Speed-Length Ratio, How controlling is it?

[WayGray]

According to Thomas Jones, his Puxe and Puxe II designs with a length of 22' and beam of 5' can achieve speeds of 12-14 knots with a 10 HP engine. According to design specs for the Russell R by Wm. Atkin, the 22'by 5'8" hull can achieve 17 mph with a 12 HP engine. According to a WB article on the 21' Handy Billy designed by Harry Bryan, it can achieve 16-18 MPH with a 25 HP engine. All these designs have a length to beam ratio of 4-5 to 1.

I have read that it takes at least one horsepower per 50 pounds displacement to plane a hull. If that figure is true, then none of these hulls are planing. According to hydrodynamic theory, wave resistance increases dramatically above a speed-length ratio of about 1.4. There is some undulation in a plot of this function, and a slender hull may be able to achieve a speed-length of approximately 2.0. An experienced local boatshop owner and also a NA on this board both stated that, for this size hull, you won't make more than 10-12 mph unless you apply enough power to plane the hull.

For the above-listed hulls, power is minimal, the hulls are not exceptionally slender or light, yet they are apparently achieving speed-length ratios of 3.0 or better. How controlling is the speed-length ratio?

[George Roberts]

"According to hydrodynamic theory, wave resistance increases dramatically above a speed-length ratio of about 1.4."

While that statement appears almost everywhere. It is garbage. I will refer to this 1.4 as "hull speed."

For small boats the wave making resistance is less than the surface resistance at hull speed making surface resistance more important.

For large boats the wave making resistance is so more than the surface resistance that .78 is the constant used.

In any case hull speed is more properly defined as the speed at which the wave making resistance equals the surface resistance. Above that speed the wave making resistance increases faster than the surface resistance.

None of this is important for planing hulls.

[imported_Conrad]

The missing piece of information is the Disp/Length ratio- for what George refers to as "light craft" with D/L ratios of less than say 150 the multiplier can increase from 1.34 to a max of around 2. Hulls with high L/B ratios (length/beam) also don't fit the rule, with boats like Hobie cats being an excellent example- they don't plane, but they sure are exceeding the 1.34 limit! A final factor is the buttock angle- the angle the bottom plamking makes with the surface of the water- flat bottomed or shallow fast semi-displacement hulls don't follow the rules either. Simple, huh?! ;)

[Tom Lathrop]

So right George.

There is nothing that prevents understanding of the planing boat as much as "hull speed". We would be better off if it was never mentioned. It served a useful purpose when all boats were fat and heavy since the so called "wall" at 1.34 times sq root of waterline length applied fairly well to these boats. As boats got lighter and hull design changed to take advantage of the lighter weight, the number kept being raised. Now, for modern planing boats, either sail or power, it is a distraction for most people.

I don't mean to imply that a hull does not have a "hull speed", just that with many hull designs, it does not do a good job of describing their performance.

Both of these can often be used to compare boats that have similar characteristics but mean little in comparing boats that are far apart in the major parameters.

I know of boats (not out and out racers) that make 30mph at 50 lb per hp. So much for that limit which may well be a reasonable measure for some class of designs.

Ignorance is not as nearly limiting to understanding as the illusion of knowledge.

[ 09-10-2003, 11:54 PM: Message edited by: Tom Lathrop ]

[John E Hardiman]

OK WayGray here is the simple explination, for a more detailed one get a good text on basic hydromechanics... Lamb or Havelock are some good ones.

The resistance of a hull is dependent on speed and comprised of two parts: skin friction and wave making (we'll ignore wind resistance for this argument, however it cannot be discounted in actual calculations). The power required to drive the hull is just the resistance * velocity/ unit constant = effective hp (ehp). Normally for a modern screw propeller ehp = 0.3~0.7 *shaft hp.

Now for the Renyolds Numbers that most small craft operate at in the real world (i.e > 1.0e6), skin friction is effectively proportional to speed squared and Cf is approximmately 0.004~0.003 (faster is lower). Skin friction would then be = 0.5*Cf*rho*S*velocity^2 where S is the wetted surface (NOTE THIS FOR LATER!!!!). So far so good.

Now lets tackle the hard part. When a object moves through the water it generates a pressure field as the water is displaced to allow it to move through( insert inviscid irrotational jargon argument here ). In the fluid there is a +pressure source at the front of the body and conversely a -pressure source that forms behind the body that formed the +pressure field. So we get what could be described as a source/sink pair. For a body near or at the surface this results in a visible wave train with a resultant wave height a period. Now a generated wave wants to move at a set speed away from the +pressure source that formed it and conversely towards the -pressure source that forms behind the body that formed +pressure; and these form the Kelvin wave pattern (two actually). Gravity is the only restoring force for these waves and it can be shown that Length of wave = 2*pi*velocity of the pressure source^2/gravity and that the power required to form these waves is proprtional to wave height^2. At a minimum a hull has two of these wave patterns (one at the bow and one at the stern), but most "ship shape" hulls have four or more.

