# Planing Trimarans

Discussion in 'Multihulls' started by Doug Lord, Sep 30, 2006.

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### ActionPotentialJunior Member

Rather an 'all or nothing' view of what happens to the centre hull when it is unloaded by heeling.
My view of planing is that when a hull rises due only to hydrodynamic forces. When another force causes the rise, be it lifting with a hot air balloon or lifting through rotating about a leeward ama,then in my view it is not planing. It is not climbing up onto its bow wave.

Now we have 6 discussions going on here.
1 is whether the Farrier 'planing hulls' can plane at all. I think they can.
2 is whether Doug Lord has proven mathematically that the Farrier 'planing hulls' can plane. I don't care, as I don't need a mathematical proof either way.
3 is whether if an owner says it is planing, that is evidence that it is planing, I think not.
4 is whether if a designer says it is designed to plane, that is evidence that it can plane, I think not.
5 is whether if a designer says it is designed to plane, that is evidence that it is planing, I think not.
6 is whether if a hull is lifted by a force other than hydrodynamic it is planing, I think not.

It is easy to prove if the hull can plane, just tow it at the appropriate speed and see if it builds up a bow wave and then climbs up on it to be supported by hydrodynamic forces alone.
Then to prove if it can do that under sail power, just make sure that there are no other forces lifting the hull, then observe it as it builds up a bow wave and climbs up on it.

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### ActionPotentialJunior Member

One problem is that many people confuse going really fast with planing.
Thus when they are going really fast they must be planing.
The genius of the Farrier designs using a 'planing shape' for the centre hull is that when sailing at low speeds, such that hull speed restrictions don't come into it (negligible bow wave) these shapes give reduced wetted surface compared to long skinny shapes, thus they go faster in these conditions than the tris with 3 long skinny hulls. Then as the wind strength rises they usually sail with the apparent wind on the beam and heeling force unloads the main hull just when it would otherwise start generating a significant bow wave. 2 speed benefits, the speed resistant bow wave is not generated and the wetted surface of the centre hull is reduced. This prompts the owner to think 'we are going really fast, must be planing, my previous boat (monohull) didn't go this fast when it was planing.

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### RHoughRetro Dude

Doug, either you don't understand my questions or are evading trying to answer them.

Let's make this simple ...

The sail force is acting 13 feet above the CB in the above situation. Using the same 200 sq ft sail and 1 pound per sq ft pressure there is a 2600 lb/ft moment at the CG. Depending on the point of sail some portion of that 2600 lb/ft moment is driving the boat and some of it is heeling the boat. When sailing to windward how much is heel and how much is drive? Say 90% heel and 10% drive?

That gives us a 2340 lb/ft moment from the sails lifting the hull and 260 lb/ft driving the hull. Do you really think that the 260 lb/ft moment acting in the direction of travel is enough to get the hull to lift and plane? Or is it much more likely that the 2340 lb/ft moment to leeward is the force that is lifting the hull?

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### WillallisonSenior Member

I enter this debate with some apprehension - I know little about sailing tri's, and I have a personal dislike of having my head verbally kicked in but here goes anyway....

I'm a powerboat guy, and since others have used them more than once to illustrate their point, I feel that I am justified in doing so....
Take a planing powerboat - it matters not what size or shape. For our example lets say it get's on the plane at 20 knots. Obviously, it is still on the plane at 30. Now let's add a couple of trim tabs. Is the boat still on the plane at 30 knots - even though the tabs are contributing to the lift? Yes, of course it is.
Now lets add a couple of outriggers. Long skinny ones. Boat's still doing 30 knots. Still on the plane? 'tis in my book.

To me the fundamental difference between the arguments you are all posting is whether a planing hull continues to be on the plane if it is being assisted by an external hullform. There's a grey area, for sure - in my example boat it is at around 20 knots - but surely not, once beyond that speed.

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### WillallisonSenior Member

BTW - Sydney Ferries Hydrofoils - foils most assuredly make contact on both top and bottom surfaces

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6. ### Chris OstlindPrevious Member

Good to see you Will

I like that whole picture of the planing power boat with the long skinny amas. Your assumption would be correct as long as the weight of the boat were not being supported by the amas in any way. Heck, I'd even be willing to grant a small exception of the outriggers supporting the smallest of the overall weight in an "every now and then" position to keep things on track.

But the truth of the matter is that your boat got onto plane without the support of the amas and it doesn't remain there because it needs the amas for the planing function to be realized.

If Farrier's boats, and all these swirling claims, could simply show a nicely photographed video of an F-Boat achieving the planing posture without the amas for any of the weight support, I would graciously back away from this table. By shifting weight around in order to perform convenient unweighting exercises, so that a vaka hull can hoist itself into a planing posture (when it most likely can not perform that function without the shifting exercise) I do not concur that the boat is planing at all. Simple.

