35728RE: [SabreSailboat] Re: Cowl Vents - 4" with a little extra work?
- Jan 4, 2012
If your analysis is correct then you’d better not use airplanes because you have just overturned Bernoulli’s principle and the related venturi effect, not to metion all the fluid flow measurements that are done using orifice plates. Basically as the velocity of a fluid increases due to a restriction in a pipe the static pressure decreases. Furthermore the total pressure of the air at the cowl inlet is not primarily a function of cowl area. A larger cowl would probably draw in more air because its opening is, on average, higher off the deck where the air velocity is higher resulting in higher average dynamic pressure. In the case of Sabre dorades there is not only the area reduction to 1.5” but the baffle inside the box which would producie a pressure drop due to frictional loss. So, in this case I would guess that air flow is primarily controlled by the dorade baffle and the 1.5” exit diameter.
As a small matter, the frontal section of the 3” cowl is 7.07 square inches, not 4.7. In the interest of science, why don’t you try using masking tape to block off the bottom third of your vents sometime to appoximate a 3” cowl and see what you get? In the meantime I’m currently sailing my dungeon with no vents, but living in California we can get away with thatJ
From: Sabresailboat@yahoogroups.com [mailto: Sabresailboat@yahoogroups.com ] On Behalf Of Jim Starkey
Sent: Tuesday, January 03, 2012 7:05 PM
Subject: Re: [SabreSailboat] Re: Cowl Vents - 4" with a little extra work?
Perhaps some analysis might help. A round 3" cowl has a frontal section of 4.7 square inches. A round 4" cowl has a frontal section of 12.5 square inches. The internal orifice is probably only 1.5" wide or 2.35", the pressure from the large cowl will be 2.65 times the pressure of the 3" cowl. Somebody else can take it from here, but the amount of air circulating through the cabin will be substantially greater with a 4" cowl than a 3" cowl.
My current (manufacturer problematic) boat has 4" cowls and approximate 2" vents. In a modest breeze, we need to close the vents (a feature unknown to Sabres) so the dinner candles don't blow out.
If you like mildew and cherish the wet boat smell, by all means go for the smallest cowls possible. Sabre was a production boat builder and $.75 per boat saved was $.75 in the stockholders pocket, market share not withstanding.
So I guess the question is whether you want a dungeon or a castle. Dungeons are a little cheaper, but...
On 1/3/2012 7:45 PM, JohnF wrote:
Peter - that's a great thought. The actual vents into the cabin are tiny. More air through a bigger cowl probably isn't going to make a difference.
--- In Sabresailboat@yahoogroups.com, "vanbeckump" <vanbeckump@...> wrote:
> I am thinking that the restrictions of the dorade and particularly the tiny vent into the boat's interior may limit the air flow to the point that the larger cowl may not be worth it. One also has to worry about the exterior size of the cowl. You don't want to fall against it and pry it loose or bend it. Big cowls usually have a stainless tubing cage around them for protection.
> Just sayin'
> --- In Sabresailboat@yahoogroups.com, "JohnF" <jl_folk@> wrote:
> > The old original Martec cowl vents were removed yesterday on our 1987 34 MkII (thank you 55 degree Maryland weather!). In reviewing the archives, it looks like most people have replaced with 3" cowl vents, often requiring a teak or starboard 'pad' to accommodate a smaller deck ring.
> > Has anyone gone to the effort to slightly modify the dorade box to install 4" vents? The holes in my deck are 3 11/16 inches, somewhat bigger than is needed for a 3" vent, and slightly smaller than is needed for a 4" vent.
> > It's tempting -- the 4" vents are only an inch taller (so I don't think they would look out of scale), but have 40% more air flow capacity (based on Vetus catalog).
> > I don't see any references to this approach, so am suspecting that there is a hidden 'gotcha !!!'.
> > Thanks -
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