Re: deliberate overpropping

[Billyfeet]

Discussing any aspect of engine/drivetrain performance needs to be limited to like kind equipment. An engine originally produced to drag a bus loaded with apacked passenger compartment and pigs,chickens and a side of beef tied to the roof over dirt roads in a mountainous 3rd World country will give significantly different performance and sensitivity to operating ranges than an engine designed with an absurd focus on emissions and light weight. The first design will last for years lugging at full throttle up a grade, overloaded, blowing black smoke from the exhaust. The latter will have to be tended to by a geek in an immaculate jumpsuit with secret software tools to get the mixture just right for optimized fuel burn/ emission ratios. Any operation outside of the factory specs voids the warranty and burns off the tops of the super light pistons. YMMV/IMHO/ETC

 

[sunchaser]

The more expensive boats I sell almost always have an engine surveyor and they always make a big deal about reaching top design RPM. They say how do you know that the engine is working properly if it will not get to the correct RPM. Is it the prop(s) or is it turbo charger, dirty fuel, bad injectors?
If you decide to over prop your boat, save the old prop to use when you put the boat on the market in the future.

Last week I did an inspection of a very large yacht with 3 propulsion engines. The Captain and engineer were anal about being able to assess overall engine performance at full rated RPM. Very detailed check lists were in place as to what to look for at various RPM when doing full load tests.

Fuel burn data was available for runnning 1, 2 or 3 engines on long passages. Guess what - best economy was achieved with all 3 running at low RPM and around 1.1 hull speed.

 

[sunchaser]

If its sucking 50% more fuel and is 500-800 RPM over what the boat requires , thats not the Mfg concern.

Not my experience. Some years ago I bought a new SeaRay. The dealer gave me spare props to try out to insure the vessel would achieve full rated RPM at the 4000' lake altitude. It was SOP for this very large dealer.

Usually wrong propped new boats are as much the fault of the buyer as the builder. One reason is the self perpetuating myth of the virtues (yes, what are they?) without any downside noted of over propping.

 

[eyschulman]

Another reason I wouldn't want to spend the money to overprop is because the idle speed is quite fast enough.

From experience I have discovered in boats with large engines that slow speed maneuvering is much enhanced by UNDER propping so that at idle speeds the boat does not lurch when put in gear. This was a real problem on my previous boat also keeping within marina speed limits was a problem had to go in and out of gear. My present boat is under propped and behaves very well around docks and in restricted speed areas. For this boon in low speed management I am more than willing to give up 2 Knots at the top end and a possible small amount of fuel efficiency. the cost of fixing hull dings was not insignificant.

 

[Nomad Willy]

Mark,
Yes .... Once the point of over load rpm is established the governor could be adjusted to limit engine speed/load to safe limits. You and I could live w that but most others here would want extra power and the option to use it.

Re the post above I liked the boat much better at low speeds around harbors and marinas. Much less in and out of gear motion and the clutches even have an easier job of it. Ideally we'd have a diesel/electric boat that can run in gear at any rpm ... 75rpm if you want.
When making a landing one must stop the boat w reverse gear and now I'm very optimized for that. I'm propped to rated rpm (3000) and I have a prop w a symmetrical shape that has very good reverse thrust. For an eight ton boat w only 40hp stopping w reverse gear is not a high performance event. But what I like most is that the engine runs much more freely or effortlessly. That's a subjective statement but the fact is that there's less load on the engine. Going at a given speed the prop sees the same load over propped or under propped but the engine sees less load per piston stroke. Less force per stroke but the same torque and power is sent to the gears and shaft because there's more strokes. So the engine is doing it's job more easily and it can be felt ... by the operator and passengers and the engine itself.

 

[FF]

The confusion is the word OVER,

An OVER loaded engine is much ungood .

Propped to match the engine to the load is good! it is not an OVER load

To not OVERLOAD an engine is very simple , run full throttle , note the RPM and pull back 10%.

At that setting the exhaust should be clean.

 

[Nomad Willy]

It must be more complicated FF. You say pull back 10%. Ten percent of what? Ten percent of max load it would seem. But then you'd need very good fuel burn numbers and if it was 10% of throttle shaft rotation how would one measure it? Baaaah ......

