Sailboats, tugboats, barges, ocean liners, submarines are all different boats with different shapes. The goal of this project is to investigate how the shape of a boat hull affects the drag force on the boat as it moves through the water.
Have you ever been to a busy marina and looked at the shapes of the different types of boats moored there?
Sailboat hulls tend to be long and narrow, with a deep keel to keep the boat from tipping when sailing in a strong wind. From your background research, which hull design do you think will produce the least amount of drag?
For each hull design, how do you think the drag force will change as the speed of the boat increases? Advanced students may want to experiment with hull design software to test hypotheses before making physical models. We also do our best to make sure that any listed supplier provides prompt, courteous service.
Proceeds from the affiliate programs help support Science Buddies, a 501(c)(3) public charity. Do your background research so that you are knowledgeable about the terms, concepts, and questions. Close off both ends of the rain gutter, but leave a small gap at one end for water outflow. Adjust the flow rate of the hose so that the rain gutter is nearly full, with a steady flow of water.
To measure the drag produced by each hull design, place the hull in the rain gutter with the bow end facing towards the hose end.
The Ask an Expert Forum is intended to be a place where students can go to find answers to science questions that they have been unable to find using other resources. Water covers more than 70 percent of Earth's surface, and marine architects design vessels that allow humans and their cargo to cross through or under those waters safely and efficiently. Ship and boat captains have the important job of commanding ships and boats through domestic and deep-sea waterways, so that passengers and cargo arrive safely. Compared to a typical science class, please tell us how much you learned doing this project. You may print and distribute up to 200 copies of this document annually, at no charge, for personal and classroom educational use. Reproduction of material from this website without written permission is strictly prohibited. Location: barbados stepped hull cat Perhaps someone can help me with this question. As for steps, there are powercats with stepped running surfaces using both symmetrical and asymmetrical hull shapes. When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Location: Sweden We studied pads on a 4 m asymmetric cat with similar proportions to your hull.
Course instability in planing, asymmetric catamarans often occur due to too low trim (too much lift aft) and absence of even the slightest keel or fin lateral area aft.
Location: Finland Here's cf on Sysser 50 at the centerline, botom is 0 and top 0,0051 on the scale.
Originally Posted by Mikko Brummer Here's cf on Sysser 50 at the centerline, botom is 0 and top 0,0051 on the scale.
In the diagram for Cf at the centerline it also looks as if the positive direction is downwards. It starts with the stagnation point at the bow (highest Cf), then there is a short part of laminar flow (low Cf), the transition to turbulent flow (a steep rise in Cf) and a gradual decrease of Cf like on the flat plate. Here's still another plot, with cf integrated (custom field1) over points in the bottom, and shown as a graf. Originally Posted by Mikko Brummer What is your velocity distribution along the equivalent body of revolution like?
Mikko, even at the risk of making you angry, I still think that there is an error in your software.
The classic similarity rules for such a distribution are the law of the wall and the law of the wake. Originally Posted by Mikko Brummer I did a basic RANS simulation without free surf effects on the Sysser 26 model (I assume it is pretty similar to the Sysser 24?). At an ambient turbulence of 0,5% the RANS code predicts much more laminar flow, and the average Cf is then 0,0034 (the second pic). Correction to above: The ambient turbulence at the inlet is 1%, at the model bow it's about 0,7% and 0,35%. The reason I ask is your skin friction plots look a bit streaky and I wonder if this could be a reason.
Originally Posted by tspeer Is this a volume-of-fluid approach, or a multiphase Eulerian solver?
Location: Finland But there is definitely an issue with the Cf plotting as Uli mentions. Location: Germany Wavemaking length I added the influence of the transom overhang to the regression and discovered a phenomenon that I can not explain. My first idea was, that it could be caused by the hollow behind the submerged transom, which would create a fictitious increased waterline. Location: Adelaide, South Australia I'm not that surprised by the worse approximation. I?m a designstudent workin on my degree doing a future vision of how we can collect algea in the Baltic Sea. I?m working on my concept now and i need some help or guidance regarding good forms to minimize waves from the machine. How can the end of the "wings" keep the flow straight back instead of pushing water towards the machine or away from it?
Second, the collectors--you may have to do some testing in a pool or towing tank to come up with a suitable design or configuration to change the tip shape.
