Question(s) on Performance, Torque & RPM, yada, yada....

C

Casey

New member
Ok, like most, I have a basic understanding of horsepower, torque, RPM, and various measures of engine output. I've looked at "performance curves" and tried to divine what they're telling me...and have even done some field trials to measure peak performance and best cruise (economy) performance. That's fine, and kind of fun ... for a while. After you collect performance data then you change something (different prop, operate at a different altitude, alter the load significantly, etc.) and it all changes!

Looking at performance curve charts I suspect they're giving me some useful information, but I don't have the technical background to interpret the data. Help.

I suspect that imbedded in the charts may be some useful data regarding how we can "best" utilize our equipment, to enhance performance, economy, etc. Airplanes come with all sorts of performance data, manuals, neat-o charts, and such. Boats and outboards come with a user manual that tells us: (1) wear your PFD, and (2) don't exceed 26,000rpm with your outboard motor. Might be nice to have more engineering info - OR a rational means of developing our own data. I suspect performance charts may be somewhere to begin. (Any good websites written for non-engineers?)

Rereading this - maybe I need to get a job with Powerboat Reports. ...naw, too much like Work.

Some questions:

(1) Where on the "torque curve" is a good place to operate, for best economy? Near the leading edge of max torque? If that rpm isn't within your prefered cruising speed/rpm, could you reach-it by making prop/pitch adjustments? Good/bad idea?

(2) I know what the "sweet spot" it; but theoretically, where is that on the performance charts?

(3) What is the optimum operating rpm for a 4-stroke (or 2 stroke) motor over the long haul? During new motor break-in they tell us to vary rpm and not to use WOT for more than "short bursts" of a few minutes. Some folks have suggested that it's ok to operate for extended periods at 80% of WOT (that seems a bit hot to me, but I come from a recip aviation background where 65% is often economical-cruise).

(4) If max horsepower is developed at the top edge of engine capability; of what use is it (unless you plan to operate at/near redline a good part of the time)? (Not my plan.)

(5) How do torque and horsepower interrelate? It seems to me that torque describes an engine's ACTUAL ABILITY to perform work - whereas horsepower is more a theoretical measure of an engine's potential.

(6) When choosing a prop - it seems that choosing one with less pitch (higher rev) might be good as it would insure the engine is not lugged (pulling too much manifold pressure...). What does that do to economy? That leads to another question:

(7) Which is better for economy - operating at slightly higher rpm with less engine loading, or letting the engine "pull" more manifold pressure by using a higher pitch prop, to capture the efficiency of prop(pitch)-travel.

(8) And finally - I've always chosen the higher horsepower avenue when choosing an engine. But that may be flawed. An engine's high-end potential isn't where most work is done. All you may be doing is feeding more horses. I think I'm coming around to respecting torque over horses (that conversion has taken a Long time...). I'm sure Tybo will set me straight in his charitable-way! (Mike: Be Gentle.)

Sorry to have gone on so long - but these are things I've pondered for a while, but I just don't have the technical background to come up with definitive answers.

Guess it's time for Tyboo Mike, Learned Les, or the Dynamic Dusty, Brilliant Bill, as well as Others to wade-in. Will be interested to hear other thoughts - if anybody bothers to read this far!

Thanks,
Casey
C-Dory Naknek
...bored in Arizona
 
Casey-

You really want to create that big a headache on a relatively quiet Sunday morning, huh?

The easiest thing would for all of us to take up a collection and send you as our official representative to some engineering school for a few years, but that's impractical and Mike's broke in advance planning mode in the horse trading market.

You've go so many questions that it would take an enormous amount to time to answer them, even if one had all the expertise, which I don't. What I would suggest, however, is a liberal application of the KISS Principle to get to a workable solution, unless the desire to get lost in the data and intricacies of the issues is overwhelming, in which case we'll send you up to spend some time with Dogon Dan until your properly lost in the Purple Haze of the Engineer's World of Delight and Delusion. (Just kidding, some of my best friends are engineers!)

Not to make fun of many of the serious questions that you've asked, but as you have already pointed out, operating conditions change so rapidly that trying to define ideal or optimal operating points is like trying to target shoot at 300 yards in a gusty 30 mph cross wind.

