AmiableDave

History....Formula 292 Fasteck. I have always had a problem with prop slip. I removed the Bravo 1's 28P 4 blade props and installed BBlade's 4 speeds. Both sets of props where not Labbed. The boat gained some mph and reduced prop slip with the BB. Although I feel the slip is still high (upper teens).
I removed about 300 lbs of exhaust manifold weight by removing cast iron and adding aluminum/SS. So the stern is that much lighter. I also added about 100 lbs to the cabin area. I am not carrying 19 gallons of potable water in the V-birth.

Problem??.... The boat at present cruises more efficiently at 3400 rpm at 42 mph consuming 22 gallons of gas per hr. Prop slip is 17%. BUT the drives have to be all the way in and the tabs all the way up. ANY deviation from that will reduce speed.
If I increase speed then I can adjust the drives out and tabs down for the best top end of the rpm range. The degree of adjustment is determined by speed.

I have my own opinion but am holding back so not to influence yours... Thank you for your time.

AllDodge

iTrader: (1)

Removed 300 from stern, added 100 forward of amidship
Best cruise with drives tucked in and tabs up

Will guess if drives come out at best cruise, the boat starts to porpoise?
What happens when you fill the water tank?

Tabs down with drives out and best speed, you need more weight forward.

AmiableDave

Yes, when drives come out at best cruise, the boat starts to porpoise. But before it porpoises, the speed decreases by 6-7 mph. I always run with a full tank of gas. That weighs over 700 lbs. I don't really want to carry around extra weight.

AllDodge

iTrader: (1)

My boat does fine until a bunch of friends show up. I'll have 8 folks (some large) and I have to tuck the drives in and keep them there to keep from porpoising. When there is just the 2 of us, then no issues.

Thought about something else, you have 28P 4 blade props, that is creating a lot of lift for a 7K lb boat. Are you reaching 5K RPM's at WOT?

AmiableDave

I am running 26P Bblades. And yes 5K WOT

AllDodge

iTrader: (1)

If this isn't a new to you boat (not original owner), then wonder how the drive vent plate lines up with the bottom of the hull

AmiableDave

On both drives, The outer edge of the inboard anti ventilation plate is 3 1/4" above the top of the setback. Top of the setback to the deadrise is another 1 1/4" The outer edge of the outboard anti ventilation plate is 1" above the deadrise.

Griff

Not sure why you are so hung up on slip percentages. Higher slip is not a horrible thing and is pretty normal on 292 Fastec's.
Higher slip %'s actually stress outdrives less.

What exactly is the dilemma other than you have to have trim and tab set a certain way to be most efficient. You are getting almost 2mpg at cruise speed.

AmiableDave

Propeller slip is thrust wasted. I don't do hole shots, wave jump, race or run WOT every single time I'm out. I ease up on plane. If I was to get lets say, 10% slip, that would equate to an additional 4 mph or a saving of 20 gallons of gas when filling up. Projected numbers..

I may run better now with the drives all the way in at 3400 rpm. I may run better if I could tuck in the drives more, but I can't. With the drives tucked in, most of the thrust is trying to keep the bow down instead of moving forward. If the stern was higher in the water and the ability to move the drives out where the propeller centers where parallel to the keel, all the thrust would be forward. Then I would have full range to adjust the outdrives to suit the conditions and efficiency. So what are your recommendations to helping me with my dilemma?

BBYSTWY

Drive spacers or 5 blades would probably help...maybe talk to brett at bblades...he can I'm sure lab your props and help this situation...have the drives been off recently? is this a new condition that just started or has it always performed this way?