Hull Speed

[rondds]

So I'm reading this article in Power&Motoryacht on a new Island Packet Trawler and the author presents this formula for HULL SPEED, which is 1.34 x the square root of LWL. He then says that, by calculation, hull speed for this boat (with a LWL of 34.75') should be 7.9 knots. He then marvels at the fact that he clocks a WOT speed of almost 11 knots, stating how the amazing hull design outsmarts physics by creating a 38% increase in speed over the theoretical 7.9 knots. So then I go to the "Numbers" page and notice that at 5.9 knots, this boat makes 34MPG (yes that is MILES PER GALLON), at 7.3 knots, it makes 16 MPG, at 11 knots (WOT), it makes 2.6MPG. Still impressive; but, by definition, isn't HULL SPEED the most efficient speed for a displacement hull? And if my understanding of this is correct, isn't comparing HULL SPEED to WOT like comparing apples and oranges?

As a follow up question, does the formula only apply to vessels with a displacement hull? OR, does every hull design, even a deep-vee, when not on plane, exhibit characteristics of a displacement hull?

If you plug my boat's numbers into this formula, hull speed for me is 7.8 knots. I can assure you that at that speed, I am not at my most efficient. My definition of HULL SPEED (going off of Floscan and GPS readings) would be more like 5 knots. Is that because I do not have a displacement hull? Or is the formula just for general guidelines purposes? Please don't make me go find my copy of Chapmans.

 

[Hampton]

Isn't his calculation the one they use for the max speed of a displacement sailing vessel? That economy is incredible. Something has to be wrong.

 

[sr360aft]

My understanding of hull speed is that it is the max speed the vessel can travel before trying to climb the wake. This may not be the most efficient for your engines but would be the threshold before fuel economy goes completely down the drain in the transition from hull/plow/plane

 

[rondds]

Isn't his calculation the one they use for the max speed of a displacement sailing vessel? That economy is incredible. Something has to be wrong.

He says that formula is for HULL SPEED. Wikipedia defines HULL SPEED as

Hull speed, sometimes referred to as displacement speed, is a rule of thumb used to provide an approximate maximum efficient speed for a hull. It is only ever an approximation and only applies where the hull is a fairly traditional displacement design. It is usually described as a speed corresponding to a speed-length ratio of between 1.34 and 1.51 depending on which of the limited sources one refers to.
In English units, this may be expressed as:

where:
"LWL" is the length of the waterline in feet, and "v" is the speed of the vessel in knots The constant may be given as 1.34–1.51 knot·ft−½, or in SI units, 4.72–5.32 m½·s–1
The concept of hull speed is not used in modern naval architecture, where considerations of speed-length ratio and Froude number are considered more helpful. It is still used by amateurs in relation to traditional displacement hulls.

I guess that makes us all amateurs, but in the article, the author brags about the naval architect background of the hull's designer, Bob Johnson.

And the numbers are correct, at least according to both the text of the article and the posted performance numbers in a table included in the article. GO FIGURE??? Maybe I shouldn't be getting my info from wikipedia?

 

[rondds]

max speed the vessel can travel before trying to climb the wake

the threshold before fuel economy goes completely down the drain

Now THAT makes sense. It would seem that when this trawler hits WOT, the economy goes down the drain, which is why I don't get his musing about how WOT is so much faster than the theoretical hull speed.

Am I thinking about this too much?

I'd like a CAR that gets 34 mpg.


Here's the boat...

and here's the car..

 

[sr360aft]

Isn't amazing the smart car actually has a pretty decent crash rating considering your butt is sitting on the rear axle? The hull speed vs. WOT seems to be misinterpreted and don't apply to sail boats either. You can hit 20kts in little Laser class 14ft with a LWL of 12.5ft. Once on plane it's a different animal

 

[Jim240]

Okay, I have a physics background, but my field of study is not hull design and what I'll share I've only learned second hand and not through formal education, so use at your own risk and if I'm wrong I fully expect to see a picture of me pasted to a... well... it would be pretty hard to find a picture of me now! But since this isn't a safety issue, here goes my interpretation of the definition of hull speed.

As a kid playing in a pool, you probably pushed water with your hand, swinging wildly about. It made a bow wave followed by a trough. The faster you did it, the longer the distance the wave stretched out.

Now take a boat and propel it forward. At first a bow wave starts, but it's small in height and length and you'll see a trough and another small wave and trough, etc... all the way down the boat. Yep, you can imagine at zero speed you'll have an infinite number of zero height waves along the length of the hull. At slow speed, there's a finite number... push the throttle forward and you'll see fewer and a little taller. Keep pushing the throttle forward and now you may see as few as two... you really can count them. Keep going faster and now you get to a speed where you have a wave starting on the bow and the next wave starts at the transom. You now have your hull speed. To go faster means the wave gets longer and you're pushing a height of water in front of the boat that isn't recaptured at the back... you've ripped off part of a wave. Your drag rise is really going up fast, pushing this water. Of course, behind you it swishes in. At least it's not as bad as breaking the speed of sound!

My guess is the equation is good for objects going through water and the range of the multiplier has to do with the density of the water, fresh to salt water. The factor would be very different if you were calculating wave lengths in say mercury, a very dense liquid.

