Confused about which prop

[Blackbart5]

Hello, I'm kinda new to the boating world. I have owned some jet skis and a ski boat. I currently have a 1988 28ft harris flotebote with a 3.0 i/o alpha one. Engine runs great and all maintainance kept up. Just tuned it up again(timing, plugs, carb, oil, cap/rotor). I know it's not a speed demon but I also know it's not performing like it should.............currently the prop is a 15x17 and wot is only 3000ish rpm's. I need help and ideas about which prop would work the best. I'm looking for a good hole shot due to pulling tubes and hauling people. And wanting to be able to hit my factory rpm range when needed. Ideas?

 

[ahicks]

Somebody put a ski boat prop on your boat. Ball park rule, 1" of pitch is about 500 rpm. I would look for a prop with something in the 13 to 15" pitch range. If you can swing it, a 4 blade would be nice on a boat that size.

 

[steelespike]

We need a lightly loaded rpm and gps speed to evaluate your numbers.And you need to try to pin down your rpm number.
Also if you usually have a gang aboard you'll want to prop near the top with a light load.
You can prop specifically to your load but will need to watch the rpm when light.
If your rpm is accurate you need to make up about 1600-1800 rpm I always follow the rule of thumb of 150 to 200 rpm per inch of change.
If we use that figure I don't think you could make up that rpm If we use 500 per inch we would be around 13" pitch.
We shouldn't make any changes until we are sure of the rpm and gps speed.

 

[Blackbart5]

Somebody put a ski boat prop on your boat. Ball park rule, 1" of pitch is about 500 rpm. I would look for a prop with something in the 13 to 15" pitch range. If you can swing it, a 4 blade would be nice on a boat that size.

That's what I was thinking per my numbers. 4200 - 3000 = 1200/300=4".......17"-4"= 13"

 

[Blackbart5]

We need a lightly loaded rpm and gps speed to evaluate your numbers.And you need to try to pin down your rpm number.
Also if you usually have a gang aboard you'll want to prop near the top with a light load.
You can prop specifically to your load but will need to watch the rpm when light.
If your rpm is accurate you need to make up about 1600-1800 rpm I always follow the rule of thumb of 150 to 200 rpm per inch of change.
If we use that figure I don't think you could make up that rpm If we use 500 per inch we would be around 13" pitch.
We shouldn't make any changes until we are sure of the rpm and gps speed.

Light loaded.....3115rpm/17 mph
Heavy load,,,,,,,2970rpm/12mph
Too, people have told me to go with a 13" four blade and to drop my diameter as well for added pitch??? I don't think in the 20's mph is a terrible goal for a boat that size. Per the specs, boat weights 2340 with empty fuel tank and no gear

 

[ahicks]

I would guess that 2340 weight as optimistic for any I/O equipped boat, though it makes no difference when you get down to it. What counts is your rpms at WOT. Set it up for the top of your engine's range for best power and you get what you get.

3000ish =3000 rpm?
4200rpm is max rating? I ask because my old 140hp was rated at 4800 if memory serves me. If you aren't sure, it's worth checking for sure!

Think you'll find that the manf's will adjust the diameter according to pitch. Not your call. Just getting the pitch right is all the challenge I need anyway....

 

[Bamaman1]

I've also had that motor on a 18' v hull boat, and it's a long lasting, fuel efficient motor. The 140 hp Mercruiser is no powerhouse out of the hole, however. If you've not noticed, but the pontoon market doesn't have any 28' boats any longer, as they're just too hard to handle docking and putting on trailers. And your engine is actually marginal for such a heavy boat expected to perform in water sports.

4 blade propellers are usually seen on high powered bass boats and not on pontoons. You should get a 3 blade, 13 pitch prop for your boat. That's about all you can do.

The most popular boats on the retail market are 22'-24' tritoons with lifting strakes on the hulls and 150 hp 4 stroke motors. They'll run about 40 mph and do just about anything you ask them to do--including towing tubers.