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Fuel burn advice please

E

Ewan

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"Hi all hope this isnt to stra

"Hi all hope this isnt to strange a question,
Boat is a 2000 bayliner 2655 with the 5liter 220hp and alpha leg

My questions is that will a 4 blade prop cause more fuel burn than 3 blade?
The boat had 17inch pitch prop 3 blade which gave a wot of 28kts at 4700rpm and cruise at 18knts at 3800rpm

the 4 blade which just fitted today gave a wot of about 29 to 30knts at 4400 rpm and a cruise speed of about 20-22 knts at 3600-3800rpm and quicker on the plane, which im more happy at over all with the 4blade

What im trying to find out is with the 4blade down on revs will that be the same fuel burn over all as the 3 blade?, Would have at the higher revs?. And will the lower max revs do any harm to the engine long term? as they are down from 4700 to 4400.

Sorry if this doesnt make much sence as its giving me one bad headache.

thanks to all who may read and help with these questions?

Ewan

on a side note would an oil change and new flame arrestor give a few extra rpms?"
 
"sounds like you are over prop

"sounds like you are over propped for the setup that you have in turn you are making the motor work harder and never reach it's specified rpms. on your side note no those will not increase your rpms any. the only way is to change the prop or in some cases you can go into the outdrive and change up the gears.. I would decrease the pitch by about 3 or 4 and try it out, you will get a better top end and with the 4 blade you should still have a great hole shot.. when doing a switch like this it's more trial and error then anything.. see if your boat shop will let you try out a couple of different props (mine lets me use some of their used props for testing) or they will let me try out the new ones as long as I don't mess them up.."
 
"I have to disagree with you B

"I have to disagree with you Bill. 4400 is in range. ( on the low end) R.P.M. should change about 200 per in. of pitch. 6-800 increase is way out of site. as for economy, probably won't see much change. test and see."
 
"I'm sorry, I did read the

"I'm sorry, I did read the whole thing wrong, I thought with the 4 blade you were staying in between 3600 and 3800 at WOT. So yes you are in range. about the Fuel burn question, The 4 blade prop does make the motor work a little harder but I wouldn't think it would be that great of a difference to notice.. Like Scott said test it out and see.."
 
"Ewan:

The big selling poin


"Ewan:

The big selling points claimed by synthetic oil manufacturers are it increases fuel mileage, or in your case, reduces fuel consumption and makes the engine run cooler. I run synthetic in my 4WD truck and 4WD SUV and cannot attest to any of the claims. All that I have noticed is that it seems to remain cleaner longer but I can attribute that to using oil filters with a smaller micron filter media than the cheap filters on the market. However, I am contemplating changing over to Mobil 1 or Pennzoil 15W40 wt. synthetic oil at my next oil change."
 
Ewan:

Concerning fuel consu


Ewan:

Concerning fuel consumption and lower RPMs improving fuel consumption and hurting the engine; that's a no-brainer. Logic tells me there is no difference between driving your car at 55 mph @ 1500 RPMS vs. 65 mph @ 2000 RPMs or driving your boat in the same manner. Fuel consumption increases at higher speeds and RPMs and vice versa.

Anyone care to counter my point?
 
"There is an an optimum ballan

"There is an an optimum ballance for maximum engine efficiency with RPM, Speed, and gear ratio/pitch combination (Prop based or car). If your engine is lugging under too much of a load with a prop that is too aggressive you will not get optimal efficiency. If your prop is way to shallow, again the engine will not be working hard, but then again you won't be getting as much thrust, and will barely be pushing through the water. (not to mention your more apt to redline your engine, as if your boat is like mine, there is no governor, the prop pitch and the properties of your boat are "the governor" for engine speed).

And you do get better efficiency driving 55 @1500 RPM for a host of reasons (wind resistance being primary) than 65 @ 2000.

I've often wondered how far a 24 gallon tank of fuel would get my boat just leaving it in gear and idling at 2-3 knots. Guess I could do the fuel consumption math, but then I've never let it idle like that for an hour.

Reminds me of getting ready to go to Kuwait just prior to OIF. Navy regulation require vehicle fuel tanks to be below something like 1/2 full. Our guys topped our trucks off before leaving Camp Pendleton for San Diego (30 miles.)

After getting their they realized the tanks were essentiallys till full. They thought surely it'd burn a gallon or two an hour, and let them idle all night (no mechs around to drain). Next day, the needle barely moved! Methinks my boat as is would probably be down a quarter tank!

Lugging the engine can be hard on it too. Can cause knocking, and other worries. But the ranges you mentioned don't seem extreme. I think my manual called for a WOT of 4400 to 4700 and to chose a prop that mainained that for optimum effeciency and best engine life."
 
"Thanks for all this input, th

"Thanks for all this input, the 3blade prop was new only on 2 weeks till hit something. I know the max rpms are down from the 4700 but the boat felt more responcive and faster with the 4blade could the 3blade at 17intch pitch been over propped? ive went thought that spreadsheet thats been posted on here and the mercury marine prop selector merc one came out at 18inch pitch for the 4 blade and the exel one on here ranged from 14-17inch pitch. Althought I think the boat is under powered at 7000lbs with gear gas and people would I be right in saying that the 5liter a little light for that type of weight? so will have to push a bit harder to get it going?

All in just wanting to make sure wasnt going to blow the engine

Thanks so much for the help everyone"
 
"ewen,

after looking up t


"ewen,

after looking up the WOT range in a Clymer manual looks like you are right on target (4200-4600) so with the 3 blade prop you were probably under propped for that application. you probably needed a 19" pitch. About the 5 liter being a little light for that boat, I think you are just about right size unless you are looking for a race boat. I have seen 4.3liter motors in that size boat before. as long as your gearing (prop) is correct.."
 
"I removed a 16 x 16 three bla

"I removed a 16 x 16 three blade and replaced it with a 17 x 15 four blade. Rpms dropped about 600 and gas mileage increased at least 10 %.

I'd be happy with what you have, and don't let anyone tell you that you're over-propped since the boat planes off better than before!

Jeff"
 
"It planes off in an instant n

"It planes off in an instant now and holds a higher speed at lower the revs.

Just glad to hear that wont do any engine damamge and also good to hear of a gas range increase aswell with a 4blade, now need some decent weather here in scotland to give it a good test.

Thanks again for the help and advice,

Ewan"
 
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