Question Hull speed.

[Manly Q.]

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.

 

[Tuesday]

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.

 

[THD]

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.

 

[Manly Q.]

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.

 

[FF]

"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.

 

[bayview]

"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.

 

[psneeld]

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.

 

[stubones99]

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

 

[TDunn]

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.

 

[Ski in NC]

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.

 

[TDunn]

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.

 

[Nomad Willy]

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.

 

[stubones99]

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.

 

[Alaskan Sea-Duction]

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!

 

[SCOTTEDAVIS]

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

 

[sunchaser]

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.

 

[AusCan]

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.

 

[Northern Spy]

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.

 

[sunchaser]

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.

 

[sunchaser]

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.

 

[QB]

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.

 

[Heron]

As others have said, hull speed is a rule of thumb and a not particularly precise rule at that.

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.

 

[sunchaser]

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.

 

[refugio]

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.

 

[Nomad Willy]

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.

 

[Nomad Willy]

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.

 

[Ski in NC]

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.

 

[Nomad Willy]

ski,
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?

 

[SCOTTEDAVIS]

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.

 

[psneeld]

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