More pitch for economy

[sjm32157]

I have a 20 foot Maxum with a 4.3 Mercruiser. I am currently using a 21 pitch x 15.25 diameter. It tops out at 4700-4800 rpm at around 48 mph. We usually just "slow cruise", 27 - 30 mph at 2800-2900 rpm. I'm thinking about trying a 23 pitch x 14.25 diameter for better economy. Rough calculating I think 2400-2500 rpm would give me the same slow cruise. Anyone using anything similar? I'm also considering the "Smart Tabs". I see so many good reviews that I'm very interested.

 

[Bob_VT]

Re: More pitch for economy

You will lug the motor, spend big bucks on a new prop........ you are propped right so, save your money.

 

[sjm32157]

Re: More pitch for economy

I bought the boat used a couple of years ago. It came with 23 X 13.75 and 18 X 14.25 props, both in poor condition. I ran it for a couple of weeks with the 18. Never tried the 23. Just thought it might be worth a try.

 

[Bondo]

Re: More pitch for economy

Ayuh,... As long as you can keep the rpms at wot at 4400 or above, you'll be Ok...
If it won't pull those numbers, you'll be luggin' the motor, tempting burning a hole through a piston, or fryin' the valves...

 

[Silvertip]

Re: More pitch for economy

While running numbers may make this seem like a logical move, theory and practice don't always come together in a plan. Think about this. Raising pitch decreases rpm at any given speed but it also requires power to twist that pitch (increases engine loading). So hole shot is going to be affected and it will take longer to get on plain. You will likely need a tad more throttle (not less) to maintain the same speed you did before. Boats have a ton of drag and to overcome it requires power. So any fuel savings are likely going to be minimal in theory and lacking entirely in practice. The more starts and stops you make the less chance there will be of any savings. If you typically run very lightly loaded you may experience some small savings. The heavier you run it may actually increase fuel consumption. There is a very inexpensive way to determine fuel economy without creating running averages. Temporarily install a simple vacuum gauge available at any auto store for just a few bucks. Hook it to any manifold vacuum source on the engine. With the existing prop, take vacuum readings at hole shot, what you consider cruise speed (not rpm) and at WOT. Now make the prop switch and repeat the test. I think you will see that under all three conditions manifold vacuum will be less with the higher pitched prop. Low vacuum translates directly to throttle opening which translates directly to increased fuel consumption. Some older cars had a fuel economy gauge which was nothing more than a vacuum gauge. Maintaining high vacuum (best economy) required that you drive like there was an egg under your foot. The same principles apply with your boat.

 

[steelespike]

Re: More pitch for economy

S covered it pretty well.Lower rpm doesn't necessarily mean less throttle opening.
And the vacume gauge is an excellent addition.Both to help figure best prop selection but also
to determine the best throttl/speed for best cruising speed.
With a little instruction you can also tell if the valves are good,rings are good,timing set right etc.

 

[sjm32157]

Re: More pitch for economy

The 4.3 runs real strong, only has about 20 hours on it (Jasper long block). I think I will try a set of Smart Tabs instead of a new prop. Since we like to "slow cruise", the tabs would probably make more sense than a stiffer prop. Thanks for all of the good input. I never thought about trying a vacuum guage.

 

[steelespike]

Re: More pitch for economy

There have been some interesting reports regarding tabs.
There is no question tabs can be a terrific help for a number of handling issues but
in the very limited info I've seen tabs don't necessarily save fuel.
And agressive use of tabs can result in more fuel useage.

 

[craze1cars]

Re: More pitch for economy

I had Smart tabs on my last boat. And I didn't put them on my newer boat...if that tells ya anything. Not a bad product, just they only do so much. Will help with holeshot and lowered my planing speed, that's about it. They hurt my wake for wakeboarding, and I didn' t like how they impacted my cornering, so they are not all 100% peaches and roses as far as I'm concerned. Oh yeah, on the pro side they did smooth rough water riding just a little bit. All that will totally depend on the individual boat.

At 27-30 mph they will be flattened out (if set properly) and will do absolutely nothing for your fuel economy. They might help economy if your "slow cruise" was more like 15 or 20 mph, right on the edge of planing. But at your claimed cruising speed I think you'll find they won't do a thing for ya.

From your description so far, I feel your money would be better spent just dumping it in the fuel tank IMO...your setup sounds pretty good.

 

[180shabah]

Re: More pitch for economy

Gonna take a different direction here - from the numbers you setup is "perfect". Assuming a 1.84 gear ratio you are running an 8% slip at WOT - it doesn't get any better.

BUT 15.25 diameter is pretty big for a small boat and is most likely hurting your low speed performance. We don't know exactly which prop you are running, but my gut tells me your pitch may be right, but with a smaller diameter and more aggressive(cup, rake, etc) blade design.

As far as the 23" goes - try it and post the numbers.

Vac gauges - probably the best choice for replacing the worthless trim gauge on the dash....