Can u get better fuel "mileage" with a certain pitch prop?

[MattP51]

I have a 16' ti hull with a J 150 v6 ob and was wonder how I can lower fuel consuption. Throw me some ideas please. PS. I normally drive at 1/2 throttle or so.

 

[Seasport]

Re: Can u get better fuel

Re: Can u get better fuel

Unfortunately, the best answer is to buy an Etec or 4-stroke! These motors, although great, aint known for their fuel economy.

You should prop the motor to reach max revs of about 5500 or even a bit higher. Experts here reckon 5800rpm is perfect. Otherwise you'll be lugging the motor and cause premature powerhead failure. You might think you could get better fuel economy with a higher pitch as you get more mph at the same revs but the motor will run sweeter when pitched correctly. My experience when dropping from a 19" to 17" was that the fuel economy didn't really change.

You'll obviously use more fuel when driving at full throttle. There'll be a sweet spot where the boat & motor work well together, usually somewhere in the 3000 - 4000rpms range.

 

[MattP51]

Re: Can u get better fuel

Re: Can u get better fuel

thanks sea, not about to buy a new motor though8)

 

[Seasport]

Re: Can u get better fuel

Re: Can u get better fuel

Me neither. A few thousand bucks still buys a lot of gas!

 

[emdsapmgr]

Re: Can u get better fuel

Re: Can u get better fuel

OMC used to offer fuel economy suggestions in their operator's manuals 30 years ago. Run the engine to full throttle, then cut back slightly on the throttle. Basically, this kept the spark at full advance, while the carb butterfly closed slightly, reducing fuel use. If you run at half throttle, it may be cutting back the spark advance.

 

[Dhadley]

Re: Can u get better fuel

Re: Can u get better fuel

Obviously the exact rpm will vary from boat to boat depending on what motor it is and what the set up is like. Look at any boat test in any magazine. Some boats will get the best milage at 3200 rpm, some at 4200.

What we do is concentrate on the percentage of slip. Once we get the slip % down as far as possible with the average load, then you're set. Lower slip means greater efficiency. And that translates thru the entire rpm range.

Run your numbers and see where you're at. Then work the set up to max it out before buying another prop or anything else. If you've never figured slip before we'll need to know the accurate top speed and rpm with an average load. Then we need the pitch of the prop. We know the gear ratio.

 

[excalibur-oc]

Re: Can u get better fuel

Re: Can u get better fuel

I have read somewhere that the fuel consumption on these older 2 strokes should equal 10% of your horse power per hour. I beleive i read it on one of these threads.
that would mean a 150hp would consume approx 15 gal/hr.
perhaps someone hirer than a cadet such as myself can validate that.
I am a newbie as far as these things but I know that formula is dead on on my 175 J
Can someone explain the slip to me?

 

[Dhadley]

Re: Can u get better fuel

Re: Can u get better fuel

You are exactly correct on the 10% rule. It might vary but I've seen it on a dyno on a 100 hp motor and a 1000 hp motor.

Props spinning in water all slip. It's not like they are turning aginst a solid and have 100% bite or forward motion. This slip is why we can get by without shifting gears.

A 19" prop will, in theory, move forward 19" per revolution of the prop. So, if we are turning a 19" prop 6000 engine rpm with a 2:1 gear ratio, we should be running right at 55 mph. Lets say the boat actually runs 45. That means there is about 18% slip. Now if we improve the set up and reduce that down to 12% then were at about 48 mph.

The lower the slip, the more efficient the rig will be.

 

[excalibur-oc]

Re: Can u get better fuel

Re: Can u get better fuel

D
that is great
thank you
I just calculated my 175 j on a 185 sport wellcraft airslot 19" prop
I am assuming that '81 egine would have a 2:1 ratio but not sure.
your formula tells me I have 18.36% slip.

Matt - try the formula on yours that will give us a good place to start

Em - i am going to try the throttle back once I get the tach working correctly

excellent advice guys

 

[Dhadley]

Re: Can u get better fuel

Re: Can u get better fuel

Your V6 has a 1.86:1 gear ratio.

Airslot, huh? Those were pretty cool boats, kind of ahead of their time.

 

[excalibur-oc]

Re: Can u get better fuel

Re: Can u get better fuel

D
Thanks, I love the boat .
But with the 1.86 ratio I an t 25% slip

What can be done to improve that & ultimately improving our consumption

 

[Dhadley]

Re: Can u get better fuel

Re: Can u get better fuel

We'll need the complete low-down on the set up. What's the top rpm with an average load? Top accurate speed? Are you running a 19" prop (probably)? What brand / style?

Can you use 100% of the trim range without the prop starting to vent?

 

[excalibur-oc]

Re: Can u get better fuel

Re: Can u get better fuel

Wow
Ok here goes
The motor is an 81(thanks Em) 175hp John with a michigan 19" prop. 3 blades not sure what style is
The boat is a wellcraft 185 sport airslot with 2 inboard 60litre tanks
WOT she pushes 38mph (GPS) 2 adults 1/2 capacity fuel - lake like glass
RPM's were in the 5800 range before my cuurent issues - see thread TACHOMETER RACES HIGH

Only once 4 hrs ago have i cavitated her at the top of trim.
I do know that I have no more capability to lower engine - on last hole

 

[MattP51]

Re: Can u get better fuel

Re: Can u get better fuel

How about installing the dolfin wings? They claim better milage after installing them,

 

[excalibur-oc]

Re: Can u get better fuel

Re: Can u get better fuel

Hey Matt,
I just posted a question on those "dolfin tales" today
Its call ..... "hydrofoil stabilizers??"

check the response I got

 

[Texasmark]

Re: Can u get better fuel

Re: Can u get better fuel

Ok ex. Get your digital camera out and take a lot of pics. Post them here and we'll go from there.

Mark

 

[excalibur-oc]

Re: Can u get better fuel

Re: Can u get better fuel

I'm trying to add pics
Muffs on = lower end of trim
Muffs off = upper end of trimCAPTAIN? - my wife is the Captain, I',m merely the Rear Admiral........Every time i fuel up I get it in the rear!!!

 

[excalibur-oc]

Re: Can u get better fuel

Re: Can u get better fuel

Geeeeeeze
Looks like I need help with addings pics also





CAPTAIN? - my wife is the Captain, I',m merely the Rear Admiral........Every time i fuel up I get it in the rear!!!

 

[excalibur-oc]

Re: Can u get better fuel

Re: Can u get better fuel

CAPTAIN? - my wife is the Captain, I',m merely the Rear Admiral........Every time i fuel up I get it in the rear!!!

 

[excalibur-oc]

Re: Can u get better fuel

Re: Can u get better fuel