Say the water flow off the tip wants to diverge outward from the track--then the spoiler would be set with a slight angle inward (toe-in) to turn the flow back in line with the track. On the other hand, if the flow off the tip of the collector is converging toward the track centerline, then the tip spoiler would be set to an angle of attack outward (toe-out) so that the downwash diverts the flow away from track centerline back to parallel with the track. We don't know which way flow is going to divert on your current design, and that is why you may have to do some pool tests. The above spoiler idea is just one of many that may be offered by others, but this is the most obvious one that comes to mind. Originally Posted by Eric Sponberg We don't know which way flow is going to divert on your current design, and that is why you may have to do some pool tests.
There are two reasons which make me believe so:The highly non-streamlined cross-section shape of the scoop will create a large area of separated flow along the whole rear part of it. Therefore, if you suck a mass of water M in front of the scoop and along its whole length, you have to expel the same mass M behind the scoop and along its whole length, like in the attached pic. Originally Posted by daiquiri I think we actually can make a fairly accurate guess about the flow direction by looking at the shape of the scoop. The lines in blue on the attached image in this post shows how the water behaves with the current design. Location: Hampshire UK Is there a reason for the long booms other than to get width? 150.17 A length of 10 m or so will be needed to minimize wave formation, and a longer length will be better.
Also, minimizing the weight of the body and therefore the displaced volume (Archimedes) will also reduce wave formation.
Originally Posted by FredrikA The lines in blue on the attached image in this post shows how the water behaves with the current design. Sensors on the leading edge of the end panel can provide feedback to the flap actuators so as to maintain the stagnation point at the leading edge of the end panel. As for waves, I'd be more concerned with the waves in which the craft has to operate than waves generated by its motion through the water. Another thing you might want to revisit is having the strainer at right angles to the flow.
When you asked about waves earlier, I think people literally thought in terms of surface waves.
Location: Italy (Garda Lake) and Croatia (Istria) IMO Tom, and with all due respect, the flap-system you are proposing is a technical overkill for this kind of boat.
The point about angled strainer with angled vanes is indeed a good one, as well as the point about insufficient inflow - which can be regulated with a variable-rpm pump. Originally Posted by daiquiri IMO Tom, and with all due respect, the flap-system you are proposing is a technical overkill for this kind of boat. I really like the idea of water coming out from the wings since youre also saying that a jetengine wont give that much of thrust at 4knots.
Algae have a negative effect in the baltic sea with to much nutrient in the water and by haversting some of the algea that are full of nutrients, the amount of nutrients in the ocean should be reduced.
Maybe i could also test the different designs and hydrodynamics in a 3d software if there is one for free? In post #63 I asked about the conditions necessary for the theoretical result of a symmetric hull having minimum wave drag. Maybe a large, bold letter warning is needed for threads like this letting folks know that not all the theory discussed is 100% directly transferable to designing boats. As I noted right way way back, the reason being the key to understanding all this, as simple as it sounds (because it is), is the length displacement ratio effects with residuary resistance. As has been noted by several here, an ?optimal? hull is often for reasons other than minimising residuary resistance.
In other words looking at one or two parameters , LCB and LCF in isolation to determine if there is a link between these two just misses the whole point of hull design. If you want to read up more on shapes of waterplanes or correctly waterlines, may I suggest you look at the boat called ?Wave? designed by J.Scott Russel in 1834. It is not always so easy to mathematically define a ?compromise?, as one finds in hull design. Location: Italy (Garda Lake) and Croatia (Istria) I have to ask a question here. Location: Australia Obviously you would have to change the lines, but you would be able to do it without changing some of the common measurements.


Location: Italy (Garda Lake) and Croatia (Istria) But would you be able to do it without altering, for example, Cp and at the same time without altering the nature and purpose of that transom-stern hull?
LCB is only one parameter to be considered in making vessel design decisions and tradeoffs, though as previously pointed out it is frequently a major one. There have been numerous plots published by a variety of authors showing resistance vs LCB, LCB for lowest drag as a function of Fn, etc with recommendations for design. I've been trying to figure out a polite way to imply futility; but so far not having any luck. Suppose you have a computer and it executes a code and this inspires you to wonder about the inner nature of the CPUs at the heart of it all.
