Drag reduction with hull tailing

[Agave]

Hi all

I was thinking about drag reduction of a planing hull in the preplaning speed range and came up with an idea of reducing the drag with a bit of hull tailing. The idea is similar that is used in reducing the aerodynamic drag of cars, missiles, bullets, (sailboats) etc.

Here the hull sides would be narrowed only from the sides, creating a slightly narrowing shape to the stern. Keel line would be straight like in a normal planing boat. sides would be fairly straight, so that chines would widen towards the stern as hull narrows. (see attached pic.)

This could work even better in a round shape hull with chines, such as Paragon Mann VSV.

Any thoughts of planing speed hydrodynamic behavior?

[baeckmo]

Nothing new, this is the classic series 62 shape. See: Clement and Blount: "Resistance tests of a systematic series of planing hull forms". SNAME 1963.

[Agave]

Thanks

[Eric Sponberg]

Originally Posted by Agave

Hi all

I was thinking about drag reduction of a planing hull in the preplaning speed range and came up with an idea of reducing the drag with a bit of hull tailing. The idea is similar that is used in reducing the aerodynamic drag of cars, missiles, bullets, (sailboats) etc.

Here the hull sides would be narrowed only from the sides, creating a slightly narrowing shape to the stern. Keel line would be straight like in a normal planing boat. sides would be fairly straight, so that chines would widen towards the stern as hull narrows. (see attached pic.)

This could work even better in a round shape hull with chines, such as Paragon Mann VSV.

Any thoughts of planing speed hydrodynamic behavior?

Agave,

Your idea is similar to what I have used on the classic speedboats that I have designed, which you can see on my website here:

Cherubini Classic 20: http://www.sponbergyachtdesign.com/CC20.htm

Cherubini Classic 24: http://www.sponbergyachtdesign.com/CC24.htm

Chris Craft Cobra, new: http://www.sponbergyachtdesign.com/ChrisCraftCobra.htm

Saetta runabout: http://www.sponbergyachtdesign.com/Saetta.htm

All of these boats are slightly narrower in the stern. However, unlike your design, the keel centerline in profile has some rocker to it; that is, the keel line is NOT a straight line. However, as a result, the deadrise from amidships aft IS constant. This makes the boat ride very well and it is very stable at high speed and in turns. If you think about it, if you pinch in the stern by making the chines narrower AND keep the keel centerline straight, then your bottom will necessarily have some adverse shape to it.

If you start with a given deadrise at the transom where the chines are pinched in, the deadrise will actually decrease going forward with a straight keel line, then increase again continuing further forward. I don't think that is good, although I do not have any empirical testing to prove it one way or another--I just don't think that kind of design would be quite right.

My speedboat designs perform exceptionally well. I cannot quantify for you how much drag reduction there is in the preplaning regime, if there is any. I suspect that if there is, it would be quite small and maybe too small to measure accurately. But generally, you don't care--you don't run these boats in the preplaning regime anyway--you either run idling to approach a dock or a downed waterskier, or you are going at high speed and planing. I will say that fitted with a lot of power--300-350 HP--these designs simply jump up on plane almost immediately. They are quite fun to drive.

Eric