Starcraft Islander 22’ with 4.3l prop recommendation

[Mi_fiveo]

I’d like to upgrade the prop on my fishing boat. It’s a 22’ Starcraft Islander with 4.3l Volvo Penta Cobra SX outdrive. It currently has a 3 blade prop and I can’t remember the size. I’m looking for a prop to get the best cruising speed with decent gas mileage. I prefer to run it under max rpm’s, which are 4200-4600 rpm. It might be 4800 max, I can’t remember. Holeshot isn’t as important to me. Thanks for the expert help!

 

[roscoe]

You are going to have to get the numbers off the current prop.
Buy the same prop, except, go up one number in pitch.
Thats the best you are going to do unless you start experimenting with all the dozens of props on the market.
Run the boat about 5 mph above planing speed.

And don't expect the gas consumption to go down enough to notice.

 

[Mi_fiveo]

Don’t people put different size props or 4 blade ones to achieve different results?

 

[Mi_fiveo]

Current prop is 14 1/4 x 21, 3 blade

 

[briangcc]

Don’t people put different size props or 4 blade ones to achieve different results?

Some will switch props when they are doing watersports. Mostly drop a pitch or two as they need a better holeshot to pop a skier out AND compensate for a little more load then normal.

Sad to say that boats and gas mileage do not go hand in hand so you may already have the best prop for your setup.

 

[roscoe]

Yes they do.
Mostly for holeshot, speed, and handling. Not so much for fuel savings.
But most predictable results will be based on the current prop and rpm.
Once you start switching to different styles, you start from scratch. Test it, get results, buy size up or down based on results. Test new prop, get results..........

Throw in variables like 3 blade, 4 blade, 5 blade, venting, cupping, stainless vs aluminum, and there are an endless number of variables and results.
Since we don't know what specific prop you have, any recommendation is foolish.

If you know someone that has the same boat, a known prop, accurate results, and precise fuel consumption records, you can save yourself a lot of guesswork and money.

Or find a shop that has a prop loaner program to let you try out different props.

Buy yourself a fuel flow monitor and gauge ($400-$600) , install it, and learn how to use it.
Then spend several days testing and swapping a dozen or more props and recording the results.


In the end, you will have spent a week or more, and a boat load of cash and gas, to get a prop that will MAYBE save you 10% a year on gas.

So is spending a $1000 or more, and 2-4 days testing, worth the fuel savings.

Now, if you live on the water, spend $2500+ annually on fuel, have access to loaner props, have lots of extra time, and plan on keeping the same boat for 10 years, it might be worth it.

But with your current performance, you are real close to where you want to be.

If you want a boat that uses little gas, you have to stay off plane, about 6 mph.

 

[Scott Danforth]

if you want fuel economy, get a boat with oars or a sail.

over-proping a boat actually burns more fuel and kills your motor.

always error on under-proping

spark ignited I/O motors burn about 0.4#/HP/HR of fuel. that is 0.4# of fuel per hp per hour. (a diesel is about 0.333#/HP/FR)

best way to reduce fuel consumption is to reduce the load (command of the throttle is the only factor you have 100% control of).

cruise speed is also about 3000-3400 RPM and not WOT which is throttle against the stops and spinning the 4400-4800 RPM

 

[Mi_fiveo]

Thanks for the explanations. There are no markings or name on the current prop that I can see, except for the size listing.

 

[roscoe]

If you remove the prop, you will likely find a manufacturers part number on one end of it.

Most props are made by a hand full of manufacturers, so posting that number will give someone a chance of id'ing it for you.

 

[jimmbo]

You don't choose a prop for a cruising speed or cruising speed rpm. You choose a prop that permits the engine to run in a specified rpm range. There will be a few props that do that, some 3 blade, some 4 blade, and some 5 blade. While these will all allow the engine to run in the proper rpm range, they may all have different top speeds, and accelleration rates. There is a small chance that a prop that meets the WOT target will provide the mid-low rpm boat speed you are looking for