balanced boat

hello gang
just thought i would start a topic. maybe get some would be desigers involved. I keep hearing that a balanced boat is a must. NOW what does this meen? the boat must weigh as much forward and aft? or the sail area must equal the about of boat in the water? or it is something different?. I would lilke to hear from most. even if you have not designed a boat. give us your opinion

long live the cup and cris dickson

clr , c 0f e, c of b, cg, etc.

Hah, this is a good one, sailed on a homebuilt/desiged 30somthing footer today that had just had the mast moved forward 18 inches to stop weaterhelm. Boat used to have to be sailed with the helm about 10-15 degrees off center to keep her tracking straight, wich aint very fast as the rudder also acts as a break. Now you can hapilly let go of the tiller in a steady wind and provinding the sails are trimmed ok she will track allong hapily by herself, cruising along at 7 knots.
The problem before was that the center of effort of the rig was too far behind the centre of lateral risistance of the hull/keel, Therefor the sails were constantly trying to spin the boat up into the wind. a Balanced boat has the two more or less in line with one another.

Luff 'em & leave 'em.

Another form of balance is balance under varying degrees of heel. This is a characteristic of a hull, in which there is no pitch caused by heeling. Without balance, the hull will pitch up or down when it heels because the underwater shape changes in an adverse way. This swings the rig fore and aft, which changes the relationship between the center of effort and the center of lateral resistance, causing the boat to turn. The effect is a boat that wanders as it goes in and out of puffs.

In 1904 Bill Daniels of the UK figured out how to calculate the balance or lack of it in a hull, and produced the first deliberately balanced boat, a 10-rater called XPDNC. Almost all free sailing model designs are balanced in his way, which makes them a joy to sail under radio – they track like they were on rails.



As most model boat sailors know from experience, the forces acting upon the boat are dynamic - they are moving around pretty much all the time. Accordingly a boat may be balanced (by which I mean a state in which the boat can sail forward, with the rudder centered, and without futher input from the helm) for a few seconds or so, and then something changes which causes the boat to go out of balance. Perhaps the most obvious example is when a gust of wind causes the boat to heel further, and there is a sudden onset of weather helm, which leads to the boat rounding up towards the wind.

Some boats seem to have great difficulty staying in balance, whereas others seem to have a wide “zone” within which balance changes are minor and tolerable. On a full sized boat we can alter the balance on any point of sail and wind strength buy trimming the mainsail and headsail separately. On a dinghy, we can also change the balance by raising/lowering the centerboard. These trim adjustments move one or both of the CoE/CLR, with the objective of bringing them back into the same vertical line. But on most of our RC sailboats (at least those limited to two channels) we are stuck with the settings made before the boat leaves the dock, and from that point on limited to sheeting both sails via a single winch.

Thus in designing an RC sailboat, I believe considerable emphasis must be put on the effects of hull shape on balance. Lester has a good article on his website about this. What we are talking about is not the position of the CoE cf the CLR - but instead the effect on the balance of difference angles of heel - through changes in the underwater shape of the hull.

Imagine your boat, upright, on a windless pond, with the helm centered. If you could give it a nudge from behind, then hopefully it will move acros the water in a straight line. Now imagine the same windless environment, but through the magic of imagination, you have the power to set the heel of your boat to, say, 30 degrees. If you could now give your boat a nudge from behind, would it still move in a straight line, or would it describe an arc across the water? Pretty much every boat will move off in a curve. Even if the underwater sections remained exactly symetrical at heel, you still have the influence of a keel fin and bulb now canted to one side.

The tighter the arc, the bigger the impact on balance through the hull shape of the boat - even in the absence of the major factors of CoE and CLR.

The hull shape for which balance is least affected by heel would be a symmetrical bulb shape, round at every vertical cross section, and also symmetrical laterally. For such a hull, the only poisoner of balance (in our windless scenario) would be the keel and bulb, and rudder. Unfortunately, such a hull shape is probably not the fastest design we could come up with - hence that wonderful neccesity in sailboat design - compromise.