If you combed through all the hull studies done since the beginning, you wouldn't be able to assemble a set that meets the analytic criteria that is even 1 percent big enough. Originally Posted by NoEyeDeer Ok, so it appears that sinkage and trim are the answers if we are talking about thinnish hulls.
Location: Australia Why do you have to go and confuse things with damnable facts? Now obviously this does not change only the LCB, but it gets presented that way when talking about it.
So, in that sort of situation, what factor or combination of factors is causing the residuary resistance to change?
Perhaps a somewhat analagous situation is the resistance of hulls with wetted transoms and how it changes as a function of vessel speed. I have an interest in understanding the hydrodynamics of boats which goes beyond that which has immediate, direct application to design. I think there is still an open question of why Leo's kayak models showed an opposite trend for drag with a shift of volume resulting in a 9% LCB change compared to the published curves for small changes of LCB.
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. It's not as smart as you are, and it may occasionally give humorous, ridiculous, or even annoying results!
If you have specific questions about your science fair project or science fair, our team of volunteer scientists can help. Some of their watercraft designs are enormous, like merchant ships, which carry huge loads of oil, cars, food, clothing, toys, and other goods, across thousands of miles of open waters. To do this, they need knowledge of the mechanical and electrical workings of ships, navigation, signaling, national and international legal rules in waterways, as well as strong leadership skills, since they supervise the work of all other crew members. It's my undestanding that the dry smooth ride of powered cats can be attributed to the cushion of air trapped between the hulls. It tends to go to zero at the end (as in my diagram), but then comes the stern wave (which I don't have in my simulation) and causes a rather high Cf on the transom overhang. The curve is not flipped, if you compare with the surface plot they correspond to each other. In other words you do not resolve the velocities within the boundary-layer, vertical to the wall on a dense gid, instead you assume a velocity distribution, typical for turbulent flow.
I do agree with you, this is odd, the Sysser 50 LES sim would have the cf values flipped also compared to the Sysser 26 RANSE plot (which again has that waterline shaped oddity at the back). It would be exiting if these were really physical, but I think they are numerical (or graphical), maybe simply due to fairly coarse resolution and projecting to a surface close to horisontal. It is a well known problem with VOF, with different correction approaches with different solvers.
One would assume that the actual wetted length, that includes the influence of the stern- and bow-wave at the given speed, would give a better approximation of the wavemaking length. So I extrapolated the section area curve beyond the transom to calculate the wavemaking length that includes the hollow behind the transom. The closer the hulls are to having identical shape and differing only in scale, the closer the wavemaking resistance vs Froude number curves will be.
The inwards-bent duct at the tips of the scoop was designed to turn the flow towards the inlet at the main body.
The first drawing had two propellers in the rear but i?ve choosen this other idea now so sorry if i mislead you. A center body Length to Beam ratio of 5 or more would be good and a ratio of 10 or more would be even better.
Too many submerged moving parts, sensors and automation for what is essentially a workboat.
You originally wanted to know about LCB because some empirical testing shows that not having it in the middle is better in some instances. Another way to express it would be hulls with an area curve which has a step at the stern vs those which have an area curve that goes to zero continuously and smoothly at the stern. Just because there is a link does not automatically suggest that it has influence despite the fact the link can me mathematically expressed. It's about a more specific topic, a question about hydrodynamics and the effect of particular hull characteristics on resistance. Considering everything I said in the previous paragraph these curves and recommendations may or may not be relevant for a particular design.
Go though the process of designing the study which would satisfy your inquiry and you will see what I mean. This means LCB moves aft to, say, 54% from the bow but displacement, prismatic, and several other things don't change. If it is not small, it may still be acceptable to include it, but perhaps then there might be other second-order eff ects which should also be included.
I was hoping someone might have ideas about the physics of what's going on, or at least know of some other information (such as how the near-field waves change as hull volume is shifted) which might shed a glimmer of light on the subject. By looking at the flow past transoms with different static immersion depths and systematic experiments a body of knowledge has been assembled which says that a Froude number based on static immersion depth appears to be a good parameter to use rather than the usual Froude number based on static waterline length or similar, and there are several different flow regimes which can be associated with different values of the Froude number based on immersed depth.