The first thing I'd do in trying to find an "answer" to many of your considerations, would be to start with engine selection. I'd be sure to have an engine that produced enough torque and horsepower in the operating range I was comfortable with so as to not be straining it at my desired cruising speed. Sure, that's an over simplification, but if I want to cruise at 18-22 mph at no more than 4600 rpm, I'm going to set up some displacement, torque, and horsepower requirements that begin to define what type of engine I need without having to go through the step by step process equivalent to designing a nuclear bomb. Put simply, get a big enough engine so it will work comfortably in your operating requirements.* We have enough empirical evidence available on this site to tell you need at least 75 hp and 1600 cc of displacement to do the job above. The exact motor choice is yours, no need to play games with "mine's better than yours" claims.

Next, get a prop that will operate at WOT in the recommended range, not vibrate a low speeds, come out of the hole reasonably, not ventilate in the turns, and produce reasonable speeds at cruising rpm and WOT. Again, not a science because of the variables, but doable within what we already know and determinable through experimentation. When in doubt, start with the middle of the road conservative accepted answers and go from there.

At this point, if you have performance issue that dictate trim tab or hydrofoil installation, take care of that to control those issues.

Now for the fuel economy issues, spend the bucks and get a sophisticated fuel management system installed that will give you instantaneous MPG figures so you can tweak your throttle, motor trim, and trim tab condtrols underway and as conditions vary. Search for that "Sweet Spot" and "Fly" that baby!

OK, we didn't build another Blackbird SR-71 for the CIA to fly around in near outer space, but We're not Kelley Johnson, either.

My two KISS the Swiss Francs for today. Fun talkin' with ya'! Joe.

* The torque vs horsepower "debate" is an interesting one for me. What really matters is not final horsepower at max rpm, but whether the motor provides the needed torque in the desired operating range. Some say, therefore, that it's only torque that matters, which for this reason is largely true. The motor will put out more total energy and therefore more horsepower when turning at a higher rpm (providing torque doesn't fall off so much as to make the torque x rpm product less), but since we really don't want to operate at max rpm, it's the torque produced at desired cruising rpm that matters.
 
Um... yeah, what he said. :thup

Really though, there is no legitimate scientific equation for getting the best performing boat/motor/prop combination. Just when you think you've figured it out, you'll add a passenger. Or empty out the water tank. Or boat a big halibut. Or add a set of shrimp/crab pots and a puller. Or slow down to trolling speeds. Or try to outrun an approaching storm. Or simply run into a stiff headwind in a decent chop.

You see, unlike cars, outboards don't have automatic transmissions or even multiple gears to choose from. You just get to pick one. But just about everything you do on or with a boat changes the load on the motor. It's all a compromise.

As a boat owner (or potential boat owner), you need to take a good hard look at what exactly it is that you expect to be doing with your boat. Then talk to other people who tend to do the same things, under the same conditions, about how they like/dislike the performance they are getting. Then try to pick a boat/motor/prop combination that seems like it'll work. And don't be afraid to try something else if you're not happy with your first/second/third choice.

Of course, swapping props is cheaper than swapping motors (or boats!). So, you'll want to do your most intensive investigation early in the process.

BTW, I had a 70hp Johnson 2-stroke on my 16 Angler. I ran either a 14-17 aluminum or a 14-17 stainless prop on it (both 3 blade). I was happy with both and noticed little performance difference. I got 4.8 statute mpg and burned 5 gallons per hour at 24-26 mph. Top speed at WOT was about 33. Not exactly a speed demon, but plenty fast for both the hull design and the captains crusing style :) .

Although I wasn't too rigorous about checking, I didn't notice any difference in those numbers whether I was loaded or empty (probably due to the fact that 70hp is a little over powered for that boat). But, it did take longer to get on step when I was really loaded down. For that reason, I'm glad I had the extra HP, no matter what the torque curve looked like. If I were ever able to load it to the point that it wouldn't get up on step :shock: , I might have started playing around with prop selection.

Wow... glad I can make things so clear with my neat, concise replies... sheesh.
 
Sorry to let you down, Casey, but in the qualifying statements included with your questions you demonstrated more knowledge and understanding on the subjects than I could muster. I will be doing more reading on this thread than talking.

