Ventilation on a boat

[freedom44]

I recently read again Eric Hiscock’s “Cruising under Sail” which is a delight.
I was intrigued by his explanation how ventilation occurs in a boat. He explained that if a forehatch which opened facing aft was left partially open that it would suck air from the boat and through the open companionway and cause it to exit from the forehatch.

As many hatches are capable of being changed from forward to backward facing, is it still better to set them up as above, or is it better to design them as “common sense” would dictate ie opening facing forward so as to scoop as much fresh air into the boat.

As many boats now have considerably more hatches than in his day when typically there was just a forehatch and companionway, does his suggested practice fall down when you have, say, a large main cabin hatch?

I appreciate the benefit of aft facing hatch from the perspective of spray not getting below. I am just interested in the ventilation aspect.
I look forward to your opinions

 

[Mirelle]

About six years ago I finally noticed that the large Dorade boxes and four inch cowls on Mirelle were practically useless, except for looking theatrically seamanlike and for catching ropes.

Off they came, and were replaced with big mushroom vents which we screw down at sea. The forehatch opens aft and has "ears" and is left part open with the "ears" turned up when on the mooring and at sea in any reasonable weather. It even works if you leave the canvas cover over it - the loose flap does not stop the air but keeps most of the rain out. The skylight is left open under the upturned rigid dinghy and the lazarette hatch is left off under the cockpit cover when on the mooring. The result is a howling draft from aft to forward when on the mooring, just as Hiscock predicts.

Ian Nicholson says in his "boat data book" that an opening of less than six square inches in area is just about useless from the point of view of ventilation and I think he is right.

 

[geem]

In the uk, most boats seem to have hatches facing backward. I think this makes sense for the uk climate. In the Caribbean, the only way to have your hatches is facing forward. You need considerably more ventilation to keep your boat bearable in a hot climate and those with rear facing hatches are at a serious disadvantage. I would dicide how to orientate your hatches based on where you intend to sail

 

[Sea Devil]

I change the hatches to face forward on all my boats. On anchorage it ensures a decent breeze, if there is any, through the boat.
Michael

 

[Blueboatman]

Yup,and in the tropics at least,sleeping in the forecabin under a forward-opening hatch rigged with an effective windscoop over it is often the only humidity free option.
..Which means you need mushroom vents too for when the boat is locked up..

 

[Sea Devil]

In Bambola I actually had mushroom vents with little solar panels and batteries - ex West marine were not too expensive and certainly helped when the boat was shut up.
Michael

 

[steve_cronin]

Most simple mushroom vents are > six square inches surely? That means a diameter of only 2.76" and even those tiny ones on old wooden boats aree bigger than that.

Steve Cronin