But the mention of Thin Ship theory in the first post may have caused some folks to think this was about thin ships. 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. This project shows you how you can investigate this question using a homemade water trough and model boat hulls. Our Experts won't do the work for you, but they will make suggestions, offer guidance, and help you troubleshoot. I've seen pictures of racing cats with steps vented to the outboard side of a single hull (asymmetrical hulls) but I don't think I've seen a symmetrical deep vee cat with steps. The reason we found was that the transverse flow into the tunnel increased, resulting in an unstable wetting of the aft tunnel sides. You should start reading the curve at about the bow knuckle, the part before that is in the bow above the air. I do think it's a postprocessing problem only, as the results for Sysser 50 compare so well with experiment, even more favorably than the validations presented in the Chalmers Sailing Yacht Transom Sterns-paper, on Shipflow.
Since this is particle based, Lattice Bolzmann, not RANS, the source of the streaking is probably not the same as in Numeca's case. Cf seems to be tricky, when I run the RANS sim for Sysser 26 again with a higher resolution, the Cf plot again looks different :-(. The three hulls with variations in rear overhang have different shapes in contact with the water when underway. The front of the machine will suck algea and water into the machine then filter befor letting the water out again. The mass of water ingested by the scoop and sent towards the central body will be a missing water mass in the longitudinal direction behind the scoop. It would be awesome to let the water out from the wings after the separation as you showed but then i cant use it as thrust? I?m super happy to get this much great feedback on the project and all of you have really great thoughts on how i can do this.
As i discussed this with marine biologist they told me that we want the outlet of water to be as far away as possible from the algae. Of course these are important considerations if designing a hull but that's not the topic here.
Based on some very limited information, the apparent contradiction between a simplified theory and experimental data it seems trim might help with the understanding.
Rules for predicting the associated drag are less well developed, and there are also very open questions about how to model the flow past an immersed transom, at least with less than full RANS model. 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. Is this because the venting of the step (inboard) will adversely affect the air cushion that the boat is running on? The boat yawed and heeled slightly until the flow along one tunnel side or the other was stable. Here cf is steadily increasing until about the max canoe body depth and then slightly decreasing towards the transom edge. The colours on the topsides above WL are in the air, they are nonsense, the free surface part is still a work in progress.
So it is not a surprise that the wavemaking resistance vs Froude number curves have different shapes.
Therefore i dont want the algeas on the edges of the machine to move in the water so its still there for the next section of harvesting. This missing mass will have to be replaced by the surrounding water, which will hence tend to rush towards the centerline of the vessel immediately behind the scoop. It appears that, even for thinnish hulls, sinkage and trim have to be included in calculations. Save for the Crude hull, which is interesting in itself, a square block of wood, with a pointy end, one would consider to be very very draggy, yet evidence is contrary to intuition.
But, and here is thing about hull design?which also relates to my question before about have you designed a hull before. 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). The surface plot is shaded by (the approximate) surface level so what you see is really under water.
Since basic thin ship theory doesn't include them, a more complex model is required if what's wanted is an explanation of why having the LCB aft of midhsips might be better at times.
But to change the LCF and keep the LCB constant results in a most peculiar (un-ship shape) variants, or one ends up using surface piercing struts only, just like a SWATH. Since what was later found (after advances in hydrodynamics) that cutting down the angle of entrance and hollowing the waterline fwd resulted in an increase of the waterline slope in a way of and aft of that point. You run it at (pulling an example out of nowhere) Fn=0.4 and get residuary resistance of X. 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.
There is a missing mass of water behind the scoop which has to be replaced by the surrounding water? This had the negative effect of resulting in greater curvature at the fwd point to bring the ?slope? back to zero at or near midships.
I'm somewhat skeptical of using the published resistance change vs LCB location charts developed for vessels without immersed transoms for vessels with transoms. Why is there a Cf-value above the waterline and why are the colors dark red, the highest values?
This may be achieved most simply by extending the step sideways to the open air at the hull’s side. You can achieve it by expelling the ingested and filtered water right where we have taken it from the ambient - along the rear part of the scoop. You can achieve it by expelling the injected and filtered water right where we have taken it from the ambient - along the rear part of the scoop.
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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