Hull speed differential between a traditional full keel boat and a modern hull

For the same displacement and length/beam, a full keel boat will have less wetted area. If it is heavier, the answer is that it depends on how much heavier. Sailboats don't always move at their hull speed, so that makes the speed dependent on weather conditions and wind angle.

 

Why wood the full keel boat have less wetted area? My understanding is the transition from "full keel" to "cutaway keel" design was in large part due to reduced wetted area with the cutaway keel design.

 

Which means flatter hull and less displacemet. For the same length it's about half the displacement but maybe 15% less wetted area. Thus the Aw/D ratio goes as Gonzo pointed out..

 

Not necessarily. You are conflating the keel design with the overall hull design. There are designs from the late 19th and first half of the 20th century with cut-away keels which have hull shapes and displacement very similar to full keel designs of the same vintage.

 

You are right, there are exceptions but generally speaking it's true.

 

To start with, full keel does not mean wooden or cutaway keel non-wood. Both types of designs are built in many materials.
Girth length | Boat Design Net https://www.boatdesign.net/gallery/girth-length.28325/

 

Hull material has not been previously mentioned. Why are you bringing it into the discussion about your claim that "For the same displacement and length/beam, a full keel boat will have less wetted area"?

 

You brought up the hull material as the quote of your post shows. I brought up the wetted area comparison, and posted a diagram, to show the error in your comment.

 

Are you refering to my mis-spelling of "would" as "wood" in

It should be obvious from the context that "would" was intended.

The diagram you posted shows only girth comparison at one cross-sections. It is a fundamental mistake to assume differences in girth at one station corresponds to differences in total wetted surface. Without knowing how the shapes vary longitudinally it is impossible to reach any conclusions about wetted surface area. Vessel A may have larger girth at the mid-section but greater wetted surface than Vessel B. (Example the keel on Vessel A may extend only a short distance fore and aft of the cross-section while the keel of Vessel B may extend most or all of the length of the vessel.

The blanket claim that

is not always true depending on how the comparative shapes of the hulls. For some comparisons it is true for others it is not. All full keel boats will not have less wetted surface area than all non-full keel boats of the same displacement and length/beam.

 

For traditional full keel boats, the wine glass cross-section would have more wetted surface than the fin keeled boat by virtue of changes to bottom shape when no smooth tapered garboards are any longer needed to transition from hull to keel.

One could easily imagine a basic round bilged hull with a deep fin keel that could then be reoriented to a full keel that followed the bottom. Thus two different keels, no change in displacement or wetted surface.

-Will

 

For a post retirement project, I intend to build a 24 inch model, which I named VICTIM. The purpose of this model is to test for various keel configurations, from full keel to very short keel.
The sail rig and the hull shape was to be kept the same. I settled on a boomed lateen sail, so the sail could be noved fore or aft if needed to get proper balance.

The area of each keel was to be determined by a formula I devised several years ago. Longer keels would have more area than shorter ones.

When sketching the profiles of each configuration, I soon discovered that the longer keel always added more whetted area while increasing the displacement nominally.

I think this myth got started when heavier displacement boats got compared to lighter displacement ones. The heavier boats almost always came with longer keels. They also came with deeper hull sections.

And then the boats were compared only by length an beam. Displacement was not considered. Also, the S/Ds were not considered either.

So, the lighter boat was not only lighter, but had more actual SA.

Now, to stand up to this greater SA, it needed more initial stability. And to get that, it needed wider, flatter underwater sections. Wider, flatter underwater sections require more surface area than narrower, deeper ones to get the same underwater volume.

This can become so extreme that the heavier displacement boat has less underwater surface area despite the long keel.