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Old 09-28-2020, 05:47 PM   #1
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Efficiency at hull speed - does weight matter?

Assuming all other things equal lets pretend you have an old 53 hatteras cruising at hull speed. Is the efficiency (nmpg) materially different if the boat is empty vs 10k# of lead in the bilge?
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Old 09-28-2020, 06:14 PM   #2
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Yes, absolutely. Weight has a direct influence on hp required to move a boat, particularly at or below theoretical displacement speed.

Your Hatteras example above probably weighs 50,000 lbs. 10,000 lbs more ballast will increase hp required and therefore fuel consumption about 20%.

Simply put, more weight means more water that needs to be pushed aside as the boat moves through the water.

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Old 09-28-2020, 06:26 PM   #3
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Assuming all other things equal lets pretend you have an old 53 hatteras cruising at hull speed. Is the efficiency (nmpg) materially different if the boat is empty vs 10k# of lead in the bilge?
5 tons of ballast in a 53' boat? Wow that seems like a lot of weight! What is it offsetting, may I ask?
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Old 09-28-2020, 06:37 PM   #4
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Oh. This a theoretical question.
It would seem to me the weight wouldn't have that much of an effect on cruise speed. Once you get the weight moving, as long as the hull isn't in the water deeper & creating more drag, your speed would be close to same without too much change in efficiency. Resistance through the water is a big factor.
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Old 09-28-2020, 06:39 PM   #5
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Once you get into the bigger boats weight won’t make as much a difference as in a small boat. Also trawlers can usually take the added weight in stride, so to soeak, than a planning hull.
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Old 09-28-2020, 07:14 PM   #6
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100# of barnacles might have a greater efficiency effect

I am no naval architect but I have read Robert Beebe's Voyaging Under Power. My understanding from that read is “it’s complicated”. Longer, narrower hulls of the same displacement are more efficient. Significant additional weight will decrease efficiency, but it also change effective waterline and wetted hull shape. I suspect a 20% weight increase would result in <20% loss of efficiency below hull speed, but it would change about 10 variables Beebe identifies and might result in other performance changes: roll, moments and others variables beyond my fluency. More weight has benefits, too. I like the momentum provided by my 50,000# to lessen the immediate effect of wind guests while docking, as one example.
100# of barnacles might have a greater efficiency effect.
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Old 09-28-2020, 07:22 PM   #7
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Great efficiency is achieved by reducing speed one knot below hull speed.
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Old 09-28-2020, 07:57 PM   #8
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. . . as long as the hull isn't in the water deeper & creating more drag, your speed would be close to same without too much change in efficiency. Resistance through the water is a big factor.
But if the boat weighs more it displaces more <water> and therefore IS "in the water deeper". My intuition is that, unless that extra weight is displaced over extra length, and not extra cross section, the power requirement and therefore the fuel consumption will increase proportionately. I agree with the math in the hypothetical above.
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Old 09-28-2020, 08:14 PM   #9
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More weight will increase power required at a given speed. At lower speeds, the difference will be fairly small. As speed increases, it'll matter more. And if we're talking about a planing boat running on on plane, the penalty from added weight will be much more significant.
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Old 09-28-2020, 08:21 PM   #10
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The Hatteras 53 comes in at about 55,000 pounds. When I add fuel I can tell. (3500 pounds if I'm down to 1/3) I can only imagine what 10,000 would do.... That's like putting 50 people on the boat, literally a bus load. It will sit lower and add wetted surface. A lot. Adding weight matters.
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Old 09-28-2020, 08:34 PM   #11
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5 tons of ballast in a 53' boat? Wow that seems like a lot of weight! What is it offsetting, may I ask?
Boom,
I’ve got two tons in my little 30’ Willard.

As Mark says much more efficiency is available one knot below hull speed. I almost always go just a tad over 6 knots w a hull speed of 7 knots.
The Hatt not being a FD boat will probably gain more efficiency that a smaller FD boat. The turbulence at the stern probably does not increase linearly so more efficiency is probably available than the percentage of speed indicates.
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Old 09-28-2020, 08:48 PM   #12
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Maybe 10k# was too extreme. It's a hypothetical question, no plans in the works. Just trying to understand loads. I was previously under the impression that load at or below hull speed wouldn't materially change efficiency.

3-4k# is the max that would actually happen.
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Old 09-28-2020, 08:55 PM   #13
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As a very general rule the maximum speed of any displacement hull--commonly called its hull speed--is governed by a simple formula: hull speed in knots equals 1.34 times the square root of the waterline length in feet (HS = 1.34 x √LWL).
adding weight sits the boat deeper into the water which should increase the LWL making the above formula suggest an increased hull speed with added weight.
I know from experience that a non displacement boat, planing or semi planing, is affected by weight if it is unable to reach plane. But why does a displacement boat get affected negatively.
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Old 09-28-2020, 09:23 PM   #14
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It's no different than a fishing boat with 5 tons of fish in the hold. The boat is deeper in the water, goes slower at the same rpm, but probably has a better, more stable ride.
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Old 09-28-2020, 09:41 PM   #15
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Assuming all other things equal lets pretend you have an old 53 hatteras cruising at hull speed. Is the efficiency (nmpg) materially different if the boat is empty vs 10k# of lead in the bilge?
Yes.
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Old 09-28-2020, 10:48 PM   #16
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Made some guesses on beam and water line length but using Vicprop came up with 109 hp for hull speed at 50,000 pounds and 131 hp for 60,000. At 47 foot waterline hull speed is 9.19 knots.
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Old 09-29-2020, 12:22 AM   #17
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Never really noticed a marked difference.
1150rpm gives us 7.5 knots with near empty tanks
And gives us 7.5 knot's with full tanks (an additional 10 tonne /22000 lb)

Ride is noticeably better with full tanks and we have been told by those that worked on her that we need an extra 10,000lb again to get her to her full load lines.
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Old 09-29-2020, 04:12 AM   #18
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There are no free lunches folks. Moving weight, in any environment, requires energy. More weight, more energy. On some platforms, a difference may not be perceived but it's there. It is just plain science. Think about the definition of horsepower. One mechanical horsepower lifts 550 pounds one foot in one second. More weight, more horsepower to lift that weight in one second. More horrsepower from a diesel engine, or any engine, requires more energy. That energy comes from burning more fuel. No free lunch.
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Old 09-29-2020, 04:30 AM   #19
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The usual rule of thumb is 3hp for each ton of displacement at normal cruise..

So 10,000lbs would br just over 4 tons so about 12 extra hp would be required.

At 15 to 20 hp from each gallon of fuel .


For most folks the weight is not that heavy , so an extra 1 ton 2240lbs, would require an extra 3hp or about an extra 1/5 of a GPH , a big deal on a 2GPH cruiser , but not easy to measure for most folks.
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Old 09-29-2020, 06:26 AM   #20
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I am no number cruncher, but my experience in towing barges suggests that while efficiency (or power/fuel) required to maintain the same speed....says yes more is needed.


Yet not necessarily directly proportional to weight...it may be more of a function of increased resistance increased by a percentage of increased displacement. Which may be a fraction of the weight.


Thus why some have said that "weight" is more noticeable in smaller boats.


So if the weight doubles the "weight of the boat" and doubles the draft...it may be more linear than if the weight only changes the draft a tiny fraction.
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