Air Dams - A Question

[Graham Humphrey]

Hey Folks:

I understand how air dams will help a catamaran keep it's nose down, but I am wondering about tunnels or hydros. Does anyone run an air dam on either of these hull types? Does it help or not?

Also, even on a cat, what vertical height should be used? Is there a width-to-height of dam ratio or something?

Thanks for any advice you can share on this topic

Graham H

Ontario, Canada

 

[Grimracer]

Graham,

I have never used an Airdam on a cat or tunnel but have installed them on sport hydro. Realy its all about less air. A blunt leading edges also helps.

Grim

 

[Martin Hamilton]

Cant speak for hydros etc but on cat hulls the dam is set 2" in from the front of the tunnel going full width. Some use things like lexan angle hanging down around 1/2" whilst i know some use auto wiper blade rubber, again hanging down held on with double sided tape. Some thing as simple as a large plastic drinking straw held on with electrical tape running along its length is also very effective & works well. Anything that breaks the air flow really.

 

[jwo]

A airdam moves the center of lift farther to the rear. As the air passes under the hull it creates lift. When an airdam is installed it efectivly moves the point where the air contacts the bottom of the hull creating lift. The deeper the airdam is the farther back the lift is. A trick thing we did to our 2.5 mod was add a foot operated airdam under the hull. slower speeds dam up higher speeds dam down. From the great mind of Ron Jones sr. It really worked well. If your hull is sponson walking at speed start with a 1/2" deep about a 1/2" behind the leading edge. You will see a dramatic difference. If blowing the rear out of the water starts to happen trim it down to get the ride you are looking for.

They dont keep the nose down rather they bring the rear up.

 

[Old Sloppy]

isn't an air dam actually "adding drag" ?

would adjusting the CG % be a better answer ?

Harry

 

[jayt]

The airdam is a spoiler which creates turbulence under an air compression hull. The turblulence it produces kills the lift generated under the hull - or at least substantially reduces it. Yes, it does add aerodynamic drag, but who cares if you can't stay on the water without it? Moving the CG does not really address the problem because it only redistributes the weight - and can make a blow off much more damaging. A certain hull setup will have a maximum speed defined by the lift produced at that speed; when the lift and weight are balanced the boat stays down and goes fast, but more speed means more lift and away we go! To go faster with that hull (and stay on the water) you need to either add weight (OMG!) or reduce the lift. The latter can be best done by re-designing the entire hull to reduce the overall lift. Obviously it is a lot easier to add an air dam.

.

 

[Jerry Muro]

What would be a good size to start with 1/4" thick across the front of the pickle or round nose? And should it go from inside of sponson to inside of sponson?

 

[properchopper]

" A certain hull setup will have a maximum speed defined by the lift produced by that speed....." Really Jay ? The next thing you're going to tell us is that the Emperor has no clothes

This concept (not the Emperor thing) is one that seems to be rarely understood by far too few boaters and is often frustratingly witnessed in the realm of the enthusiastic ranks of the growing scale offshore cat sport builders. Pushing these hulls past the "maximum (by design) speed " often is an excercise in futility yet persists in spite of gentle (but equally futile) admonishons to that effect.

Still, to be competative, or to set records, the air dam "band-aid" becomes a necessary evil. I've used wiper blades (my favorite), metal bars with drilled-out holes, or the quick "pit-fix" tape job [which enabled me to set the curent NAMBA P-Ltd Offshore SAW record]. To determine the optimum location for the air dam I use the proven "inter-ocular trauma" technique - I stare at the tunnel and when the correct location for the air dam hits me between the eyes I stick it down.