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Old 07-13-2015, 01:09 PM   #21
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Originally Posted by stubones99 View Post
Has anyone ever done a formula on the cost to carry extra fuel on a full displacement boat?
Rough back of the envelope calculation:

All the empirical formulas have power required strictly proportional to displacement (when speed and waterline length are fixed).

Diesel is about 300 gallons per tonne.

So if you have a 20 tonne boat with 300 gallon tanks, operating from half-full to empty vs. full to half-full would make about half a tonne, or 2.5%, difference in displacement, and therefore in power, and therefore in fuel required. This would be about the difference between 4.0 and 4.1 gallons per hour, for example.
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Old 07-13-2015, 01:24 PM   #22
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As others have said, hull speed is a rule of thumb and a not particularly precise rule at that.
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Sounds like a lot of math to me.........I just go and set the throttles which seem good to me and the boat.....Its all about having fun!
I tend to agree with both these observations! My 8500 lb boat with a 26' waterline length and SD hull shape seems happiest at about 7.5 knots. Slightly above the calculated hull speed. At 200 HP, I am obviously WAY overpowered, but at 2K RPM and 7.5 Knots, the boat just feels right there. More and I'm pushing a lot of water. 2GPH at that RPM "seems" pretty efficient to me. While "Feel" is a pretty esoteric concept, we all know where our boat feels just right.
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Old 07-13-2015, 01:25 PM   #23
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So according to QB, hauling my friends and relatives around this summer has cost me 35 cents per hour. Also, I did pay the airfare and travel meals for the grandkids. This is turning into an expensive hobby and lifestyle.
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Old 07-13-2015, 01:26 PM   #24
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A CPP (controllable pitch propeller) would add $7,000 to perhaps $12,000 to the initial price and perhaps save 10% to 15% of the fuel bill, bit it would take a generation to see a saving.
Well...the CPP on my old Romsdahl allowed me to rotate the boat (ccw only!) in little more than the length of the boat. Just rocking forwards and backwards on the pitch adjustment. So with careful planning it could eliminate a bow thruster. But the beauty of the CPP was that it allowed me to tune the engine load to a specific EG/cylinder head temp.
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Old 07-13-2015, 02:16 PM   #25
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bones,
Never heard of that. Seems extreme to me but re the formula you ask for ....
If hp per ton is a common expression re power for a boat then it would follow that if you reduced your fuel weight by "X" that reduced your boat weight be 12% then power required to drive the boat would be 12% less.
My boat has 100 gallons fuel capacity. Way too much and I tried to reduce it when I had new tanks made so I asked for two 35 gal tanks. They forgot all about it while I was in Alaska so I've still got two 50 gal tanks.
But re your question I've never thought of running my tanks low to reduce drag and save fuel. But I have an experiment starting and that is to run at about 1/2 or 50 gallons monitoring the tanks more carefully to see if there is an increase in water found in the bottom of the tanks. Common knowledge says water in the fuel will increase. I've never had a water in fuel problem w motorcycles, ultralight aircraft, outboard boat engines cars or inboard boat engines. I'm not going to carry an extra 50 gallons of fuel around because of an old wife's tale.

Tom,
You didn't use the word "trawler" but you used the word "magical". That's VoDo compared to trawler. And I fail to see any magic in 1500 to 1800 rpm. If you think it's magical go for it. The the things I see that could possibly related to magic would be smoothness or noise or both. That has nothing to do w 1500 to 1800 rpm. If the same magic your'e talking about occurred at 800 to 1100rpm or 3100rpm to 5300rpm there would be no difference. No magic. Just vibration and noise. And every engine has it's own favorite speeds. One engine I looked at selecting an engine for Willy developed it's power at 2500rpm. That engine would have driven Willy 6 knots at your "magical" engine speeds of 1500 to 1800rpm. But what would have been the difference ....... basically nothing. Willy's Mitsu makes it's power at 3000rpm and I cruise at 2300. No difference. No magic.
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Old 07-13-2015, 04:19 PM   #26
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Did you know that waves have a hull speed, that's why a tsunami can travel over 500mph. Something I just learned recently. Think how fast a 28000 ft hull could go!

Tsunami
Yes and the hull speed of an an aircraft carrier is about 30 knots. The natural speed of waves on the water is the basis of the formula "1.34 ect.

A sailboat about 45' long can romp right past my little Willard even though it would likely be more of a FD vessel than mine and much heavier. Length is the biggest biggie for FD.

Heron,
You can go much faster than me because of the hull shape and the fact that the Willard is twice as heavy has much to do w it but even w four times the power and half the weight the Willard couldn't go nearly as fast as your Cape Dory. Unless the Willard was given a straight and mostly level run aft.
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Old 07-13-2015, 04:23 PM   #27
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Tsunamis are pressure waves, similar to speed of sound in air or other materials. Hull speed is defined by surface waves, where speed depends on amplitude. Math is quite different.
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Old 07-13-2015, 04:45 PM   #28
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Speed of waves depends on amplitude? I question that.

