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Published 09.12.2013 | Author : admin | Category : Wooden Skiff Plans

My thinking is that a formula along the lines of the one below could be used to establish whether a hull is limited to a hull speed or whether it is able to break free and continue accelerating in displacement mode e.g. The waterplane coefficient, Cw, could be used in place of angle of entry of bow, ideally just Cw of forward half of hull would be used. For hulls that do not exceed the critical k value, factors other than bow wave formation dominate drag, so the concept of relating the hull's maximum speed to the square root of its length is of little practical importance. Alternatively you could take the opinion that there is no such thing as hull speed, hence trying to calculate it is pointless. Location: mallorca Displacement Hull Speed calculation Juan Baader the writer of the book?Cruceros y Lanchas Veloces? introduced a very simple way of Displacement hull speed calculation. Location: mallorca The displacement hull is a hull with narrow stern and the speed is limited to the waves the hull is producing. Location: USA I tend to think it's probably impossible to come up with a single, simple equation that would be accurate enough for all hull forms and sizes. The predictive value of such equation would tend to considerably diminish in extreme cases e.g. The problem seems to be that numerous factors affect R, and under different circumstances they don't always interact exactly in the same way with other factors.
A more useful approach could be to find for each boat category of general size and form an equation that best describes empirical findings, and therefore would have a good predictive value for design purposes.
Location: Dubrovnik, Croatia If it would be that easy to calculate resistance what would be doing all these towing tanks and institutes.
Location: mallorca Using the R-Value formula it is very easy to calculate boat speed for Displacement hulls. When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. When I was a lad, I was taught that no boat could go faster than about 1.4 times the square root of its waterline length, in knots. What stops a boat which is not 'planing' from going faster than its max hull speed, regardless of whether its motive power is wind or engine and regardless of 'horsepower', is the boat's inability to ride up and over its own bow wave. Although this one is merely repetition of some family legend there are some sites that give the impression of reputability but still put forward the same nonsense even though they may cloak it in more pseudo scientific bafflegab. Nowhere did Froude make reference to a maximum speed or an unattainable speed based on the waterline length. When a vessel proceeds through the water it creates a wave train at the bow and another at the stern depending on the speed and the length on the waterline.
The shape of the wave is assumed to be a trochoid which gives a good relationship between calculations and observations.
Put the straightedge on the paper lengthways and the coin on the paper near one end resting against it. There are mathematical expressions giving the results of energy transformations of collapsing waves and the result is that all waves travel at a speed determined by the wave length independent of wave height. The early experimenters pointed out that the resistance was exponential and it was increasingly difficult to achieve higher speeds (so what else is new!). The only limit to a boat?s speed is power and does the system fit, it is physically impossible to install sufficient hp to drive it at any imaginable speed. This is not a law on the maximum attainable speed but only the formula giving the hull speed at which the created wave length equals the hull length on the waterline. In the quote at the beginning of these notes is the notion that a boat can climb up and over its bow wave. The first two charts below show the case of a 32 foot LWL trawler type, displacement 26,000 lb with a fuel tank of 100 US gallons of diesel fuel. There should be enough power installed to overcome a high wind and yet maintain a reasonable speed ensuring you reach your destination. A quick, simple way to get a handle on the effect of wind is to use a formula based on a combination of Adm.
A quick example; the transverse projected area of the example boat is about 100 square feet. Suppose a boat speed of 7.5 knots is chosen and the wind is neglected then 40 hp is needed. It seems a helluva waste of money to include power that may not be used so a great deal depends on the where you are sailing and the general, prevalent weather conditions. If you make long trips in inclement weather it is also advisable to derate the engine generally so that it cannot operate at more than say 90% output, ie a maximum continuous rating (MCR) of 90%.
Excellent post, really informative with just the right number of equations to get my head around - thanks!
There is very little difference, if any at all, in the hydrodynamics of displacement hulls compared to semi-displacement hulls. This system gives the best of both worlds; a pair of high power gas turbines for very high speed operation and a medium speed diesel of around 15,000 hp for cruising up to about 17 knots or so. These two ship types also illustrate another consideration in design ? how do you take best advantage of the internal space? The net result is that small, single-hull boats, whatever their type, all tend to break the fast but long ship notion.
