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Old 07-24-2014, 07:01 AM   #1
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So called Overpropping

The latest issue of Pro Boat Builder has a good article by N. Calder on proper prop selection for max efficiency.

No surprise that a larger diameter prop works better , up to 5-7% if properly sized, the hassle is the deep reduction gears ,< 4.5-1> not usually made for small ,,,100, 150hp engines.

Most interesting was the test tank results from a slight over prop , where the results were a 15% better fuel burn.

This was with the std red. gear on a Volvo 75 HP rated diesel.

The top speed was identical with no overload .

15% is no big deal to most trawler owners , as the use is too few hours to pay for the new prop.

15% of 2 or 3 gph doesnt add up to much .

However to me the prop would easily be worth the bucks as the RPM at most normal cruise was reduced 200 to 400 rpm, and at some speeds more.

I enjoy the smooth QUIET created by lower RPM , and even at only a few hundred hours a year , the time enjoyed is worth far more than just 15% less diesel .

The old concept of propping for full rpm is fine for folks that do cruising /operating AT FULL RPM.

Calder suggests a rethink of this for displacement boats.

Any NYC 7th grader could paper his way into a Free subscription to PBB , perhaps its worth a try?

Many folks like contemplating prop diameters and pitch , the online stuff is easy but does not usually allow for anything but the built in concept of what the programmer though was best.

Skeenes elements of yacht design is at most libraries and will allow anyone to prop their boat.

A fuel map and real HP/ Torque info from the engine builder is also needed.
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Old 07-24-2014, 11:40 AM   #2
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FF wrote
"The old concept of propping for full rpm is fine for folks that do cruising /operating AT FULL RPM."

Propping to max power rpm is'nt "old" or new and it's accepted in the industry as the proper way. Overpropping is old.

Your'e right though the bigger the prop the greater thrust can be had. BUT that's usually only at nearly full rpm and the advantage drops off fast as one approaches cruise rpm even on a boat that isn't overpowered. Who needs 5% more efficiency at an engine load and speed you'll never use for more than a few minutes? At cruise rpm you'll eat up the efficiency of the big prop by excessive blade area and the resulting parasitic drag.

As always over-propping is too much lost and too little gained. But it's the stuff you dream about Fred and as long as you don't overload your engine all will be fine and not much lost ...... IMO.
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Old 07-24-2014, 11:46 AM   #3
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Thanks FF. I'll have to read this article as an owner of said diesel in the test. I'll be interested to see what is considered WOT rpm as VolvoPenta allows for a range of 2700-3000.
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Old 07-24-2014, 11:55 AM   #4
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Quote:
Originally Posted by manyboats View Post
FF wrote
"The old concept of propping for full rpm is fine for folks that do cruising /operating AT FULL RPM."

Propping to max power rpm is'nt "old" or new and it's accepted in the industry as the proper way. Overpropping is old.

Your'e right though the bigger the prop the greater thrust can be had. BUT that's usually only at nearly full rpm and the advantage drops off fast as one approaches cruise rpm even on a boat that isn't overpowered. Who needs 5% more efficiency at an engine load and speed you'll never use for more than a few minutes? At cruise rpm you'll eat up the efficiency of the big prop by excessive blade area and the resulting parasitic drag.

As always over-propping is too much lost and too little gained. But it's the stuff you dream about Fred and as long as you don't overload your engine all will be fine and not much lost ...... IMO.
Not according to Calder (and others) in that article..thus my previous thread on this article.
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Old 07-24-2014, 12:44 PM   #5
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Not according to Calder (and others) in that article..thus my previous thread on this article.
Yes, a re read of that thread from last week is a good idea. As best I recall, Calder's comments were based upon an article by O Gulbrandsen which if read in its entirety does not support just a simple change to a larger prop.
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Old 07-24-2014, 01:41 PM   #6
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Calders comments come from the graphs displayed in his article.

I wish I could copy and paste the article , but worry about copyright laws.
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Old 07-24-2014, 02:06 PM   #7
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Yes, a re read of that thread from last week is a good idea. As best I recall, Calder's comments were based upon an article by O Gulbrandsen which if read in its entirety does not support just a simple change to a larger prop.
And not all boats or the way hey are used...etc...etc....I know..

But just as clear as that is..is the data that under the right circumstances overpropping does work.
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Old 07-24-2014, 03:17 PM   #8
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Scott,
I'll stand by my "too much lost and not enough gained".

But agree w you on "But just as clear as that is..is the data that under the right circumstances overpropping does work." But rather that say "work" I'd say "has benefits".
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Old 07-24-2014, 03:27 PM   #9
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Spy,
I'll bet that for all practical purposes 3000rpm is where it's at to use the present vernacular. But it would be better if it was 2900. That would be blessing continuous operation 100rpm over and 200rpm under. I wouldn't bless the bottom 100rpm though. Have you ever asked a Volvo technician?

With my boat 1" pitch change changes rpm 200. So I effectively have a range too rather than a bulls eye to hit .. that may or may not happen. I've been lucky though and always managed to get it within 50rpm of max power (3000).
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Old 07-24-2014, 05:42 PM   #10
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Scott,
I'll stand by my "too much lost and not enough gained".

But agree w you on "But just as clear as that is..is the data that under the right circumstances overpropping does work." But rather that say "work" I'd say "has benefits".

