What makes a sailboat go fast?

hello people.
in an effort to help the young designers out ther. i thought a post like this one would help. What is more important to a design? we all know sail area provides horsepower. but we also know that with too much sail we heel over, SO we put a fin and bulb on. this how ever causes drag? so we give one for another ie. more sail= more drag. if we build a wide beam boat. the boat sits on top of the water and does not have that much hull restance but the boat slides sideway. if we build a narrow hull the boat sits in the water and does not slide?
so we draw from this, a hull sitting deep in the water causes restance but allows forward motion.
so after all this. we come to wieght. a light boat accelerates fast. but i find it also losses speed fast(tacking) where as a heavier boat keep the speed up and interia takes over and we swing tough the tacks?
basicly what do we trade off? less sail area and less fin lenght? we go to a wide beam boat and a heavy hull?
jump in and say your piece. i am no expert but i am intrested to see what you all say.

long live the cup and cris dickson

I’ve given this a lot of thought and I think one big point is symmetry. If you have a hull,a keel,a rudder and a bulb that are perfectly symmetrical and have them set up so as to go through the water straight with no input from you then I beleive you will have way less drag than the guy whos boat is not perfect. If anything is out of line-even a hair-you will be doing something to correct it and that causes drag. I know perfection is rather tough to attain but the farther from it you are the slower you will go. Thats why CNC milled everything is so popular. Stuff that is straight is fast. I can’t afford a CNC mill or my stuff would be fast too. I wouldn’t-I would still be a klutz-but my boats would be as good as they could be. I love making everything myself but have realized that I will never produce a fin as fast as Sails Etc. Thats why their stuff sells. You asked and thats what I think.

Vancouver Island

hey cougar…I just have to …but we all know that i dont mean it:

now to respond to your question…well easy…a non Canadian skipper LOL

Cheers from the old continent


_/ if it isn’t broken, don’t fix it! _


one part good hull,one part good skipper,one part good sails,well trimmed and whatever’s below the waterline


The first thing that you need to know is that sailboats do not go fast. :slight_smile:

Now if you want to know what makes the fastests sailboats go, then you are taking about some pretty exotic boats designed to defeat the laws of sailing that you laid out. Sail Rocket for example has positive stability. Yellow Pages Endeavor uses extreme beam to gain righting moment with minimal weight.

Windsurfers and kiteboarders use dynamic balancing of the ballast (the sailor) with the driving force which are fundamentally decoupled from heeling to achieve very high speeds.

Working toward the mainstream, you have Multihulls which gain righting moment from beam for a mimimal drag deficit. To increase the speed of a multihull, foils would be in order. But not looking for a fight, I will simply acknowledge that foils have some limitations. While they produce a faster boat in conditions where they can be used, they can snag weeds (as do traditional keels and rudders) and can increase drag if they are not retractable in extremely light wind.

If you are interested only in heavy ballasted monohulls, then I think you have aknowledged in your opening post that the keel and weight slows you down. You need to carry more sail area to overcome that. You can work that equation in your favor with canting ballast where you get more sail carrying righting moment for less weight, but you pay the price of needing extra appendages if you plan to sail upwind.

If you are only interested in recognized classes of heavy ballasted monohulls as most of us are, then you must pretty much throw out everything I just said because they are all considered illegal. You are now left with the minding the details. Balancing ballast and sail area withing the constraints of the rules usually pushes you to a corner of the envelope. From there, you are talking about fine tuning the hull shape, fine tuning the balance and fine tuning the sail shape, trim, etc.

I just looked on my bookshelf here at the office and I have 6 books on these subjects - not the least of which is the 730 tome by C. A. Marchaj “The Aerohydrodynamics of sailing”. If I were to summarize that book in one post on this forum, I would do both you and Marchaj a disservice. At home I have perhaps a dozen more. In fact the vast majority of the content of this forum (at least until recently) has been basically trying to answer this very question. If it were possible to answer that question in a simple answer, then the entire naval archetecture industy would go belly up and the americas cup would cease to exist.

So, there really is no way to answer your question.

  • Will

Eidted to be more clear on what I meant to say about foils.

Will Gorgen

Light as possible, big sail area, large enogh righting moment to hold down the sail area. But, it must also be controllable! Ok, thats the very very basics, you can go on for ever.

