Hull speed - really?

Took our boat out for a run today as it had been a couple of weeks. My mind wandered around all sorts of things, finally settling on hull speed - probably because I was looking out the side door at our wake.

I have followed - sort of - the discussion re theoretical calculation of hull speed, - all very interesting I am sure, but how do I know I am at hull speed by looking at the wake? - can anyone describe what hull speed looks like (a photo would be fantastic).

I guess I should have taken a picture and asked...."is this it" .....but we are all wise in hind site.


Looking forward to hearing the discussion.

Cheers

George

Be very careful...hull speed may be one thing...but a point in space....a lot of opinions are just that...not facts.....


Hull speed has a definition very similar to this...


Hull speed or displacement speed is the speed at which the wavelength of the boat's bow wave (in displacement mode) is equal to the boat length.....

waves will appear different to a point based on hull shape


https://en.wikipedia.org/wiki/Hull_speed

Here is a diagram that makes the point pretty well. This calculation was done for a boat with a 32' waterline length. The widely used hull speed formula would give the hull speed as 7.58 knots. I would suggest that a better measure is where wave making resistance (red line) becomes greater than skin friction/viscous resistance (black line). For this boat that occurs at about 7.2 knots.

Should have included the rest of the beginning of the article...


From a technical perspective, at hull speed the bow and stern waves interfere constructively, creating relatively large waves, and thus a relatively large value of wave drag. Though the term "hull speed" seems to suggest that it is some sort of "speed limit" for a boat, in fact drag for a displacement hull increases smoothly and at an increasing rate with speed as hull speed is approached and exceeded, often with no noticeable inflection at hull speed.
The concept of hull speed is not used in modern naval architecture, where considerations of speed-length ratio or Froude number are considered more helpful.


Too many people dwell on hull speed or other singular naval architecture terms to try to convince others hey can from afar tell you a lot about your boat....well, unless they have all the design data for your particular hull...they are just guessing....a LOT!


If it was all that easy...naval architecture would be an 8 hour internet course and we all probably would become one. It's not.


Aside from "theoretical hull speed" which is what many do call it...you can do your own testing for efficiency. Figure a way to calculate fuel consumption and then distance covered in light winds and current. Then you will have something useful opposed to a subject that many unqualified amateurs claim to know something about.


Not being one of them...I prefer the actual over theoretical....the numbers that affect my credit card and not my keyboard.

You can tell what a good efficient speed for your boat is simply by using the throttle.

You will find a speed at which further small advances in throttle do not yield a proportionate increase in speed.

That is what I as a layman would call your hull speed.

On my sailboat I could easily look at the wave I was creating directly behind me. As I approached hull speed the wave became a very pronounced standing wave that wasn't visible at lower speeds. Now I'm not steering from the stern of my boat I don't get to see it on a regular basis.

Richard

psneeld said:

The concept of hull speed is not used in modern naval architecture, where considerations of speed-length ratio or Froude number are considered more helpful.

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I agree totally. The transition from frictional/viscous resistance being dominant to wave making being dominant is generally assumed to occur at a Froude number of about 0.45. Calculating resistance/horsepower at a given speed does require quite a bit of information about the hull form. Simply using the waterline length will only give you an approximation. That said, that approximation is not all that far off what the more sophisticated calculations give.

If it was all that easy...naval architecture would be an 8 hour internet course and we all probably would become one. It's not.

Click to expand...

Actually the calculations are not particularly difficult and it doesn't take all that long to learn the basic principles particularly if you have graduate level experience in fluid dynamics .

Overall, the simple hull speed calculation (hull speed in knot equals 1.34 times the square root of the waterline length in feet) does provide a reasonable approximation for a pure displacement hull. Basically you know that your fuel consumption will start rising rather rapidly as you go above about 0.85-0.9 times hull speed and then will rise very quickly above hull speed. However, note that I said full displacement hull. Semi-displacement hulls, which most trawler style boats have, behave differently, although the difference is not all that great until you get above nominal hull speed.

As far as a visual estimate of hull speed, look at your wake. Up to about 0.85 times hull speed your wake will be minimal. From 0.85 times hull speed on up, the wake will be come much more pronounced.

Hull speed is not theoretical. It's an exact calculated speed .. 1.34 X the square root of the water line length. I'm not so knowledgable re the froude numbers but believe them to be more accurate. But accuracy is not very valuable here as the effects of hull speed don't change suddenly. The effects of hull speed happen over a range of speeds in the vicinity of hull speed.

