Air Flow

[John Seybold]

Hi,

Just wanted to get some thoughts on whether or not holes are needed in cowls? What have you guys found out as if they’re necessary or not?

For those of you that do cut holes in the cowls, do you have a cool way of doing it? I’ve seen some that just had a drill hole in it, triangle cut. I’m sure some have come up with same nice ideas.

Thanks

 

[PaulHail]

If you have experience making molds it would be fairly easy to make a mini naca duct and glass it in place. Dimensions aren't super critical and they look cool.

 

[Dennis Somers]

Model sized naca ducts are available on the net, and work very well for fresh air IN. But your exh. heats and expands the air under the cowl, so you need plenty of EXIT area relief, for the heat to get out.

 

[Grimracer]

They are.. you would not be leave the true amount of air coming in to the carb.. a NACA duct is not necessary but I have made them in the past and there are rules to it to make them work properly. Ether way you need an air inlet.

Grim

 

[Hydro Junkie]

I think Grim has it, air to the carb. The modern turbines aren't a problem with that big scoop.
The old boats, on the other hand, are an issue. Lack of air intakes will starve an engine. Depending on the cowl shape, you could get by with just exhaust stack holes to let excess hot air out though some boats have had extra vents "creatively" added to help with the air flow. Heat isn't really the problem, with the modern water cooled engines, it's air to the carb without the carb getting showered with water

 

[Mike Cathey]

I saw a gas scale Oly lose the cowling over the motor in the chute and the motor immediately picked up several hundred RPM. Looked like it grabbed another gear.

 

[julianconde]

There’s a reason a lot of saw boats don’t use Cowlings on boats with the exception to a few

 

[John Seybold]

Thanks everyone, been searching for some naca ducts and can’t seem to find any of various sizes. If anyone knows of a place, it would be greatly appreciated.

 

[Keith Halverson]

Hobbyking has these,,or did have.
https://hobbyking.com/en_us/naca-style-inlet-duct-set.html

Bought a big set/kit of various different sizes and shapes a long time ago. Have used most of them up except a few of the very small ones. .060 lexan material. They were made in the US,,but that company has long since set sail. RC Works was the company I think. Kit came with a large square piece of tight weave SS grille screen material. Used them mostly for relieving electric speed controllers of overheating on EDF jets. They worked equally good at letting air in,,as well as reversing them to suck hot air out as per the readings.

 

[John Seybold]

Thanks Keith, those seem to be the only ones I could find, to a bad there not in stock.

 

[XtremeRcBoats]

John not sure what size you are looking for
https://www.mhz-powerboats.com/shop/en/hulls/air-scoops/386/naca-duct-air-inlet-gfk

Same as what Dennis wrote, I always keep as rule of thumb outlet area shall be 3 times inlet area.

 

[Hydro Junkie]

John not sure what size you are looking for
https://www.mhz-powerboats.com/shop/en/hulls/air-scoops/386/naca-duct-air-inlet-gfk

Same as what Dennis wrote, I always keep as rule of thumb outlet area shall be 3 times inlet area.

It's interesting you say that. That is the same basic rule that R/C aircraft builders use to prevent an engine from overheating

 

[Timbo Whalen]

Hi,

Just wanted to get some thoughts on whether or not holes are needed in cowls? What have you guys found out as if they’re necessary or not?

For those of you that do cut holes in the cowls, do you have a cool way of doing it? I’ve seen some that just had a drill hole in it, triangle cut. I’m sure some have come up with same nice ideas.

Thanks

This scheme is working well on my D hydro, with no water impingement coming from other boats. Of course, this

is for a down draft carb setup. We added bleed ports that aren't shown in underside photo.