|Posted: July 28 2006 at 07:57 | IP Logged
Assuming the numbers you provided are MPH, I've converted my own knots to mph for comparison (this is with the old 454's NOT the new diesels I have now) 4300 RPM WOT = 31 mph ....... 3400 RPM = 23.3 mph. I was swinging 4 blade props. My data was put together with a digital tach/sync and gps.
I am assuming your 1990 vessel is a mid-cabin model. If it is, your engines are further aft than mine, and your props are smaller, but running faster with different ratio transmissions than mine.
Assuming your tachometers are accurate and also assuming you are using a GPS there are several things to address.
First of all, from my own experience with the gas engines, if I had 8 or 9 people oin board, the livewell full and the extra on board freezer on deck as well, yes she would be ass heavy in the water and not ride well. My engines were a bit worn though, and she just didn't have enough stuff. The factory tabs are 9" front to back. I upgraded mine to Bennett 12" with a drop fin which was a direct factory replacement, didn't have to change the actuators or anything. The additional lift with these made a huge difference for the vessel especially when she was heavily loaded, and changed the running angle. With these changes and also opening up the exhaust deflectors, increasing the size of the engine breathers and lubricating the spark advance mechanisms I was able to find a sweet spot economical cruise speed @ 17.3 mph with a flowscan reading for both engines of .93 mpg !
My gas engines were a bit tired so my data will be different than for fresh 454's and I babied them, rarely running her above 3000 RPM. Also if yours is has different transmissions, props & shaft angles, comparisions will be different as well.
The diesels changed everything. Now the boat runs like it should, the props provide enough lift that I never have to touch the tabs. She remains at plane at 12 mph (no tabs), has a top speed of 32 mph and I can cruise all day at 25 mph and go twice as far on a gallon or tank of fuel which costs about .50 cents less per gallon.
Your problem may not be any one thing but could be a combination of several. How much diferent is it when your fuel level is low? How much drag is being created by a dirty bottom ? Are your tachs accurate and if so, why are you not reaching 4400RPM? Are you carrying around excessive bilge water in and around the liner (aft)? among other things.
You can get an inexpensive bubble type incline level device West Marine/Boat US and attach it to a side window while at rest in the water with full water & fuel tanks. Then take the boat out and check your running angle. With the gas engines I ran 4.5 degrees @ 2700 RPM and 5.5 degrees @ 3200.
It would be interesting to compare our results.