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Old 07-12-2015, 11:24 PM   #1
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Question Hull speed.

Hi All.

I have seen many times on the forum a discussion about how to work out hull speed by using a formula but the formula does not seem to take account of the various forms a hull can take.

For instance a Sharpie hull, typical trawler hull form is squared off at the transom whereas a Carvel hull is similar to that of a sail boat, more rounded and smoother through the water and I suspect that it requires less power for more knots.

I may have missed some point in using the formula though and am happy for any of the learned boaties here to learn [sic] me.

Also the discussion about how much H P is needed to achieve hull speed appears to neglect the power drawn away from the engine by power systems on the boat which do not add anything to propulsion.

I hope I have kept the question simple enough so that the answers do not drift too far.

Thanks

David.
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Old 07-13-2015, 12:44 AM   #2
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The equation isn't intended to yield solid numbers. Unique factors on each vessel's configuration make the numbers slushy +/- a bit.
It is just a guideline for ballparking rough expectations.
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Old 07-13-2015, 02:00 AM   #3
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Manly-Tuesday is right on the hull speed being approximate for any given hull the formula factor, 1.34 is at the lower end of the scale, hull speeds generally being between 1.34and 1.51 of the square of the waterline length of the hull. However, "hull speed" has a very precise definition: It is the point at which the wavelength of the bow wave, from crest to crest, is exactly the same as the waterline length of the hull. Of course there are hull designs where the hull can exceed its theoretical "hull speed." It does have some uses, although in modern naval architecture, speed/length ratio (actually speed/square root of length) and the Froude number (a fluid dynamics measure of the resistance of a hull moving through the water) are more commonly used. Either, or all three measures do have some uses the chief being the decision on how much power a given hull needs. In a full displacement hull, as Eric mentions often, why would you put more HP in the hull than is needed? In determining engine size, of course builders are going to look to the ancillary HP draw of ship's systems, i.e. hydraulics, alternators, etc. They are gong to take shaft HP needed to drive the boat, HP of the ancillaries plus some measure of extra power to determine pwer needs. Of course, as I believe Eric has mentioned here many times, the vast majority of FD boats are overpowered.
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Old 07-13-2015, 03:55 AM   #4
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Thanks T H D.

For your fulsome and precise explanation. I was concerned because so much column inches have been used to explain that this boat or that is / was overpowered.

Regards.

David.
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Old 07-13-2015, 06:17 AM   #5
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"For your fulsome and precise explanation. I was concerned because so much column inches have been used to explain that this boat or that is / was overpowered."

A multiplier of 1.3 vs 1.6 does not change the hull speed much.

Few cruisers will go hull speed as pulling back to a multiplyer of .9 to 1.15 will save 1/2 or 2/3 the fuel burn.

Another way to look at cheap power cruising is by realizing that most boats will cruise at under 3 hp per ton (2240 lbs per ton) maximum underway.

Most boat engines and the drive train is not optimised for cruise speeds so for many 15 hp per gallon of fuel is a close rule of thumb.

The Sq RT of LWL was measured on old fat boats L/B of about 3 , once the hull has a L/B ratio of over 6-1 the wave making becomes less important , with weight and skin friction surface area being more a factor.

Everything is a compromise , mostly to nital coat.

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.
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Old 07-13-2015, 08:44 AM   #6
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"hull speed"is oftem misunderstood as an efficient speed of operation. As THT points out it is the speed where wave making is great. wave making is a waste of fuel. So economical operation of boats of interest here will be with little wave making and a S/L closer to 1.0.


Theoretical hull speed is also IMO not a defining number for power required as power required to achieve it in still water will be found insufficient when pounding into head seas where the boat is almost stalled by each wave.
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Old 07-13-2015, 10:07 AM   #7
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the 1.34 number is an "averaged" number for displacement hull shapes. They can be greater or less...changing the end result...not a lot...but some.


there seems to be a lot of disagreement as what the range of numbers is...so the amateurs argue a lot about what is possible or not out of a "displacement" hull.
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Old 07-13-2015, 11:04 AM   #8
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I have a theory about full displacement boats and fuel consumption.

I think that most full displacement boats will be most fuel efficient when the engine is run at their peak torque, not peak HP. On long stroke (slower diesels), I know that this results in most output for the fuel burned. I have not verified this theory in other boats.

I also know it depends on the transmission and propping of a boat to determine the best speed / economy. Find the power curves of your engine(s) and see if you don't go farther when they run at or near peak torque.

If you want to see what this does with your boat, I would appreciate any feedback, right, wrong or otherwise.