EGT temp and rpm observation seems far simpler and much more accurate.

 

[bayview]

Last week I did an inspection of a very large yacht with 3 propulsion engines. The Captain and engineer were anal about being able to assess overall engine performance at full rated RPM. Very detailed check lists were in place as to what to look for at various RPM when doing full load tests.

Fuel burn data was available for runnning 1, 2 or 3 engines on long passages. Guess what - best economy was achieved with all 3 running at low RPM and around 1.1 hull speed.

Did you mean 1.1 *sq rt of water line?? Hull speed term means so many different (wrong) thins to people.

 

[cafesport]

Given that a slow turning prop has more slippage and is less efficient many elect to increase pitch in order to gain back some of the loss. Since diesels are most efficient under load and that only happens during acceleration or at WOT we clearly have oodles of horsepower headroom available. We generally pitch the prop to the intersection of two curves namely the engine load curve which is linear and somewhat convex and the propeller load curve which is exponential and concave. All the area between these curves represents safe combinations of pitch/load and rpm/hp/fuel burn.If we had controllable pitch props we could optimize and fly closer to the flame in order to gain peak efficiency. If you decide to play around with this a pyrometer can give you an indication of how hard your making the engine work. One other reason for propping to WOT is that any performance issues the engine may be experiencing will be revealed at WOT.


Via iPhone.

 

[sunchaser]

Did you mean 1.1 *sq rt of water line?? Hull speed term means so many different (wrong) thins to people.

Yes. 1.1 by my less than clear meaning would be about 20% below sq rt 2.

 

[bayview]

% reduction in load requires use of the prop curve not straight line RPM. The prop curve is generally assumed to follow al a 2.5 to 3 exponent curve.


as in Load =100*(RPM/rated full load rom) raised to the 2.7 Pwer.
like this:
100*(1600/2200)^2.5= 45%


so a 200 Hp engine with 2200 rated and achievable RPM operating at 1600 is producing about90 HP.


Take the above formula and substitute your numbers for top rated and running and past it into google search and the math majic is done for you. []
Then use 18 HP per hour per gallon for a SWAG of fuel consumption.. Anything trying to be more accurate is like putting a micrometer on the end of a yardstick. Just gives more precise inaccuracy.

If it is over or under propped the curve doesn't work as well. Significant over propping produce more slip IMO unless you are going very fast where the water is passing the prop at speed.

 

[ghost]

Ski, well said.

Am I the only one who already sees members of the planing set torturing data until it tells them what they want to hear?

With the majority of trawler owners operating planing hulls (semi displacement included) and operating them as such (see the current thread on this topic). I see this trend going back to the future of mfg over propped boats to make specs look better and little more. Recreational boats do tend to tolerate some degree of over propping in practical use, but there is also a pretty strong history of garden variety engine failures when inevitably pushed. One of the few routine causes of recreational engine failure outside of cooling system failures. Those in the planing hull set would very rarely be able to objectively measure a significant decrease in drag at a common speed or at least no more so than those who "objectively" show benefits from adding acetone in their fuel. So ultimately, the problem is a wildly unfavorable risk/reward ratio. I don't mind consciously taking risks, but I do want to see clear benefits when I do so. I gamble when the upside is significant. The benefits to be gained here, when they exist, are small in nature.

For those in the displacement set, I see benefits, no argument from me.

Anyway, for most here, I just don't get it. I'll try to keep an open mind, but objectively I'm not getting there yet.

 

[Marin]

A previous owner installed a good EGT system on our boat. While I was used to using EGT in the planes I was flying, I didn't think it would have a lot of value in a boat like ours. But it has proven to be a good check on how hard our engines are working and as one more indicator which, if it starts reading differently, it's a good idea to find out why.

 

[jleonard]

Marin, what do you typically see for EGT at cruise? At WOT?

 

[Marin]

Marin, what do you typically see for EGT at cruise? At WOT?

We have only taken our engines above 1800 rpm in the last 16 years once, and that was to WOT to get rpm data for the prop shop. We didn't look at the EGT then because we were only at WOT for a minute or so.

At our cruise rpm of 1650, the EGT for each engine reads about a bit under 600 degrees. The gauge showed 600 degrees before we had the props reworked.

