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Old 07-18-2013, 08:03 AM   #41
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City: Miami
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Join Date: Jul 2013
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I am weak on prop understanding - is over propped increasing the diameter, pitch, or a combination of the two?

Proper propeller pitch just means that you are always running on or below your engine manufacturer's designed power curve. Because most boats don't have multi speed transmissions or variable pitch propellers, propeller size and pitch has become the go to tweak to accommodate some of the dynamics affecting our boats. Simply put a properly propped boat can handle the changes in loading without creating excess loads on the engine/engines. Some of the dynamics you might want to consider when propping are heavy loads, foul bottom, towing, running on one engine for the twin engine guys, and big seas. If you can run your engine's max rpm under all these scenarios then your good to go. If not then your making your engine work harder at all rpms, it's kind of like starting up a hill in 2nd gear.

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Old 07-18-2013, 08:04 AM   #42
City: Pensacola
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Originally Posted by Ben View Post

Well, Blue Herons data, my interest, and it was confirmed by BH to be each. Since I drive a single screw with a similar engine as BH's twins, it's of interest.

So my disclaimer is to not use this as an exact measure, only a reference in determining optimal points for diesels of the 120-135 hp variety. Prop size will also figure in, load on engine will figure in.

These data won't be applicable to a 435hp Caterpillar, but there are a lot of trawlers out the sporting Lehmans, Perkins, and possibly some Volvos where it might be useful. Use only as a benchmark as I will for your own optimal point and understanding of your boat's performance characteristics.

Good grief. I sound like an engine manual now.
Concur. It's a reference. Your "mileage may vary "... Hehehe

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Old 07-18-2013, 09:46 AM   #43
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City: Concrete Washington State
Country: USA
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
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Mark wrote;
"I don't agree Eric." Great ... that means we've got something to talk about but I'm short of time these days.
"to have more power"? Well your boat is powered correctly I believe. Something of a rarity around here it seems. So what if I bought a Coot and "wanted more power"? Can you imagine a Coot w 160 hp??? Or try 40. Stupid either way. Boats (FD or close to it like a DeFever) can only gracefully use X amount of power. In your case it's 50hp so an 80hp engine is perfect. But a 50hp engine would be shy for you and a 120hp engine would be something you'd leave in the boat let it run it's course but nothing a responsible NA would specify for a build.

Semi planing hulls that are closer (dynamically) to planing hulls have a wider speed range and some can effectively use more widely varying power. Observe the newer GBs and many others. The legitimate "go fast go slow" crowd. However that's what a planing boat is.

"More power" is stupid for boats close to FD design and can't be used except for marketing (how many people chose 6cyl Chevy's over V8s?). It clearly goes back to our automotive culture. And our overall culture leans heavily toward "more" and "bigger".

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Old 10-03-2013, 02:49 PM   #44
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City: Valdez, Alaska
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Vessel Name: M/V Infinity
Vessel Model: Universal Litton 36
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I'm pretty new to this boat, just got her this spring, but have been taking pretty detailed notes on fuel fills, sightglass readings, RPM, speed, etc..

I don't yet have a Floscan, but I'm finding my "happy spot" is at about 1600-1650 RPM doing about 7 knots, which burns about 1.5 GPH.

I hope to install a fuel metering system sometime this winter to give me more detail.
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Old 12-02-2014, 07:40 AM   #45
City: Ottawa
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fuel economy President or Med Yacht

I am looking at a 45 Med Yacht with a pair of FL 275s and a 41 President with a pair of FL 225s both enjoined are turboed. What can I expect for fuel economy at 7 knots on either. Also I do not need the speed would I do better on fuel if I removed the turbos?
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Old 12-02-2014, 08:58 AM   #46
City: Carefree, Arizona
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The hand on the throttle determines fuel consumption per NM. Expect about 1.5 to 2 gph per engine in either case at 7 knots. There is no fuel economy gain in removing a turbo. The remaining life of 30 year old engines will be at the whims and care the POs have bestowed.
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Old 12-02-2014, 08:58 AM   #47
City: gulf coast
Country: pinellas
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the fuel difference is not worth considering. Neither boat is an efficient way to travel. Buy what is best condition with best features. Larger is always better as they do shrink after owning them for a while.
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Old 12-08-2014, 07:05 AM   #48
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The sweet spot for any boat (if low fuel burn is the goal) is about at the SQ RT of the LWL in knots,( SL.)

SL x .9 to 1,15 is the efficient range.

What ever RPM gives about that speed is the place to be in terms of MPG .

Pushing water aside costs money.

The faster you push it aside the faster the fuel tank empties.

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