Install mid ship hatch open forward or backwards ?

[dfelsent]

Our bow hatch opens aft. The two midship hatches open in opposite directions, one aft, one forward. The aft cabin hatch opens forward.
Both companionways open aft.
There are canvas covers that help keep the rain out of the opening portholes. And canvas to help keep the rain out of the aft opening hatch.
I have wind scoops that work wonders as well.
It’s all about choices. And sometimes fans. [emoji1]

[GafferMate]

Pilar's skylight opens both ways, fore or aft, with custom hinges, pivoting from center.


There are over-center latches at each end of the hatch to secure skylight when needed, with thumb screws to tighten at the middle. If I do manage to upload the photo, the fore/aft latches had not yet been reinstalled when the photo was taken.

The lifting rails were easy to make from brass barstock, their end-fittings cut from brass rod, soldered to a plate and drilled and threaded for an acorn nut to encapsulate the end of each slotted rail. Works GREAT, and adding fitted 'side screens' of triangularly cut plexiglass adds further draft, as mentioned in another post.

[dfelsent]

Originally Posted by GafferMate

Pilar's skylight opens both ways, fore or aft, with custom hinges, pivoting from center.


There are over-center latches at each end of the hatch to secure skylight when needed, with thumb screws to tighten at the middle. If I do manage to upload the photo, the fore/aft latches had not yet been reinstalled when the photo was taken.

The lifting rails were easy to make from brass barstock, their end-fittings cut from brass rod, soldered to a plate and drilled and threaded for an acorn nut to encapsulate the end of each slotted rail. Works GREAT, and adding fitted 'side screens' of triangularly cut plexiglass adds further draft, as mentioned in another post.

Very very nice!

[Alan Mighty]

Originally Posted by Knotical

Alan, I am curious about this thought - I have been on several boats and whenever the forward facing hatch is open I always feel the wind coming from the bow going towards the back, I have never felt the “reverse” (this is only on boats on swing moorings or at anchor).

My point is that decisions about passive ventilation should be driven by evidence, in the spirit of seek truth from facts. Not assumptions or guesses.

So anchor the boat in a location where it will be ride to the wind, not the tide. Check what happens with below-deck air flow. That may differ at different wind speeds - in stronger winds a forward facing wind scoop may direct flow below decks; in calmer conditions, reverse flow below decks might be the rule.

Passive ventilation is generally not a problem on a windy day - you'll easily achieve your target whether it be in volume change rate (VCR) or air changes per hour (ACH).

A calm day with oppressive tropical conditions might call for a very different strategy.

Check out the direction of below decks flow in the diagram accompanying this Practical Sailor story by Drew Frye:

https://www.practical-sailor.com/blo...at-ventilation

Dave Gerr's Boat Mechanical Systems Handbook has an excellent chapter on ventilation including how to set VCR or ACH targets. Gerr includes Rod Stephens's 'vent rule' for tropical/oppressive conditions.

The only draw-back to Dave Gerr's chapter is his reliance on diagrams from Marinco/Nicro Ventilation Systems. Those diagrams of course suggest that one ought consider adding Nicro vents.

Drew Frye's story in Practical Sailor provides a better whole-of-boat view. But not all boats fit that general scheme (of below decks air being drawn forward by low pressure generated by wind speed over the fore deck).YMMV is the rule. So seek truth from facts.

[GafferMate]

...am reminded of another ventilation idea for those with round portholes...though square could also be fabricated. We found some thin bronze plate (scrap) for cutting and rolling to fit PILAR's portholes, which again can be rotated to exhaust or induct wind (additionally, a slight bit of rain protection, too):

...hope this broken image heals when posted...

[Alan Mighty]

Originally Posted by Alan Mighty

Dave Gerr's Boat Mechanical Systems Handbook has an excellent chapter on ventilation including how to set VCR or ACH targets. Gerr includes Rod Stephens's 'vent rule' for tropical/oppressive conditions.

The only draw-back to Dave Gerr's chapter is his reliance on diagrams from Marinco/Nicro Ventilation Systems. Those diagrams of course suggest that one ought consider adding Nicro vents.

I add that I have no problems with the fine products of Marinco/Nicro Ventilation Systems. I have a couple of solar-powered vents that are more than a decade old and still work quite fine.

I further add that vents are important - when rain is falling or the boat is buttoned up for whatever reason, hatches are likely to be closed and that's when vents and the passive or active ventilation provided by them is way more important that whether the hatch hinge is forward or aft of the hatchway.

[DefinitelyMe]

Originally Posted by Alan Mighty

I add that I have no problems with the fine products of Marinco/Nicro Ventilation Systems. I have a couple of solar-powered vents that are more than a decade old and still work quite fine.

This has not been my experience at all. I've tried the solar ones about three or four times on different boats and they've all died within six months. Even the totally passive ones (we have three cowl vents with water traps) lasted about 5 years before the glue failed, the plastic yellowed and then cracked. In my view they are over-priced, provide little ventilation and don't last nearly as long as they should.