Now on to your question.... smile.gif

Hull speed is normally technicaly defined as the velocity where the length of the individual Kelvin wave systems is the same as the length of the waterline. This would be where the bow and stern are on crests from the dominate bow wave system and the hull has sunk into the trough in the middle. This would occur at a Froude Number (V/sqrt(g*L), see the discussion of Kelvin wavelength above) of 1 which is equal to a speed-length ratio (V/sqrt(L)) ~3.355. Now as the heights of this wave system is proportional to the amount of water that must be displaced out of the way by the hull; a higher length/displacement ratio (L/D^1/3) means that the wave train heights are smaller and therefor require less power to generate. Additionaly remember what I said about S? S is based upon the true wetted surface, not the still water line wetted surface. Furthermore, remember what I said about there being more than one Kelvin wake set? Through careful choice of hull shape, the wave train can be made to cancel each other out ( not quite and insert another inviscid irrotational jargon argument here ). For most "ship shapes" (i.e. a wedge/pmb/wedge) there is a "hump" in the wave resistance curves at V/sqrt(L)= 1.0-1.6 (Fn ~0.3-0.5) BUT THERE DOES NOT HAVE TO BE.

Now what does this mean?

What it means is that with careful manipulation of shape and displacement, the power required to drive a hull can be minimized (No sh*t, Huh!!!) If we take an extreme of L/D^1/3 = infinity (i.e. a flat plate on the surface of the water), there would be no wave making and resistance would be proportional to V^2 only to all speeds; i.e. no hump. As L/D^1/3 decreases, wave making increases so that for the same power you make less velocity until the hull shape becomes just an infinte plate going faceways through the water (i.e. L/D^1/3 = 0) where skin friction is zero and it requires infinte power to move the shape.

Now what happens when you go above a Fn of > 1 (V/sqrt(L) > 3.355)?

Basicaly, you have to start climbing a hill. As speed increases above a length-speed ratio of 3.4, the stern begins to sink into the trough of the Kelvin bow wave. S increases as the stern sinks, gravity begins to resist forward motion, propulsion is no longer ahead but ahead and up. Now again, careful choice of hull shape can minimize this, but you can never get over your own bow wave because wherever you go, there it is :D . Does this really matter? Not if you have enough horespower.

Now... what does planing do?

What planing does is effectively make the displacement less AND reduces the wetted surface. A double bonus?....well not quite. Remember what I said about the Kelvin wake formed by a pressure source. The same pressure source (i.e. the lifting surface of the hull) that reduces the displacement and wetted surface also generates a larger Kelvin wake. Additionally, the lifting surface also has a lift drag associated with it ( and insert another inviscid irrotational jargon argument here ). Furthermore, as almost an aside, as the hull is lifted, the length of the waterline decreases and therefor the speed length ratio goes even higher! Again, does this really matter? Not if you have enough horespower.

The truth of the matter is that there is not one "best" hull shape. We have not even touched on other important boat qualities such as seakeeping, stability, ride quality. IMHO the rapid growth of high hp-low weight marine engines led to a boom in planing boat development in the inter-war years. In the final analysis, the "need for speed" in a small afordable boat that could be trailered and stored in a garage/boathouse won out over the long, lean express cruisers that most people could not afford. Bainbridge Island did an article for SBYD a few years ago that just about summed that up.

To sum up; speed-length ratio is meaningless by itself without other prameters to further define the hull shape in question. Like the Admiralty coefficient, it is just part of another tool used early in the design to "put you on the paper".

[Big Red]

So John, let me see if I have got this straight. :confused: The faster a boat goes, the longer the waves it makes gets. Eventually (if its not a planing hull) the trough of the wave gets to the back of the boat, and for the want of a better phrase, she sinks?

That is why a longer hull is "faster". Because the boat can go faster (make longer waves) before it falls in its own hole? :D

Now the tricky question: How can I enjoy the benefits of a planing hull when the water is smooth, and not get that bogged down feeling some planing hulls have below planing speed when it gets a bit choppy and I want to slow down (to keep the fillings in me head?) :D

[mmd]

Bravo, John! :cool: Good explanation of a tough subject. When does your book get published? :D

[Popeye]

How can I enjoy the benefits of a planing hull when the water is smooth, and not get that bogged down feeling some planing hulls have below planing speed when it gets a bit choppy and I want to slow down (to keep the fillings in me head?)

you need more lift than drag? such are the questions that have plagued mankind since then dawn of time.

when you invent this hull, give me a call and i will invest.

