rpm range

[muskyfins]

I have been working on some prop improvements (in another thread) but I keep reading about guys running 5000+ rpms.

I have 1993 stingray with a mid 70's 188/888 ford 5.0 and she is screeeeeaaaammming at 4200. What gives? Should I have replaced with a newer motor? The one I have can't have many hours on her. She runs like brand new. Super stong, but I'm a little tired of reading about guys with 4.3's or even 3.0's running 50 or even 60 mph.

FWIW I run a 19P SS Vengenance and while we're on the subject-my drive has a 1.5 ratio and I understand some of the newer ones are higher-reducing some of the difference in my WOT rpms.

How the hell do these guys do it?

(not trying to start a real technical thread-I already haveone somewhere on iBoats-just doing the head scratch thing and wondering what my motor WOT rpm should be)

 

[steelespike]

Re: rpm range

I guess you could build up any motor so it can turn 5000 rpm and more ,and develop horsepower beyond the factory specs.
But in my opinion the motor wan't be happy for long. Figure todays family sedans capable of 5500 to 5800 rpm
Keep in mind these run at those speeds for a few seconds at a time.
I don't think any 3.0 could do 50 and be good for anything else.A stock 3.0 in a 16-18 ft boat could do about 43 with a 21"
prop. If it could get a 23" up to 4800 would do about 47.And not be good for much else.
9 times out of 10 The typical 3.0 runs about 40 lightly loaded.I'm betting some of those claims are with inaccurate gauges
and some optimistic observations. Again you can build up any motor but don't expect it to have the same longevity of
a stock motor.Your 5.0 is pretty old technology by todays standards.
There are some areas that gps speeds could be off by 3 to 6 mph running down stream unless they figure the current in the equation.

 

[Sunnyjen]

Re: rpm range

Gear ration not HP make the difference. Here's the equation: pitch x RPM divided by gear ratio x 1056. For example 13 pitch x 6000 RPM divided by 2.27 gear ratio x 1056 = 32.53. Depending upon your boat there is a slip factor. Planing hulls have on of about .80 while pontoons have one of about .72. So if you have a planing hull = 26mph. HP comes into the equation when running higher pitch props. You need the HP to keep the RPMs up in the operating range. HOpe this helps.

 

[muskyfins]

Re: rpm range

I guess I'm just venting, and I understand the actual calculations involved, but look around here on iBoats. You'll see plenty of guys discussing these rpm levels and speeds. steelspike has a point too. Combine some amount of inaccurate speedos with a dose of denial and 48 on the dial becomes 55 on the internet when it's 42 on gps. LOL

That being said, I boat on a chain of lakes crammed full of knuckle heads. It is the busiest inland waterway per acre in the US. The main lake I use has an average of about 5 feet of water and there are no less than half a dozen 30-40 foot offshore boats out there at any given time not to mention hundreds of various go-fast boats. I will add pics to this thread of two of them. The white skater has twin blown alcohol motors running about 2000 HP apiece!! On a lake 3 miles long!!! I get walked all day by guys running 18-22 foot class runabaouts. So if I'm running 42 mph they've got to be in the low to mid 50's.

Could be like my brother says. "Those guys" have d@*k envy and are trying to keep up with the go-fast guys by way over-propping with no regard for the fact that they drag their hull through the water for 200 yards before planing. It hard on the motor, the drive and not comfortable for the passengers.