I had a theory many years ago that a boat reached hull speed when the bow started to rise. Over time I realized that happens at a considerably lower speed. So that theory turned out to be false. It may in fact indicate some fraction of hull speed but I know not.

A theory of mine that I consider to be basic fact is that with a FD hull the bottom of the boat aft is built at an angle up to the water level (or nearly so) and this provides a surface shaped much like the following wave the boat is riding on and the boat will surf down the following wave to some degree putting some energy back into the boat pushing it fwd .. or helping .. again to some degree. Kinda like the "Bart" subways in SanFrabcisco when they go down a hill and turn their motors into generators using same as a brake.

The stern of a SD hull experiences much less advantage from riding on the following wave and is a bit like or nearly like a planing hull (depending on it's hull shape aft) so the submerged transom causes even more drag being deeper in the water near the crest of the following wave. Like the FD hull the SD hull probably gets a little push from the following wave but the push is largely eaten up by the deeply emerged transom. So the resistance of a SD hull is probably much more like a planing hull in the vicinity of Hull speed. However some are shaped much more like a FD hull and share the performance advantages of that type to a much greater degree. It depends largely on the bottom angle aft as viewed from the side.

ksanders said:

You can tell what a good efficient speed for your boat is simply by using the throttle.

You will find a speed at which further small advances in throttle do not yield a proportionate increase in speed.

That is what I as a layman would call your hull speed.

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That's about all I read into it as well, Kevin. Roughly speaking, when 200 extra rpm only gives me a 0.1 knot speed increase, I figure I am at or very close to hull speed. Maybe not theoretically exact, but for all meaningful purposes its close enough for me.

I've tried to distinguish it by a wave at my stern, but the water is fairly undisturbed behind me compared to most boats. Possibly due to a canoe stern and a round bottom?

Hull speed should be erased from the vocabulary of boaters.
It provides an estimate of a very inefficient speed not an efficient of desirable speed. The above idea of bow down is actually correct as an economical speed. Beyond that a greater percentage of the additional power is bring used to make waves rather than increase speed.
.

Brisyboy said:

I have followed - sort of - the discussion re theoretical calculation of hull speed, - all very interesting I am sure, but how do I know I am at hull speed by looking at the wake? -

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To try to answer your actual question...

Look at the forwardmost crest of your bow wave, at your bow. Now look at the next crest aft. The distance between the two is the wavelength of your bow wave. (You can estimate your boat's speed through the water from that: speed in knots is 1.34 times the square root of the wavelength in feet.) When that second crest coincides with the first crest of your stern wave, that's "hull speed", where bow wavelength equals waterline length.

Here's some more which seems to agree with my readings and understandings...


Hull design implications[edit]

Wave making resistance depends dramatically on the general proportions and shape of the hull: many modern displacement designs can easily exceed their 'hull speed' without planing.
These include hulls with very fine ends, long hulls with relatively narrow beam and wave-piercing designs. Such hull forms are commonly realised by some canoes, competitive rowing boats, catamarans, fast ferries and other commercial, fishing and military vessels based on such concepts.
Vessel weight is also a critical consideration: it affects wave amplitude, and therefore the energy transferred to the wave for a given hull length.
Heavy boats with hulls designed for planing generally cannot exceed hull speed without planing.
Light, narrow boats with hulls not designed for planing can easily exceed hull speed without planing; indeed, once above hull speed, the unfavorable amplification of wave height due to constructive interference diminishes as speed increases. For example, world-class racing kayaks can exceed hull speed by more than 100%,[1] even though they do not plane. Semi-displacement hulls are usually intermediate between these two extremes.


https://en.wikipedia.org/wiki/Hull_speed

That's fine if you are cruising on an olympic kayak or a racing shell. However most of us have displacement or semi-displacement hulls with fairly small length to beam ratios (under 3 in most cases). For the types of boats we have the simple hull speed calculation provides a reasonable estimate of the transition from low fuel consumption to much higher fuel consumption.

Note that the calculation I showed the results of above was based on a few hull parameters including waterline length, waterline beam, wetted surface area, displacement, canoe body depth, prismatic coefficient, surface roughness, location of longitudinal center of gravity relative to the longitudinal center of buoyancy, etc. That is the sort of input you need to get a reasonable estimate of the resistance to movement of a hull. To determine power required to drive the hull and fuel burn you also need to know propeller efficiency and the specific fuel consumption of the engine as a function of power output. The calculations are a bit complicated, but not hard (i.e., no higher math).