Cougar; If you want to make a fast mono, ditch the fixed keel for starters! You can then build a lighter boat, whic will naturally be faster (better sail area to weight ratio, similar to the power to weight ratio of a car say) also you can make a narrower hull, that ils less impeded by wave making drag, or a wider flatter planeing skiff.

Luff 'em & leave 'em.

ok gang this has started off good
most of us here race monohull boats. so you are right in saying i was refering to monhull. what i am gathering here is that balance is the most important thing. we cant shorten our keel because of rules. BUt we can lighten our boats to a degree. and even play with sail area. maybe a bigger jib? matt pointed out wave drag. that is another thing we have to consider. does a narrow boat have less form drag and a skiff have less quaterwave?. we are designing boats and maybe we could help each other with what we have found, i like narrow beam boats, because it seems to me that a hull in the water tracks better than one sitting on it. i dont know why a ts 2 is so fast yet it sits on top of the water. so maybe i am wrong. i think will is right everything we do is a trade off. but what we are trying to do is get a better trade. maybe 1/10 knt faster. but where? manuverabiliy?( just got up sorry about the spelling) striaght line speed? acceleration?.
long live the cup and cris dickson

Be careful about the generalizations you make here Cougar. what class rules are you talking about that don’t allow you to shorten the keel? Many classes are one design but since you are talking about making changes to the hull shape, I’m guessing those classes are out.

In the US1M class, you can shorten your keel all you want - there is only a maximum depth in the rules, not a minimum. In fact, Hal Robinson has a short keel for his latest US1M “patriot” that he uses in light wind to reduce the wetted surface area in conditins where the need for righting moment is not so great.

Sometimes the tradeoffs are not as obvious as it seems. Look at that new boat that Todd Brown just finished. It is very narrow. Todd was looking for a light wind advantage, but is finding that it works well in stronger winds as well. I’m not sure that everyone is convinced that the TS2-style skiffs are the way to go. There was some discussion a while back that indicated that Graham Bantok does not believe in skiffs. I beleive he managed to win the US national champs this year with a non skiff boat. Was it last year’s IOM worlds where a triple crown placed very high.

I think if you are looking for ways to improve, you have to focus on the details. It seems like the majority of the big changes have already been tried and most of the designs today have settled on a good all around hull shape. But most boats are not sailed up to their full potential because the sail shape is not quite right or the boat is out of balance more than it needs to be. So if you work with what you’ve got, and figure out how to sail it better you will be able to get more speed improvement than any change to your hull shape. Keep in mind that even if you did get 1/10th of a knot more speed (which is a lot more than you think) if you make one bad tack, your advantage is blown!

  • Will

Will Gorgen

lol will
you are so right. maybe this post was to general. i design IOM so the fin is bacicly one lenght. but what i was trying to get at was what do you all think about the hull designs. you are absolutly right is saying. 1 bad tack takes away the 1/10 of a knt. but if we sail perfectly we could still end up middle of the pack. i would love to see todd browns design. i like the idea of a narrow hull like bantock. but i see the advantages that the skiff design does. what do others think. talk it up. and see what poeple think. is ikon or a ts2 the best design? . i am just promting a dicussion over designs and ideas. is there any realtionship to sailarea over overall boat wieght?
long live the cup and cris dickson

<blockquote id=“quote”><font size=“1” face=“Verdana, Arial, Helvetica” id=“quote”>quote:<hr height=“1” noshade id=“quote”>Originally posted by cougar

what i was trying to get at was what do you all think about the hull designs<hr height=“1” noshade id=“quote”></blockquote id=“quote”></font id=“quote”>
Hi Cougar

Stepping back a loooong way, what makes a boat fast (or more likely, slow) is the point at which its drag is exactly balanced by its drive. So reduce the drag and you have a faster boat. Easy, really… !

Zooming in a little, there is a fundamental difference between sailing downwind (on the run), and sailing upwind (close hauled), and different things affect drag and drive. (Different things happen again reaching, but I’ll ignore 'em here to keep it simple.)

Downwind, the bigest deal is whether your design can or will plane. The A class is traditionally considered a “pure” displacement boat, but the modern examples plane easier than an IOM. Astonishing sights at the UK A class champs last weekend, 40 lb, 7’ boats planing in winds of around 25 mph and reaching what looked like 20 mph in the process. To get the boat to plane, you need a hull that will lift up and stay controllable while carrying sufficient sail area to do it.

