Detroit 8v71 recommended rpm

Naturals need to turn at least 1800. TIs need at least 2250.

I'll crack my books when I get back to my desk.
That TI number may be 100 low.

Afaik both the N and TI are rated at 2300 rpm so a max cruise of 1900 to 1950 is reasonable.

You didn't say what size both you have but running between hull speed and on plane is just about the absolute worst speed you can pick. You re pushing a big wake, loading up the engines and burning as much fuel as you would if you were on plane

Hull speed is typical the sq root of waterline length times 1.3 so for a 50 footer you re looking at about 9kts.

My 53 Hatt burns about 10gph (both 8V71Ns) at 9kts. If I push to 11kts, that jumps to around 18gpg, 12 kts is more like 25gph... Makes no sense.

If the boat will get on plane then it is a much better speed. Make sure your trim tabs works and set them full down to help.

Again not sure what size year Chris you have, but back when naturals were commonly used in the 60s and early 70s fuel was cheap so burning twice the fuel for a extra 2 or 3 knots wasn't a concern.

So you re better off either running at hull speed or on plane, but never in between

TI's spec is 2450 no load, 2350 loaded but I would not push a 40-year-old-plus motor. Old Detroits rarely turn up to spec at WOT. TI's are in the turbo boost at 1750. 1750-1850 is an ideal RPM for the 8V71 to cruise however in your case I doubt you plane at 12 ...maybe at 14 though. 1100 RPM should give you +/- 9 knots and way better fuel economy.

Actually I had not seen Pascal's comments above when I posted. Basically saying the same thing except I would not cruise the old girl above 1850.

You need to see 1800 to 1850 WOT. 1450/1500 would be the long life throttle (approx 80%).
I used to run straight 671Ns at 16/1700 all weekend per owners instructions on one boat.
Many years later, Yep, it's still running with no problems.

As K1W1 stated above, if you still have the tags on the valve covers, those are the ultimate numbers to go by.

Huummmm
Would luv to get the serial numbers. Those numbers are no-load and similar to a T or TI.

8V71s are rated at 2300, you need to see 2300rpm at WOT loaded. If you don't get that, or close to, it means you are over propped, something is wrong, the boat is dirty, etc...

When I surveyed my 53, the engine surveyor noted the engines only turned 2200 under load and warned against running them hard until the problem was fixed but running at 1500 for hull speed woudl be fine.

One thing I can't stress enough is how even 1/2 knot over hull speed will greatly affect your fuel burn. On your boat, your hull speed is about 9.5kts. Running at "around 10kts" may cost you 3 or 4gph or m ore if you are actaully running closer to 10.5. When trying yo find the best rpm for hull speed, take into account possible current and wind and obviously use GPS not paddle type instrument

Your tags State over 2000 rpm. That kinda proved me incorrect. I maybe remembering old 2 valve heads before the '70s.
Either way, you still have lots of drag or your over propped.
Still would luv to look up the factory specs with your serial numbers.

Have you preformed a no load high idle max test yet? This will tell you quickly what the governor is set for.

Pascal is correct on any little throttle adjustment can change angle, mpg and speed greatly.

Paddle wheel senders are not accurate but repeatable. Gps is accurate but over the ground, not thru the water.
When measuring boat performance, make two passed with & against the current and average the speed recordings.
If you have a paddle-wheel speed instrument, learning the errors and variations would be of benefit. also for other task when DR-ing.

Sadly, build records before 1972 are not available to me. Sorry I can not deliver any DDC factory build notes.
Do you have any info on 2 or 4 valve heads and injector size?

I'm sure we have the same thing.
My DD shop can pull up better detail on lots of extra data. Sadly, they hit the '71/72 wall in the DD data base..
,rc