Planing Hulls: Redux

So I was thinking about some of my previous ventures to this site and how there was some discussion as to whether or not model boats will plane or not. Now, I remember Will Gorgen saying that its simply the amount of weight for the size of the boat that keeps it from planing. I also remember He Who Will Not Be Mentioned :wink: saying that he had sailed multiple RC boats based on full size boats and even with “powerful power-ballast” systems, they would not plane. Then it hit me:

Scaling effects would affect a planing hull just like they affect a displacement hull. Now, a model hull make look similar to a full size counterpart, but that’s usually where the similarity ends. The huge appendages, the mass of sail, aren’t they all a result of scaling effects? So, wouldn’t it be completely possible to build a model boat with a HUGE SA/D ratio and employ some method of moveable ballast to produce an extreme righting moment with a minimum of weight? It seems to me the boat would end up looking disimilar to most other things around, but that’s fine-- all it would need was a good looking transom cuz that’s all anybody would ever see:-)

Last comment notwithstanding, do you guys believe in the possibility (or reality if its already out there) of an RC boat that can plane on a reach and perhaps even upwind?


Hi Graham

From what I can see on the pond-side (not currently owning a top-spec one), Marblehead (and 10 Rater) class boats will plane happily on a reach.

Lester Gilbert

The MG30 planes quite easily on a reach.
I do it all the time with my demo model, several customers have sent up glowing reports and pictures of their models on a mad plane.

There are many on this board who will say there is no way a model sailboat can plane, but I ask them to give us one (1) scientific reason why it can not.
Planing is simply the point at which a hull no longer remains in the realm of displacement, and rises up in the water, compressing the medium under it.
Any hull that overcomes this displacement weight will plane. It is simply a matter of speed.
Even a solid lead bar will plane if you get it above the speed where its displacement is less than the water under it.

A perfect example is a flat skipping stone. By itself it sinks. Give it a bit of momentum and it bounces off the surface of the water. Take a look at the bottom of the MG30 hull sometime. Look familiar?

Peter Richards


A human body, if towed fast enough behind a ski-boat will eventually plane. And barefoot water skiers certainly don’t have much surface contact with the water in comparison with their “displacement weight”. I think the key is speed.

A human body, after loosing its ski while being towed behind a ski boat will plane until it eventually stops. The contact with the water can be substantial and can leave a decent mark. The key is definitely speed!

Will Gorgen

ok…so next question:
if, lets say I have a boat A…what speed does it need to “plane”? and eventually, how to make that A boat faster?


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

Will and Lester will probably have some really cool formulas to determine planing speed versus displacement. As to the other part of the question … I would suggest flatter hull bottoms, big rigs, lots of wind and lighter weight boats. [:D]

<blockquote id=“quote”><font size=“1” face=“Verdana, Arial, Helvetica” id=“quote”>quote:<hr height=“1” noshade id=“quote”>I would suggest flatter hull bottoms, big rigs, lots of wind and lighter weight boats.<hr height=“1” noshade id=“quote”></blockquote id=“quote”></font id=“quote”>

Pretty much sums up the MG30 sailing in a nutshell.[:D]

Peter R.


Warning - longish post.

Well, we know that in the big boat world, an broad indication of the displacement hull speed limit of a hull (in knots) is given by the formula 1.4 x sq root of waterline length (in feet). However we also know that the shape of the hull has a big effect on what happens to the boat when force causes it to exceed its hull speed. The archetypal “planning” hull will do just that - get up and plane quite quickly once force pushes the hull to limits of its hull speed. Not so the “displacement” hull, which reacts initially by creating a bigger wave and deeper trough. The bow rides up the wave, and the stern sinks. Considerable additional force is required to get such a hull over the wave and onto the plane.

Two examples. 1. A deep V power boat and a shallow V power boat. As the throttle is applied steadily, the deep v boats creates a big wave, squats downs, and eventually rises onto the plane. The shallow v seems to move onto the plane effortlessly.