Although it would take less power if the small trawler was a bit longer it may not necessarily be more economical overall.
Because boats are rather small anyway an unacceptable power increase on a large ship is normal on a boat. Any compromise will result in a distinct change in the hull form to suit both the speed and function of the boat.
I modified the spreadsheet I posted for planing boats to make it for displacement hulls and semis.The image appended to this posting shows the output. The old formula was created by looking at fat boats of the time , and doesn't work well for skinny fast boats.
I wish I could answer your question but unfortunately, I don?t know the definition of hull speed used by AYRS to come up with their formula. Since my first posting, I have now skimmed through over 800 sites or forum threads that use the term ?hull speed.? Only one site used it correctly and it was a US Coast Guard paper on yacht safety where it was discussing the speed of a yacht compared to the water without any reference to LWL. This ratio allows you to determine hydrodynamically similar speeds for yachts with different waterline lengths. There are always practical limits to the speed but the limits are only a case of power and the means of application. As far as displacement hulls and semi-displacement hulls are concerned using a factor of 1.34 is merely coincidental to the boat length equally wave length. The attached image below is taken from Basic Naval Architecture by Barnaby and it was used to demonstrate how resistance increases disproportionately with speed and the number of waves in the vessel length. The numbers in the top left of the chart are the number of waves occurring in the vessel length, which is invariably taken as the length of the waterline with the vessel static in still water.
Similarly, it gives a very high coefficient for a fast, slender hull so that in both cases it points in the right direction for speed but is not an absolute canon.
So as you see the WSA varies with the square root of the length, it is easy to verify that the total area will increase and so the weight increases and you?ll pay more.
In the image below, Residuary Resistance means Total Resistance less Frictional Resistance.
To begin with, you cannot possibly design an appropriate hull shape without an accurate estimate of weight and center of gravity. Once we have displacement and center of gravity scenarios tied down, the finessing of hull shape can begin in earnest.
Get this right and you have a wide range of storm tactics at your disposal, the ability to average much higher boat speeds in all conditions, and a significantly more comfortable vessel. On the other hand, if you lengthen the waterline, reduce displacement, and then moderate the beam, you suddenly have a relatively shallow hull which is still knifelike forward, enough so it will slice comfortably through the waves, yet shallow enough to be easily steered. In the context of heading uphill in moderate breezes, say up to 20 knots, this hull is so powerful, yet fine-ended, that it will barely feel the waves. Steps are breaks in the hull intended to reduce the amount of hull surface in contact with the water. Short, wide (high-aspect) surfaces are more efficient than long, narrow (low-aspect) ones in terms of frictional drag on water. And the popular notion that any added speed from a stepped bottom is due to a layer of bubbles undoubtedly reduce frictional drag to some extent, but the real saving is in minimizing the hull area in contact with the water, specifically by presenting two or three wide and short surfaces to the water instead of one long, narrow one.
As I have explained in a previous article on hull shapes, the lift production is more efficient for a surface, with a small length to beam ratio. Builders often provide large inlets to the areas behind the steps, and a few even provide air paths through ducts that lead to the trailing vertical edge of the steps. Since the lift is now spread to several surfaces along the hull (see drawing) the longitudinal stability becomes very large. Stepped hulls should be used by experienced drivers who know what the hull is likely to do in a seaway and in hard cornering, and who know how to react to the unexpected. In general, the higher the number, the heavier the boat for its length and the slower it is. The length-to-beam ratio gives an indication of how long the boat is relative to its beam and allows you to compare two boats of different size.
We find that the larger boat has a length-to-beam ratio of 4.167 whilst the smaller boat has a ratio of 4.
When I design a boat, I use the power-to-weight ratio to indicate whether the boat has sufficient horsepower for its weight. This is merely an indicator of the amount of horsepower a boat needs to push its own weight through the water. Multiply the waterline length by the boat’s beam and depth (from bottom of the hull to the deck edge), you get a cubic number (CN).
