I don't disagree that I should Be pulling more rpm and the prop is a little tall for the set-up but my fear is that I may sacrifice the mid-range which is the majority of my boating......4100rpm 72 - 73mph is a pretty big increase. Jeff at hering has been very good and has offered to work them a bit......it was the first time doing this set-up on a 35 so nobody was expecting perfect. I am on the fence as to what I should do.......until then I will enjoy the water and fun. :)

A thought. I could be wrong but here it is.

Having too large a prop causes extra load on the engine.
You'll have to give it more throttle to maintain any given speed and RPM than with a smaller prop turning more RPM to maintain the same speeds.
More throttle ALWAYS means more fuel. So much for efficiency.
A smaller prop will out accelerate the larger, meaning if you accelerate at the same pace as with the larger prop, you'll be giving it less throttle. = Better MPG. Of course we'll accelerate faster so go ahead and throw that one out!
Having the engine spinning a little faster keeps it in the torque curve, it's most efficient area, where it won't bog as often and you won't have to throttle to maintain speed. = Better MPG

Something to that effect.

There is a balance, but I think you're way off.

Another thought. Cut 2" off your skeg?
The Sportmasters have really long skegs. It'd be similar to putting a -2 IMCO on. You have a full stagger 35 that I'm betting the props are still in the water plenty so who needs all that drag? The skeg brings the stern up along with having all those blades. At some point the nose will be getting heavy and you'll be plowing. More throttle required to maintain speed = Bad MPG

Now lets get stupid. Too late. We all own boats don't we! Anyways, how about stupider???

If you spin your engine 20% less RPM it should last 20% longer. Say that's an extra 100 hours/season. 100hours X 70MPH = 700 miles driven at 1 MPG (at 4000RPM) = 700 gallons X $4 per gallon = $2,800. Save this number, it wont matter later.
Had you run a smaller prop and burnt less fuel you would have saved: $2,800 X 20% less fuel = $560 X 4 seasons = $2,240 in fuel saved.
Engine rebuild at $10K divided by 500 hours/5 seasons = $2K per season. Divided by 4 seasons = $2,500
4 seasons = engine at $10K + fuel at $8,960 = $18,960 divided by 4 seasons = $4,740 per season.
5 seasons = engine at $10K + fuel at $14,000 = $24K divided by 5 seasons = $4,800 per season.
This equals $60 savings per season running a higher RPM.
Higher RPM = less stress on drives + better acceleration = more fun.
I wont go into the drive costs right now. Just figure a larger prop will eat a drive sooner.
I hope you're pulling your hair out right now.

Basically, higher RPM = more fun!
So you may as well just send me those props and I'll dispose of them for you.

MAIDEN........all I can say is WOW!!!!!! if your thinking is right, you make prefect sense.....I will be calling hering in the morning to arange for the props to go back.........hahaha......thank you very much for the input / advice.

Wow I guess. IMO there is no way that cruising with smaller wheels turning at a higher RPM is going to yeild better fuel milage. I would give exception to a boat that was waaaaay underpowered and under great load to even maintain speed. Hardly the case with the boat in this discussion.

A given engine will burn a given amount of fuel per hour at a given RPM. Where the efficency, MPG as we refer to it, comes into play is in how much distance can be covered in a given amount of time, or during the hours worth of fuel burn. Therefore if you can cover more distance at the given RPM in the hour with larger props you will see a higher resulting MPG. Smaller props at the same RPM will not cover the same distance.

Here's the deal,,,,,,,,,,,,, I have a sport boat with 496's and cruise at 50 MPH at 3500. My friend has a family cruiser with 496's and cruises at 25 MPH at 3500. Both boats are twins, 496's burn 18 each or 36 total gals per hour at the 3500 RPM. We both take the same 50 mile trip, takes me 1 hour and 36 gallons of fuel, takes him 2 hours and 72 gallons of fuel. There are variables that may skew the end result by a small margin one way or the other. But, the end result is totally based on fuel burn per hour VS distance traveled. End result is that the further you travel on an hours worth of fuel burn the more efficient you will be, in this case translates into better MPG, and larger props that increase your cruise MPH at the cruise RPM will do this, smaller props will not. The result will also be skewed by the prop design, number of blades and the slip factor ( directly related to efficiency of the set up ) of the props on a given hull. Seldom on a boat can you get the best of speed and cruise efficiency with the same prop design.

Wow Rob, I'll bet that set the calculater on fire.

They sure look good though!!! :evilb:

That is allot of material to swing !!!!

Just messin' around Jim.

The RPM vs MPG vs MPH could have an effect I'm talking about if you are propped way wrong. "single w/twins" (my old license plate frame BTW) sounds to be way over propped and could run into this potential problem.

Example: 77 AMC Pacer 258CID 3.07 gears, 4speed and 38" tires (what? you guys wish you had this car!)
To get anywhere will take FULL throttle.
Now put 25" tires on it.
To get anywhere will take FULL throttle. OK Bad example.
How about driving up a hill at 3K RPM with no load?
Now put a trailer on (with the Pacer on it) and go up that same hill at 3K RPM. Same gear. Which do you use more fuel?
More load = more fuel.

So at some point there is a balance point as to under/over propping a boat.

Basically, if a large prop is X and the small prop is Z, then the fuel would be C if used with X but when you go to Z it would be D. Therefore X ta C is directly disproportionate to Z ta D when Mercury is in alignment with Uranus.:bsflag: Something's starting to smell.
Simply put: Boat = fuel3/fun2 X (gas/mass)
Synopsis: Owning a boat will always cost a butt load of fuel. Ask Freddy
I seriously need to go boating!!!!