 

[sunchaser]

For those in the displacement set, I see benefits, no argument from me.

I am in the displacement set with current vessel. It was propped at the factory to achieve full rated RPM. I would not buy any displacement vessel that is otherwise propped. The current builders and engine suppliers of the largest displacement boats by numbers being built today agree, good enough for me.

Why the fuss and worry, pretty simple it seems.

 

[Nomad Willy]

Bayview,
With your prop curve is the power listed for engine speeds achieved at WOT or at some point along the curve that represents a pre-programmed typical load for a typical boat?

 

[BaltimoreLurker]

Did someone mention overpropping?

 

[Twidget]

Did someone mention overpropping?

Lets see. 106,000 HP, 505 feet long, Displacement 9,166 short tons. Max speed 32+ knots.
Over propped? Who cares

 

[deepsix]

To over prop a new boat is so dumb! Its just to decieve potential buyers, so they think the boats faster than it is, plain and simple! Then you load all your gear ,dinghys davits ,tops arches,full tanks , electronics ect... It woun't show up right away...but you will do "serious damage" to some very expensive Hardware!! any good surveyer will recognize this problem during the sea trial and if your buying "new"check the motor specs (the broker probably woun't mention it)

 

[psneeld]

To over prop a new boat is so dumb! Its just to decieve potential buyers, so they think the boats faster than it is, plain and simple! Then you load all your gear ,dinghys davits ,tops arches,full tanks , electronics ect... It woun't show up right away...but you will do "serious damage" to some very expensive Hardware!! any good surveyer will recognize this problem during the sea trial and if your buying "new"check the motor specs (the broker probably woun't mention it)

dumb?

depends if it is the dealer/manufacturer because the engine installed didn't match the hull...or the owner fixing the dealer/manufacturer blunder.

 

[Nomad Willy]

deep six,
You're a brave fellow. I've said many many things about 1/10th as direct as that and in that direction and got every kind of tomato and egg on my face for it. There are also many here that do it right. Right IMO that is .. and theirs of course.

But of course overpropping is fine if you know the limits and have the discipline to stay there. You can see I've moderated a bit on this issue but I try to be 50rpm underpropped.

 

[eyschulman]

This issue of propping seems to hit raw nerves with some. I have my opinion on what's best and it does depend on the boat and how its used and what safety stops are in placed if over propped. truth be told I really don't care what others do its their boat and their engine and money and not a safety issue that affects others. There is a point when the issue would fly in my face and that is if I were shopping for a used boat. It would take a lot to convince me to buy an over propped boat no matter what power curves the owner showed me particularly if an other same boat were on the market where the owner documented that the boat was always propped to and beyond manufacturers safety margins. What it comes down to with over propping is playing with the engines tolerance and safety margins which the manufacturer set in an area they are comfortable with. Some one who over props is tinkering with that and maybe they know what they are doing and maybe not. Maybe if Nigel Caulder(spelling?) sells me a boat with tinkering I would go for it for he is an authority on such things but the average boat owner a crap shoot not for me.

 

[DavidM]

I am late to the party on this thread. There have been dozens of posts but only two: FF's and Ski's addressed the benefits of overpropping. FF said that the fuel consumption would drop 50%. Ski said that fuel consumption would improve slightly and the main benefit is noise reduction.

I looked at the prop curve fuel consumption and the wot fuel consumption curves for a NA JD 4045 and reported the results here on the CP prop thread. There was no difference in the fuel required to produce 40 hp at the wot rpm (about 1,000) or at the prop curve rpm (1,800 as I recall). Admittedly the JD curve has a horrible almost unreadable scale. But there has to be some difference. Maybe in the middle (about 1,400 rpm) you would see a difference, but not much I suspect. As FF noted the manufacturer's don't give you the fuel maps to compare accurately.

So until someone with two different props, a Flowscan and extra time and money for prop change outs does a controlled study, we aren't going to really know for sure. I suspect the fuel consumption benefit will be minimal.

Would 1,400 rpm be quieter than 1,800 rpm? Sure but it depends on engine installation and soundproofing whether it would be meaningful.