To simplify, if you took a block of wood, the length of the waterline and shaped to the underwater portion of the midships section, then carved it away to model the ends of the boat, the Cp is the remaining percentage of the original midships-shaped block. The optimum Cp ratio varies in direct proportion to the hull resistance and the boat speed. In my previous article, I briefly discussed and touched on the different hull shapes available, how they work and how to distinguish these hull forms from one another.
A planing hull uses hydrodynamic lift to rise up and out of the water to reduce resistance. In order to plane the hull must achieve an appropriate angle of incidence to the water flow, trimming up by the bow to generate lift.

The need to generate hydrodynamic lift places constraints on planning hull designs such that all true planning monohulls share a number of features in common.
Look at many power boats from the side and you will see more or less a sharp corner on either side where the hull bottom meets the Topside. A hard chine is intended to throw spray to the sides of the hull and to prevent water from rising up the hull sides where it will increase drag.
For most planing hulls the chine should be immersed below the waterline from midships (more or less the mid section of the boat) towards aft at a depth roughly 1.5% to 4% of the maximum chine beam. Deadrise is the angle a hull bottom makes with the horizontal plane viewed from ahead or astern.
For inshore crafts, deadrise can be about 10 – 12 degrees from the midships aft, increasing from midships as you go forward towards the bow. For coastal craft, deadrise should be 15 to 20 degrees from midships aft, increasing as you go fwd towards the bow. For offshore boats, deadrise should be 20 to 25 degrees from midships aft, increasing as you go forward. In general, the deadrise angle determines at what speed and seastate a planing boat can best power. The number and location of spray rails, as well as their run along the hull, is a subject on which there isn’t clear agreement.
In other words the spray rails were dead straight if you look from them from directly beneath the hull. Generally, the modern thinking is the spray rails are dead straight (follow the buttocks) aft of station 4 to 5 (5 is generally midships), but curve in (in plan view) as well as sloping gently up, rather that following the buttocks as they run forward.
Hot at the heels of the bright Magic Vee micro racer from Joysway is the enjoyable new Magic Cat!
The Flash is a great handling boat and just a few seconds of full power will get her skipping on top of the water and awaiting your next command. For the background to this thread see Humpless Planing in the Sailboat section of this forum.
Froude was a scientist who worked in the second half of the 19th Century doing much research work for the British Admiralty. He observed that when models were run at speeds in proportion to the square of their length they created similar wave patterns. If there is an interaction the resistance is higher than the smooth line ignoring all the interferences.
Some years ago the children?s toy Spirograph allowed the construction of beautiful wave patterns among them the true trochoid.
Make a mark on the coin where it touches the straightedge and then roll the coin along one complete revolution.
What we all tend to forget is that they were thinking of coal fired boilers and steam engines. A practical limit is reached when the boat speed is high enough to travel fast enough to get there yet is slow enough so it is not too thirsty. If it is a sailing vessel then the boat speed is quite obvious but if the boat is powered then a propeller travels through the water at the speed of advance. Agreed that a planing boat may rise and may travel very fast but the wave is still there at the position where the point of contact is. But what is just as important is the power setting of the engine and sufficient power in reserve to overcome bad weather. It is risky to simply double the power or triple it because very small boats need a higher proportion add-on than larger boats.
That is, the average hull width above the LWL times the freeboard amidships plus the house width times the height. Against a wind of 20 knots the boat would make about 6.5 knots ? slide left along the 40 hp line until you hit the 20 knots wind speed then read down to the boat speed. If there is always a strong wind you must make allowance but if this occurs only a few hours a week then it would be costly to allow for it. I tried to delete the posting intending to reduce the image size and re-post but I kept getting a message to the effect that I wasn't logged in or that it wasn't my posting. I tried to delete the posting intending to reduce the image size and re- post but I kept getting a message to the effect that I wasn't logged in or that it wasn't my posting. Once someone else posts, only the admin can delete the first post of the thread (which deletes the thread as well). Destroyers were originally quite small and were termed Torpedo Boat Destroyers with the purpose only of destroying fast torpedo boats in the late 19th Century. The typical modern destroyer is around 400 feet long with a top speed of about 34 or 35 knots. Keeping the displacement constant and increasing the length will increase the hull surface area and the deck area. But the basic question still remains ? how fast is it and what operating conditions are the norm. Because of the potential reduction in power it can be economical to opt for a planing boat rather than a semi-displacent PROVIDED you repeatedly operate at such high speeds. It will also be tailored to where the boat will operate as ocean goers are heavier than river goers and require more stability as well as more power for the worse weather. My next post will be notes about the spreadsheet Then the spreadsheet itself I hope it answers your questions, Kevin. Maybe someone will develop an ultra-lightweight engine of incredible power but how could you use it in a small boat?