Upwind, two issues kick in. One is optimising the hull shape to minimise drag while beating. This hasn’t yet been cracked as far as I know. No one actually knows how to distribute the hull volume along the hull’s length so as to minimise drag for specified conditions. All that is known is that there are a bunch of parameters, and you get to vary them in a trial and error way until you stumble over something that makes you happy.

The other is the little detail of VMG, and the fact that the first boat to the windward mark could well be the slowest that just happens to point very well. Your ability to point depends upon your leeway and your “drag angle”. The drag angle isn’t too much talked about, but is pretty fundamental. It is the ratio of your rig’s lift to its drag, so anything to make your rig more efficient is a good thing.

Lester Gilbert

Hi Lester -

I had this into an email format to you, and then decided to post it here, as others may also have thoughts. Being I am not a naval architect, I have found the thoughts about fast sailing rather enjoyable to muddle-about.

With my background in small dinghies (Laser and Force 5) as well as multihulls, I would like to set a bit of basics and then pose my question.

Both of the dinghy classes mentioned, and also boardless (beach) catamarans offer conflicting designs, yet still allow for a fast ride (sail).

The dinghies use centerboard, and crew weight to off-set the sail area/mast height. Both also use a flat planing hull of a very low angle “V” bottom and small vertical sides.

The multihull, uses a quite deep “V” hull shape, but is boardless, and the hull is asymmetrical - the inside of the hulls being curved, and the outside slab and vertical which along with the rudders creates the lateral resistance. Beach cats without boards sit pretty deep in the water compared to the circular sections below the waterline for daggerboard equipped boats.

V-bottom planing hull type boats may not need the weight of a keel to provide resistance to heel, as the flat bottom surface provides a very high amount of initial stability to heel. When the boat does heel going to windward, basically 1/2 of the hull surface is creating drag (leeward side of keel) and perhaps a much smaller portion on the windward side of the keel. At the same time, the flat, slab sides are creating lateral resistance along with the daggerboard/centerboard, and the moveable ballast of the crew is proviiding resistance to further heeling of the hull.

Why then would a boat of similar size, with a keel and much heavier weight in the bulb, but a displacement hull, be (in general) faster around the course that a planning, V-bottom, flat hull?

With the flat bottom, and less displacement compared to a circular displacement type hull, it seems contradictory. It would seem, that a flat bottom (V-bottom) boat, having hard chines, should require less keel bulb weight, be much less “tender” and sits atop the water, and theory be faster than a hull that weighs more, sits deeper in the water and is very susceptible to heeling - even in light winds.

With the current design of “skiff type” hulls in the IOM class, there is added resistance to heeling. If one were to continue to the next logical step, having a flat plaining hull with sharp chines to add initial stability would be the logical step, allowing for reduced weight in the bulb, a stiffer hull, and the added possibillity of getting downwind faster than a displacement hull.

Somewhere I am missing something - but I’m not sure what it is?


Long post warning…

I guess we?re talking about models here. So in the context of monohull models, the first problem is that we don?t have the benefit of moveable crew weight to use as ballast. Although you are absolutely correct that the beam and hull shape contribute to stability, there are two key factors (and a third perhaps less obvious) that prevent us designing sufficient stability into the hull of a monohull model to compensate adequately for the removal of a ballasted keel. The first is that the improved stability obtained from beam, a flat hull and hard turn at the bilge is ?initial? stability only. It resists the healing moment when the boat is upright, but that resistance decreases as the heel increases. We’d have to be quick at easing the sails, or they’d capsize in the gusts and would not be self-righting.

The stability curve can be plotted, and if you haven?t seen it, you?ll enjoy reading the report that came out of the Fastnet disaster of 1979, which showed some interesting comparisons between the leading boats of the time (which, under the IOR Rule, favored wide beam, light displacement and ballast high up in the keel), and the more traditional (for the time) designs. The light displacement racers showed higher initial stability than the traditional heavier displacement designs, but reached an angle of heel where stability was lost quite suddenly. The center of gravity was quite high in these boats, as it is in a dinghy that uses a centerboard to provide lateral resistance, but crew weight to provide ballast. The greater initial stability was a result of their beam and hull form.