Interesting note that our boat with a 28 foot water line hull speed is just over 7 knots using the traditional calculations. Using Dave Gerr's calculations it is 7.9 knots. Actual performance the boat fuel usage claims exponentially over about 7.2 knots. Like others on here I vote the hull speed with my wallet.

TDunn said:

That's fine if you are cruising on an olympic kayak or a racing shell. However most of us have displacement or semi-displacement hulls with fairly small length to beam ratios (under 3 in most cases). For the types of boats we have the simple hull speed calculation provides a reasonable estimate of the transition from low fuel consumption to much higher fuel consumption.

Note that the calculation I showed the results of above was based on a few hull parameters including waterline length, waterline beam, wetted surface area, displacement, canoe body depth, prismatic coefficient, surface roughness, location of longitudinal center of gravity relative to the longitudinal center of buoyancy, etc. That is the sort of input you need to get a reasonable estimate of the resistance to movement of a hull. To determine power required to drive the hull and fuel burn you also need to know propeller efficiency and the specific fuel consumption of the engine as a function of power output. The calculations are a bit complicated, but not hard (i.e., no higher math).

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I know...just pointing out it is not always as simple a ONE calculation, ONE picture, ONE guess., ONE post.

and even more so....just learn your own boat or someone else's that may be similar to the one someone is looking to buy. Sure there's a lot of similarities out there...but each boat is a little different usually.

QB said:

To try to answer your actual question...

Look at the forwardmost crest of your bow wave, at your bow. Now look at the next crest aft. The distance between the two is the wavelength of your bow wave. (You can estimate your boat's speed through the water from that: speed in knots is 1.34 times the square root of the wavelength in feet.) When that second crest coincides with the first crest of your stern wave, that's "hull speed", where bow wavelength equals waterline length.

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Now we're getting somewhere.
Well put QB and all can see he used no numbers at all. As it should be.
The bow pushes the water aside with power and the Vee shape of the hull. Once set in motion the water has energy in it's mass and velocity. It moves against the static sea water, is repelled by it and bounces back toward the boat. While this is happening (one can see that it is by observing the water level dropping below static amidships) the water from both sides collides under the stern an pushes the boat fwd w a force dependent on the shape of the stern.

Depending on the hull shape and speed (among other things) one can estimate the speed of slow boats fairly well (as QB pointed out) by observing the relationship of the stern wave relative to the hull that made it. Observe the pic of a sailboat in the vicinity of hull speed in a previous post in this thread by psneeld in post #2.

It's all physics, hydraulics and enertia.

bayview is right hull speed expressed as a number is misleading. But it's very valuable expressed as a SLR (speed length ratio). FF uses SLR a lot here and relating to various speeds of various boats/hulls it is a very valuable tool. A SLR of 1 (1.34 ..... ) may be a very good speed for a SD boat like a GB but a very bad speed for a FD boat like a Coot.

TDunn said:

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,,, Basically you know that your fuel consumption will start rising rather rapidly as you go above about 0.85-0.9 times hull speed and then will rise very quickly above hull speed. ...

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My fuel consumption rate approximately doubles when going from 6.3 knots (normal cruising speed) compared to 7.3 knots (assumed hull speed). During this one-knot increase, the bow rises a little.

This discussion needs a real-life naval architect to chime in and say 'I am a licensed/registered naval architect and my position on this is ... " in order to either answer the question or add fuel to the discussion.

Wow, great discussion so far, although my question was how to physically recognise when I am at or near it, not how to theoretically calculate it. It seems to be the sweet spot that we all want to be able to peg for fuel burn.

So....... as I said I should have taken a picture however if I can describe it - the second wave was coming from the stern quarter, with the third being generated just aft of the transom. IF, I was in smooth water and I played with speed, should that second wave be generated from the edge of the transom, just like psneeds picture?

Interesting discussion. As a sailor, I am always thrilled when I can hit "hull speed" under sail. Under power, I cruise at a very sedate 6.8 knots running my littl Yanmar at 2000rpm. Waterline is only 34.8 ft. I can go a knot faster, but at a much, much higher fuel burn.

This raises a question for me as a prospective power boater. I am interested in fuel burn at a typical cruise. However, "typical" varies from captain to captain. Not everyone is ascheap as I am. I imagine there are owners of SD hulls out there who place a premium on time as opposed to fuel cost. So as I compare different boats, how would you suggest I evaluate fuel economy? If they list a fuel burn of 4gph at 8 knots, does that tell me much about what fuel consumption would be at 7?