I am kind of in the same boat with Mr. Red Fox on this one. Slap all the power you can haul around comfortably on the transom. Prop it for best WOT with a light load, and know that when the boat is loaded heavy the motor will not be. My CD25 has 130 hp, and whatever torque and other factors that comes with it. I can't really pick and choose all that much in that regard - manufacturers make motors and sell them. About the only real variable the customer has is which HP to select. There are certainly different tweaks under the cowl that give you a somewhat superficial range of wise choice, but the fact remains that a 75 hp Yamaha is going to perform on a 22 CD hull pretty much the same as a 75 hp Honda, Suzuki, or Scott Atwater. Sure there are some big differences, but you will not notice them in 95% of your use. I am to old and too busy with other things to beat myself up over this stuff.

The 130 hp on my CD25 is inadequate. If I were to repower, there would 200 hp hanging in the back. The configuration doesn't matter at this point. I would go for the maximum power using the standard quantifier - horsepower - to guide my selection. I have noticed that the CD25 hull is much more comfortable at higher speeds in poorer water conditions than the CD22 I used to have. Maybe I could not do it on a 22, but on the 25 I would just go way too fast if the extra power wasn't needed for the load. As for fuel economy, I am of the simple school that says you pay for the work you get, and if you don't have to push the gas pedal down as far with a bigger motor, your ultimate fuel use will not be that much different. That statement is, of course, offered knowing that we are considering a small range of differences in the motor, and not thinking of hanging some massive powerplant that takes more fuel to idle than our motors use at speed. My present economy sucks, and there is no doubt in my mind that it would improve if I were feeding a more powerful motor.

And that's all I have to say about that.
 
Good topic Casey but I have to agree with Tyboo and Sea Wolf. The difference between brands in torque and HP curves of motors with the same HP rating is so small that it would be a minor consideration in deciding wha to buy.

However understanding torque and HP has always been a bit of of mystery to me. These days I look at it like this, more of both is better within reasonable limits.

Like Tyboo when I repower it going to be with a 200 hp motor.

stevej
 
Agreed on horsepower 90 is 90, etc. Where the differences are is alternators, trim limits, etc. As for economy, our boats have 3 speed transmissions, essentially: 1st gear -- hull in water; 2nd gear: on the step (9 -12 mph) just barely planing; 3rd gear: 18-22 mph where the nose drops down and the sternmost portion of the hull is now skimming the water instead of pushing it, and the motor is working easilly.

Remember also that horsepower to handle the boat slow is just as important than horespower to drive it fast . . .

The Merc, powering the boat 3 miles across the flats at low tide:
4337514f5ef427f2.jpg

This shallow:
433753ee6566870b.jpg
 
In my case I took they advises of some of you a while back. Ask my dealer to supply me with props until I could achieve maximum rpm. took 4 set of props, all test where done full fuel, 2 adults, one large dog ,dinghy. The set of props that I choose let me reach my maximun rpm 5800 , cruise at 3900 rpm between 3.8 to 4.2 gph at about 14 to 16 mph on my fuel management gauge. Cruising at 4100 to 4200 I get about 4.3 to 4.9 at 17 mph , I am happy with this result my spare set will be either 4 blade adjustable composite blade or the new Ring Prop don't know yet. Sorry it might not answer any of your questions but in my case I am sure glad I don't have to worry any more about torq peak , rpm to slow to fast , fuel this and fuel that,,,,! All I have to do know is what are my spare prop going to be ? Gary
 
Thanks for the replies, and I hope other folks will chime-in with their comments, opinions, or whatever.

Don't get me wrong - I certainly don't lay awake at night worrying about this stuff, but it does tend to being out a geeky side in me. I thoroughly enjoy the C-Dory (and I'm not planning to repower with Anything), and find it interesting trying to tweak performance here and there.

Then again - maybe it's just be more to learn how to fish.
...naw

Thanks....
Casey
C-Dory Naknek
Lake Montezuma, AZ
 
OK Casey, I'll chime in... with zero answers and still more questions that I think fit this thread. However, I will say that I don't know squat about all that torque and WOT stuff. So thanx to DodgeRam for bringing the issue down to props, cuz I have no intention of changing oats nor engines!

