Prop for best fuel economy at cruise speed

[nphilbro]

Evinrude 200hp ('78) (1.86 Gear Ratio)
20ft Fiberglass Deep-V
14 1/2 x 19 Aluminum
CMC Jackplate 6"x"6 Currenly set with AV plate 1" above bottom of hull
*All speeds on GPS, freshwater

Since I upped my HP from 140 to 200 I'm getting better performance all around, but I think I can do better for cruising on dry-plane which is 22-26mph depending on weight and water conditions with this 200hp. 22 seems to be the low cruise speed with two people 26mph steady control plane. I haven't taken this out on the salt water yet.

First, hole shot is powerful, no complaints, more than I need actually. If I punch the throttle just a bit it responds and will knock you back in your seat.
I've messed with the height of the Jackplate and this is in the rough area of best performance on a small busy lake where I need to make frequent turns.

I'm currently getting 22mph at 3200 rpms which is about 29% slip
I opened it up a bit and got 50mph at 6800 rpms :24% slip
I've got another 700rpms before peak rated 7500 but soon there were two many PWC on the lake and didn't feel like opening it all the way up. Anyway, top end is not what I'm seeking.

I'm wondering if I can move up to a 21 pitch prop and lower my fuel consumption from point to point (5-75 miles) running 3200 rpms. Keeping in mind, I can raise/lower motor (at the dock) and adjust the trim easily on the fly . According to the calculator, at 29% slip I would gain 2mph but I don't know if I higher pitch would lower or raise my slip at cruise speed.

My 140hp was running about 4200rpms on a 13 1/2 x 19 at 22 mph plain so it was burning 6 gallons an hour and running faster than I cared to go for more than 20 minutes at a time. I'd like to cover more distance with the same fuel or decrease time between points with the same consumption.

Any thought? I know going to stainless would help but I would like to get another opinion on pitch before doing that.

I'm also going to wax the hull and get a couple rough spots out this winter.

I'm not worried about it so much on rough water since I actually "drive" through that based on conditions.

 

[Georgesalmon]

Re: Prop for best fuel economy at cruise speed

I think the important thing is that your motor needs to run within the recommended RPM range at WOT to avoid overworking it. So, with that said it looks like you will be able to do that with a 21 pitch prop. As far as fuel consumption I think that it takes a certain amount of HP to put your boat on plane. Which depends on the hull design and water conditions. Since saltwater is denser than fresh water it might be a bit easier to do that than it is on freshwater. Other than the weight of the two motors if it took 104 HP to plane with the 140hp then it should also take 104hp to plane with the 200hp everything else being equal. If that was equal to the amount of fuel burned as in so many hp per BTU (not likely?) you should use the same amount of fuel at planing speed with either motor. Not sure if this makes sense and I am no expert just thinkin'

 

[steelespike]

Re: Prop for best fuel economy at cruise speed

My info indicates your max rpm is 6000, 6800 seems really high.Especially when you still; have more throttle.
Your slip seems really high as well.A deep V will have slightly higher slip but I think 24% is close to what a pontoon might have.
I would suspect your tach is not accurate, reading high.For the sake of your motors health I sure hope its high.
At a more reasonable slip of 10% 50 mph = 5750 rpm a much more reasonable number.

 

[Texasmark]

Re: Prop for best fuel economy at cruise speed

On increasing pitch, for a given medium speed and hole shot, increasing pitch will load up your engine which is not what you want to do for best fuel economy with a carbureted engine. You want the engine loafing which says reduction of pitch. From the hole shot you mentioned, you have enough HP to power your way through the hole shot so that is a moot point.

I agree with Steel that your slip is high, but on that engine with that boat I don't think that an Alum prop is the right choice. Why? Most aren't cupped (less slip with cupping) and with thin blades SS are more efficient (think of swinging a baseball bat and machete through the water....exaggeration I know but you get the point). Also agree that based upon his comments, which are usually right in my book, your tach is not accurate; which means your slip is probably not accurate.

You didn't mention having trim tabs. They do a super job of keeping the transom up at lower speeds. Some people report as much as a 5 mph difference in their minimum planing speed with and without.

Test after test has proven that "just on plane" is your most efficient operating point.

