# Planing Trimarans

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

1. ### Doug LordGuest

voodoo

Bethwaites formula says that Sail Carrying Power is righting moment divided by the distance from the CE to CLR. The ratio "SCP divided by total weight" only has meaning, in the case of a tri with a planing main hull, IF the load carried by the ama is subtracted from the total boat weight giving the load actually supported by the planing main hull.
If you recheck THE BOOK you'll find that the statement above: "...and use it's weight to create SCP" is meaningless: SCP in the book is the RM(weight of crew times distance to CB) divided by the CE-CLR distance. On the tri, in my example, it's not weight that creates RM but the buoyancy
of the immersed ama equal to 600 pounds of force acting upward(as opposed to downward in the mono case) times about half the beam.

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### RHoughRetro Dude

So you want to look at it backwards and call 600 pounds of buoyancy times half the beam as the righting moment?

OK, we'll work with that.

A moment requires a force and an arm.

A moment is force in a direction about a fixed point.

In your description the ama is creating a 600 pound force upward acting on an arm equal in length to the half beam. For this to become a moment one end of the arm must be fixed. What keeps the end of the arm at half beam fixed? The only way to create the moment you describe is to have the hull fixed. That would be the weight of the hull balanced by it's buoyancy. Therefore, although you don't see it, you are using the weight of the hull to create your righting moment.

What creates RM is lateral distance between the CG and the CB. Ignoring moving the crew, the CG does not change, so the maximum righting moment will be when the CB is furthest from the CG. If the boat is sailing on the ama only (the hull and windward ama are not providing buoyancy) the RM can be calculated as the weight of the hull times the half beam plus the weight of the windward ama times the beam. The fixed point on the RM arm is not the centre of the boat, but the CB of the leeward ama.

I think you would be less confused if you used the leeward ama CB as the fixed point of the arm and did the calculations from there.

Hope this helps.

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### ActionPotentialJunior Member

I agree completely. Need we say any more?

4. ### Doug LordGuest

planing tri

Well, on a tri the CB moves between the hulls and at relatively low angles of heel with partial immersion of the ama it is accurate and convenient to use the buoyancy of the ama times half the beam to come up with righting moment-which is way less than maximum . Max righting moment would be almost exactly as you say: the distance from the CB of the immersed ama
to the CG of the boat.
A more complicated, slightly more accurate and less convenient way to illustrate my point would be to find the actual CB of the boat in the 1lb. per sq. ft.SA pressure case mentioned in the first post and above which lies somewhere between the main hull and ama with the ama partially immersed.
In my example, the ama is displacing 600 pounds
and the main hull is supporting 1200 pounds so the CB of the boat has moved one third of the distance between the main hull CB/CG and ama CB to leeward or .33 X 8.5 = 2.8' . So the righting arm (2.8)times the total boat weight(1800) =5049ft.lb. which is very close to the figure obtained in the simpler(and I thought, clearer method: 600 X 8.5 = 5100 ft.lb.)- less than 1% difference.
==========================================
And so if, as you say, Bethwaites ratio "SCP divided by boat weight" is proof of planing AND you subtract the displacement of the ama from total boat weight(1800-600 =1200) to arrive at the actual loading of the main hull then this is true:
SCP=5049 (RM) divided by 12.75(CE-CLR distance)=
396. The ratio "SCP(396) divided by total weight( load on the planing hull=1200)" = 396/1200= 33%.
According to Bethwaite anything above 30% is capable of upwind planing.
It is simply absurd to use any other figure for the load on the planing main hull than the actual load; if the total weight of the boat was used it would not plane(in addition to being completely inaccurate). You must deduct the displacement of the partially immersed ama to arrive at the actual load on the planing hull....
Once you understand that the numbers all fit from the old axiom "500 sq.ft. per ton" to the
Bethwaite formula ,to the SA/D. And the numbers for the F22 match the actual experience on the water of other F boats and Mr. Farriers comment that in order for the boat to plane part of the load must be taken by the ama.
FINIS

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### RHoughRetro Dude

Thanks, at least one other person understands.

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### VegaSenior Member

Will, back on post 77 I have said practically the same.

Looks that after all, I am in good company, in what concerns what sailors call a planning trimaran.

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### Dan SJunior Member

I think only one person doesn't , and who really cares about that person anyway.

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### ActionPotentialJunior Member

If planing means skimming across the top of the water supported by hydrodynamic forces only (as per 18' skiff), then whenever there is another supporting force it is not planing.
If planing means skimming across the top of the water, partly supported by hydrodynamic forces, then the trimaran centre hull skimming across the top of the water partly supported by another force (heeling force if you like) is planing.
If planing means skimming across the top of the water, partly supported by hydrodynamic forces, then the catamaran windward hull skimming across the top of the water partly supported by another force (heeling force if you like) is planing.
It al depends on your definition of planing but you can't have one for catamarans and another for trimarans.