(the short answer is "semi displacement" hull, a stepped, deep Vee., if you will.)

[ 09-11-2003, 10:03 AM: Message edited by: popeye ]

[Tom Lathrop]

Great job of covering the basics of hull speed John.

As implied from his question, Big Red was not able to dig the real meaning out of it though. This is the real reason for what I said earlier. It is easy to get trapped in the belief that the math causes the result. The math is only a means of representing physical stuff on paper so you can calculate the efects. It often gets in the way of understanding what is going on.

Bolger uses his "sea of peas" to visualize the action and reaction of hull and water to the pasage of the boat. I like to think like a water molecule. Whatever works. After a life working as an engineer, I know that a lot of bad product gets out because the engineer understood the math very well but could not visualize how the thing actually interacted in the real world.

I would prefer to start a discussion of planing with the simplest of cases and introduce the math only as needed. You got to this, but it is at the end rather than at the beginning of the explanation.

[Ian McColgin]

How quickly do you want to go on the flat?

How slowly will you accept for rough?

How big a boat? How much can you afford for fuel?

And what are your normal conditions?

There's not best hull.

Say you're into sport fishing in Nantucket Sound and surrounding waters. You'll want a boat that can get you there reasonably quickly, even against a steep chop. You'll want a boat that once there can troll or drift and still be comfortable. So you start with a sharp bow, moderate beam, moderate power and you end up with the Crosby Striper.

If, on the other hand, you want a boat that's got really high speed in rough conditions, like you plan a life of across the bar rescues, you might blow half a mil or so in an RBI with a couple of 1,000 horse MTU's attached to jet drives.

Or you want to do some light lobstering within a 5 mile radius coupled with the occasional run over to the Dog for Sunday breakfast . . .sharp bow with a V coming aft and flattening at the transom, narrow for good speed/power, OB for ease of building and maintenance, and presto - a Culler file bottom emerges.

The boat that's terrific for fishing off the beach in Oregon might not be a good choise for running the Oregon Inlet in NJ.

You see 'mission creep' in all sorts of civilian and military brfight ideas.

You know, let's have a plane that is a stealth interceptor for the airforce. Add some bomb capacity and better slow speed ability so the Marines can use it for close air support. And short take off and landing with hinged wings for the Navy.

Let's have a boat that can plane when light, carry cargo at speed, be cheap to run, and sail home if the engin breaks . . .

The more you get involved in different boats, the more you will know what you really want and what features (who really needs a tuna tower?) you won't even miss.

There are a lot of different things to do with boats. Very few are happy with really single purpose boats. I like my sea kayaks, which are definatly a little limited in their purpose but virtually unlimited in the application. I love Grana most of all, but for a day sail I'll often go with others and hope by next summer to have Il Pipi going. The dory Leeward has been wonderful for sailing, longlining off Oregon, and for rough weather I really need to get home transportation, but she's not the only boat in the universe.

Maybe that's why we sailors as such promiscuous types.
* * *

Fidelity

The sailor is loved by and loves one vessel
Or dares not voyage where they may live or die together.
The terms of fidelity are for the voyage, at least,
But most sailors sail different ships at different times.

We who are professionals may be as good whores,
Giving good service for the voyage's length.
The Corinthian is a bit of a slut
Skipping from yacht to yacht for pleasure.

The sailor may love with mechanical competence
Or with lyrically delirious abandon
Or bound in chains of bitter duty
Or just a summer sunshine thing.

I pray that every sailor takes the chance
To love one vessel without stint or measure.
* * *

Enjoy

[Popeye]

yo ian, you forgot to mention submarines.

[John E Hardiman]

Big Red:
The answer to your question is to use a VBG (Variable Bottom Geometery)hull. (I just thought this up so (C), copyright,(R), Pat. Pend., Manufacturing Licenses available.... :D ) However nobody can afford it....yet. Actually a SES (Surface Effect Ship) is sort of this type of thing.

mmd:
Thanks redface.gif , but there are MUCH better people out there than I to do that.