Thanks!
stu
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Old 07-13-2015, 11:17 AM   #9
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As others have said, hull speed is a rule of thumb and a not particularly precise rule at that. Using the 1.34 time square root of waterline length formula give my boat a hull speed of 7.6 knots. Reducing the multiplier to one gives my boat a speed of 5.6 knots. My fuel consumption at 7.6 knots is about 1 gal/hr while at 5.6 knots it is about 0.4 gal/hr. So slowing down from a multiplier of 1.34 to a multiplier of 1 gives me a 60% fuel saving. In other terms, my nautical miles per gallon goes from 7.6 at 7.6 knots (nominal hull speed) to 14 at 5.6 knots. Wide open throttle on my boat lets me go 9.1 knots, but used 2.5 gallons per hour or 3.6 miles per gallon.

So I am quite happy to run at around 6 knots with the knowledge that I can jump to to 7.5 knots if I want to. In addition to reduced fuel consumption at lower speed, there is a lot less engine noise.
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Old 07-13-2015, 11:27 AM   #10
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Each engine has a BSFC fuel map, but mfr's don't like to publish them. Annoying. There are some on the net. Just google BSFC- brake specific fuel consumption.

In general Stu, you are correct. The "sweet spot" or best BSFC is usually found near peak torque rpm, but at a higher load than on the typical boat's load curve.

If I was to design a propusion system to maximize efficiency, I would determine the hp needed for the design speed, then spec an engine that would make that hp at the sweet spot on it's BSFC curve.

But there are other considerations that usually trump efficiency. The engine may not "like" running at that spot, i.e., load too high at too low an rpm, which has warranty issues. And then boat is really designed for one speed, like a freighter. Ok for them, not so much for pleasure boats.

I just wish engine mfr's would publish their BSFC data. Cat and Cummins do, albeit in table form, but many do not. I think I know why, too. Some data are nothing to be proud about.
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Old 07-13-2015, 11:27 AM   #11
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Quote:
Originally Posted by stubones99 View Post
I have a theory about full displacement boats and fuel consumption.

I think that most full displacement boats will be most fuel efficient when the engine is run at their peak torque, not peak HP. On long stroke (slower diesels), I know that this results in most output for the fuel burned. I have not verified this theory in other boats.

I also know it depends on the transmission and propping of a boat to determine the best speed / economy. Find the power curves of your engine(s) and see if you don't go farther when they run at or near peak torque.

If you want to see what this does with your boat, I would appreciate any feedback, right, wrong or otherwise.

Thanks!
stu
It all depends on what you mean by most fuel efficient. If you mean the point where the engine puts out the highest horsepower per gallon of fuel consumed you may be correct. However if you are talking about miles the boat can move per gallon of diesel you are 100% wrong. Fuel consumption versus boat speed depends on lots of factors including, but not limited to LWL, beam, wetted surface area, displacement and hull form. How that relates to engine rpms is determined by things like transmission gear ratio, prop diameter and pitch, number of prop blades and their form and the disk area ratio of the prop. Changing any of those parameters will change the boat speed at a given engine rpms and the mpg at that engine rpms setting.
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Old 07-13-2015, 12:19 PM   #12
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David,
I have a saying about hull speed that applies here .. I think. Sometimes I use the expression "effective hull speed". Boats w full ends start pushing serious volumes of water aside very close or right at the cutwater .. the point where one would measure water line length .. WLL will have speed characteristics common to boats w a slightly longer WL. Other boats are rather hollow at their ends (like sailboats as you posted). On a boat w a hollow bow the angle of entry would be less at the cutwater than somewhat aft of that. So the major force pushing the water aside will happen later .. closer to amidships. Boats of this type have IMO a shorter "effective" WLL and will be driven w less power at and under a speed length ratio of about 1 than a boat having very full ends (blunt bows). The boat w fuller ends probably has speed advantages because her "effective WLL" is longer. She'll ride on a wave w crests a bit further apart and probably have a bit more speed (given enough power to do it) than the hollow boat w the same WLL.

I place great value in the lbs per hp method of estimating power required to drive a FD (full displacement) hull. Partly because I've made many observations and comparisons of actual boats knowing their power and disp. For typical trawler hulls like a KK or Willard 3 to 4hp per ton fits nicely. Probably more like 2 for sailboats (as an example).

Hollow ends are an extreme element of hull fullness (or lack of fullness) at the ends of a hull. Boats w full ends have a high PC (prismatic coefficient) and those that have a low PC have slack ends. PC is expressed as a number (for those that like numbers) and the number describes the percentage of displacement at the ends v/s the middle of a boat hull. Sailboats are rather extreme (PC wise) and as a result tend to be a bit diamond shaped. Visualize an extended or stretched diamond shape. High PC hulls have most of their beam rather close to the ends of the hull. As you can see a boat 12'X40' w a high PC will support much more weight than a boat 12X40' that has a low PC. So for a given disp and length the high PC boat will be narrower w the same displacement.

