98 crownline prop?

[98crownline182]

I have a 98 Crownline with a 3.0 volvo penta with an sx cobra drive with a 17 pitch prop and at 4000 rpm the speedometer reads a little over 20 mph the prop has a little dings and a little wear im wondering what pitch and size prop I need to run to bring my speed to about 40 the boat mostly just cruises occasionally pulls a tube now and then thought my speed was awfully slow like it is. Maybe the hub is slipping? Someone help me out plz. Thanks Matt!

 

[Philster]

Re: 98 crownline prop?

ESSENTIAL DATA REQUIRED
When posting a question about propellers, you cannot get a reasonable answer unless you provide the following information in your post: Items 1-4 are essential for meaningful response to most questions.
1) Performance issue you are trying to correct.
2) Current prop manufacturer, model, aluminum or stainless as a minimum.
3) Current prop diameter and pitch (required).
4) Wide open throttle RPM and speed with an average load (very helpful)
5) Engine/drive make, model, year, and HP
6) Boat make model, year, length and weight

 

[Maclin]

Re: 98 crownline prop?

When I put your numbers into the prop calc formula I get 37% slippage which means new prop time. I am assuming a drive ratio of 2.0

Is 4000 RPM all the higher the engine will rev to now at WOT (WideOpenThrottle) under these conditions? If so then right now you also have a problem or two in the power department. That engine in good tune should reach 4500+ rpm's.

Going to a higher pitch prop will not work well at all until you sort out the engine problems.

Having typed all this in, by the numbers a 21 pitch prop at 4500 rpm with a 2.0 drive ratio and only 11% slip calcs to a theoretical top speed of 39.8 mph. But as I mentioned before, if your engine cannot rev past 4000 now then going up in pitch will not work.

 

[steelespike]

Re: 98 crownline prop?

Without the info requested Its difficult to tell much but it does appear there is a power loss of some sort.Get us as much info as possible.
Its not unusual to find small Volvo's in much bigger boats than normally expected.
This would certainly change recommendations.

 

[98crownline182]

Re: 98 crownline prop?

Im unsure what the prop brand is it is aluminum and the patent number is 967.751 and it also has a number (7751) on it. The prop is 14-1/4 by 17. The boat is a 98 crownline br182 its a little over 18 feet, not sure on weight. The motor is brand new from michigan motorz its a 3.0 gs base marine engine i think its 140hp. Im pretty sure there is no performace issue it would have turned more rpms but i didnt want to over rev it. Thanks Matt!

 

[98crownline182]

Re: 98 crownline prop?

Oh and the drive ratio is 2.18 Thanks!

 

[98crownline182]

Re: 98 crownline prop?

And also the speed of 20 mph at 4000 rpm was just me in the boat at 160 pounds

 

[Maclin]

Re: 98 crownline prop?

Refer to Philster's post, item#4 is one of the imperatives to find out before any troubleshooting can be productive.

Your prop slip now though is really high, it may make sense to get a run of the mill 19 pitch aluminum prop in good condition first then go get the performance numbers.

 

[steelespike]

Re: 98 crownline prop?

Unfortunately 4000 rpm and 20 mph doesn't tell us anything.Slip will be abnormally high at reduced throttle.
Get us all that info together and we can tell if your setup works and what you may need if anything.

 

[98crownline182]

Re: 98 crownline prop?

Ok finally got to get the boat on the water this weekend and at 4600 rpm the boat was running about 26 / 27 the max rpm is 4600 but i rarely run it that high i like to not go over 4200 or 4300. Im thinking about a 3 blade 14.3"x21" does this sound about right any other opinions? mostly just a cruising boat hardly ever pull a tube or anything but it will occasionally. Thanks Matt!

 

[98Shabah]

Re: 98 crownline prop?

FWIW, 1998 Mariah 18' Mercruiser 3.0, 14.25x21 Michigan alum 3 blade, beach gear, 2 adults and 2 younger kids I hit 38mph at about 4650rpm with a full fuel tank.

 

[steelespike]

Re: 98 crownline prop?

Ok finally got to get the boat on the water this weekend and at 4600 rpm the boat was running about 26 / 27 the max rpm is 4600 but i rarely run it that high i like to not go over 4200 or 4300. Im thinking about a 3 blade 14.3"x21" does this sound about right any other opinions? mostly just a cruising boat hardly ever pull a tube or anything but it will occasionally. Thanks Matt!

When we nag about max rpm its to be sure the motor can rev freely within its rpm range.You can drive it at any reasonable rpm of your choice once we know its setup right.You do have some issues. With a 17" pitch your motor should be close to or actually over reving.
With a 21" pitch it should run about 40. But the 21 won't work unless you figure out why it lacks power to make speed.
I know you don't want to go fast but your best economy comes with a motor that can rev freely and have the highest speed for its setup.Don't assume cause the motor is new that it is setup right.Throttle may not open all the way,timing may be set wrong or not advancing as it should.Is the bottom clean and smooth?There's even a slim chance its on 3 cylinders.
Note 98Shabah's load and speed with a 21.With just one it must really crowd 40 mph.