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??

[spareparts]

thought i had edited my post earlier, must not have taken, Last nite, after thought, the 19 pitch Mirage is probably a better prop for your set up, its a large blade surface wheel with much lift, either bow or stern, this prop is extremlly efficient, i had a customer that had one on a performance boat, we calculated it was within 7% slip at 101 MPH, that type of efficency is allmost unheard of. We ran 21( it had been worked on) pitch on my old roommates boat, it had a 3.7l Mercuiser 4 cylinder, it was rated at 165 hp and has simular torque band as the I-6 250 GM engine you have, we got that 4 cylinder pushing a 17 ft boat to 52 mph( on radar), not bad for a 4 cylinder. The original Mirage had too much blade surface for an outboard( Mercury only recomended its use on the 3.4L 275), it was designed for the 454 sterndrives. most smallblocks wont even turn like it needs to be, but some engines, due to gear ratio in the drive( I-6 like yours, I-4)and their low rpm torque curves are able to make use of this prop. Some of the go fast crowd(Fountain for one) was trimming 1/4 inch of diameter to get a little less bite, to allow their engine to turn up in the rpm band they wanted it to be in, Mercury saw what they were doing and applied some fine tuning to the Mirage, allowing a litle less bite, making it an excellent prop for outboards.

Mercury has a prop selector on their web site, i have never used it, but several people i know have and have had excellent results with it.

Improperlly set up trailers can ruin a boat, they can warp the hull ( especially if it has set a long time), putting in excessive hook or pockets. This keeps the boat wet, it won't let the hull lift like it was designed to, roller trailers will do the same thing, just not one hook or pocket but several, at each place a roller contacts the hull. Of course it depends on how good of a hull you're talking about, there are some old trihulls out ther that you could'nt warp if you parked a truck on them.

As far as Volvo goes, they make the best sterndrive availble, they also have some of the most expensive( yamaha may have them beat), hard to find parts around. they now offer 2 day deliverd parts out of the wharehouse in Chesapeke anywhere in the continental US, it just seems the parts i order are allways nla

Was down at the ramp where Cummins Mercruiser test their boats, they have a new 150 hp Diesel that looks good

[ThePHNX]

Have you ever bought parts for a Volvo??

I've heard the horror stories!-) Truth is in over ten years of living aboard and cruising the Atlantic and The Mediterranean all I needed was the usual spare impellers, a consumable anyway.

Props are another story: I carry an inventory to select for range versus maneuverability - the SnowGoose had a sterndrive as well - but they were not terribly expensive.

As to diesel, there is a weight consideration in a planing craft, and while us blown boaters prefer the safety of diesel inboards, like the idea of keeping the SeaCraft original as possible.

On safety, do all of you with inboards make a practice of waiting while your bilge blowers run a while before turning the key?
When my sister got her first cabin cruiser, gas not diesel, silly girl, I gave her a key ring timer for the ignition keys, instructing she and her crew to wait, after starting the blowers, till the 5 minute timer had rung before starting engines.
HBH

[Bigshrimpin]

As to diesel, there is a weight consideration in a planing craft, and while us blown boaters prefer the safety of diesel inboards, like the idea of keeping the SeaCraft original as possible.

Man . . Phnx you're one difficult complex dude. I'll bet that diesel is lighter than your setup now. You'll get better range and similar performance . . . especially if your 1976 motor is getting a little tired. I had a great time trying different props on my boat to find the right one. Each prop was very different and caused the boat to handle differently. Personally I really liked the Highfive, so if you try out one of the five blade hillmarine props I'd be interested to hear how you like it. The high five was super super smooth and I could turn the boat sideways at 45mph and still hold water. Where are you located?

[ThePHNX]

Have set much store by your info, has the `ring' of knowledge and veracity to it. Wasn't it you recommended the Hill 5-blade which I find very interesting.
Another thing mentioned that has our attention is the stern lift factor you and others have pointed out.

If we assume the Seacraft, properly trimmed, is run at best power of 3-3200 RPM through the 1.65 drive and gets on step at just around 30 mph or better, and that little is gained by adding much power beyond that except raw speed in exchange for raw fuel consumption, than I find myself leaning toward a 21" pitch.

Trim will have to be changed of course when returning with 600 pounds of fish!-)

On the Outer Banks of North Carolina, Kitty Hawk Landing, plenty of dock space should you want to drop by and tie up.