boat speed

How fast do different classes of boats go in knots?(nautical miles/hour) I have had a soling 1 meter do 6 knots once, and that looked reasonably fast for that type of boat since its heavier than comparable 1m. what to boats like IOMS, Marbleheads, and AC’s achieve for a top speeed?

Careful. No matter what we answer here, the multihull boys will give us a hard time. Then the iceboaters will get in one the act and remind us how pedestrian our displacement monohull models really are. [}}:-|>>]


Darn it. Why didn’t my little smiling devil show up?!? :slight_smile:


Ok, ok, I’ll try to restrain myself from any chestpounding about the thrills of hard surface sailing.[:D][:D][:D] One difficulty with any speed documentation is the hassle needed to set up gates & accurately record time over a course. We’ve had good results using a small onboard GPS which will record maximum speed, distance, average speed etc. Of particular interest is the Garmin Foretrex 101 which weighs only 2.6 oz

ps; I’ve got my models packed (hard shell golf case & ski bag) for my flight to Las Vegas this afternoon. We’ll be holding speed trials & races on Dry Lake Ivanpah on the Nevada-California border in conjunction with the NALSA ( America’s Cup event. Here’s some pix from 2003 that some of you might appreciate:

Speed trials…so thats how you call it : [;)] [:)] [;)]


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

HOLY COW !!! [:-bigeyes2]

Bill - you only said how much fun sailing an r/c landyacht would be. You <u>NEVER</u> said anything about “fringe benefits” - or were you keeping this information to yourself?! Is it too late to catch a flight? [:D] [:D]

Good luck out there - with the sailing, I mean !!!

6 knots on a displacement boat… O well the best i have seen Live is around 8 knots on a f48 trimaran fully foiling. The absolute best i have seen on video is sail rocket model doing 18+ knots and foiling. I have both videos if someone wants them.

  • HJ

“Expertice is gained trough mistakes. However repeating
same mistake is not learning but stupidity.”

Getting back to displacement speeds. We know that, in a full sized boat, maximum hull speed through the water (in knots) can be estimated as 1.4 times the square root of the waterline length (in feet). Somebody either here, or one one of the other forums explained once that this formula did not hold true for models because of problems with scale. But I can’t remember the detail. Does anyone else know?




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

Yes - that’s the same formula as I stated (give or take the rounding of the multiplication factor). The question is to do with application of the formula to model boats. What further errors are introduced by the small scale, and why?


the boats are a lot faster than they’re full scale counterparts because of the scale factors. If you are sailing an IOM, say it was going 5 knots. Now, this probably isnt a very good example, but take a boat which is 30 times the length of an IOM-the australian maxi Skandia- and you will find that for its length, it isnt as fast. If it was as fast as scaling up would suggest, than it would be sailing at 150 knots. The wind power is not proportionate to the size of the boats. I dont know how to exactly explain that, but 4 knots of wind seems like a lot more to a boat that is a meter long than one that is 30 meters.

Same with wave size and wind gradients as one leaves surface of water … ie: 5 foot mast versus 30 foot versus 100 foot - big difference in wind strengths and even directions.

I’d suggest that, whether we are talking a one meter model or a 30 meter super-maxi, the wind strength has little effect of speed limitations in displacement configuration. It does of course have an effect on breaking through the barrier and getting the boat onto a plane.

The limiting factor is the wave generated around the hull. The boat is “held” by the wave and cannot sail faster than the wave will move - other than by moving out of displacement mode and onto a plane.

A modern, light displacement racing keelboat will plane readily - even the super-maxi. The ability to get onto a plane is a function of power v weight and the hull shape.

We know that the rules do not apply to a very long skinny hull - think multihulls.

What has been said so far suggests that the scale effect on a small displacement hull, such as an IOM for example, means that it is less constrained by the speed of the wave around it, than it’s full sized cousin. I’m not talking a TS2 on the plane - I’m talking a boat that is not planing.

Some heavy boats (in the real world) never achieve the necessary power/weight ratios to get up on the plane - and their hull shapes are not condusive to planing. Anyone who has sailed an early 1970s era IOR racer will know what I mean. As the power gets stronger the boat creates a bigger and bigger wave. It may surf beyond hull speed, but never planes. A light displacement boat, such as, for example, a Mumm 30, a ULDB designed for the Transpac and any number of other light racers, move effortlessly onto the plane as the power comes on - just like sailing a Laser dinghy.