I have a little 24 inch sailboat, which I built off a plan many years ago. When first launched it showed wild swings in balance between lee helm and weather helm, with every gust of wind. Over time, I modified rig design, mast position, mast rake and rudder size. I got rid of some of the problems but the boat was always “unbalanced”. This is because the underwater hull shape changes a lot as the boat heels. The Center of Boyancy moves a long way, and the underwater hullshape becomes very asymmetric.

So, in addition to talking about CoE and CLR, any discussion on balance must include hullshape - especially in models.

There’s another influence too - but I’ve already written too long a post. Think about the turning effect created when the lift generated by the rig, is moved away to leeward as a boat heels. Even if the CLR and CoE are in the same vertical plane when viewed from the side of the boat - they are not when viewed from above. Consider this against the advantages (aerodynamically) of a high-aspect ratio rig v a low-aspect ratio rig. But that’s another story.


Things to look for in my opinion.Only considering the Canoe body at this point
Symetrical waterlines on heeling.
Small movement of LCB and COB on heeling.
Amount of pitch on heeling.
Nail these 4 suckers and your hull should be fairly well balanced.
Then there is appendages and rig,but thats another story.

I have gone through this exersise lately with a commision for a new IOM design.


any chance of a “sneak preview” of your latest creation?

This is a great thread, I have been R/C sailing for 20+ years and could not understand how a difference in backstay length of 1/8 inch with the same jib tension could make such a difference in how the boat could track straight or round up. This is data that I tried to use to get max speed and now I should be able use and understand. Big thank you to all, Clyde

I agree completely with Brett’s comments.

Just to add another observation - in big boats we can also alter trim by moving crew weight forward and aft. Even on the very big boats, crew weight is an important variable. We can’t do that on (2 channel) models - hence Brett’s observation about pitch on heeling is important. The software tools we use, like Hulls, Hullform, Maxsurf etc, are able to predict pitch changes for different angles of heel.


do i check the clr with or without appendages attached?

Hi ed,
Sorry,not much chance of seeing any drawings of my latest IOM. A first timber example is being built by the owner of the drawings.We will see how it performs.

Not sure what you are trying to find out there.
The methods of finding CLR are wide and varied.
The geometric method is sometimes used…but do you use the full underbody,fin,bulb and rudder in finding it?
There are as many answers to that as designers designing.
Other methods are trying to locate the Dynamic CLR using the centre of pressure of the lifting bodies.
eg finding the centre of pressure on the fin itself…ignoring rudder and underbody.

Normally the designer is interested in estimating CLR so he can estimate where the CE of the sailplan should be. often keel and mast positions are changed as the design is tuned,even in AC design teams the keels and masts are shifted.So clearly not even the very best people in this area know exactly how to find these values.
Most designers develop there own rules of thumb for various types of yachts and merely estimate where things should be.This is the “Black art” of yacht design.

Pushing your hull sideways in the water with all appendages in place might tell you something.But this is only a static test.


thanks for the reply. the reason for my inquiry is because i’m currently scratch building 2 ioms. the first one has a wide beam and i am still trying to determine the best keel fin location. 2nd one will be a narrow beam, currently building a plug out of foam at the same time. i am planning to build the two hulls with interchangable appendages, meaning i can transfer the fins/rudders/rigs/electrics from one hull to the other. i am using the “bathtub, push the hull with your finger” method to determine the static clr. i’ve tried both with and without appendages with different results. since we all sail our boats with appendages attached, :wink: i will be using the results from that test.

I would use centre of pressure on the fin only myself.
What designs are you building to?


i played around with something that i downloaded from lesters page, excel actually, he made for hull lines. the wide beam is 290 mm ( ala ts2 i think), the narrow one is 240 mm( cockatoo ish). i figured that it would be better for me to invest on good keelfins and rudders, rigs/sails and winches rather than get an expensive hull. the hull i can always scratch build. i also have the triple crown and a danish design call laerke in the pipline. i’ll just transplant the major components to the other hulls when required.

Good on ya Ed for having a go,
I know Lesters spreadsheet pretty well,used it myself for ages.
Tom Thumb was actually designed with it.
My latest designs are fairly narrow,around 210 -220 beam