[Alan Mighty]

Originally Posted by DefinitelyMe

This has not been my experience at all. I've tried the solar ones about three or four times on different boats and they've all died within six months. Even the totally passive ones (we have three cowl vents with water traps) lasted about 5 years before the glue failed, the plastic yellowed and then cracked. In my view they are over-priced, provide little ventilation and don't last nearly as long as they should.

I've heard similar reports. My Nicro vents are from older batch, more than 10 years and perhaps older!

[GordMay]

Drew Frye’s ‘PS article, “Simple Tips to Improve Boat Ventilation” rightfully says:
“... The companionway area has the lowest pressure. Open a hole anywhere forward and air will be sucked out the companionway...”
➥ https://www.practical-sailor.com/blo...at-ventilation

As we all know, air/wind will flow from higher pressure, to lower pressure.

See also: ”Ventilation Can be Improved in Almost Any Boat”
➥ https://www.practical-sailor.com/boa...lmost-any-boat

[GordMay]

More air exchanges improve Indoor Air Quality [IAQ].

I agree with the PS articles, that suggest a minimum ventilation level of 1 air exchange per hour.

The rate at which outdoor air replaces indoor air is described as the air exchange rate.
ASHRAE (formerly called the American Society of Heating, Refrigerating and Air-Conditioning Engineers) recommends (in its Standard 62.2-2016, "Ventilation and Acceptable Indoor Air Quality in Residential Buildings") that homes receive 0.35 air changes per hour, but not less than 15 cubic feet of air per minute (cfm) per person, as the minimum ventilation rates in residential buildings.
ASHRAE also suggests intermittent exhaust capacities for kitchens and bathroom exhaust, to help control pollutant levels and moisture in those rooms.

[Alan Mighty]

Originally Posted by GordMay

More air exchanges improve Indoor Air Quality [IAQ].

I agree with the PS articles, that suggest a minimum ventilation level of 1 air exchange per hour.

Dave Gerr, if my reading is correct, aims higher than that minimum.

My reading was that his aim was to state guidelines for cruisers in conditions including Florida and the humid tropics.

I'm not interested in infringing Dave Gerr's copyright. I'll attach for just 24 hours a table from the work of his I cited, giving the Volume Change Rate for various spaces on a cruiser.

[GordMay]

Originally Posted by Alan Mighty

Dave Gerr, if my reading is correct, aims higher than that minimum.

My reading was that his aim was to state guidelines for cruisers in conditions including Florida and the humid tropics.

I'm not interested in infringing Dave Gerr's copyright. I'll attach for just 24 hours a table from the work of his I cited, giving the Volume Change Rate for various spaces on a cruiser.

I wouldn't presume to disagree with Dave Gerr.

The ASHRAE recommended ventilation rates for schools, offices, shops, restaurants, and homes varies from 0.35 to 8 air changes per hour.

When dealing with places that may contain viruses, the recommended air changes per hour are higher, approximately 6-12.

As I said, previously, more is better.

[Knotical]

Originally Posted by Alan Mighty

My point is that decisions about passive ventilation should be driven by evidence, in the spirit of seek truth from facts. Not assumptions or guesses.

So anchor the boat in a location where it will be ride to the wind, not the tide. Check what happens with below-deck air flow. That may differ at different wind speeds - in stronger winds a forward facing wind scoop may direct flow below decks; in calmer conditions, reverse flow below decks might be the rule.

Passive ventilation is generally not a problem on a windy day - you'll easily achieve your target whether it be in volume change rate (VCR) or air changes per hour (ACH).

A calm day with oppressive tropical conditions might call for a very different strategy.

Check out the direction of below decks flow in the diagram accompanying this Practical Sailor story by Drew Frye:

https://www.practical-sailor.com/blo...at-ventilation

Dave Gerr's Boat Mechanical Systems Handbook has an excellent chapter on ventilation including how to set VCR or ACH targets. Gerr includes Rod Stephens's 'vent rule' for tropical/oppressive conditions.

The only draw-back to Dave Gerr's chapter is his reliance on diagrams from Marinco/Nicro Ventilation Systems. Those diagrams of course suggest that one ought consider adding Nicro vents.

Drew Frye's story in Practical Sailor provides a better whole-of-boat view. But not all boats fit that general scheme (of below decks air being drawn forward by low pressure generated by wind speed over the fore deck).YMMV is the rule. So seek truth from facts.

Thanks for the response, Alan. We get a lot of those calm conditions in western Long Island sound during summer, I will make sure to pay attention to the air flow direction down below.

[Amapola]

Forward for airflow

[MAJICDAN]

Forward and midship hatches should open forward to pull in the air when anchored.
These hatches should never be open when at sea.
The rear hatch on a center cockpit boat faces backwards and can sometimes be open at sea.