Options
Categories
- All Categories
- 8 Non-Rinker Boats
- 2.9K General Boating Discussions
- 293 Rinker Custom Projects
- 134 Trailer & Trailering Discussions
- 470 Generator Discussions
- 172 Rinker Photo Gallery
- 363 Rinker Sport Boat Forum
- 3.9K Rinker Express Cruisers Forum
- 1.2K Engine Discussions
- 1.2K Electrical Discussions
- 658 Rinker Support Q&A
- 589 Rinker Marketplace & Classifieds
- 66 Rinker News & Updates
- 102 RBO Forum Archives
- 174 Region-Specific Questions
- 31 Canada
- 135 United States
Comments
Boat Name: King Kong
"Boat + Water = Fun"
The engine doesn't limit the output based on load (as measured by vacuum signal) but on revolutions.. it will NOT rev beyond 5400 even in neutral. The engine in neutral could likely nail the limiter at half throttle. The engine in gear and pushing the weight likely isn't capable of nailing the limiter.. at 4800rpm, that engine is likely nearing or at full throttle, and to the point if it was capable of producing proper power it would be revving beyond 6kRPM, or in neutral without a limiter nearing the mechanical maximum (if it was stupid balanced and valve train could handle it, that would be at or near 12kRPM, but there are only a handful of carefully crafted v8's that can handle that)...
The 4800rpm he can nail is not producing the power it should be at 4800rpm, because the air to fuel ratio is all buggered up- its trying to rev more but it can't due to load. You'd be wanting an easy 5k on a magnum which produces power, which means anything under that is property trimmed (fuel trimmed, not hull trimmed), and the engine isn't struggling, and is producing the expected power a proper fuel ratio gives... He may be dumping 20% more fuel than an identically loaded and duplicately equipped boat running alongside him, working harder, and producing less power. Oh, and burning more oil to boot..
In a shorter response: his horsepower is there (work), but his torque isn't (sustainability of work or difficulty to stop it from spinning)... Or, maybe the torque IS there, but its being spent in slip instead of bite..
I'd be curious to see the outdrive gears teeth.... I'd also be curious to see the props blades.. I ain't gonna get hung up on those, though, because if they we slipping, revving wouldn't be a problem, slipping would be, and it wouldn't have slipped all season (or since the birth of this thread) without the gear box grenading, or such unbalance he'd have looked at the prop by now..
In cars: forward speed, overall gear ratio, and rpm are written in stone.. if you know two, you can determine the other.. on boats, the overall gear ratio has a variable almost impossible to standardize, which is displacement of water, and manifest in slip by most calculators, but doesn't account for slippage based on the boats attitude, or planing surface, which whittle at speed and increase load... Load is the great unknown, and THE thing to manage when tuning an engine and pairing an engine to an application... Thats why big blocks rock in boats. :-)
An engine capable of producing the power will be limited only by safety margin of maximum revolutions before it flies apart.. that's why rev limiters exist.. they won't let you float the valves...
An engine capable of pushing a load up to the limiter is wasting capacity.
A prop may be too steeply pitched which puts too much load on an engine and limits its revs while it allows slipping at the same time... Where as a shorter pitch will allow the prop to spin up and slip because there is too little load on the engine due to leverage... A proper balance of power and pitch to load ratio will allow the engine to reach its maximum sustained load index, and limit slip. He seems to be properly propped if it is an OE prop selection from the factory, he's just slipping it because he doesn't have the proper power at an rpm to push the boat further out of the water, promoting slip and not decreasing load as expected if there was less boat displacing...
If he planed more hull, the load decreases, which allows more power translation to screwing water as opposed to pushing it, which equates to less slipping as it bites..
This is same concept as to why a dirty hull won't allow rpms or speed.. its loading the engine AND its promoting slip.
Has anybody asked about the condition of the bottom? Even a little slime/crust can/will kill performance.
Maybe take an experienced marine mech/pilot friend for a ride who can conclusively say it needs more/less throttle/drive angle/speed to get fully on step at an efficient speed and then carefully find (and note) the sweetest trim settings at cruising speed(near 26mph). Our 270 with a perfectly clean bottom, OD trimmed slightly up/aft from parallel to the keel and no tabs is happiest at 3800rpm and 26-27mph.
21mph is NOT on step. 4k rpm should be very near 26-30mph.
If it's not that simple (sure hope it is) then there's plenty of detailed possibilities elsewhere in this thread.
Good luck. Mike
We have very nearly the same boat and here's what a well trimmed 26mph looks like fully loaded full of fuel/water and with 4 souls on-board....
It is worthwhile to note when I use the trim tabs when I am on plane My speed is reduced. I only now use my trim tabs to help go on plane and/or to balance my boat when people move around. At time I needed them when water conditions warranted them to balance again to balance the boat.
Boat Name: King Kong
"Boat + Water = Fun"
I wish more people would post up graphs of speed (gps), rpm, and fuel burn for each style boat. I think it'd help alot of people when they arent sure if their boat isnt running how it "should". This weekend should be near perfect conditions up here, so I'm planning on getting a bunch of speed & corresponding rpm readings.