Which brings us to point number two, - they used the wide beam, and the high center of gravity, because the rule they were designed under encouraged it. Beam is slow (according to the IOR rule) and less stability is not good, so the designers were rewarded in other ways, by being allowed (for example) more length or more sail area if they designed in some of these ?slow? features. But if we take a boat like the IOM or the US One Meter, our key performance factors of length and sail area are fixed (or at least the maximums are). With the IOM, no matter what concept of boat we go for, it?s must weigh in at 4kg or more. That?s actually quite heavy for a one meter long boat. It?s not really practical to design a flat-bottomed planning hull within the parameters of the rule. The ?skiff? designs amongst the IOMs are not really planning hulls, but their wider beam allows them a shallower hull for the same displacement. So they are easier to get up on top of the water than a narrow IOM in those situations where they have enough power ? i.e. downwind in a strong breeze. The higher drag of these hulls is evident in the light airs downwind stuff. It?s horses for courses.

When you sailed the laser (the dinghy ? not the RC Laser) you will have worked hard to keep the boat flat when sailing to windward. Lightweight racing dinghies, and light displacement performance keelboats, sail as upright as possible on the wind. Excessive heel gives the boats a very different underwater profile from that they have when upright. I?ll never forget my first experience as a kid onboard a trapeze dingy planning to windward ? I still love sailing trapeze dinghies almost 30 years on. But to get the boats to plane on the wind we need power (hence the trapezes to compensate for the high sail areas) and we need to keep the boats upright so that the planning hull retains the correct underwater shape. To do this without moveable ballast (a careful choice of words), we?d need to go very wide indeed. The boats would end up as barges with a very large wetted surface relative to the power available from the sails. They?d sail like barges too.

As the IOM skiff designs rely on ballasted keels, AND they are sailed at quite an angle of heel on the wind, the designers have put considerable thought into the underwater shape of the hulls at heels angles applicable to windward sailing. Hence the dishy shape of the hulls in the aft sections. If I recall, Lester Gilbert has a great article about this on his website.

Finally, the effects of scale come into play. The smaller the boat, the less inherent stability it will have. Larger boats can achieve a greater length:beam ratio for given inherent stability than smaller boats. Scale comes in with fin area two ? but so does the speed at which a boat is designed to sail. To put it another way, the speed at which a foil is designed to move through the water, or the air. That?s why fast jets have a much smaller (relative) wing area than slow flying aircraft. We use the lift to resist leeway. More speed equals more lift from a given foil, area and angle of attack.

So we can design a fast, flat-bottomed planning hull that will burn the pants off a ballasted keel hull of similar length so long as we have the power to drive it and the ability to keep the correct underwater shape upwind. Assuming we want the boat to sail on all points, then to do that we need moveable ballast. Enter the canting keels. Other ideas seen over the years (and in some cases still very much in evidence) are water ballast, and rigs that ?heel?. Add a channel or two on an RC model and we could have a ballasted model crewmember on a sliding seat (or something similar).

With the surf cats you mentioned, what you really have is two heavy displacement hulls with a high ratio of length to beam. Narrow is fast ? wide is slow. The lack of a centerboard is possible not only because the leeward hull has a sizeable area underwater to provide lateral resistance, but also because these hulls are moving fast. (More speed ? more lift again.) That?s also why fast catamaran sailors have prehensile toes (like the tails on certain species of monkey) ? so they can hang on while trapezing at 25 knots.

By ?heavy? displacement I mean that the hulls are designed to sail deep in the water, not that the hulls themselves are heavy. When a fast cat is flying the windward hull, the entire weight of the boat is carried by the leeward hull in the water ? double it?s upright displacement.

OK ? many simplifications and omissions here, but in a nutshell, this is what we?ve got. Beam gives stability but increases drag, and planning sailboats are sailed upright through control of power in the rig and via moveable ballast. Of course with water ballast, weight can be reduced when the power is not required ? i.e. off the wind.

I haven?t talked about the differences in the drag, and the change in importance of the various factors contributing to drag, that occur when a boat gets up on the plane. The old, hard chine, planning dinghies I used to sail, such as the Cherub (in NZ, Aussie and the UK), where quick on the plane, but I suspect that a good round bilge design of the same size and power would have beaten them at displacement speeds. The sharp edges of the hull and turns at the chine create more drag than a design without chines. But that?s another story?

So its less about what makes a boat fast, and more about what makes a boat slow. Rules make boats slow ? and the need to sail both upwind and down. So the trick is to design a boat that is least slow under a given rule. Ah, there?s the challenge, and the fun, of rules like the IOM.

