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Old 01-21-2015, 11:20 AM   #41
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% 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.
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Old 01-21-2015, 01:24 PM   #42
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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.
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Old 01-21-2015, 01:52 PM   #43
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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.
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Old 01-21-2015, 02:27 PM   #44
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Marin, what do you typically see for EGT at cruise? At WOT?
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Old 01-21-2015, 03:55 PM   #45
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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.
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Old 01-21-2015, 04:48 PM   #46
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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.
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Old 01-21-2015, 05:07 PM   #47
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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?
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Old 01-28-2015, 11:34 AM   #48
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Did someone mention overpropping?
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Old 01-28-2015, 11:56 AM   #49
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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
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Old 01-28-2015, 07:50 PM   #50
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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)
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Old 01-28-2015, 07:54 PM   #51
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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.
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Old 01-28-2015, 08:12 PM   #52
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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.
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Old 01-28-2015, 08:28 PM   #53
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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.
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Old 01-28-2015, 08:34 PM   #54
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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.

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Old 01-28-2015, 09:08 PM   #55
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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.
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Old 01-28-2015, 09:41 PM   #56
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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!!
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Old 01-28-2015, 10:55 PM   #57
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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.
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Old 01-28-2015, 11:43 PM   #58
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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.
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Old 01-28-2015, 11:55 PM   #59
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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 ?

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Old 01-28-2015, 11:57 PM   #60
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Thanks Marin, you beat me with a response Thanks, Al
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