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Old 03-27-2023, 01:00 PM   #41
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On a new diesel powered boat, we'd aim for 50-75 rpm over the rated rpm at WOT. That's with a "light load", strategy being that with a full load, rpm's would be at manufacturer's max rated rpm at wide open throttle. Cummins, for one, would not approve an installation if this wasn't followed during sea-trials. You're a used boat at full load, so in an ideal world you should be propped to meet rated rpm. Things change as a boat accumulates hours of course so you have some latitude. This isn't a ski boat, you don't adjust the prop to get a certain speed. It is what it is...

Get Dave Gerr's "Propeller Handbook" for an in-depth discussion of props. It's incredibly informative - it was on my desk when I was spec'ing out props.
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Old 03-27-2023, 04:21 PM   #42
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Originally Posted by markpierce View Post
You will increase efficiency (reducing fuel consumption by 50% or more) moving one knot below hull speed. At hull speed, my fuel consumption goes from 4 gallons an hour to 1.7 at one knot below hull speed.
So it is reasonable to assume then, based on what I have been reading to this point, that my 48 ft ( 43 ft LWL ? ) should economically cruise at about 8 knots? I am powered by a detroit 671 rated at 165 hp. THe boat tips the scales at 41,200 pounds.
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Old 03-28-2023, 09:05 AM   #43
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Good point. A photo tach is fairly cheap to buy and will allow you to calibrate the tach and make sure it's accurate.
Even cheaper is a permanently mounted digital rpm with magnetic sensor and readout. Less than $20. Easy install.

I have installed on both engine and shaft. Takes the guesswork out.

Disregarding the possible faulty Tach, i think the OP's speed data is fine. Very comparable to my Monk designed 37, just below 7 knots is where the wake size increases and diminishing returns start.

Speeds beyond that are pay to play.
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Old 03-28-2023, 09:46 AM   #44
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Originally Posted by Tinman View Post
So it is reasonable to assume then, based on what I have been reading to this point, that my 48 ft ( 43 ft LWL ? ) should economically cruise at about 8 knots? I am powered by a detroit 671 rated at 165 hp. THe boat tips the scales at 41,200 pounds.
Your theoretical hull speed (and remember, it is just "theoretical") is a bit under 8.8 knots. So 8 knots should be a reasonable cruising speed. Of course, slower will be more economical.

The displacement of your boat has no bearing on its theoretical hull speed. What the displacement DOES have a bearing on is how many horsepower are required to reach that speed.
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Old 03-28-2023, 10:42 AM   #45
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Blackinblue wrote;
“Also, probably better to be a little under propped than over propped”

rslifkin wrote;
“There's no real harm in being under propped, but if you see 3000 rpm the tach is wrong.”

You can only harm your engine from being overpropped. That’s a fact unless you run at a lower rpm. Lower than rated rpm. Like 2300rpm w an FL.

IMO 100rpm above rated is ideal.

Rated rpm is NOT a never exceed engine speed.
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Old 03-28-2023, 11:13 AM   #46
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Hull Speed

Typically hs is 1.34 x the square root of the WLL.
But it’s not cast in stone.

For example;
OAL is not a factor. It’s strictly water line length.
Somewhat harder to understand is that a full shaped bow or stern (or both) has a significant effect on speed (or lack of) when considering boat speed and the formula.

A FD boat w a full bow (under water) will be ultimately faster but require more power. A boat w a very pointy forefoot (bow) will act (hs wise) differently. Her power requirement to run a knot below hs will be less.
A boat’s normal loaded WLL should be considered re hs matters.
Some boats have (mostly sailboats) a long overhang aft and will have a considerably variable WLL depending on boat load.

There’s lots of variables.
But generally a FD boat should usually run (like in cruising mode) quite close to one knot below hs. A Willard 30 (FD) is a good example and mine ran at about 6.15 knots w a WOT speed of a bit (very small bit) over hs … as in 7 plus a tad knots. Rpm was 2300 cruise and 3100 rpm (rated at 3000) at WOT.
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Old 03-28-2023, 11:20 AM   #47
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Even cheaper is a permanently mounted digital rpm with magnetic sensor and readout. Less than $20. Easy install.

I have installed on both engine and shaft. Takes the guesswork out.

Disregarding the possible faulty Tach, i think the OP's speed data is fine. Very comparable to my Monk designed 37, just below 7 knots is where the wake size increases and diminishing returns start.

Speeds beyond that are pay to play.
Thanks for the input. I had not considered using a digital tach, but it sounds like a great idea. For the sake of calculating fuel consumption, I will go with 2 gph at 7 knots.
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Old 03-28-2023, 02:45 PM   #48
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In post #46
I wrote;
“ A FD boat w a full bow (under water) will be ultimately faster but require more power. A boat w a very pointy forefoot (bow) will act (hs wise) differently. Her power requirement to run a knot below hs will be less.””

Most musta thought I was nuts .. not yet haha.
A FD boat operates in the trough of a wave created by the boat itself.
With a fuller bow the bow wave will be created sooner and w a full stern (like my Willard) will be a bit further aft and the stern wave further aft. With the two waves further apart the wave will be longer. A longer wave will move faster carrying the boat along w it.

The opposite is a sailboat w both ends fine. The wave will be shorter w both ende fine. The boat will be slower and w considerably less power requirement. They cal this hull a diamond shaped hull. The slowest and most easily driven.

Hope that explains a boat w full/blunt ends will be faster. It’s very rare in the US but a boat of this description is common in the Netherlands. I think they call them barges.
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Old 03-28-2023, 07:41 PM   #49
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Had a 210 Cummins in my 38ft Marine Trader, only ran at 1600rpm for average 7.3 knots fuel burn about 2 1/4 gph.
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Old 03-28-2023, 10:02 PM   #50
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I love theories. When the builders develop that curve of RPM, speed, fuel consumption curve, they are an empty boat and we all aware, no one cruises in a empty boat.
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Old 03-29-2023, 06:18 AM   #51
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Originally Posted by Tinman View Post
So it is reasonable to assume then, based on what I have been reading to this point, that my 48 ft ( 43 ft LWL ? ) should economically cruise at about 8 knots? I am powered by a detroit 671 rated at 165 hp. THe boat tips the scales at 41,200 pounds.
8-kts is right at 1.2 S/L for 43-foot waterline. 1.2 S/L is a good speed for most boats in this class as it's usually enough to get the engine(s) well into their comfortable operational temps. In my opinion, 1.34 S/L, the generally accepted 'hull speed' is the speed at which the boat starts transitioning ----- and efficiency starts dropping like a rock as you can hear/feel the engine(s) and prop(s) starting to load-up. Stay at 7.5-8.0 kts and you won't lose much time and your wallet will thank you.

Just my opinion.....

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