Air dam lose speed?

[Beaux]

I think I'll try that on mine,

[trialsguy]

My Genesis had issues flipping, mainly because we run in ocean water that has a mixture of strong current and wind. I had to drop the strut to the point it was slowing down the boat to prevent flipping. Even with the batteries far up in front in the sponsons and the ESC all the way to the front, I still had this issue.

However, once I stuck an airdam on the boat, I was able to put my strut up to even neutral, and actually picked up about 5-7 mph while no longer flipping.

So whatever speed I lost because of the airdam, I gained back in stability and better trim which resulted and even higher top speed (low 70s).

Here is picture of my airdam, its just a Kintec carbon fiber airdam for UL-1 ($10), which I cut down to fit my Genesis. I used 3M double sided outdoor tape to install it, has been there for over a year in salt water, hasn't come loose.

[ATTACH=CONFIG]141085[/ATTACH][ATTACH=CONFIG]141086[/ATTACH]

Very cool. I noticed that your air dam is farther back from the front of the tunnel (as compared to what I've seen on full size unlimited hydros). Does it make much of a difference how far back from the front you place it? Or just put it anywhere near the front?

[SD Eracer]

Its a 37" hull, and rather narrow, so farther back probably isn't an issue. Normally, I think on on a hydro you put it about 1.5 inches from the front, on my Genesis its about 2.5 inches.

It's also hard to see in the photos, but there is a pretty strong curve on the hull just before the air dam, part of the reason why I put it there was because that is where the hull leveled off so the airdam would seat flush with the hull.

It made a strong difference in how that boat performed. SAW runs went from flipping almost once every single 2 minute run, to never flipping at all. And that's after I was able to raise the strut back to neutral. Higher top speed and more stability because of this little piece of plastic.

I tried a straw earlier, taped it on there, but didn't have quite have the strong effect as square piece of carbon fiber. I almost gave up on the idea of an airdam, glad I decided to give that carbon airdam a try.

[SlickZERO]

Any advise where to put a dam on an Aquacraft Lucas Oil Cat? With M445 prop (well S&B) and strong batteries it will run 45 mph but usually ends up blowing over.

[fweasel]

Any advise where to put a dam on an Aquacraft Lucas Oil Cat? With M445 prop (well S&B) and strong batteries it will run 45 mph but usually ends up blowing over.

Normally, I think on on a hydro you put it about 1.5 inches from the front

I'd start there... he seems to know what he's talking about.

[Fluid]

Lots of misunderstanding about the use of an air dam. There is very little "lift" generated by the deck. Just like an airplane wing, lift is generated by air pressure under the wing, not on top. Hydros and catamarans are air-compression hulls which rely on increased air pressure in their tunnels to keep the hull out of the water. An air dam normally does not prevent air from entering the tunnel (it is placed too far aft for that) but it does cause major turbulence particularly in the front of the tunnel - which is where the lift causes problems like blowovers.

We started using dams over 20 years ago on sport hydros, back then speeds were low and hatch tape air drams worked wonders. At today's speeds a stiff dam is required.


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[SD Eracer]

Any advise where to put a dam on an Aquacraft Lucas Oil Cat? With M445 prop (well S&B) and strong batteries it will run 45 mph but usually ends up blowing over.

My Lucas Oil is mostly stock, but running an Aquacraft 2030kv motor and X642 prop, it will hit speeds of 52 mph in saltwater. As Fluid mentioned a stiff airdam is needed but because of the slightly rounded hull where you would attach the airdam on the Lucas Oil / Motley Crew hull, it makes it hard to put one on there.

I have a Genesis and UL-1 Hydro that have carbon fiber 1/2" by 1/2" block airdams. Does wonders for their performance, especially at 70+ mph on the Genesis.

However, I found a solution for the Lucas Oil right here on OSE. Its the Hugs Racing air foil: http://www.offshoreelectrics.com/pro...g-motley-lucas


I did a write up as well: http://forums.offshoreelectrics.com/...-by-Hug-Racing

[TheShaughnessy]

Bernoulli's law of lift http://www.physicsmyths.org.uk/bernoulli.htm . If lowering the nose is the goal then you will want the air traveling over the hull to be going the same velocity (or slower for downforce) as the air going under the hull. If air going over the hull is traveling faster then the air going under then a low pressure area is created on the deck resulting in a lifting force from below.

[kfxguy]

Allow me to update with some good info.

I first tried an airdam on my 27" mini Rivercat. It was doing allow 70's and it was unstable. Basically it was grossly over powered. I really couldn't give it all it had because it would blow off the water. Trimming it negative is something I didn't want to do and was a last resort because the boat is so small and thin that I was afraid it could easily dive into the water if I brought the bow down too much. So I put a piece of carbon fiber tubing in the front of the tunnel. I think it was 6mm in diameter. Next outing I was able to get it to full throttle. Result? A stable and unflighty 92mph.

Next boat. 32" Rivercat. Best speed was a fairly stable 98mph. After the airdam it went 100 and seemed glued to the water. It actually almost looked slow it was so stable.

Then I tried it on my 37" shocker. Best speed was a consistent 98 mph. Never faster. Add air dam and it went 99.9mph. Pretty consistent to what happens with the 32". Was stable either way but I was thinking I may pick up some speed because the 32" did. It did. All of them have the same carbon tubing. Like this:


At this point it was glued on to some tape. It's now permanently attached.

[Fluid]

Bernoulli's law of lift http://www.physicsmyths.org.uk/bernoulli.htm

Even aeronautical engineers argue over exactly causes lift. I prefer NASA as a reference:

https://www.grc.nasa.gov/www/k-12/airplane/wrong1.html

Nice video of air flow:

http://www.cam.ac.uk/research/news/how-wings-really-work

But boats aren't planes. Ground effect and the related air compression are a lot different than normal flight. And air dams work far far better at killing lift than stall strips on the deck, this is what matters.


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[TheShaughnessy]

thanks for the read Fluid, guess it's true, school is where they teach us lies. I'm gonna have to bring this up to my physics professor on Tuesday since he lectured on Bernoulli's principle last semester. It's a simplification anyhow. I was just questioning this the other day because i was thinking about symmetrical airfoil and interestingly that is brought up in one if the articles.

[boredom.is.me]

Bernoulli's law of lift http://www.physicsmyths.org.uk/bernoulli.htm . If lowering the nose is the goal then you will want the air traveling over the hull to be going the same velocity (or slower for downforce) as the air going under the hull. If air going over the hull is traveling faster then the air going under then a low pressure area is created on the deck resulting in a lifting force from below.

As long as you understand that Bern's principle applies to open areas. It's a completely different game when an "airfoil" is traveling within close proximity of a surface. So pretty much your entire statement is invalid as you aren't accounting for a physical change in pressure, not just an induced.

ugh...late again...

[MarkF]

Travis
Your boats are blowing over because your cg is too far back. Our fast cats had the cg at 40 to 45% to keep them on the water without adding air dams. CG on an oval boat is different from a saw boat.

Mark

[TheShaughnessy]

As long as you understand that Bern's principle applies to open areas. It's a completely different game when an "airfoil" is traveling within close proximity of a surface. So pretty much your entire statement is invalid as you aren't accounting for a physical change in pressure, not just an induced.

ugh...late again...

yup i made an over simplification

[pistol18]

Orange crayon air dam!!

On a good note, the boat would blow off at 45-46 and hasn't gone over since the crayon LOL