Tom:

Bolger uses his "sea of peas" to visualize the action and reaction of hull and water to the pasage of the boat. I like to think like a water molecule. Whatever works. I like magnetic marbles, you can explain cavitation to people that way ;) . I did not start with planing because as I see the world, planning is just a specialized subset of general resistance, not the beginning. That may be because my mind starts from a deeply immersed body and works toward the free surface rather than placing the body down into the fluid like an aero guy would go at the problem.

popeye:
Subs have their own set of ploblems, one of the more important being that they have no waterplane to keep things simple and stable. ;)

[George Roberts]

And if you really want to go fast with low power you put your stubby airplane wings in the water and fly.

1hp, 250# = 20knots.

(I bet you could power it with sails.)

[John E Hardiman]

Originally posted by George Roberts:
And if you really want to go fast with low power you put your stubby airplane wings in the water and fly.

1hp, 250# = 20knots.

(I bet you could power it with sails.)Hydrofoils are just planing boats with the hulls sunk and the flying bridge out of the water :D .

As for the sailing part, see this web page (http://home.earthlink.net/~stevensm/sailing.html#SH) for a good run down.

[Aramas]

Planing hulls are more analagous to aircraft than to displacement boats.
More beam is like wider wingspan = more lift and efficiency. A wider hull can carry a bigger load for a given power, and plane at a lower speed. Power to weight ratio is everything. Despite all the contrived 'cleverness' of the aviation pioneers, it was the power to weight ratio of the engines that got the first planes off the ground. An engine with enough power and light enough weight would make brick fly.
However you look at it, a planing hull is a pig at displacement speeds. Double ended hulls like the bartender get around that to an extent, but most planing hullforms are more efficient on the plane than even at very low speeds.

[Popeye]

see also:

SWATH (Small Waterplane Area Twin Hull Ship)

(Nav Archs make great drink'n buddies :>)

[Tom Lathrop]

Originally posted by John E Hardiman:
[ I did not start with planing because as I see the world, planning is just a specialized subset of general resistance, not the beginning. That may be because my mind starts from a deeply immersed body and works toward the free surface rather than placing the body down into the fluid like an aero guy would go at the problem.
)[/QB]Point taken John but, to me, planing is so removed from the rules that govern displacement motion that I think it is easier to start with planing of a simple flat plate and then go to the transition zone rather than the other way round.

No matter, the point you made about the necessary lift of a fast planing hull requiring the normal reaction force from the water is well taken (Kelvin waves). We must not forget that a hull in equilibrium must always have a reaction force from the water exactly equal to its weight. This force being the sum of the buoyancy and dynamic lift.

[George Roberts]

Thanks for the link John.

No one has mentioned active stability. It seems rasonable that a planing boat would have enough power to provide for active stability at low speeds.

I guess we all hang our hats on different posts.

[John E Hardiman]

Originally posted by popeye:
SWATH (Small Waterplane Area Twin Hull Ship)
SWATHs are so specialized as to be almost non-starters for any generalized design. They make poor boats, only OK small ships, and never a large ship. While they have the advantages of very low motion in up to "rough" waters, they perform poorly in "very rough" waters and long swell. They are subject to high structural loads, poor arrangements, and weight problems. They have very large deck areas, but very little tolerance to draft changes. All in all they are only good for the few things that that have been used for already.

And I'm not talking out of my hat here; my college Design Project was a SWATH (which turned out to be about the same size and 3 years ahead of the T-AGOS-19) and it taught me a lot. Also I had available to me all design data on SWATHs up to that date, and I've followed the ups and downs of several designs since then. A nice concept, but with issues when too small or too large.

But to the point, what a SWATH does is to manipulate the demihull and strut Kelvin wake interactions to slightly increase the size of the first resistance hump and cause a large hollow behind it at higher speeds. This allows the vessel to achieve high speeds for the same power needed to clear the first hump. If we follow on increasing speed, the second hump is also increased and the second hollow is likewise deeper.

Remember the discussion of S and it's relation to total resistance above in my first post? Basicaly S and D do not change for a SWATH, and by making a big hollow after the first hump the resistance will only grow with the square of the speed less the drop in wave making resistance. If the drop is steep enough, the power curve is flat or even negative! As an aside, SWATH are also difficult to control in speed after the first hump because of this, it will "hunt" as there are several speeds that require the same power. Some small SWATHs can achieve the second hump due to high power/low displacement considerations (i.e. gas turbine engines), but they begin to become succeptable to wave impacts due to structural air gap considerations.

[WayGray]

Thanks to all for your replies. This discussion went way beyond just the significance of the speed-length ratio. Your views generally coincided with what seems logical to me. However, when I was just told, by a person I would expect to be knowledgeable, that a hull of this size would probably need 50 HP to get beyond 10 mph, I thought it advisable to get other opinions from your combined experience.