Second example - has anyone had the experience of being in a displacement sailboat being towed at speed by a faster motor boat? I remember an incident here a few years ago when a 32 foot sailboat was taken in tow by a small naval vessel after a call for help. For reasons not relevent here, the naval vessel steamed at 10 knots. The sailboat had a LWL of about 26 feet, and so a theoretical hull speed of just over 7 knots. The force required to pull the sailboat through the water at 10knts did not get it up on the plane. But the drag created set up a big wave, and major loads. These damaged the pullpit and mast of the sailboat (to which the towline was anchored), broke the towline and forced the abandonment of the tow.

So in conclusion, the form drag associated with the hullshape impacts (together with other factors including LWL, displacement and parasitic drag-inducing items) on the level of force needed to get a hull beyond its hull speed and onto a plane. At speed approaching hull speed, wave drag is the major component. It’s a bit like puching an airplane through the sound barrier. The displacement of the hull in turn effects the underwater hullshape and therefore the form drag. An IOM is not light for its size, whereas the MG30 is. The MG30 floats higher in the water, has a flatter underwater shape, and pushes up onto a plane relatively easily.

We can all design a sailing model that will plane downwind. The trick is having the same boat sail well to windward or on a reach in a fresh breeze. Enter the canting keels etc. It’s all good fun.


PS - having just read my post above, I should say that I am in no way implying that an MG30 does not sail well to windward. I only meant to draw attention to this aspect of compromise in sailboat design,



given enough horsepower(sail area), even a brick will fly

and then flip over when it rounds the leeward mark;-)


who said anythin bout rounding a mark?

I see said the blind man to the crippled nudist who put his hands in his pockets & promptly walked away.

we need to talk about rounding a mark . because striaght line speed is ok. but what about bringing the boat back? we have to look at the all around aspects of a hull. we are talking about a hull planning downwind. fine. with enough power , “even a brick will fly”. i dont think that is right too. for a sailboat to plane . i think it would have to have a shallow bottom . ( sort of like a skiff). then over power it with the sail. then the boat would get up.
long live the cup and cris dickson

given enough horsepower,even a brick will fly, just like the mcdonell douglas f4 phantom,whose lines defy aerodynamics,but with 2 general electric j79’s, it rocks.any how,on big boats, we normally have a downwind set of sails, and an upwind set of sails, so no worry there of flipping over when we round the mark:-)

I personally think alot of models are built too heavy and too srongly, mainly for practical durability reasons, which limits their plane-ing ability. I mean how many times have you seen two fullsize sailboats of any type collide at full tilt and not do alot of damage? Models tend to bounce off and cary on sailing.

The lack of any moveable ballast has alot of effect, If you put a fixed keel on an 18 foot skiff and fitted r/c gear, you wouldent get it to plane upwind, it would probally heel lots and sail like a dog. Down wind it would probally plane for a second if you’r lucky, before the bow goes down the mine followed by a nice big roundup and then it will be quite stable on its side. Ok extreme example but even on say a Mumm 30 sportsboat everybody stands next to the helm at the back downwind in a blow, you cant do that with a model without some thing being made to move about onboard.

Alot of the battle with going fast is control, with a model you have just two controls, my typical position when sailing in big winds downwind on a J-80 sportsboat is with one hand on the kicker (vang) ready to dump in the gusts and pull back on in the lulls, if I wasnt doing that the helm would really be strugling to keep control. At the same time you got somone on the spin sheet, someone helming and another person just looking around calling the gusts and checking out what the oppositions doing.

Yes in short models can plane, but most of the established class rules limit alot of the factors that would make it a much more regular occurence, I’m not going into that.

Luff 'em & leave 'em.

hey hey hey, if you put a fixed keel on an 18, it would probably sink cause it aint designed for keels, but you are on the right track, movable ballast, build boats less strong & I also think that builing them with more downwind sail area would also help.

I see said the blind man to the crippled nudist who put his hands in his pockets & promptly walked away.