more motor, lifting strakes and a center toon log.

Thank you. So, I would be going to a 15 1/2 X 13. Correct? Should I go back to aluminum on that?

 

[alldodge]

Stainless has less drag due to thinner blades, aluminum has more drag. The overall issue won't have much impact on performance in your case

If aluminum prop hits something hard in most cases it just damages the prop. If stainless prop hits something hard in most cases it will ding the prop but also damages the lower unit

 

[Scott Danforth]

Thank you. So, I would be going to a 15 1/2 X 13. Correct? Should I go back to aluminum on that?

before you buy a prop, make sure the compression is good.

 

[tr@v1s]

before you buy a prop, make sure the compression is good.

I will definitely check that out.

 

[cyclops222]

What is your Prop Efficiency ?

 

[airshot]

Yep....gotta be sure your engine is running at peak performance or your wasting time and money. You need to adjust the pitch to achieve max operating rpm at wot. SS props are mostly for performance boats and typically do little to nothing for barges like a pontoon boat unless you have big engines with lots of power and speed. Most pontoons are not any kind of performance boat, so high performance props are a waste of money.
Pontoons are not speed demons either, gaining a few mph takes a lot of hp.
If you need to gain 400 rpm to reach your rpm max then subtract two inches of pitch on next prop. The general rule is 200 rpm for each inch of pitch and 100 rpm change for each inch on the diameter. If you carry heavy loads or tow toys, some folks adjust prop pitch to increase wot rpm beyond the max
( Mabry to 4600 rpm) so they get a better push ( hole shot) when loaded or pulling....but you need to watch your tach when loaded light so you don't over rev that engine.

 

[tr@v1s]

What is your Prop Efficiency ?

I don't know how to get (figure?) this. Sorry. I'm pretty ignorant when it comes to props, but eager to learn.

 

[Scott06]

I don't know how to get (figure?) this. Sorry. I'm pretty ignorant when it comes to props, but eager to learn.

Google prop slip calculator you put in pitch rpm and mph it compute % slip

 

[tr@v1s]

 

[jlh3rd]

"If you need to gain 400 rpm to reach your rpm max then subtract two inches of pitch on next prop. The general rule is 200 rpm for each inch of pitch and 100 rpm change for each inch on the diameter. If you carry heavy loads or tow toys, some folks adjust prop pitch to increase wot rpm beyond the max"

this is what I did. 24' pontoon with a 115hp CT merc. I ran a prop that would exceed the recommended WOT when I was by myself. This allowed me to stay in the upper rpm WOT range when towing teens on tubes. Top speed suffered, but that wasn't what I needed.
I am not a supporter of worrying about what my prop efficiency is. I believe in propping for hitting max recommended rpm @WOT. Prop efficiency then is what it is.

 

[mr 88]

A SS prop is not going to flex or slip as much as a aluminum prop would , especially with a pontoon boat . Not sure why anyone would tell you to stick with aluminum. The only positive I hear is ,well if you hit a rock , the prop shaft may bend with a SS prop where as the aluminum prop takes the brunt of the hit , sparing the shaft . I'll believe it when I see it .

 

[Scott Danforth]

I have sheared the prop shaft hitting a rock with a stainless prop. I have sheared off prop blades on aluminum props

 

[mr 88]

The last time I hit a rock I was 13 years old and no depth finder or chart . Pretty hard to fathom in this day and age smacking the bottom , especially at enough speed to take out blades and shafts . Ain't happening with all the info on the lake at my fingertips with today's electronics .

 

[Scott Danforth]

Lakes are easy

Local channels here move with every tide and every storm. GPS means nothing, even your tracks from day prior

Run aground, pick up rock in the muck, rock wedges between anti-vent plate and prop at 3000 RPM

Most passes here have this sign.

 

[jlh3rd]

agree. Garmin with updatable map. Depth finder. helps.

 

[airshot]

A SS prop is not going to flex or slip as much as a aluminum prop would , especially with a pontoon boat . Not sure why anyone would tell you to stick with aluminum. The only positive I hear is ,well if you hit a rock , the prop shaft may bend with a SS prop where as the aluminum prop takes the brunt of the hit , sparing the shaft . I'll believe it when I see it .

Because the aluminum prop is one third the cost of a SS when the prop is damaged. SS props give little to no performance increase on barges like a pontoon boat. That amount of flex on the aluminum prop is so tiny, you could not measure it ! Now....if someone has deep pockets and enjoys spending money or they have a performance boat and motor, then that is another story. Most prop mfgrs recommend an aluminum prop for regular pontoon boats, ask one of them.

 

[cyclops222]

I had just taken out the brand new Chaparral 186 with a 5.0L engine . Doing about 50 GPS. I swing way around a big island. I look down into the water, I am in nothing water.
MOMMA save me !!
Later I looked at my bottom depth and what it is made of. chart. Solid rock ledges. with weeds. 2' mean depth. My prop is a 23". I was living my rule of life........... Luck beats skill every time.

 

[mr 88]

Because the aluminum prop is one third the cost of a SS when the prop is damaged. SS props give little to no performance increase on barges like a pontoon boat. That amount of flex on the aluminum prop is so tiny, you could not measure it ! Now....if someone has deep pockets and enjoys spending money or they have a performance boat and motor, then that is another story. Most prop mfgrs recommend an aluminum prop for regular pontoon boats, ask one of them.

Cost was not mentioned , and a few hundred more in the boating world is pocket change . Pontoon slipped next to me does 55 mph. Not bad for a " barge " is it ? Yep , he runs a SS prop , told me he gained a few mph over aluminum as the mfg can do more tweaking with a SS prop over aluminum . And the SS have hubs in them , so when you hit something the hub gets blown out ,before transferring that hit to the prop shaft . The SS will take small hits with nary a mark vs Aluminum that gets bent with the lightest of hits . To each there own , you think aluminum is better then stick with them . I will stick with my SS and Nibral props , thank you .