If you drop a ball of concrete in the water the speed and length of the wave will be determined by the diameter of the ball .. is that true? I think it is. But I don't know if the speed of the ball or it's weight has anything to do w it. One would think so but then gravity and weight given the air drag of it's size and shape is probably directly involved. Anybody know how the variables relate?
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Old 07-13-2015, 04:51 PM   #29
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Tsunamis are pressure waves, similar to speed of sound in air or other materials. Hull speed is defined by surface waves, where speed depends on amplitude. Math is quite different.

Thanks ski, I was dreaming of a 500knot long skinny ship, was gonna call her the t-salami. Now my dreams are dashed.

You and your math stuff.




The formula for wave speed for a deep water wave is:



To figure out whether it's a deep or shallow water wave, you need to find its wavelength.

a) The formula for wavelength vs. period is



T, the period, is in seconds.



Every good surfer should know this crap.
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Old 07-13-2015, 05:19 PM   #30
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Here's a tidbt to see where the trail will lead....


Tsunami waves do not resemble normal sea waves, because their wavelength is far longer. Rather than appearing as a breaking wave, a tsunami may instead initially resemble a rapidly rising tide.....


https://en.wikipedia.org/wiki/Tsunami
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Old 07-13-2015, 05:35 PM   #31
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I like the ballast effect of full tanks too.

Interestingly, while doing a crossing of the gulf a week or so ago, I observed on my AIS the usual tug and barge combos doing 8 or 9 knots. While looking out the window, I saw a pusher tug and barge combo that looked like it might not be a conflict, until I checked the AIS and discovered it was doing 13 knots and was certainly going to be conflicting! The tug and barge combo in the pusher configuration provides an effectively huge waterline and therefore the hull speed was way higher than a conventional tug-and-tow config. Beware!
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Old 07-13-2015, 05:48 PM   #32
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Thanks ski, I was dreaming of a 500knot long skinny ship, was gonna call her the t-salami. Now my dreams are dashed.

You and your math stuff.




The formula for wave speed for a deep water wave is:



To figure out whether it's a deep or shallow water wave, you need to find its wavelength.

a) The formula for wavelength vs. period is



T, the period, is in seconds.



Every good surfer should know this crap.
Having worked my way around this stuff a bit there are several missing links, the most important being propagation and directional changes. In other words winds cause waves and a passing storm has winds of varying strength and directional changes.

As computer models become more sophisticated, short term updates to wave height and amplitude have become more accurate. Great if you are the predictor but with more finger crossing if you are a pleasure boater outrunning a storm or if you are the commercial fisherman trying to make the the fisheries window a payday.

Lots of data and forecasting is available for us to make decisions, most but not all even 3 day accurate. Therein lies the conundrum.
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Old 07-13-2015, 05:53 PM   #33
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Edit #28,
Re the falling ball example I think the speed of the ball determines the wave's amplitude or height while the size of the ball determines the wave's speed ... outward.
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Old 07-13-2015, 05:56 PM   #34
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are most waves created by wind?....or concrete balls?
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Old 07-13-2015, 06:32 PM   #35
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I vote wind.
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Old 07-13-2015, 06:45 PM   #36
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Wind or mechanically generated I think they have the same chacteristics. A bigger/longer hull or a greater dia falling ball = a wave w greater distance between crests. And greater velocity true or no?

Actually wind is mechanically generated.

Scott your question above is stupid.
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Old 07-13-2015, 07:07 PM   #37
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Research and report back....


http://www.enchantedlearning.com/subjects/tsunami/


The Speed of a Tsunami:
A tsunami can travel at well over 970 kph (600 mph) in the open ocean - as fast as a jet flies. It can take only a few hours for a tsunami to travel across an entire ocean. A regular wave (generated by the wind) travels at up to about 90 km/hr.


http://earthweb.ess.washington.edu/t...teristics.html


Tsunamis are unlike wind-generated waves, which many of us may have observed on a local lake or at a coastal beach, in that they are characterized as shallow-water waves, with long periods and wave lengths. The wind-generated swell one sees at a California beach, for example, spawned by a storm out in the Pacific and rhythmically rolling in, one wave after another, might have a period of about 10 seconds and a wave length of 150 m. A tsunami, on the other hand, can have a wavelength in excess of 100 km and period on the order of one hour.
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Old 07-13-2015, 07:33 PM   #38
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And Tsunamis have what to do w hull speed?
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Old 07-13-2015, 07:36 PM   #39
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Quote:
Originally Posted by SCOTTEDAVIS View Post
The formula for wave speed for a deep water wave is:


That's the speed of an isolated deep water surface wave (the group velocity of a water wave packet). The speed of a deep water surface wave series (the phase velocity of a plane wave, more like what the bow of your boat generates) is twice that.

Then converting meters to feet, and meters per second to knots, you would get

V = 1.34 sqrt(L),

the familiar formula for "hull speed", the speed at which the wavelength of your boat's bow wave equals its waterline length.
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Old 07-13-2015, 07:36 PM   #40
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MBs....


Nothing...just like a lot of what is posted.


Just refer to post #36 for similarities of mechanically or wind generated waves...that may be one of the reasons I am linking info.


I didn't start the comparison...just providing links to support what I already know.
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