24ft Suntracker 3.0 I/O with 40% prop slip

[meantobe63]

Bought the boat a year ago and tachometer didn’t read correctly. Mercruiser with Alpha one 2.0 gear ratio. With the prop that came on it, a 16x14p, I’d only get 16-17mph with 4 adults and 3 kids. This year with the new tach, I’ve ran this prop again and getting 4000 rpm. At that rpm and speed, calculator showing a 40% slip. With a 14.5x15p at 3800 rpms I got 20mph, and a 16x16p at 4000 rpms I got 18-19. With all 3 of those props I’m seeing the same 40% slip. Now I understand I should be at 4800 rpms and am not achieving that, (going to address that this week), but either way I have extreme prop slip. I’ve also had a lot of issues with blow out, but this last time out shifted all the weight I could to the rear. When I got the boat, from water line down the pontoons were primered and extremely rough, and even the parts that weren’t painted were extremely rough. In the last couple weeks I’ve sanded ALL the rough aluminum and all the paint off to silky smooth. All except for bottom that is sitting on bunks. Took it out yesterday with the original 16x14p and saw no difference. Looked smoother in water, but speed not changed. I understand that the very bottom 12” has the most drag in water, but still can’t see a 40% slip. Pontoons aren’t in greatest shape, so I have installed drains to make sure they aren’t full of water.
What else could be the problem?

Oh yeah, original “top speed” for boat was listed as 26mph. I’m not trying to make this in to a Playcraft, but would like to get closer to what it’s supposed to be.

 

[meantobe63]

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[meantobe63]

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[meantobe63]

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[ahicks]

I can just about guarantee you that when they say 26mph, they're talking about a lightly loaded boat, likely with just a driver aboard. NOT a boat with 4 adults, 3 kids, 2 coolers, beach gear, and a full tank of gas!

If this were my boat, it would be much less about prop slip and mph, and much more about a prop that will let the engine turn at max rated rpm - probably a 4 blade - when lightly loaded. Then let whatever else fall where it may. It is what it is.

 

[meantobe63]

And I could almost do that, except when I see the same size boat with 20” pontoons (mine are 26”) with more people on board than I have, pontoons barely visible front to rear, blow by me with a 70hp on the back. What was that boat rated at empty, 40mph???? Haha

 

[porscheguy]

To increase rpm you most decrease pitch. Try a 12 or13P prop and you should start to find some of your missing rpm.

 

[ahicks]

And I could almost do that, except when I see the same size boat with 20” pontoons (mine are 26”) with more people on board than I have, pontoons barely visible front to rear, blow by me with a 70hp on the back. What was that boat rated at empty, 40mph???? Haha

What you are working against is the weight of that I/O, and likely the fact it's been installed with the lower unit set much to deep to be quick. I/O pontoons are about putting along for hours. They've pretty much gone extinct due to the availability (and popularity) of much lighter 4 stroke outboards.

A 70hp 'toon that will crack 20mph would be pretty unusual. Not saying it can't be done, just that there aren't many that are capable.

With the right prop, you should be able to crack 20mph pretty easily. 26mph when lightly loaded isn't hard to believe. Enjoy it for what it is!

 

[meantobe63]

Is there a common number prop slip that most pontoons exhibit? Two toons, let’s say 3000lbs of people and equipment?