Creating lift in displacement hulls

[JordanMay]

- Yes, Im going to find out by placing similar ribs on the stern extension of the Roughwater 41.
- The oblong buoys are nearly same length and weight...2.2 feet.... but the ribbed ones are a bit fatter with more rounding.
- I like comparing "dissimilar bodies in water" because they produce dissimilar results. According to interceptor threads on this website they are being placed midship to create lift forward of transom. On ridged buoys...it shouldn't work but it does and thats why Polyform sells thousands of these "Low Drag Buoys" in the commercial fishing industry. Before these buoys came along all the gear would be under water at max ebb and flood. Now the trailing buoy stays on the surface. The poly line doesn't stretch at all before the skipping starts and it happens with a light pull right up to 10 knots. The ridges don't create drag at least in the form of slowing the body or creating downward force. But yes it's erratic-doesn't track straight. I'll have to put a little motor on one of these buoys to determine if the front is forced down.

The Hydraskegs on stern of 400 foot barges increase speed/reduce fuel consumption at low speed. If the ridges don't work I may build one aft.
Hydralift Skeg Use Gaining Speed https://www.marinelink.com/news/hydralift-gaining-speed300960

Thanks for the feedback all.

 

[Ad Hoc]

.... Is it possible there's another dynamic that can affect water flow?.......

No, simple hydrodynamics, no magic.

....But tow the oblong buoy with rings around it and it skips over the surface....

Reread my post about flow separation and the reasons why…

Just to be clear, you need your ducktail to be a near vertical transom, and for the large bilge radius to come to a near chine, like so:

Keeping the bottom flat, and as near horizontal as you can, or a simple extension of the keel line.

 

[HelmutSheina]

Are the oblong buoys roughly the same size and weight? It's an interesting issue.

I'm thinking the ridge acts in a similar manner to a stepped hull, and the models are mass produced, just need a YouTuber to do a slick video.

 

[JordanMay]

Ahhhh....thank you Ad Hoc! A light just came on after re reading flow separation. Soo...the water flows aft along the straight hull but then it's pulled up by the shape of stern. Correspondingly, water's heavy ... so it is also pulling the stern down...considerably. so I will square the stern next haulout.
I still need to identify what interupting that separation zone does physically. Does placing ridges across that zone, like the Low Drag Buoy, cause pressure in front of ridge?....or a vortex behind the ridge. Or both?

 

[fallguy]

My 2 cents on the buoy, based on intuition.

The “low drag” buoy is not low drag per se.

it is all about the vectors of force on the buoy

if you are towing a buoy; the rope drags on the water; then the force acting on the buoy is downward

if the buoy has a few ridges; they increase the drag on the buoy and work to pull the line or bow up, and modify the direction of the force at the leading edge, but calling it a slightly more drag buoy wouldn’t sell

Please pay attention to Ad Hoc, you probably do not realize the good fortune you have with him contributing freely.

 

[rxcomposite]

Seems a little too late but you might want to investigate several semi displacement hull forms. If you have your hull lines, superimpose it in one of the hull form attach for "best fit". These are tank tested hull lines and the working parameters are either in the net or available (for a fee) from a guy who posted here sometime ago.

Be prepared for some LCG shift.

I have seen some done in Dubai but all mods were above waterline. No change in hull shape. It was sort of a deck extension for lounging, not to gain power efficiency or added speed. It is much like adding a swim platform, only larger.

 

[rxcomposite]

My question with pics.....for anyone with an answer. Tow this round buoy at 5 knots and it dives underwater. Pull the oblong buoy and it dives partially under....But tow the oblong buoy with rings around it and it skips over the surface. It can't be just the shape or the oblong without rings would skip.

The first one is like a highly prismatic hull like a tugboat. The harder it pulls the more water goes around it and it sinks deeper. For the buoy/fender, pull it harder and the third law of Newton takes over. It bounces up, pushed upward by the water.

Remember the dimpled golf balls? the dimple is there to create disturbance, lets the airflow adhere to the ball much longer, reducing drag. There are other forms of this "turbinators".

In aircraft, they attach "vortex generators" near the leading edge of the airfoil. "small, aerodynamic vanes—often found on aircraft wings, wind turbines, and cars—that create tiny vortices to re-energize the boundary layer of air flowing over a surface. By mixing high-energy air into the slow-moving boundary layer, they keep air attached longer, reducing flow separation, stalling speeds, and drag while improving control effectiveness."

In water tank test, they attach "trip wires" in the bow of the ship model. It introduces turbulence and is used as a corrective method for errors introduced by the model tank. The same principle used in the ribbed buoy/fender. It keeps the boundary layer attached much longer.