Realize that our boats climb a little and the LWL probably gets a little shorter near hull speed as we're already climbing up just because of the sloped shape of the bow is always trying to lift you up.

Now different hull shape can have different efficiencies. In aerodynamics, from an elementary practice at slow speeds... a teardrop shape is most efficient. Displacement boats have these great transoms that cause separation of the flow and that leads inefficient flow dynamics (drag). The sail boats push the water aside and then let it gently come back together. A lot of trawlers look like sail boats below the water line.

Should I continue or am I causing heads to hurt?

 

[NorCal Boater]

Ron,

You need to move to a climate where you can actually use your boat all year. You have way too much time on your hands.

Shawn

 

[Four Suns]

Should I continue or am I causing heads to hurt?

Please continue. I'm all ears...

 

[Sea Gull]

JIm - this is a great explanation. Just to add a bit, I believe that the hull shape efficiency you are referring to is known as the prismatic coefficient and should be factored into the formula. A canoe would have a high prismatic coefficient and a square coal barge would have a low one.

Also, I believe that if you towed a displacement sailboat hull upright in the water it would squat down as the speed increased rather than rise up as you say. Of course the ends of the boat eventually become buried in the bow and stern wave and the rest of the boat is in a giant trough.

 

[mawyatt]

Jim,

Please continue. Very enlightening, thanks.

 

[tmhudson2]

Jim, a good explanation. To further discuss hull shape and its effect on fuel economy, a seagoing full-displacement trawler hull (consider a Krogen: http://www.kadeykrogen.com/ ) has a hull that does resemble that of a cruising sailboat below the waterline. If enough horsepower were applied to try to push it beyond its hull speed, its stern would drop as its stern wave was no longer beginning at its stern; these boats can be very efficient on a fuel burned per ton-mile basis. They are more efficient at speeds a little below their hull speed; hull speed is not a vessel's most efficient speed.

A semi-displacement hull (such as a lobster boat-type like the Mainship Pilot series: http://www.mainship.com/ ) has a sharp, deep vee bow with a deadrise transitioning to a nearly flat deadrise at the transom. They are not as efficient at or below hull speed as a full-displacement hull, but handle the transition between hull speed and planing speed fairly gracefully.

Then we have true planing hulls. The deep-vee hull favored by Sea Ray was developed by Raymond Hunt, I believe. It allows planing performace and safety in moderate seas. The sled-like planing hulls used by Hacker in the 1930's are the direct ancestors of the hulls used on ski boats like the Mastercraft. They are very efficient and fast on relatively calm waters, but can't take the waves as well as the deep-vee.

Full displacement doesn't equate to slow on large vessels. Consider a large container ship or nuclear aircraft carrier with a LWL of 1000'. Its theoretical hull speed is over 42 knots! The container ship may not go that fast, but can be very fuel-efficient at 30 knots: http://www.emma-maersk.info/

 

[rondds]

FWIW, Island Packet is predominantly a sailboat manufacturer and from the website it seems this is their only powerboat. It is powered by a 100hp Yanmar diesel and it does have a 5000lb keel. I couldnt find any photos of the underside of the hull but the article describes it as being sailboat-like (yet it'd draft is only 3'8")?

Great explanation. Shawn, you just pipe down back there or you're going to the principle's office. Why dont you just go boating or something!

 

[redneck joe]

wow - I didn't even know I was interested in this stuff until now...

 

[HIFI]

To go faster means the wave gets longer and you're pushing a height of water in front of the boat that isn't recaptured at the back... you've ripped off part of a wave. Your drag rise is really going up fast, pushing this water. Of course, behind you it swishes in.

Jim, I want to understand this.

So as the bow travels through the water at the multi wave speed. This is not effecient because the bow is propelling through the waves it created itself. You need to get to a speed that creates a wave length where the LWL does not see the secondary wave untill it hits aft or just aft.

Is my thinking on or off?

Can you explain "Recapturing" at the back for me please?

 

[mrsrobinson]

Wow, all these numbers has my had spinning. I feel like I am back in HS Calculus class.....continue, this is interesting.

 

[billandamy]

When I push the throttle forward the boat goes at varying speeds depending on how far forward I push the throttle until I cant push the throttle forward any more.

Any other q's?

 

[sebagoman007]

Jim:

Your explanation is exactly as I understand the concept of hull speed, i.e., one wave at the bow and another at the stern. Thanks for taking the time to explain it in such an understandable fashion!

I would further think that, for the most efficient boat one would want the max. efficiency of the engine to occur approximately at or near hull speed. Right?

Cheers,

Bill

 

[redneck joe]

When I push the throttle forward the boat goes at varying speeds depending on how far forward I push the throttle until I cant push the throttle forward any more.

Any other q's?

yeah - how variable are your speeds?

:grin::grin:

 

[tmhudson2]

I would further think that, for the most efficient boat one would want the max. efficiency of the engine to occur approximately at or near hull speed. Right?

This really is not applicable to Sea Rays, all of which are designed and powered to exceed hull speed, but a displacement hull boat usually will have an engine (or two) of more than sufficient horsepower to push it through calm seas at hull speed. This is because it may have to push into a head sea or strong headwind, and need more power to overcome those forces just to make way at or near its hull speed.