In your clear example, you added component parts to illustrate an element of the argument. So, for the extended aspects of the discussion, let's take away a few component parts to produce a counter point.

For the sake of the discussion, let's just say that an F-Boat is "planing" along at 18 knots with the leeward ama immersed rather firmly. If one were to take away the leeward ama and the supporting aka beams, would the boat remain on plane? Or, would it, in fact, not only fall off plane, but fall completely over because virtually none of the dynamic support achieved by the vaka shape is doing any substantive work at all to keep the boat upright and on a plane.

It's that fact, simply, that make these planing claims dubious and wishful.

I could just as simply claim to be able to swim at 40 knots by conveniently hanging onto a rope tied to the stern of a ski boat while moving my feet in a sprinter's flutter kick. Hey, I'm swimming and most likely planing as well. I'm just conveniently using another means of support in order to do it.

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### RHoughRetro Dude

Welcome to the loony bin!

In my mind the debate is not if the main hull of a trimaran lifts out of the water or not, but the most likely cause for it to do so.

In the case of a power boat, given enough power and a decent hull shape the boat will plane. In the case of the sailing trimaran, more than 50% of the available power is acting through the lever of the ama to lift the hull out of the water. The remaining power has to get the boat up on plane.

Think of a power boat with two engines 700 HP driving a rotor blade to lift the hull and 300 HP driving the boat forward ... the hull comes up out of the water ... is it planing? or is it supported by the 700HP rotor blade?

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### WillallisonSenior Member

Geez - you guys are good at this
Chris - removing the ama's, the boat would, of course fall over - they are there after all to stop it from doing just that. Pitty really, 'cause maybe then we could settle this!
Don't get me wrong - I get your argument. I'm just not sure about the conclusion.
Same with the proposition put fwd by RHough. True, the boat is being assisted by the lifting rotors. I still think it's on the plane though.
Having said that I will concede that it is quite clearly not being supported entirely by dynamic lift. Then again, neither is a 'normal' powerboat - otherwise it would leave no 'hole in the water' at all....

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### ActionPotentialJunior Member

looks to me like it is still rising.

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### ActionPotentialJunior Member

No we can remove the leeward ama and put it on top of the windward one then we can see if it will lift by hydrodynamics alone.
If the hull being lifted by the lifting rotors is planing then so is the windward hull of my catamaran when it is being lifted by pivoting around the leeward one, reacting to the heeling force.

11. ### Doug LordGuest

planing tri or try planing?!

Hello, Will and welcome!!

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### RHoughRetro Dude

Fair enough ...

One definition of planing is that the weight of the boat is supported by dynamic lift. Since this is not the case, how can the sailing trimaran be planing?

When the main hull is in an attitude that could be explained either by the physics of dynamic lift or by the physics of levers and the force on the lever is several times larger than the force providing dynamic lift, is the logical conclusion that the dynamic lift causes the attitude or that the lever causes it?

It would be possible to use a crane to hoist a boat partially out of the water so that it has the same attitude as when it is on plane. Is it planing? Of course not, it is not moving and cannot create any dynamic lift, it is hanging off the crane. Now move the crane so the boat moves through the water, it is still not planing, it just looks like it is.

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### grobwww.windknife.com

With something small like a moth it should be fairly easy to tell weather its planing by performing some towing tests and measure the drag at various speeds. If the hulls are not planing they should follow a fairly predictable drag speed relationship once you have moved away from the wave drag speeds.

To go a little more high tech perhaps you could first predict the drag with something like michlet and compare your results to that. It would be interesting to prove once and for all wether long thin hulls plane. With enough data you might then be able to predict at what speeds a new design would plane.

Gareth

14. ### Chris OstlindPrevious Member

It's been fun...

It would appear that with all ideas and convenient mythology long since exhausted, that this discussion is now completely dead.

One sure sign of that condition is when one of the denizens resorts to extensive, cut and paste procedures to promulgate less than creative solutions. It's as if the internal Internet in their techno-soul has reached the end of it's cyber capacity, long ago having left its humanity outside the discussion. A crackling, whipping, loose wire, sending its electron stream into oblivion for no purpose.

I bid you farewell and hope to see some of you again in another thread.

Remember to get that video, Mr. Lord. It's your only hope.

Chris

15. ### Doug LordGuest

planing

Again, the mental block seems to be conceiving of a high beam to length ama operating in a displacement mode AT THE SAME TIME and AT THE SAME SPEED a relatively low beam to length main hull is planing. Which is exactly the way a Farrier tri works according to the science, the designer, and the owner/sailors.

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