Proper Performance Engineering

[ThePHNX]

The Good, Bad, and Ugly:
Quote from Mercury Marine:
........ " How do I obtain horsepower curves, torque curves, or fuel consumption data?

Horsepower and torque curves are not published because they are considered confidential company information. Fuel consumption data is not published because it is affected by factors beyond our control, such as boat hull type, temperature, humidity, altitude, gear ratios, propeller, etc. In some cases, fuel consumption or other performance data is available from your boat manufacturer for a specific boat/engine combination."

What absolute nonsense, keeping torque diagrams secret. Did not know marine engineers were required to be all wet!-(

Anyway: to properly determine best props (wheels), throttle settings, etc. one NEEDS to know the engine dynamics and torque curves.

For instance, one post read recently said there was NO delta between 2 stroke and 4 stroke engines, ah but I beg to differ: and the difference is readily distinguishable when the torque curves are shown.

Some two-strokes are notorious for their icicle (stalagmite) power curves where there is hardly any power until a certain high point of RPMs and the power multiplies so rapidly it totally changes the engines personality.

Who here can at least help with engine RPM stats, High, low, and cruise (typically 75%).

The engine I'm working with is the Mercruiser GM I-6 165 I/O. Knowing the proper throttle / RPM settings (again, the POWER is a direct function of engine design, volumetric efficiency and RPM) and the planing speeds for the SeaCraft hull we should be able to compute the proper - theoretical - pitch and diameter. Once the design P & D are determined things like harmonics, slip, etc. would play into the final determination. Right now I find the five blade Hill <http://www.hillmarine.com/> interesting.

Thanks, HBH
Oh yes, the craft is a lovely 1976 SeaCraft SF20

[digreg]

hp = torque * RPM / 5252

[ThePHNX]

Am familiar with this relationship.
Note that it requires knowledge of one or more performance values for the specific engine.
It does little for analyzing the best performance from a given engine.

For instance, a rule of thumb is that engines are generally most efficient at their 75% power settings. That 75% position is NOT linear.

Several things are known about vessel throttle/power management - whether aircraft or planing boat - the amount of power needed to get `on step' (and that phrase applies to a hull without a geometric step, as it does to an aircraft wing) is greater than the amount of power required to keep it there. The skilled pilot will back off throttle while monitoring speed as will the skilled helmsman. Advancing throttle(s) after getting `on step' may increase speed but disproportionately increases fuel consumption.
It is a given that aircraft manuals provide extensive information on the powerplant. It is a shame on the boating industry that these same standards are ignored.

Am wondering if some power boaters add engine instrumentation; vacuum, manifold pressure, fuel flow. I know that on my Snowgoose Motor-Sailer I found the fuel flow meter (costly because for diesels a return line is required) VERY useful.

Here's a question then: What kinds of engine instruments are some of you using, and how. BTW, had full manual from Volvo and it was excellent. Guess I'm as much a Volvo fan as I am a Merc' denigrator!-)

HBH

[spareparts]

put a 21 pitch old style Mirage on that boat, get it to turn 4400, and you should be right where you need to be. Theres no reason to complicate a simple setup

[ThePHNX]

Raw speed holds less interest for me than efficiency. For me the challenge is to return to base with more fuel left than anyone else.

Read your comment - elsewhere - on hull distortion from improper trailering, can you explain more?
HBH
BTW: the manual suggests 19 pitch, would tend to go with your recommendations as the impression is you go beyond basics.

[Bigshrimpin]

For me the challenge is to return to base with more fuel left than anyone else

The only thing the prop will change is the efficiency of your setup. Laser has nice thin blades, smaller diameter, and is a fast prop. I personally like the mirage plus because of the stern lifting characteristics, but folks here will run both. You've got a 20ft CC with a heavy I/O. The 20cc is light in the bow to begin with . . . without the 600+ lbs of motor/outdrive. But if you're truely after efficiency, get the Diesel, have someone blueprint the hull, have the prop labbed and be done with it.

Page 1 has the torque diagram for the cummins diesel.

http://www.cmdmarine.com/PDFs/4081824_1104.pdf

Guess I'm as much a Volvo fan as I am a Merc' denigrator!-)

Have you ever bought parts for a Volvo??

[oldbluesplayer]

hmmm, well, returning to the core of this post...

As most Mercruiser IO powerplants are GM, I would bet you could get the torque / horsepower / rpm curves from them - I know in my old hot-rodding days these numbers were available - and often were published in the factory shop manuals (not the owners manual), but the last set I have of those is for old Ford V-8's (think Boss 302.... yeah, I'm an old fart)

quite true on horsepower to get to plane, vs horsepower to maintain - same with a car, what it takes to accelerate to 60mph, vs, what it takes to cruise at 60mph - much lower.

Phnyx, sounds like you have heavy aircraft background - safety requirements called for many more guages to monitor in-flight condition of engine - on small, pleasure type boats you will mostly see fuel flow, maybe voltage, temp, oil pres if an inboard or IO - but would probably get more involved on a big bucks sportfish with diesels. My experience on manifold pressure, is only on piston engine aircraft, and military / commercial vehicles, with turbo or superchargers (and hot-rods with same)- so the boost point kick-in, and max pressure, could be monitored. Any other old farts out there, with any Army experience, remember the M551 Sheridan light tank - had a 6V53 Detroit Diesel, with twin turbos, and a supercharger - on full boost it was quick - I worked on that puppy at the Detroit Tank Arsenal.

stray thoughts...

Bill

[spareparts]

Bigshrimpin, for what its worth, i just changed out a set of impellers on the water taxi, twin 150 four stroke Yamahas, 2005 model, 1900 hours, they were worn, but they were still pumpin'