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Old 11-05-2013, 10:58 AM   #21
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SS that's a nice feature of SD hulls and one that is dear to some or most.

I'm just trying to say you can save more fuel from going even slower as some of us do all the time and you will still get to Bremerton. But this time of the year it may be in the dark Haha.
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Old 11-05-2013, 11:09 AM   #22
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We've been down this road before and there comes a time when simply having the motors both running works against you. I don't have Floscans but my logic is somewhere just below hull speed any boat (sail, one motor or two) will operate at optimum efficiency (trading MPG for GPH). Where those two S-curves intersect is the sweet spot and my gut feel for mine is at about 8-8.5 kts.

Your LWL is a bit less and 7 kts would be a good guess from me as well.
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Old 11-05-2013, 01:43 PM   #23
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After all this talk, I don't feel so bad about my 18GPH at 18kts!!!
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Old 11-05-2013, 01:53 PM   #24
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After all this talk, I don't feel so bad about my 18GPH at 18kts!!!
Exactly! And it didn't take you all day to get to wherever you were going! I fully understand that going slower & slower will increase your mpg. Some of us, however, would actually like to get somewhere at a decent time!

Going 50mph on a freeway is going to save hundreds of thousands of gallons (if not millions) of fuel but sometimes there is a pressing need or desire to get to your destination faster.

If your boat is getting 4 mpg at 4 knots, just think what your mpg would be at 1 knot! Fantastic, huh!
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Old 11-05-2013, 03:43 PM   #25
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After all this talk, I don't feel so bad about my 18GPH at 18kts!!!
Indeed.

I almost bought a slow boat and am glad I didn't. I can poke around like everyone else(see avatar) but have gotten out of more trouble than I've gotten into having the speed available.

Ironically great anchorages are about a half mile apart where I'm at so slow is not really a "penalty"
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Old 11-05-2013, 04:43 PM   #26
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"4mpg at 4knots"

Walt's got it! Go w a 3knot tide and you'd be going faster than we generally go on Willy. I wonder if I'd burn a quart an hour at 4 knots?
Fuel burn was a big thing w me when I was shopping for Willy and I out did myself. I don't need to limit my fuel consumption to 1 gph. I went overboard. Life would be just fine w twice as much burn or thereabouts especially if I could find cheaper moorage.

But I don't think we'll kick Willy out of the slip for not consuming enough fuel. If we ever go on a long trip again it will come in handy.
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Old 11-05-2013, 05:57 PM   #27
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some of us are on the other side of the "fuel's no big deal fence"....

some of us when we cruise have no schedule or need for speed, boat in an area where there's something to see and do every couple miles so travelling only 10-15 miles a day is just fine, burn more than 1000 gallons of fuel a year so economy and price of fuel can start to make an impression and are on a tight budget overall...

add those things up and yes some of us give a darn about 10ths of a knots and small increases in NMPG....

even small differences can mean the difference in how many times or where one can dine out or fix a broken system immediately instead of putting it off.

a new member even has a cruising kitty on her blog so anyone can help out another cruiser enjoy....
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Old 11-05-2013, 07:38 PM   #28
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I agree with psneeld. The voyage is the primary purpose when I boat (I don't fish). So, why be in a hurry. Destinations are more often best approached using car, plane, (sometimes a bus), or commercial ship if the voyage isn't important. Trains are also an option if the voyage is primary.
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Old 11-05-2013, 07:45 PM   #29
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some of us are on the other side of the "fuel's no big deal fence"....

some of us when we cruise have no schedule or need for speed, boat in an area where there's something to see and do every couple miles so travelling only 10-15 miles a day is just fine, burn more than 1000 gallons of fuel a year so economy and price of fuel can start to make an impression and are on a tight budget overall...

add those things up and yes some of us give a darn about 10ths of a knots and small increases in NMPG....

even small differences can mean the difference in how many times or where one can dine out or fix a broken system immediately instead of putting it off.

a new member even has a cruising kitty on her blog so anyone can help out another cruiser enjoy....
I fully understand. I honestly look forward to the day I slow down. My last boat was my first power boat with a bit of speed(15kts@6GPH....not bad for a planing 30 footer). It kinda got me hooked on speed. Being that I still work and like to cover ground when I am off. But one day, I will slow back down!!!...
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Old 11-05-2013, 10:07 PM   #30
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The best boats for fuel economy are of course FD boats. And of course light long and narrow is also by far best.

People that are really pressed to cover their fuel costs clearly have the wrong boat.