It most certainly is not a limit and if it were then there would be no such things as planing boats. However, the boat weight will increase if the design uses a long length but displacement is constant.
Naturally, if you add icing you?ll pay more again but a long icing boat will cost more than a short one for the reasons cited above. FPB 97-1 is underway, and the folks at Circa are hard at work, getting ready for the day when they start to cut metal. In terms of fore and aft trim and vertical center of gravity, how the tanks are laid out and used have a substantial impact on calculations.
While interior is obviously an important topic, in a yacht that is meant to entice its owners to cross oceans, to meander about the world on its own bottom, the design equation has to emphasize sea keeping and safety first, to which the preceding list refers. Can you imagine how cool this will feel, surfing down tradewind swells, watching the speed gauge climb from 11.7 knots to the high teens, while the autopilot barely blinks? As seas increase and perhaps steepen, one needs to make sure the increased longitudinal stability that comes with length does not force the bow through the larger waves.
A very famous design was Maple Leaf, built in wood in 1912, and since then many successful racing hulls have had this type of bottom.
Steps can run straight across the hull (although these are structurally weak and not often seen today), or they can be V-shaped, with the vertex facing forward or aft. More specifically, this means that a petrol-powered family cruiser with steps should be able to cruise fully loaded at 30 knots, not just reach this speed at full throttle.
When the air supply is lost, a backflow occurs behind the step causing an excessive increase in resistance. Hundreds of boats are sold every year and every salesman has his or her own sales pitch trying to perswade you in the brand they are selling.
If using these tools, don’t let the spraed between boat sizes get too large otherwise the comparison will be distorted.
At sea, the heavier the boat, the likely to handle waves better than a similar lighter boat. A smaller ratio indicates a boat with greater transverse stability, making it a better for trolling or drifting in beam seas. When comparing boats, make sure that you use the same horsepower number whether its is brake horsepower (bhp) or shaft horsepower (shp). In other words it is 45 percent larger and all other things being equal, should cost more to buy and maintain.
It is based on the fact that the quikness of motion or corkiness of a hull in a choppy sea is what cause discomfort and seasickness. The ratio ranges from 10 or less for lightweight day cruiser to thehigher 50-60 such as a old sailing pilot boat.
Designers use their experience and knowledge of other designs to select the best Cp for the style and speed of the boat they design. As the generated lift approaches the weight of the boat, the hull rises from the water and start to plane. Chines with a wide flat area (called chine flats) contribute significantly to create lift in the moving boat.
The ultimate in reverse chine hull is the classic Boston Whaler (not regularly seen on the waters in SA), in which the chine forms a tunnel on either side. The chine should run parallel to the DWL (Design Waterline), from the transom forward to about midships.
The right amount of deadrise gives a boat directional stability, a softer ride and reduces wetted surface drag as the boat rises on a plane. Some very high speed offshore boats use deadrise in the afterbody as high as 26 – 30 degrees. They are usually triangular in cross section with the bottom face parallel to the water surface. Different designers and builders each have their favored system and each is sure that their system works best. The steering is direct and just a small input will give you a slight change of direction, the more input the tighter she will turn. A 45 footer with a 36 foot waterline length has a max hull speed of about 9 knots, which it cannot exceed unless it can 'plane', or is actually falling as noted below. The work was for warships so any references to sailing and Froude are probably distortions even though he did like yachting. In 1876 he gave his famous Law of Comparison which states that the resistances of similar ships are in the ratio of the cubes of their linear dimensions when their speeds are in the ratio of the square roots of those dimensions. If there is no interaction the resistance is less than the smooth ?average? line so there is a series of humps and hollows. This represents the length of the wave and the line the point makes would be the wave form where the wave height is the coin diameter.

