Prop sizing for Eastern 22 with i/o

[E22]

I have a '92 Eastern 22 lobsterfisherman with a Mercruiser 3.0 i/o that I'd like to squeeze a little more mid-range performance out of. My buddy has the center console version of the same boat with a 115 OB that blows mine away from a cruise and max speed standpoint. Granted I'm carrying more weight with the pilothouse and heavier motor but he's cruising at 20 kts vs. my 15 and topping out at 34 kts vs. my 24.

Here's my stats:

Current prop: Michigan Wheel 15.25x15
Boat weight: not sure but guessing 4200+/- lbs fully loaded
Motor: Mercruiser 3.0, 135 hp. 2.0 drive ratio. Good compression.

Current performance (mostly flat, 3 adults, full tank):
3000 rpm = 12 kts
3400 rpm = 15 kts
3800 rpm = 18.5 kts
4200 rpm = 20 kts and on finally plane
4600 rpm (WOT) = 24 kts
Just me and lightly loaded I briefly hit 4900 rpm and 26 kts...

I'm not really getting onto a plane until I hit 4200 rpm, then I can trim out a little and back off the throttle to 4000 before falling off. I'm generally running in saltwater with some swell and chop so top end speed is not all that important to me. I'd prefer to be able to get on plane earlier and hold plane at the motors preferred cruise rpm of around 3300 +/-.

I think a pair of trim tabs are in my future as well, but for now I'm focused on getting the prop right.

Was thinking about going a little up in pitch but then started reading up on 4 bladed props and how they're better at keeping heavy boats on plane at lower speeds due to their stern lifting characteristic and less slippage. I care less about top end speed than about getting and staying on plane at a reasonable rpm so it seems like a 4 blade might be the way to go....

Any prop experts or someone out there with experience with this boat and setup that can chime in?

 

[alldodge]

I would weight your boat; I'm finding should be between 907 to 1224 Kg (2000 and 2700 lb)
Specs I find show the stern drive the same weight as the outboard model (907), but the inboard as (1124). Don't fully agree with this, but just what I'm seeing.

Also find the center console model weight is 757 Kg (1670 lb)

There is some weight difference and this could be all there is

 

[E22]

I'm slipped right now with no easy way to get the boat weighed but will try to make it happen in the next few weeks. My weight estimation is based on the following:
2,500 lbs dry weight from on old brochure I found
800 lbs +/- motor, drive, batteries, wiring, etc.
200 lbs for 30 gal fuel
500 lbs for 3 adults or 2 adults and a couple kids
200 lbs miscellaneous fishing and other gear

Deck and pilothouse are solid so I don't think I'm carrying around a bunch of extra water weight but who knows, maybe I am so I will get it weighed, just might take a little while.

 

[alldodge]

The dry weight of an inboard or I/O boat includes the motor. Now if the boat normally comes with a 4 cyl, and options for 6 and 8 cyl, this will increase the weight. An OB boat also includes the OB motor but only the motor it normally comes with. These are just reference numbers and not actual.

So what I saying, is yours weight is 2500 lb, and more then likely your buddy's CC weight is probably 500 to 700 lb less. Even if your not water logged and bone dry, this could be your issue. Weighting the boat will help take away some of the guessing.

Another thing to note, is if your tachometer is accurate. Have found some think there turning 4800 when were only turning 3600 or so. Not saying this is an issue either, just something else to look at

 

[E22]

OK my bad, I thought dry weight did not include the motor.

Assuming the tach is accurate and the weight is close to the original design (believe they both are but will verify)... I ran some prop slip numbers based on speed, rpm and pitch and am getting a calculated slip of 28% at 3500 rpm which seems high and is right where I'd like to maximize the efficiency. It goes down to 15% at WOT but that doesn't do me any good for my purposes. Any guesses on what direction to go with a prop change? If any? Starting to believe tabs might give me the best bang for the buck...

 

[alldodge]

Anything in the water creates drag, tabs will also create drag and can help the ride.

You have high slip until the boat starts coming up on full plane. IMO what you need is less weight or more HP. If your motor is outputting close to full HP, then there isn't much more you can do with it. A V6 is an option but can be costly, unless you find one in a donor boat.