What the hell - you knew this already, but I’m sure there’s something here to attract other opinions.


Putting them in your car?

19 out of 7 people have trouble with statistics

I always wonder why I don’t see a class of boat that’s got a skiff-style planing hull and retractable centerboard. Something like a Lightning class sailboat.

Any particular reason that I don’t hear much about this style of boat? Is there something wrong with the performance of a smaller RC model-sized sailboat?

I would think that this type of boat would be very exciting and fast to sail with lots of room for experimentation.

Hmm, anyone care to comment on the retractable centerboard idea for a model “planing hull” sailboat? It seems like a solid idea for making a boat go pretty fast, no?

i think that the only way a retractable centerboard would work. is if the hull could accept the lenght of the fin. i am not sure how long you would expect. but i will comment on IOM. that is what i design.
my fin is 3 inch wide and 12 inch deep. th eproblem i have is where am i going to put it? i can fold it back but the i have a problem with the bulb? so the other option would be bring it up into the hull. but that meens the fin come into play with the boom? again not a good idea. i agree with your idea of getting rid of the underwater restance. but i cant seem to find a way around it.
what goes right with the big boat sometimes does not go right with the small boats. IE the laser. it looks almost idenatcal with the real thing. but the real thing does not have a big dagger board. we use a human for ballast. the model has to realy on the dagger board.
i was just wondering if andbody has an opiniion on how far forward the deepest part of the roccker pannel should be? i have been putting it around 50% of the boat. but aftre hearing some thing here? making the hull a lifting surface i was thinking of an airplane wing. the camber is at 30% so mabe the deepest part of the boat should be there?.
what do you think?
long live the cup and cris dickson

I think you misunderstand me. I’m not talking about an existing IOM or US1M style of boat, but rather a boat that is specifically designed for a thin, unweighted (no lead bulb) centerboard, very much like a skiff-style full-scale racing boat. A Lightning class boat is a good example.

So for a 1-meter sized boat, maybe a centerboard of around 6-8 inches would be suitable, of a shape that could be retracted into the hull below deck. I haven’t heard or read anything about this on any online forums, so I’m curious as to whether or not there’s a specific reason that this type of design wouldn’t be desirable. For someone who wants to have a fast boat, it would seem that this style of craft would be ideal.


How would you keep the boat from capsizing?

  • Will

Will Gorgen

Andrew -

in your example, the Lightning sailboats use a steel centerboard (as opposed to a lead bulb) but there was still a lot of weight there.

I started as part of the crew on one back in highschool days (many years ago).

A model could be built to use a solid steel centerboard with lever arm that would allow the board to be retracted into the trunk, and the keel/trunk would need to be properly scaled to provide sufficient lateral resistance. Retracing can be accomplished using a servo and a multiple part purchase to rotate the board into the trunk.

As Will points out, there were also three (3) moveable ballast crew to augment the steel board, so that is the area to which one would need to address a “how-to”. A larger/heavier board is a possible answer, but you also need to be able to raise the board a specific distance for off wind sailing. Something difficult to do from shore without a programmable Tx.

It certainly could be built, but performance and sailing issues would be questionable until one or two were sailing to see how they behave.

On the other hand, a vertical retracing keel (Hobie 33 as example) could be done, but you run into problems with the top of the board, sticking up above deck and being hit by the main boom. The Hobie uses a short keel with large bulb that extends up into the cabin, but not above the cabin roof. Again keel length and righting moment are the unknowns.


Seriously, I don’t know. It’s an open-ended question. I’m looking for some good reasons why this hasn’t been done before. That might be one good reason, but I wouldn’t say that there isn’t a solution to preventing it from capsizing somehow. Maybe moveable ballast has a good application here. Maybe a tilting mast arrangement to reduce heeling?

Also remember, while the centerboard doesn’t have a lead bulb on it, it can still have a good amount of weight if it’s made out of steel (possibly with some lead poured into it). But the simple fact that it can be retracted eliminates a considerable amount of drag and allows a better chance of planing downwind.

Believe me, I don’t have all the answers. In fact, I don’t have ANY of the answers. I’m just looking for some feedback about the idea. Maybe someone HAS tried this concept and found some sort of fatal flaw. And maybe that flaw can be worked around with some ingenuity, who knows.

But since this is a discussion forum, I don’t see why we should be hesitant to at least talk about it. Throw out some crazy ideas!