When I bought my trawler with twin 85HP 4.236 Perkins, the PO told me he ran at 8 kts and 4 GPH. That's what I got for the first year or so. Then I decided to play with the power settings a little bit and sought the sweet spot.

I found that at 7.4-7.5 kts, I was running at 200-250 rpm less than the 8 kt power setting and my resultant fuel flow dropped to 3.2-3.3 gph. This has been consistently replicated after many hundreds of hours at the new, reduced power setting. Calculations after my fill up last week averaged 3.2 gph after 50+ hrs of operation.

7.5 Kts:



10 Kts:

Brisyboy said:

Wow, great discussion so far, although my question was how to physically recognise when I am at or near it, not how to theoretically calculate it. It seems to be the sweet spot that we all want to be able to peg for fuel burn.

So....... as I said I should have taken a picture however if I can describe it - the second wave was coming from the stern quarter, with the third being generated just aft of the transom. IF, I was in smooth water and I played with speed, should that second wave be generated from the edge of the transom, just like psneeds picture?

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I dunno whether you could tell much about your waves -- very precisely -- from onboard...

I've found it easier to start with the calculation and then modify RPMs from there, comparing speed deltas per 100s of RPMs. Lots o' more RPMs with little gain in speed on ours pretty much indicates we're at the threshhold.

Modified somewhat by wind and current.

We gain much fuel economy by running at about 1 kt below our "calculated" hull speed.

-Chris

My boat uses 2.1 GPH, 2000-2100 rpms at 7.2 knots which is slightly above calculated hull speed. At 1900 RPM's (7 knots) hull speed, it will use less than 2 GPH. You need to find the sweet spot

I let my wallet tell me. I do the largest amount of my boating on the Delaware River and Bay. Both have a fairly consistent 3 kt current that changes direction twice a day with the tide. I use my tach. My most efficient rpm is 1900. depending on the tide and the direction I'm going, 1900 rpm will give me roughly 7 kts or 10 kts.
John
2003 MS390

This has been one of the better threads as far as good technical analysis of the issue of “hull speed” is concerned. No one claimed to be an official naval architect but several posters certainly have good engineering insight and knowledge. The graphs presented by TDunn really say it all. There is not any exact speed beyond which the boat will go no faster, but the resistance increases nearly exponentially near hull speed and any prudent skipper will operate somewhere just below that steep rise. It does, of course, depend on hull shape and length-to-width ratio among other things.

The water is undisturbed before the boat enters it and leaves large waves in its wake. Those waves require energy of be formed and the energy comes from your fuel tanks. Look at the wake behind a sailboat operating below hull speed (which is almost all the time) and you will see why sailboats can cruise nicely with very small engines.

johnma said:

I let my wallet tell me. I do the largest amount of my boating on the Delaware River and Bay. Both have a fairly consistent 3 kt current that changes direction twice a day with the tide. I use my tach. My most efficient rpm is 1900. depending on the tide and the direction I'm going, 1900 rpm will give me roughly 7 kts or 10 kts.
John
2003 MS390

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One of the first tasks after acquiring my boat was to equate engine speed (RPM) to speed over ground. Did this by going up and down river in Mare Island Strait and averaging to cancel out tidal current effects.

ksanders said:

You can tell what a good efficient speed for your boat is simply by using the throttle.

You will find a speed at which further small advances in throttle do not yield a proportionate increase in speed.

That is what I as a layman would call your hull speed.

Click to expand...

Also, if the throttle push lifts your bow, back off and listen to the engine(s). Even the rattles around the galley tell you something - it's a calming trim operation.

so why such a large disparity between hull speed and best mpg's?

FlyWright said:

... I found that at 7.4-7.5 kts, I was running at 200-250 rpm less than the 8 kt power setting and my resultant fuel flow dropped to 3.2-3.3 gph. This has been consistently replicated after many hundreds of hours at the new, reduced power setting. Calculations after my fill up last week averaged 3.2 gph after 50+ hrs of operation.

7.5 Kts:

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Al, what's your fuel consumption at six knots? Bet it's around two gallons an hour.

what_barnacles said:

so why such a large disparity between hull speed and best mpg's?

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Similar to the question "why do automobiles get better mileage at 55 mph versus 70 mph."