Deck hole size for dorade box

[sanibel sailor]

My current Dorate boxes are shot. They have 3 inch Cowls. I'm building new ones fitted with 4 inch cowls.I am planning on enlarging the hole in the deck as well. I'm going to use PVC pipe for this with a threaded fitting on the inside. This will allow me to shut it off with a plug if needed. I'm considering whether to use 4 inch or 6 inch pipe. Four-inch matches the vent. Six inch has less restriction and might let in in more air and has a higher lip, perhaps preventing water intrusion better.The 6 inch just looks awful big however. Any thoughts on an oversize deck pipe?

[Sailormantx]

The size of the cowl determines amount of airflow. I have installed the same on my boat and it works well. I have 5" cowls and 4" pipe fittings as I could not find any in 5" and thought that 6" was too big. The part projecting above deck is also a good place to hose clamp some bronze screen to keep bugs out.

[georgelieber]

Use the 4". If you want greater airflow, install a solar powered ventilator cap.

Marinco, 4" Day / Night Plus Vent Solar Light w/ Stainless Steel Cover, N20704S

[JPA Cate]

Hi, John,

The 4" will do noticeably better than the old 3". The cowl size (as mentioned above) is the limiting factor.

For underway in hot, steamy places, I'm a greaty fan of fans, strategically placed, for making life below decks better.

Also, for at anchor where it rains a lot, we made a hatch dodger that works on the dorade principle, and can leave the hatch open up to about 25 knots, after that, the wind blows the bouncing rain drops in, below that, it allows a lot of air in.

Ann

[Cadence]

Sailormantx had a great suggestion with the screen If you have the room go with screen and a six inch since the screen will inhibit air flow.

[Sojurnr]

[QUOTE=Ann T. Cate;1871866]Hi, John,

"The 4" will do noticeably better than the old 3". The cowl size (as mentioned above) is the limiting factor."

Roger that, the difference between the 3" and 4" diameter pipes results in nearly twice the area (12.6 vs. 7.1 square inches) and thus nearly doubles the potential airflow.

Sojurnr

[sanibel sailor]

And I just pulled the trim ring and looked at the old deck fitting which measures about 2 1/2" ID. Four inch will be almost 3 times the area.

The limiting factor will be the size of the cowl, but restrictions are cumulative. Six is still an awfully big hole in the deck. I will go with 4" and can always make it bigger if needed. Bronze screen hose clamped to top is a great idea. I had been playing with ideas to make the screen pretty from the inside. Outside seems best.