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### WillallisonSenior Member

Is a powerboat planing when it is assisted by trim tabs?

From elsewhere - a quote from Savitsky himself -
This clearly rules out any suggestion of a windward displacement hull being on the plane

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### RHoughRetro Dude

I'll agree that the issue is like trying to define shades of gray.

In the very clear case of an 18 or a power boat, it is obvious that no other force is acting to lift the hull when the boat planes.

In the not so clear case of the trimaran, the two ama's sailing as a catamaran have enough power to get a centre hull weighing as much as the entire boat up on plane. In other words, the F-22 could tow or push a 1700# hull fast enough for the hull to plane. The heel of the boat is not needed to unweigh the centre hull. The fact is that the centre hull is attached and is supported in part by the heel. However, the key point in my mind is the support is not needed. If the effective weight is reduced, the hull planes at lower speed, but the power to get the hull on plane without reducing it's weight with heel is there.

On a catamaran, if the boat can produce SCP greater than 30% of total weight without using the windward hull to add RM, it also has enough power to plane a windward hull. If it cannot produce that much power, the windward hull can only skim the surface, lifted by heeling moment of the sails.

Like I said, shades of gray.

Since a high performance cat or tri has enough power to fly the windward hull(s) completely free of the water, the value of planing one or more of the hulls is another topic. Farrier clearly says that his use of a planing form is to provide larger accomodation, without incurring a speed penalty. That seems logical and very clever.

As far as I'm concerned, I've proven to myself (I have a very thick head) that Farrier Tri's can plane the centre hull. Whether what they do in fact is "true" planing or not is of no interest to me. The portion of weight of the centre hull not adding to SCP through RM must be supported either by displacement or hydrodynamics. One uses no power, the other requires a substantial amount. The power is there, the boat could plane, that's all I was trying to figure out.

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### RHoughRetro Dude

The trim tabs are just modifying the angle, not changing the location of the stagnation point ... no?

Can a chineless hull be planing at all?

The sailing attitude of a Farrier Tri certainly satisfies the yachtsman's definition ... of faster than hump speed at lower angle ....

Honest questions, not a troll.

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### ActionPotentialJunior Member

Quote:"If planing means skimming across the top of the water supported by hydrodynamic forces only (as per 18' skiff), then whenever there is another supporting force it is not planing."

Is a powerboat planing when it is assisted by trim tabs?

I don't know much about power boats of trim tabs but I thought trim tabs would be considered part of the hull or hullshape and not be considered another supporting force, don't they work on hydrodynamics?

From elsewhere - a quote from Savitsky himself -
Quote:""To the researcher dealing with the hydrodynamics of
planing craft, the inception of planing is associated with complete
separation of flow from the chines and transom and full development of
the stagnation line at the forward edge of the wetted bottom area. This
will occur prior to the so-called hump speed when the trim angle is
maximum. My 1964 paper in Marine Technology discusses this.
To the average yachtsman, planing is taken to occur at
speeds higher than the hump speed when the craft runs at trim angles
smaller than the hump trim. They refer to this as "getting over the
hump and running on plane".
As a hydrodynamicist, I use the first definition."

This clearly rules out any suggestion of a windward displacement hull being on the plane.

I would say that this clearly rules out any suggestion of a windward hull being on the plane.
__________________

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### WillallisonSenior Member

RH - In changing the trim angle, you also change the stagnation point

And yes, a chineless boat can most certainly be planing - the point of separation just becomes a bit harder to define. Most planing, round chined powerboats incorporate at least some kind or spray rail at or near the turn of the bilge to assisit in flow separation. Otherwise they tend to be very wet boats.

AP- Trim tabs may or may not be an integral part of the design. They are often added as an aftermarket item for instance. Yes they do operate on the basis of lift, but the point I was trying to make is that they assist the main body of the hull in doing so.

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### WillallisonSenior Member

If you have a catamaran with two planing hulls and the leeward hull is planing, what's the windward hull doing (assuming it's not in the air!)?

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### ActionPotentialJunior Member

I never doubted Ian's logic nor his cleverness. I never doubted that a farrier centre hull could plane. I even suspect that if the leeward ama was unbolted and laid upon the windward ama, in the right conditions the centre (now leeward) hull would plane.
My only point is that I cannot accept that a hull being lifted by any force other than hydrodynamics can be described as planing.
Sure the 'planing' shape gives better accomodation without incurring a performance penalty, in fact I suggest it brings a performance advantage, as explained previously. It's just that it doesn't do it by planing (according to my definition).

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