HTH,
Mark

 

[nphilbro]

Re: Prop for best fuel economy at cruise speed

You didn't mention having trim tabs. They do a super job of keeping the transom up at lower speeds. Some people report as much as a 5 mph difference in their minimum planing speed with and without.

Test after test has proven that "just on plane" is your most efficient operating point.

My motor is rated to 7000rpms for the full 200hp to be realized - I found this extremely high, myself. I have another tach in the garage I can try. At one point I thought it was burned out but it might have been failing due to intermittent rectifier problems I was having with my last motor.

I've been eyeing those "smart" tabs quite a bit. By its nature, this boat is designed to ride with the bow pretty high in order to cut large waves. I've noticed in the past that anything I try to do to better its perfomance backfires on me - porposing, sluggish hole shots, cavitation, ventilation, resisting plane, etc.

I'm in 100% agreement about going to stainless on the prop. I was just trying to see if there was any consensus on the pitch. I guess I'll try to verify the tach reading and report back.

 

[Don S]

Re: Prop for best fuel economy at cruise speed

It's very simple. Prop the motor to the high side of the manufacturers specs as suggested by the mfg. Then figure out the best MPG at different rpms.

Over propping lugs the engine and makes it work harder (at all rpm/speeds) which decreases your MPG, under propping means you don't go far enough for the rpms' you are turning, and loosing MPG no matter what rpm you are at.

 

[jestor68]

Re: Prop for best fuel economy at cruise speed

My info shows the 1978 Rude 200 having a WOT rpm range of 5000-6000 rpm.

A motor's HP rated rpm is usually right in the middle of it's recommended rpm range; which in this case would be around 5500 rpm.

 

[Texasmark]

Re: Prop for best fuel economy at cruise speed

My info shows the 1978 Rude 200 having a WOT rpm range of 5000-6000 rpm.

A motor's HP rated rpm is usually right in the middle of it's recommended rpm range; which in this case would be around 5500 rpm.

On rpms, I have never heard of a 2 cycle stock OB being rated up to 7000. 6000 is as high as I have seen in 50+ years of boating.

Agree on the rating being in the middle of the hp band. That doesn't mean you can't run on the high end which is where 2 strokes like to run......at least my Mercs do.

Mark

 

[Texasmark]

Re: Prop for best fuel economy at cruise speed

. By its nature, this boat is designed to ride with the bow pretty high in order to cut large waves.

You are realizing that you pay for this convenience. Not being sarcastic, but if you need the rough water capability then you just gotta pay the bill. I certainly would. If you don't need it, I'd opt for a flatter bottom, like a semi-v mono-hull which has a flat(ish) transom area and is easy to get on plane and hold it at lower speeds with a lot less hp.

Mark

 

[steelespike]

Re: Prop for best fuel economy at cruise speed

On rpms, I have never heard of a 2 cycle stock OB being rated up to 7000. 6000 is as high as I have seen in 50+ years of boating.

Agree on the rating being in the middle of the hp band. That doesn't mean you can't run on the high end which is where 2 strokes like to run......at least my Mercs do.

Mark

I fully agree 7000 is out of the question for a stock outboard. The only stock outboard I've seen at 7000 is certain years of the omc 15 hp.
My very limited research has indicated that the best outboard cruising speed could be from just on plane to about 1000 rpm higher.
About the only way to accurately check cruising speed economy is with a flow meter.

 

[nphilbro]

Re: Prop for best fuel economy at cruise speed

I fully agree 7000 is out of the question for a stock outboard. The only stock outboard I've seen at 7000 is certain years of the omc 15 hp.

You are probably right there. I had a Johnson rated for that. I'll double check this one. I've normally never pushed passed 5k

My very limited research has indicated that the best outboard cruising speed could be from just on plane to about 1000 rpm higher.
About the only way to accurately check cruising speed economy is with a flow meter.

This is precisely what I'm shooting for. Anyone know how acquire/borrow a flow meter? This would help me answer most of my questions while actively running vs. crunching the numbers after the fact.

Here's the tag :

I'm also sure my Tach is off because there is no way it was going that fast (rpms) with throttle still left to go (as I said - I hit 50 on GPS and it wasn't even WOT) using a factory stock prop.

I'm also more than willing to pay the price for high bow rider for the ride it gives in big water.