Near the coin edge it would be a big wave and near the coin centre it would be a small wave. The very earliest steam plants weighed in at around one ton per shp, by around 1900 this fell to about 400 pounds per shp then dropped again when steam turbines appeared. The speed of advance is the hull speed times (1 ? Taylor?s wake) so it is nearly always less than the hull speed. A gas engine has the characteristic of efficient operation at almost full power whereas a diesel should not run at more than about 70% output. The chart shows the power needed to overcome a headwind with the boat travelling at various speeds. Suppose you included the effect of 20 knots wind speed then you would need about 65 hp and you would do only 6.5 knots against a 30 knot wind.
Because of the wide range in power the propeller selection is critical and it explains the increasing interest in controllable pitch and, to some degree, electric propulsion motors. This is the extreme case solution as it gives economical diesel ops up to cruising speed, so-so economy in the mid range but high speed when it is called for in emergency situations.
This will add cost and as there will be some kind of insulation on the skin or thickness of framing then the internal area and volume will shrink.
The only way to find out is to conduct a series of cost benefit studies making use of an operational profile which is a table of duration at various speeds. It is easy to see that there is nothing special about 1.34 except that it is about half way up the steepest stretch of the curve. It is very rare to see a designer quote the wetted surface area for his designs but SNAME?s Principles of Naval Architecture has many references to boats of all types. Distortion of an optimum hull shape for the sake of accomodations is a topic banned in our presence.
The ideal wave penetrating hull shape is deep forward, narrow above the waterline forward, and narrow aft. Put another way, it is desirable for the bow to lift to the seas before the waves begin sweeping continuously down the deck. We are concerned with this at rest, at speed in an upright attitude, and as the boat heels from wave impact.
A complex witches brew, not easily dealt with computationally, in spite of super sophisticated CFD codes, etc. They will have large apertures on the outboard side of the hull to allow air to be sucked down into and ventilate the step.
So, the idea behind a stepped bottom is to reduce wetted surface by allowing the hull to plane on two or three high-aspect planning surfaces rather than one large, low-aspect surface. Otherwise, the extra cost of tooling and the added time and cost spent laying up a stepped hull is wasted, and the stepped bottom is just a marketing gimmick. The speed thus drops momentarily – a dangerous situation, which may even cause injuries to the crew. This is no problem in smooth water, but in seaway the hull may tend to follow the contour of the waves.
At a speed roughly equal to 1.5 times the waterline length, if the hull is designed to plane, it will move into a transitional region where it is neither planning nor operating in the displacement condition. Because life is not as simple, chines comes in different forms – Hard chine (angular), Soft chine (rounded) or a reverse chine. Much softer than a hard chine but top speed on soft chine boats is however not as high as hard chine boats.
When the boat is underway, water thrown out by the center hull is deflected downward by the reverse chine to provide additional lift and gives an extremely dry ride.
From Midships fwd to the stem, the chine sweep up higher, to the height above the DWL about equal to a distance 0f 20% - 25% of maximum chine beam. These lines are the curves that result from slicing the hull from top to bottom and front to back – like slicing your loaf of bread from end to end along the side. The reasoning was that water low straight aft along the spray rails which generated added lift with minimal added resistance.
Features a mains operated AC Charger (for 7.2V Energy %) and a 9V Dry Battery (for Transmitter).
After a few turns you will find Flash turn tight or with less steering input she will carry a lot more speed throughout the turn.
Although the LWL is the usual length used in resistance calculations the length actually depends on the pressure variation at the ends and it varies so that LWL is a kind of average used to simplify the problems.
The so-called hull speed is a reasonable compromise and is often exceeded especially by small boats as the power needed is still relatively low and fuel is relatively cheap. Naval architects and designers of motor vessels take care to differentiate between the hull speed and the speed of advance when calculating propellers and making power calculations. It is unfortunate but it means a gas engine is inefficient at lower, cruising speeds whereas a diesel is more suited.