John Baker wrote " My last boat was my first power boat with a bit of speed(15kts@6GPH....not bad for a planing 30 footer)" I suspect it's "not bad" because it's light and it's light because it's wood. That planing boat w a small diesel would be at least as economical as a FG FD boat just because of it's lightness. Build a 40' canoe w plywood and power it w 30hp and cruise at 8 knots perhaps. Light, skinny and FD. Can't beat it unless you go the catamaran route.
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Old 11-05-2013, 11:03 PM   #31
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I fully understand. I honestly look forward to the day I slow down.
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Old 11-06-2013, 11:39 PM   #32
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When I retire....when I have a bit more time and don't have to go fast....but we will see...
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Old 11-07-2013, 01:37 AM   #33
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We've been down this road before and there comes a time when simply having the motors both running works against you. I don't have Floscans but my logic is somewhere just below hull speed any boat (sail, one motor or two) will operate at optimum efficiency (trading MPG for GPH). Where those two S-curves intersect is the sweet spot and my gut feel for mine is at about 8-8.5 kts.

Your LWL is a bit less and 7 kts would be a good guess from me as well.
If you have a 42 then that is much higher than your mpg sweet spot. Your going to be much closer to 6-6.5 kts. If your bow starts rising your past your sweet spot.
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Old 11-07-2013, 08:35 AM   #34
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Not if you do the math. Max hull speed is 1.4 times the square root of the waterline in feet. At 42 feet it comes out to 8.75 kts.

That's why I feel best at 8-8.5kts. I'll have to try it at the 6.5 you're talking about and see how that feels some nice summer day.
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Old 11-07-2013, 10:53 AM   #35
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Not if you do the math. Max hull speed is 1.4 times the square root of the waterline in feet. At 42 feet it comes out to 8.75 kts.

That's why I feel best at 8-8.5kts. I'll have to try it at the 6.5 you're talking about and see how that feels some nice summer day.


That should be 1.34x... and if your OAL is 42' then your LWL is what, maybe 40? A little less?

And of course "max" still isn't necessarily the most fuel efficient... even though there's something to be said for "time efficiency" and slight differences in burn rate might not always be significant enough to incite change.

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Old 11-07-2013, 11:14 AM   #36
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that 1.34 is not truly a constant....it varies with hull shape....the more slippery the boat...the higher the number.

many of our hard chined, vertical transomed boats are probably closer to 1.1...

My Albin 40 has close to a 36 foot waterline and the 1.1 works pretty well with my "guessing" of all these cool hydrodynamic rules ....that's because I have no idea what they are because I didn't compare the line drawings with tank test results and the NAs ideas for my particular boat.
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Old 11-07-2013, 11:25 AM   #37
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I was under the impression that 1.0 or 1.1 was the best recommended(efficient) speed for a FD boat....according to Mr. Buelher.
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Old 11-07-2013, 11:38 AM   #38
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I was under the impression that 1.0 or 1.1 was the best recommended(efficient) speed for a FD boat....according to Mr. Buelher.
Every boat has it's own number based on it's design, how it's built and whether it's sitting on it's designed lines or not...

here are some highlights of the same old stuff I have learned and used for 45 years+...many opinions on this forum are only tastes of the whole concept of boat design. I'm getting ready to sell my library of design books if anyone is interested...keep an eye in the classified section.

http://www.sponbergyachtdesign.com/T...N%20RATIOS.pdf

Nevertheless, what this tells you is that most displacement boats travel most of the time
at Speed/Length ratios of at least 1.0 and slightly above, so you need enough volume to
support the hull at those speeds. If volume is either too much or too low—that is if Cp is
too big or too small—your hull drag is going to go up. Either the boat is going to have to
push too much water out of the way (Cp too big) or it is going to sink into its own waves
(Cp too small).

Speed/Length ratio Cp
1.0 0.52
1.1 0.54
1.2 0.58
1.3 0.62
1.4 0.64
1.5 0.66
1.6 0.68
1.7 0.69
1.8 0.69
1.9 0.70
2.0 0.70

see how the s/l ratio is tied to the prismatic coefficient....the first 2 numbers are the s/l ratio...followed by the Prismatic coefficient

Definition: Speed-length ratio is the speed of the vessel in knots divided by the square
root of the vessel’s waterline length in feet = V/Lwl^0.5. At speed-length ratios less than
1.34, the vessel is in displacement-mode motion—that is, the hull is simply moving the
water out of the way as it moves forward. When speed-length ratio is between 1.34 and
2.5, the vessel is in the semi-displacement or semi-planing mode—that is, it is trying to
rise up over its own bow wave to get onto plane. Some boats are designed to operate
at these speeds. Above speed-length ratio of 2.0 to 2.5, the vessel is planing and relies
on dynamic lift to raise and hold it out of the water so that it can skim along the surface
of the sea.
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Old 11-07-2013, 12:00 PM   #39
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I was under the impression that 1.0 or 1.1 was the best recommended(efficient) speed for a FD boat....according to Mr. Buelher.
The disclaimer "Your mileage may very" comes to mind.
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Old 11-07-2013, 12:05 PM   #40
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JB wrote;

"I was under the impression that 1.0 or 1.1 was the best recommended(efficient) speed for a FD boat....according to Mr. Buelher. Today 10:14 AM"

Absolutely! Guys are still hung up on the "hull speed" thing.
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