Here's what I know concerning my CD22 powered by twin Honda BF40s. The props are 3 blade aluminum 03x11.1x13. At full throttle I get just under 5K RPM and run at just over 20 MPH. I usually back the throttles off to 4500 RPM which cruises at just over 18 MPH. This is quite a bit lower than what is shown on the C-Dory site at http://www.c-dory.com/22 performance.htm... First I don't get anywhere close to 6K RPM and second I can't do over 21 MPH no matter what.

Also, I noticed something else. A week or so ago I saw a message in another forum that was referring to the rooster tail that the engines can produce. As I recall the writer wrote said something like 'adjust the engines so that the rooster tail just disappears'... or some thing similar. So while I was running a few days ago, I got a good rooster tail then lowered the engines until it just disappeared. Well that was fairly close to where I always run anyway... maybe just a little higher. The strange thing is I was doing about 20 MPH when I started the engine adjustment. When to rooster tail began to show my speed dropped to about 18 MPH. I lowered the engines back down until the rooster tail disappeared and was still at 18 MPH. I lowered a bit more to where I was always running and stayed at 18 MPH. Never got 20 MPH back! No changes to throttle or trim or anything else except engine up/down. I just don't understand how I can loose 2 MPH and not be able to get it back...

I guess I have two questions:

1) I procured the boat in California and brought it to Utah. Plus 4K on the elevation. Do the BF40s need to be adjusted for altitude?

2) Which way do I go with the props? More blades? More or less pitch?

Oh and two other notes:

1) Either engine with the current prop propels the boat just right for trolling at idle right now.

2) How will I ever keep up with other CD owners should I happen to make it to a gathering if I can only cruise at 18 MPH?
 
Ken I did not really want to tell any of the C-Brats what size props I am using. Cause What work for on boat might be quit different for another. All I know is with some trial on all the other prop that I try, I could not get maximum rpm and also top speed of 30 to32. Now I can achieve those speeds and also down to trolling speed , good fuel economy, but nothing will ever be perfect. I do at time have some ventilation but nothing serious. What I have on my twin 50 hp are size (10 5/8 x 12 G) AL 3 blades. GARY
 
Thanx Gary... I will contemplate the information. It is just that I don't know a dealer who will let me keep trying props. But I do have a set of comp props that came with the boat. As soo as I find them I will let you know what pitch they are.

What I find interesting is that what you describe is very sililar to what I am seeing.

Guess that's why we have to hang together!
 
The altitude will definitely affect the performance. You may want to have the motor retuned(change the jets in the carbs...it is probably running too rich) for the altitude if you are always running it at that elevation.

IMHO it sounds like you are running too much prop since you can't get the RPM's up to 5800 rpms. So it sounds like you need to hit up the dealer for some props to try. Get the max rpm about where it should be and you will likely see your fuel mileage improve and your top speed improve as well.

Also, I remember reading a while back that someone had two sets of props for their boat. One for low altitude and one for high altitude. You might need to do the same.
 
Sounds like a good idea Gary... I can make the attempt. Biggest problem here in Utah is that all the Honda dealers are more concerned, by orders of magnitude, with keeping their motor cyclists and snow mobilers happy than they are anyone with a boat. I don't understand it... boats just seem to take an extreme back burner in the desert!

I'm waiting for the next big quake to hit... I'm gonna sit in my CD22 and watch while liquefaction sucks all the cars into the sand. Just hope they don't contain peeps.
 
Ken,
I have a 22 with twin 40 Hondas. My props are Honda 3 blade aluminum at 11.4 x 12 pitch. I get 5600 rpm at WOT. With normal loads and smooth water I cruise at 4500 rpm at 15Kts. Elevation here in Spokane is about 2000ft. I have not noticed any difference at sea level. Hope this helps.
 
Ken-

Interesting problem!

Here in California, the Honda, Yamaha, Evinrude, and Suzuki dealers are totally separate for marine and motorcycle operations.

Not sure about jetskis and snowmobiles throughout the state, but here in Redding a third type of dealership deals only with those two products, one a winter and the other a summer business.

In Redding, there are three different marine dealers that offer Yamaha outboards (and I bought mine 170 miles away in Santa Rosa because of the better deal offered).

And this is the least populated end of the state! Sometimes there are
advantages to living in more populated areas, although...........(you know).

Joe.
 
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