So the formula (1.34 X the square root of the WLL) tells numerical hull speed but as you seem to suspect David there's more to it. IMO a boat w a high PC will generate a wave a bit longer than a boat w a low PC. Speed could be greater. A boat w a low PC will generate a wave a little shorter in length that will limit her speed near hull numerical hull speed. But she could be considerably more easily driven at a speed length ratio of .08 of numerical hull speed. My FD Willard has a hull speed of 7 knots and I cruise her at 6. I can cruise at 6.5 but it takes a lot more power. Somewhere south of 6 knots my fuel consumption will be half of that it is at 6. Probably not that far from 6 knots. Perhaps 5.4. I don't know but as you go slower efficiency goes up, of course but it's always amazing how much more efficient a FD boat gets from even fairly small reductions in speed. My Willard has 4hp per ton and has only an average hull for efficiency.

If you have a semi disp boat and need to determine power required you will probably intend to exceed hull speed. If not you're looking at the wrong boat.
You will need (w a SD hull) more power to achieve hull speed and more power yet to get above HS so the required amount of power goes up considerably. And that amount is widely variable depending on the disp and hull shape. The shape in the bow has very little to do w the difference .. it's the shape of the stern that counts.
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Old 07-13-2015, 12:28 PM   #13
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Has anyone ever done a formula on the cost to carry extra fuel on a full displacement boat? I've been told by several experts in the field that sea conditions will make more difference in the fuel burn rate at reasonable speeds (below 10 kts) on a full displacement boat.

It would be handy to know for those with boats with bigger tanks, is it cheaper to fill up all the way vs 1/2 tank when it is cheaper with higher purchase volumes?

I know planing vessels pay a penalty for every pound of extra weight they carry, since they have to push it over the top of the water.
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Old 07-13-2015, 12:31 PM   #14
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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!
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Old 07-13-2015, 12:38 PM   #15
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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
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Old 07-13-2015, 12:42 PM   #16
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Some of us cruise in big waves from time to time. Extra power comes in handy, even on a FD hull shape. Installing and maintaining a modicum of extra power is cheap, and allows one to keep the RPM in the magical 1500 to 1800 RPM range. Note - I have not used the word "trawler" - generally less descriptive than "boat" IMHO.

If one is really concerned about fuel burn, leave your vessel at the dock. Seems like many do, even those with true sippers.

Now back to hull speed, perusing Tad Roberts' written facts on this subject is recommended.
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Old 07-13-2015, 12:52 PM   #17
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Quote:
Originally Posted by stubones99 View Post
Has anyone ever done a formula on the cost to carry extra fuel on a full displacement boat? I've been told by several experts in the field that sea conditions will make more difference in the fuel burn rate at reasonable speeds (below 10 kts) on a full displacement boat.

It would be handy to know for those with boats with bigger tanks, is it cheaper to fill up all the way vs 1/2 tank when it is cheaper with higher purchase volumes?

I know planing vessels pay a penalty for every pound of extra weight they carry, since they have to push it over the top of the water.

I don't notice any difference at all after I fill up the fuel tank; although its usually only an extra 100kg (220 lbs) if one tank is empty.

The biggest factors affecting my maximum boat speed seem to be sea state and wind.
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Old 07-13-2015, 12:53 PM   #18
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For those interested in learning instead of speculation and opinions, Eric Sponberg did a wonderful tutorial a few years back on design ratios that still may be found with a little Googling. Might still be on his website.

Dave Gerr also has the knack of educating the lay person in the arcane craft of naval architecture, without too much math.
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Old 07-13-2015, 12:56 PM   #19
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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?

It would be handy to know for those with boats with bigger tanks, is it cheaper to fill up all the way vs 1/2 tank when it is cheaper with higher purchase volumes?
I love extra fuel, ballasting effect in big seas is noticeable. An extra ton or two down low quiets things down. And the range possibilities when making long and meandering voyages is much appreciated. But, that is just me and my cruising style.

Boating and saving money seems an oxymoron. Like cars, houses, motorcycles, and airplanes boating provides opportunity for all forms of excess.
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Old 07-13-2015, 12:59 PM   #20
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For those interested in learning instead of speculation and opinions, Eric Sponberg did a wonderful tutorial a few years back on design ratios that still may be found with a little Googling. Might still be on his website.

Dave Gerr also has the knack of educating the lay person in the arcane craft of naval architecture, without too much math.
As usual Spy is right on.
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