# Quantifying Drag of Rocker

Discussion in 'Hydrodynamics and Aerodynamics' started by mitch184, Mar 10, 2023.

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### mitch184Junior Member

Have an an oddball question and seeing if anyone could help.

I have been designing a few full bottom extension outboard brackets for fiberglass boats. I/O to outboard swaps. Based on my experience and input from very experienced bracket builders local to me, Washington, there are a couple unique details we've done.

Many of these boats are used to cross bars and return to Port with large following seas. One thing we've incorporated is adding a slight bit of rocker, with respect to the keel of boat. Usually its only 0.5 to 1.5 degrees. This seems to be a good compromise of helping out in a large following seas and not causing any porpoising or reduce the ability to keep the bow down in a head sea.

I've always known that rocker can increase the overall drag, but I havent been able to see a real world difference. Is there a calculation or method to give a ROM guess of how much drag is increased from a known rocker and surface area?

For example, 22 degree deadrise boat, 8' chine width, 19' to 16' of wetted hull length. The bottom extension would be 27" long by 36" wide with 1 degree of rocker. This equate to just under 0.5" of rise, with respect to the boats keel, over the extension.

I can find the volume of water in that rocker. I was imagining that possibly the weight of the water, roughly 11lbs, in that volume might give me an idea of "suction" force, but I'm sure there's a more technically correct way of doing this. I just haven't found it.

Also, kind of related, but I was wondering what effect this amount of .ocker would have on the boundary layer of water in this area and how it might be impacting prop mounting height.

Thanks

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### jehardimanSenior Member

Rocker in an extension bracket for OB won't give any measurable or operational difference. This is because it is all buried in the thrust reduction (unless you are using very long shaft OBs) nor is it significantly wetted when on plane (assuming a well designed original transom and proper loading).

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### mitch184Junior Member

Thanks for the reply. Just to clarify the extension would be flush to the existing transom, no step, then rocker upwards. 30" shaft outboard with the vent plate even with, or up to 2" above the bottom edge of the new transom on the extension.

How exactly would the bottom of the extension not be wetted? I think maybe I explained it wrong. I can attach a picture.

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### mitch184Junior Member

Back view

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### jehardimanSenior Member

With a well designed planing hull, and even a semi-displacement transom hull at high speed, the water should leave the transom parallel to the bottom then the wake surface curves up to form the stern crest behind the vessel at a distance correlated to the speed. If the bracket curves away from this surface faster than the water surface curves then there is no significant impingement; so no significant pressure drag or friction (all assuming smooth water, etc,etc.)

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### mitch184Junior Member

Ok yes. I understand that. Misunderstood you. If it breaks free, all is good. If it doesn't break free, then it creates suction and extra drag. Correct?

So is there a calculation for the angle and rate at which water climbs? This boat will cruise at 30, WOT 45. Very heavy, 8000lbs, for its size. 26' x 8'6".

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### DolfimanSenior Member

The influence of the aft buttocks shape on high speed hull performance was investigated at model scale and reported here attached : see the kind of profiles tested in Fig. 2 , i.e. various afterbodies with each a constant rocker, straight or hook radius , this radius versus model length ratio being used as adimensional parameter. No clear info was given for the length itself of the afterbodies used, seems roughly at same order of magnitude of the hard chine beam. Experimental results on calm water and sea states are presented, but no theoritical approach is associated.

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### mitch184Junior Member

I did read that study. I even converted the different radius into flat rockers and tested them. By my calculations the sweet spot was around 1.25 degrees. But I know its not a direct 1 to 1 and wanted to be on the conservative side to make sure I didn't have any negative issues.

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### CocoonCruisersJunior Member

Hi, sorry i can't help you with the math of the optimisation problem,
but i'd think of the angle of the extension as a kind of trim tab:

The trim effect of the extension and the outboard trim combine.
Result is a given pitch of the vessel, and the changes in pitch may well have a much larger influence on drag than the extension itself.
(On a sidenote, if the two trims fight each other, you create additional drag by directing the flow up then down.)

From there, i'd think that the 'right' angle will depend heavily on usage of the vessel (loading, desirable pitch for the local bar etc), and that your best bet would be to eyeball what kind of correction the individual boat may need, or to somehow make the plate adjustable, at least for a few test runs before welding it into place definitively.

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### mitch184Junior Member

Yes, I understand your point. If I add rocker, I would also add a larger angle of retract to the trim tabs to ensure they aren't fighting each other.

The intent of the rocker, is to ensure I can still get enough bow lift for running in a following sea with 6 people in the pilothouse. On the flip side, while keeping enough bouyancy at the stern to keep the bow down going into a head sea.

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