So, Are They Planing?

[SailDesign]

Originally Posted by ErikG

hey now Steve, be nice now he's new around here and he's swedish. Wether that is a redeeming factor... I do not know

sheez i can't spell worth a sh*t...

Not trying to be nasty, Erik, just wondering... Although Mighetto does not sound very Swedish.
As for spelling, when I can write Swedish the way you write English, I'll start complaining. Don't hold your breath.

Steve

[asathor]

How about a definition that matches the way the rest of the world talks:

Planing: When the hull is lifted out of the water and supported by a substantially smaller surface area than its usual wetted surface in displacement.

Surfing and Semi-Planing: When the hull is gradually and partially (incrementally) raised somewhat in the water either through power or wave-action reducing its wetted surface and thus friction therefor allowing it to move faster than in displacement mode.

True PLANING is the result of a catastrophic* change in how the hull interacts with the water. This is clearly illustrated in powerboats when the boat "climbs out" and level off before beginning to plane.

* in the Physics sence of the word.

[FAST FRED]

Most of the formulas were for boats with 3-1 or 4-1 LB ratios.

When you get a fine hull with 8-1 lb ratio (or better) 3 times the SQ RT of the LWL is still NOT plaining , just easily forcing over the tiny bow wave.

Its still in displacement mode , with most of the LWL supporting the boat.

FAST FRED

[Skippy]

asather: True PLANING is the result of a catastrophic* change in how the hull interacts with the water.

I would think of it in terms of the interaction, not the change. Bethwaite showed that triangular hulls don't have the hull-speed hump. They just start displacing poorly and lifting better. Almost any hull will plane below its hull speed if you reduce the boat's weight enough. The hump is common because hulls are often made too short to eliminate it, or are weighed down. That's just a practical matter of convenience. I wouldn't base a definition on it.

[asathor]

Thank for clarifying Skippy

[SuperPiper]

Here's an observation:

A 12 foot fibreglass fishing boat will plane easily with an 8hp outboard, provided the boat only has 1 or 2 passengers. With a little extra weight, the boat has problems climbing up onto a plane. It is dragging its transom and throwing up a tugboat wake. The usual reaction is to move some of the ballast (passengers) forward. This action causes the bow to plough through the water and to lift the transom clear. The boat slowly achieves a plane.

So, should I be able to perform the same trick with my bathtub-shaped pocket-cruiser? If the ballast is shifted to keep the transom high, or the deck level, is the boat likely to accelerate even though the bow may be ploughing a ton of water?

We have all seen that classic diagram of a keelboat not being able to plane becuase it is inclined up its own bow wave. If that boat could be forced level, would its speed increase? On a small boat, it is easy to affect the longitudinal trim. I know from experience that the boat may average 1-2 knots faster if I can keep the transom dry.

[CT 249]

Originally Posted by Skippy

asather: True PLANING is the result of a catastrophic* change in how the hull interacts with the water.

I would think of it in terms of the interaction, not the change. Bethwaite showed that triangular hulls don't have the hull-speed hump. They just start displacing poorly and lifting better. Almost any hull will plane below its hull speed if you reduce the boat's weight enough. The hump is common because hulls are often made too short to eliminate it, or are weighed down. That's just a practical matter of convenience. I wouldn't base a definition on it.

Has FDB "shown" anything? He's SAID a bit, but what has he SHOWN? He uses graphs comparing a Tasar and an IC to a 49er, but the three boats are so enormously different that the graphs are not very useful. It has to be said that some of Frank's other propositions (ie windsurfers don't go upwind, tacking downwind started in Australia, cats are slow in light airs, etc) are simply wrong.

Frank doesn't mention that some of his boats are fairly slow in very light airs (ie 29er) and no other designer I've spoken to says that the triangular shape has anything to do with the lack of a "hump". It's fairly simple, they say....the 49er and 29er are fairly light for their length so as you said, they plane below hull speed. Consensus (from some guys who have more succesful records than Frank as skiff designers) is that the triangular shape has nothing to do with it.

Frank is a very smart man, a great designer, but not infallible.

[Andy P]

Originally Posted by SuperPiper

Here's an observation:

We have all seen that classic diagram of a keelboat not being able to plane becuase it is inclined up its own bow wave. If that boat could be forced level, would its speed increase?

This is why a T-foil in int moths has been successful in all conditions.
A narrow transom hull form which has lower drag than a traditional wedge planing hull in light winds can also be used to success in strong winds. The lack of support due to the narrow hull aft ( which could also be compared to lack of lift due to lots or rocker aft on a keel tub ) is compensated by the lift ( or trim angle control ) given by the T-foil.

However the speeds, lift and forces needed for a big slow heavy boat might mean it doesn't work so well!
Some outboards motors have a horizontal fin so that there is an effective T-foil for trim angle control.

re FB - don't forget his idea of the 4th mode!
in spite of which, 29ers, b14s still have poor low speed performance even with the crews on the foredeck.

[Skippy]

CT249: ... He's SAID a bit, but what has he SHOWN? ... the three boats are so enormously different that the graphs are not very useful. ... no other designer I've spoken to says that the triangular shape has anything to do with the lack of a "hump".

That's interesting CT, the smooth-transition argument made sense to me. So what's the point of the triangular hull?
As for using different boats, I thought that was the whole point, to show a range of behaviors. I don't want to pile on Bethwaite too much, but I agree with some of your points, especially the catamaran.

[gggGuest]

Originally Posted by Andy P

...re FB - don't forget his idea of the 4th mode!
in spite of which, 29ers, b14s still have poor low speed performance even with the crews on the foredeck.

People seem to get very excited about this 4th mode bit, which is just a very small part of his book, but all I read him as saying is that when you sail wedge shaped boats like that in lights airs they perform better than they would do otherwise, not that they suddenly become light airs flyers, which clearly they aren't!

[Andy P]

Originally Posted by gggGuest

People seem to get very excited about this 4th mode bit, which is just a very small part of his book, but all I read him as saying is that when you sail wedge shaped boats like that in lights airs they perform better than they would do otherwise, not that they suddenly become light airs flyers, which clearly they aren't!

Well - that is what I meant really ... and also the 4 mode thing has come up before somewhere else on these forums.


And the triangular shape works cos you get narrow entry combined with wider flatter hull aft for fast planing. - but there are other shapes that work too!

For future reference Pierre Gutelle in his book "Design of Sailing Yachts" has a small treatise on sailboat planing p113-117. More info than any other book I've yet found but still not enough...