hey everyone, just bought a cowl vent for my dorade box but I'm wondering should the opening be facing forward or backward?
it is located about midship above the head
it is located about midship above the head
dorade box cowl vent
- Thread starter fred1diver
- Start date
Because of the water drains in the box part it will not pull air out of the boat. It is an in for air, not extraction, so as above face it forward.
I think this is one of those "sounds good on paper" but in real life it isn't so. Dorade box drain holes tend to be very small, maybe a quarter inch, large enough to drain the occasional wave or wind driven rain. The cowl has a large opening that funnels air into a slightly smaller opening, this pressurizes the dorade, with the bulk of the fresh air going through the 3 or 4 inch opening and into the cabin.
Besides, if a lot of green water is coming over the bow, turn the cowl around to prevent water from coming out, the vacuum will still pull some air out.
There used to be ventury vents with a smaller cone intake to windward and a larger one down wind. Those would pull air out of a cabin, but ours sure won't with just the curved cowl pointing aft. We do indeed turn them around when going sailing, but that is to keep water out, not to get air out.
This certainly isn't the case on my boat; the Dorade scuppers are quite large, could get rid of all of the water the cowls could swallow, in my opinion. I would guess there's at least as much scupper area as cowl input area. I guess it will vary by boat.
If that was the case, then your dorades would be ineffective. There needs to be enough aerodynamic resistance in the drain holes to force the air to flow into the cabin. Looking at the photos @capta posted, it appears the drain holes are fairly small, perhaps ¾" x ⅜". That would yield roughly 1 square inch of drainage. A cowl with a 4" diameter (and a very small cowl at that) has about a 12.5 in^2 opening to allow air to flow in. I think it is a safe bet that the outlet for his cowls is at least 3" in diameter, which would give it about a 9.5 in^2 diameter opening into the cabin.
For this semi-hypothetical example, if we add the drain hole openings to the exit hole openings we would have about 10.5 in^2 of exhaust and about 12.5 in^2 intake. With any pressure at all on the cowl, a large amount of air will be forced down into the cabin. Since I'm not an engineer I won't try to calculate a precise amount, however, the air lost to the drain holes is a fairly small amount. Based on surface area alone, only about 10%.
Way back when, S&S had a formula (I believe) of the size of the scuppers. Like Dave said, size is important as the scuppers exhaust air under pressure as well as water. Plus the height of the standpipe or divider and the volume of the box were taken into account. We still haven't improved on them for a boat out at sea taking spray and green water over the decks.
As I've built several dorade boxes, I played around with the idea of a floating flap inside. The idea: Simply a small piece of wood held with one, not tight screw in an enlarged hole, that would simply float as spray and green water would fill the box, and open the scupper.
This was for a foredeck mounted dorade box that is over the anchor locker and doubles as access to a hose for anchor washing. I really appreciate that 4" cowl vent blowing fresh air into and through the anchor locker. The rode and chain dry quickly and there's never an odor. Plus the deck plate to access the deck wash is clear and shines daylight into the locker below.
In the end, I just kept the scuppers to the same design as the originals knowing that I've never overwhelmed those.
If dorade boxes have plastic tops, as well as providing full time passive ventilation, they act as day lights below, nice in our boat as it brightens the head and passageway forward. This is the foredeck box over the anchor locker.
As I've built several dorade boxes, I played around with the idea of a floating flap inside. The idea: Simply a small piece of wood held with one, not tight screw in an enlarged hole, that would simply float as spray and green water would fill the box, and open the scupper.
This was for a foredeck mounted dorade box that is over the anchor locker and doubles as access to a hose for anchor washing. I really appreciate that 4" cowl vent blowing fresh air into and through the anchor locker. The rode and chain dry quickly and there's never an odor. Plus the deck plate to access the deck wash is clear and shines daylight into the locker below.
In the end, I just kept the scuppers to the same design as the originals knowing that I've never overwhelmed those.
If dorade boxes have plastic tops, as well as providing full time passive ventilation, they act as day lights below, nice in our boat as it brightens the head and passageway forward. This is the foredeck box over the anchor locker.
I have a stainless steel guard mounted on my dorade box to keep the sheets from snagging. Did that after loosing two cowls.
They must be in the right place (no act on my part), Roy because I don't recall sheets catching them except maybe once or twice. The cowls do have a small rod from the lower lip to their base which leaves less places to hang up.
I think the location is why. The 2 on the house are just abaft the mast. The headsail sheets don't reach those and the the mainsheet isn't slack in that area. Then the forward cowl and box is mounted forward of where headsail sheets flog (they DO catch on mast winches, too often,...).
Here's an old ocean racer with the same locations and no guards.
After a hundred years of yacht design, the old stiffs in the Alden office probably knew ALL the wrong places to mount cowl vents on dorade boxes.