The worst effect is at about 30 degrees off the bow depending on the boat speed and the true wind speed and angle. High speed is required from the port to the fishing grounds to save time and both engines are in operation.
Long, narrow spaces in the hull are not economical use of space and the ship cost rises with length anyway. If you run at planing speeds 99% of the time the decision is obvious but what if you run at that for only 60% or even only 40% then what? The process that brings us to this point is long, painstaking, and involves a mix of scientific analysis and black art (also known as gut instinct born of experience).
As an example, we know quite precisely the area of twelve and eight mm hull plate, and head and hull liner panels as well as their construction weights. Towards this end the FPB 97 has a touch more volume up high in her forward sections than is the case with her smaller siblings. At the same time, there needs to be sufficient lateral plane, in particular aft, to prevent the hull from squatting as it increases speed. In general, the speed increase of about 10 to 15 percent can be expected from a stepped hull over a non-stepped hull with the same power train. Some runabout builders even carve out a little scoop at the chime amidships, which I suppose is meant to suggest that the bottom is stepped, when in fact the bottom is as straight as an arrow.
Larger hulls may acquire a tendency to bump into the next wave, making the ride very uncomfortable. I have touched on this a bit in my previous article of Leisure boating explaining the different modes of planing.
In extreme reverse chine design, one could almost say that the hull for is a cathedral hull (see previous article of leisure boating). After a very short time you should now have her under complete control and feel comfortable at the controls. The last US battleship with reciprocating steam engines, USS Texas, had 28,000 shp, the machinery and boilers weighed 2,350 tons.
It is rarely exceeded by ships except mainly by fast warships that top 1.5 or more at flank speed or Government large patrol boats around 200 feet that often exceed 20 knots. I am not aware of any other meaning of the term hull speed used by professional naval architects.
The Whitehead torpedo was recent and could sink a battleship, so the tiny Torpedo Boat suddenly turned naval tactics on its head with this new-fangled weapon. On the grounds the boat will slowly search for fish using the small engine and will switch to the bigger engine when trawling. There will be some variables from detailed engineering, but these are allowed for via various safety factors. This leads to a locked-in bow heading down the waves, and that can turn into a broach and capsize situation.
Where this extra buoyancy would create pitching in the FPB 64, the 97 is large enough that the smaller waves will not feel this added volume in the upper reaches of the topsides.
But as you add beam, you quicken the roll period, and that is both uncomfortable and possibly dangerous. Normally this is not a problem since the pressure is very low, but it’s extremely important that the air supply is not cut. When it breaks through the hump to a true plane (thanks to hydrodynamic forces), its speed increases and trim levels out. If you do manage to turn too tight and Flash ends up upside down then a blip of power will have her upright and ready to go. Strictly speaking, the power should be reduced by the effect of the boat moving at its own speed without any wind but this will be very small in proportion and can be ignored unless the boat speed is high compared to the wind. Other items like batteries, engines, deck winches, window glass, and even light fixtures find their way into the calculations. Not fun to contemplate, so the prudent tactic as the waves build is to slow down, which increases your exposure time, or turn around and head into the waves. Four diesel engines each 40 tons, two gearboxes about 15 tons each, two shafts and propellers just under 200 tons and say add 30% for miscellaneous auxiliaries etc giving a grand total of about 500 tons. Jogging uphill balances a reduction in broaching risk against being caught by an oncoming storm.
The FPB handles this with slight change in the aft lateral plane and volumetric distribution. This may be achieved most simply by extending the step sideways to the open air at the hull’s side. The same happened on small ships and boats and now an outboard hanging over the stern replaces the old huge engine and boiler as well as coal or wood bunkers.
For now, let us just say that we have an excellent stability curve both upright at anchor and underway, and positive stability through two and a half times the angle any conventional motor vessel can achieve. At very high speed some designers have chosen to take advantage of the aerodynamics of the above-water part of the hull, using wing-like devices to keep the hull upright.

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