So are we saying that, planing aside, the effect of scale is that a model is able to exceed the comparable hull speed limitations imposed on a full-sized hull? The multiplication factor (1.4 in my example, 1.34 is the link above) does not apply to every hull, and is therefore a variable. Are we saying that the variable should have a higher value for models?

Come on - where are the naval architects? Lester - are you reading this? I’m guessing that, when tank testing is done for full sized designs, a result has been an understanding of scale factors on wave generation. I recall that in Americas Cup design programs, models at 1/4 scale and 1/3 scale are used for tank testing - the later to minimise scale effects. I’d be very interested (for no other reason that simple curiosity) to know how far science has gone in this field.


Well, think of it this way. If say you have a boat that is 12 times smaller than the prototype it is based on, than 1 knot of wind for the big boat would be equivalent to 12 knots of wind for the small one. Or something like that. Im no naval architect, but if the boats get smaller, than the wind speeds increase prortionately to the scale of the boat. That is why most R/C boats are not exact scale models, because an exact scale model’s keel will be too short to provide enough righting moment for moderate winds. EC-12s are relatively close to being scale boats, and they suffer because of excessive leeway, which is due to them having the shallow keel.

Ahhh - now there’s another very good question. The relationship between leeway, lateral resistance and scale. Again wind is not the sole factor here - though of course you are quite right in your point that 12 knots is a light breeze for a maxi, but a stiff breeze for a model.

The other factor is boat speed. Other factors being equal - the faster the boat is travelling, the less area it needs in it’s fin to generate sufficient lift to resist leeway adequately. Ever wonder why a windsurfer can sail to windward with such a tiny centreboard - while a sailing dinghy of the same size needs more area? It’s partly the smaller sail area of the board - but also partly the speed at which it is designed to operate. Speed generates lift. (The same reason that a 747 increases it’s wing area so much when it slows below 200 knots IAS). And that’s before we get into discussions about high aspect ratio feel fins v low aspect ratio keel fins, planform shapes, and foil sections.

Also, most models - even scale - have high aspect ratio keels compared to their big cousins. While this has (amongst other benefits) the effect of lowering the ballast, and therefore increasing its power to right the boat when heeled, the high aspect ratio keel lowers the centre of lateral resistance, and therefore the point through which the lift generated by the keel acts. This increases heel. Catch 22 eh? Increased heel makes the boat seem tender and increases leeway in the model.

Now here is a question I’d like to research. I must check the web when I’ve got time. Imagine a boat sailing to windward. As the wind strikes the sails we can visualise the force of the wind (the “action”) resulting in three “reactions”. These are a heeling force, a driving force (thrust) and a drag force. How does the split between these forces change with (1) increase in scale and (2) increases in the wind force? For example, as the wind increases, does the ratio of thrust v heeling v drag remain constant?

Using the EC12 as an example - is the leeway you speak of due to them having a shallow keel (and therefore a shorter righting arm) or is it due to a lack of planform area (of what combination of the two)? How does planform area required (other things being equal) change with scale?

Woah - now we are getting very technical. I wish I had time to study this stuff - it’s fun (well I think so any way).


Quote; “These are a heeling force, a driving force (thrust) and a drag force”

No thrust, lift, lift is the heeling force too, foils produce lift and drag, thrust is a resultant force. The ‘thrust’ that drives a boat to windward is the resultant force of the lift of the foil under the water (keel, daggerboard, windsurfer skeg etc…) and the lift of the rig, and also the drag of both of those. Heeling is an unfortuate side effect of this as the lift of the two forces creates a turning moment on the whole boat.

Luff 'em & leave 'em.

Thanks for that Matthew. Any thoughts on how these forces alter ralative to each other with changes in scale?


I’ll have to check in Marchaj’s Aero-Hydrodynamics of Sailing and other books and think a bit but I’m pretty sure the forces relative to each other won’t change with scale. What will change is the magnatude of the forces (obviously) and what you have to do to get the relative magnatudes the same (models generally have keels larger in planform area with fatter sections than their fullsize countaparts for example. Reynolds Numbers come into this as a model is operating at a much lower number than its fulsize counterpart, so to get the same lift/drag ratio blah blah blah…).

Bit technical for me off the top of my head to remember and make into somthing understandable, give me a chance to read up about it! Sailboats are far more complex than you would ever think…

Luff 'em & leave 'em.

This seems to be a good challenge for your College to answer!!.
Go for it!, make them work for a change, always helps if YOU are the one who asks the “Awkward Questions”.
Push a few Teachers buttons! Get some answers!!.
Jaydee. [^]

Yeah. Write the thesis and we will “peer review” it.