David

 

[Marin]

I don't think one can make a blanket statement about overpropping because there are so many variables. As I mentioned early on in this thread, Grand Banks boats were all deliberatly overpropped by the manufacturer during at least the first couple of decades of production.

But.... this was when the boats were mostly powered by FL120s, and overpropping by an inch or so didn't put any undo strain on the engine(s). These engines are (or should be) typically cruised between 1500 and 1800 rpm The overpropping gave the boats a little more speed--- probably a knot or so--- which over time and distance can make a bit of difference in one's time-to-destination. So in this case, it was beneficial to the owner and not detrimental to the engines.

As Grand Banks boats started to be fitted with more powerful engines for more speed, I do not know if the deliberate overpropping continued. If it did, I suspect it was like it was with the older boats; enough to give a bit of benefit but not so much to threaten the integrity of the engines.

 

[Al]

Marin has blown the clouds from the basic subject. His having experienced just what several of the posters including myself has in a couple of simple paragraphs lays out a level of common sense owners should apply when dealing with intent to over prop.
His statement should give concerned owners a level of comfort going forward
Good post!!
Al

 

[Nomad Willy]

Marin how are you going to gain speed by loosing power?

An overpropped engine can't make even it's rated power much less more.

 

[Marin]

Marin how are you going to gain speed by loosing power?

An overpropped engine can't make even it's rated power much less more.

All I know is what Grand Banks as well as people like Bob Lowe on the GB owners forum who for years owned a yard in Oak Harbor that specializes in GBs and who knows more about these boats than God have said.

And it makes perfect sense to me. I'm making these numbers up, but if I get 7 knots at 1700 rpm in a cruiser with a 24" prop pitched at 17 inches, if I put a 24" prop pitched at 18 inches on the boat and run the engine up to 1700 rpm, I'll get 8 knots.

Why? Because the 18" prop is moving more water back at 1700 rpm than the 17" prop did at 1700 rpm. And for every action there's a reaction, right?

I know you know all this, Eric. I'm just giving my explanation.

Now there's no such thing as a free lunch, so what's changed? The throttle position at 1700 rpm has changed.. I have to feed the engine more fuel to make it turn at 1700 rpm with an 18" pitch prop.

So I'm using more fuel, and if I had an EGT gauge on the engine I would see that the exhaust was hotter with the 18" prop at 1700 rpm than it was at 1700 rpm with the 17" prop. So the engine is having to work harder. But the boat is going a knot faster.

So if moving my boat through the water a knot faster is important to me, I can do it with a coarser prop and making the engine work a bit harder and still keep the engine within its normal cruise rpm range.

And this is what Grand Banks did. They made no secret of it; in their promotional material they said that by using a slightly coarser prop their boats would go a bit faster at cruise power, which is all their owners cared about.

If fuel economy is more important than speed, then I can reduce the work the engine is doing by putting on a flatter prop, and thus reduce the fuel I have to feed the engine to get my 1700 rpm.

In the first couple of decades (or more) of Grand Banks production, fuel cost was not a consideration, and the FL120 was more than capable of doing a bit harder work at the cruising rpm people used--- 1500-1800 rpm (this is a 2500 rpm max engine but nobody in their right mind runs them up near that on a continuous basis).

So overpropping by an inch or so gave the owner a bit more speed through the engine's normal rpm range and the engine didn't care that it had to work a bit harder to do it. The fuel tank did, but fuel cost was not anything anyone worried about back then.

 

[Al]

Did I miss something? I took Marin to say that in the RPM range of 1600/1800 RPM with a over prop a increase could be expected. It doesn't appear that Marin is speaking to WOT or full power as a consideration. If the matter related to less than manufactures WOT performance due to overprop conditions then Eric may have a point.

Is torque playing a part in these assumptions? To my way of thinking, if the throttle is retained within the range that produces the desired reasonable speed (Speaking to FD/SD (Grand Banks? ) of 1600/1800 RPM and there is still an amount of available RPM even if the total is under the manufactures WOT RPM setting what is the point of debate.
Eric perhaps has a valid point to what end?

Could FF or Djmatchand pull this together with a confirmation or opinion ?

Al

 

[Al]

Thanks Marin, you beat me with a response Thanks, Al