hull bottom vents

I was looking over some high performace sites and notice lots of hull vents, can anyone explaing how they work and if the benefit is a function of the boats speed or will they work on any planing hull?

 

If you had a look at a few sites you will have seen there would be some major reconstrucion to be done to make this work . Most hulls are specially made . put simply you are wasting your time !! But if you want to spend a load of money and are really interested the performance gained could be disapointing !!, performance is all about reducing wetted area changing a existing boat is a wast of time .
Lots a power with the motor set proprerly and a light weight balanced boat trim it so it just touchs the hull on the water on the last one foot of area and make it fly !! then you are going fast .

 

Brokensheer, I'm assuming that you are asking about stepped hulls. As Tunnels pointed out, speed is about reducing the wetted surface and hence drag produced. With a stepped hull, water rushing along the bottom at speed comes off the back of the step and reforms again further aft on the hull leaving an air gap, effectively reducing the wetted surface. This "unwetted" area needs sufficient vents to remain unwetted, or to not produce a low pressure area (sort of vacuum) which would negate some of the performance gains and possibly produce some funny handling at speed. Stepped hulls are meant to go fast ("fly") with absolute minimum hull touching the water and are not too happy at low speed because of the extra drag/turbulence created by the immersed step.

Would this work on any hull? ...any hull that was designed to use steps, yes, but you'd better know what you are doing if you want to add steps and vents to an existing hull. The same step principle is generally used on aircraft floats, with the aim of getting as much of the float out of the water as quickly as possible (within the shortest distance) so that the aircraft can reach take off speed before the end of the runway/waterway comes up. Again, these are specifically designed with the aircraft weight and balance in mind. I haven't ever seen additional vents on aircraft floats though, except for the natural vents on each side.

 

Venting a hull bottom typically makes it sink.

http://www.boatdesign.net/forums/al...voyage-costa-concordia-cruise-ship-41313.html

 

Theres one born every day in your case you must have been twins !!

 

Couldn't help it.

 

Thanks Tunnels, & X19, you confirmed my thoughts!,,

 

Post some photos of what you are looking at. Typical are steps which are "vented" at the chines. Search the forum for "step" and there are a number of threads on steps.

There is also: http://www.boatdesign.net/forums/boat-design/air-ventilated-planing-hull-design-38431.html

 

I hadn't really considered it until I read what FMS posted, but I suppose that some (if not all) power tri-hulls and cats would qualify as an air ventiilated hull. I remember an old boat designer explaining about his his tri-hull designs being very efficient due to the amount of "foam" formed between the hulls that produced the required lift with much reduced drag. He was a big fan of reverse chines too I suppose for much the same reason.

 

The thing of skin friction and air ventilated hulls etc etc is all about going fast ,so unless you are doing 60 mph plus anything below that is a waste of time and effort !!. The bubble thing i keep hearing about getting to the propeller is just a matter of lowering the motor 5 mm maybe so the bubbles in the waters surface are just ablove the tips of the prop!! Personally no one really knows !!its all speculation and here say !.
The most wonderous thing i did to my own boat was to push the motor 450mm back on a new transomcome bracket with a differance and i was able to raise the motor 20 mm above the botton of the hull line !! wow that made it go !! and the trim is beautiful and positive to use .Now i can watch the chine spray out the side of the hull shift from the front to almost the aft corner of the hull at 55 knots plus .its given control to the trim button and can raise the bow effortlessly at just the touch of the button .
I have only done stage one to my boat so far and when i get back to nz later will do stage two which will be a whole of bottom (chine to keel , bow to stern) modification with wide lifting strakes to get the hull even higher out of the water and go even quicker and stable at speed !!. No i dont want a flat ski pad on the bottom !!

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Here is an excellent thread about a stepped-hull construction.

http://www.boatdesign.net/forums/powerboats/actual-real-life-boat-being-built-30490.html

 

Recent articles in Professional Boatbuilder magazine have alluded to allowing air into the surface on boats by Buzzi.His record would suggest that it is a topic worth further investigation.

 

Here is another thread on the topic of stepped hulls: http://www.boatdesign.net/forums/boat-design/stepped-hull-resistance-35492.html#post418688