Don't discuss it with me...write a letter to Calder at Pro Boatbuilder...I'd love to see his response.
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Old 07-24-2014, 07:15 PM   #11
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Here is the letter I sent to Editor Pro Boat as soon as I read the article:

"Although late to the party, I'm glad to see that Calder has awoken to the fact that running a diesel power plant in the "sweet spot" on the fuel map will yield more fuel savings than all the high tech, high voltage, high cost, complications he can dream up. The problem is; and I have tried to do this twice, once with over-propping, and once with a 2 speed transmission; you can't get an engine manufacturer to sell you an engine configured this way with a warranty. Not sealed and limited at the factory nor limited in the field. They have every excuse in the world why they won't do so, even though they do it every day with the same engine if attached to a generator. If you do so on your own, you have no warranty and the value of the warranty can exceed the value of the fuel savings."
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Old 07-24-2014, 08:09 PM   #12
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FF wrote;
"No surprise that a larger diameter prop works better , up to 5-7% if properly sized, the hassle is the deep reduction gears ,< 4.5-1> not usually made for small ,,,100, 150hp engines."

If you went with a Lehman from 7 knots at 1700rpm w a 2-1 gear then switched to a 3-1 gear you would need to run more than 1700rpm to make 7 knots. Assuming the boat was propped to 2500rpm. Anyone agree?
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Old 07-25-2014, 01:14 AM   #13
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Quote:
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Spy,
I'll bet that for all practical purposes 3000rpm is where it's at to use the present vernacular. But it would be better if it was 2900. That would be blessing continuous operation 100rpm over and 200rpm under. I wouldn't bless the bottom 100rpm though. Have you ever asked a Volvo technician?
The installation and owners manual both give the WOT as 2700-3000 rpm.
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Old 07-25-2014, 09:14 AM   #14
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If you look at major engine makers warranty specs they require propping to max rated rpm. They do this, of course, because they are expected to pay for any in warranty engine failures and their past warranty experience. I have never seen a warranty that said anything different about trawlers.

So who do you want to believe? A magazine writer, boat salesman, forum wisdom or the engine makers??
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Old 07-25-2014, 09:21 AM   #15
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Tank testing is just not real world conditions. In the real world with windage and waves you will need extra power to counter those effects.
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Old 07-25-2014, 10:05 AM   #16
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Bayview wrote,
"In the real world with windage and waves you will need extra power to counter those effects"

It would seem so. It's hard to imagine it wouldn't. But in my real world I've always reduced power (usually 300rpm) in heavy going especially in head seas. If I was to keep driving hard it would just be too uncomfortable. On numerous occasions I've done this for hours and probably made 4.5 knots most of the time in 35 to 40 knots of wind.

If the seas were not there and the water flat as in calm and if I was to continue along at 2300rpm (my usual) I'd loose 1/2 a knot or so but I'd get where I was going just a bit later.

If I had a FB and a forest of rigging I'd probably loose more speed. On our trip south from Alaska we bucked the tide in Knight Inlet making 3 knots and on occasion only 2. In a few hours we were in a narrow channel making 4 knots over our normal of 6.

I've heard this "need extra power for rough going from wind and waves" but I've never had that experience and think many think that from an automotive mindset well developed in our extremely automotive culture. It's a bit like trucks going over the mountain pass .... they always get to the top but later than most cars. To get better mileage on a trip w my cars I usually go faster down the hills and slower up the hills. I occasionally downshift a gear going up the grades on the freeway (new Jetta is tall geared) to lessen the load on the engine. On a boat we don't have the option to downshift. As FF says at times it would be nice. But w/o that option (or a vairible pitch prop) we should prop to max power. And power only to what we actually need.
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Old 07-25-2014, 10:20 AM   #17
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The installation and owners manual both give the WOT as 2700-3000 rpm.
Spy,
My bad. I should not have said "continuous" in what your'e quoting.
Several conclusions could be derived from that specification. Perhaps they give the range to accommodate different hull shapes. That may be applicable on a 20hp OB but not w your engine (probably a 70hp (or so) 4cyl diesel). Or perhaps ther'e actually blessing a small amount of overpropping ... as in 2800 max power, 100rpm overpropped or 200 underpropped. Or Max power is at 2700 and the're saying underpropping 300rpm is acceptable. I've had Willy underpropped 100rpm at times and rather liked it.

Can you find out what the max power rpm is?
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Old 07-25-2014, 10:47 AM   #18
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Fred Wrote re post #1 ...
"The top speed was identical with no overload"
I don't think that's possible. Overpropped any amount at WOT would be overloaded. Is that not correct? In black and white terms of course.

"The top speed was identical with no overload"
Any overpropping will result in a loss of power so that's not possible either.

But if you are talking "for all practical purposes" and the engine in question has an extremely flat power curve (and some do) only overloaded 100rpm for example could produce the results you claim. It looks to me like you're talking slight tweaking. Overpropping 50 to 100rpm w a flatish power curve or similar. I'd rather underprop 50 to 100rpm as I like the way my engine runs there but that is subjective.

Fifteen percent fuel reduction? Not likely as several have reported here on TF that the savings is more like 5%. However I think that's possible w an extreme overproping but then that's not tweaking is it?
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Old 07-25-2014, 01:19 PM   #19
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Current is not a consideration because it is speed through the water that's limited on a FD hull. More power against the current cant get the SOG any faster than max speed through the water minus current.

Windage surely makes a difference and I can personally vouch for the desire for a bit more power when I was trying to punch through some waves off the CA coast. The waves were stopping a heavy boat that would do hull speed nicely in calm conditions. Each wave stopped the boat then it would accelerate again to the next wave. The conditions were not a big deal just not enough power to accelerate from each slow down. More power and I would have rounded the small point and been in better conditions. Nothing automotive about that at all.
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Old 07-25-2014, 02:33 PM   #20
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Has anyone here reported actual numbers they derived from overpropping?

I would love to see some real data.

I have seen data on running one eng versus two...but not sure I have seen any on overpropping in the last couple years.

Anyone???? or anyone remember enough I could search more specifically for it (say member or a good keyword)?????
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