Vent location question.

Can someone give me advise?
I have a 25' Wellcraft Cruiser that I have been rebuilding and I'm looking for some input on how to vent the engine compartment. Is there a basic rule that applies? I understand that I need air in and I need air going out (with a blower attached) but is there a specific spot each vent needs to be in? I have two rectangle holes directly above the transom and two rectangle holes on each side of the boat about 4 inches forward of the transom. Here is the big question, can I put both vents on the side as long as I route the hoses to the right place? Does the exhaust air vent necessarily need to be above the transom? Sorry if this sounds like a dumb question but before I cut two 4" holes in my new deck in the wrong place I want to make sure I ask. Any help would make my weekend. I'd be able to move past this part of the build with more confidence.

 

I think, you need the intake facing forward and the exhaust
with the blower facing aft! You need to run the blower
prior to startup and then the motion of the boat with keep
it vented!!
They need to be on opposite sides of the boat to get the
best coverage!

Some one will give you the exact regulations I am sure!

 

Thanks BHOFM. I understand that the blower has to be run for a specific time prior to startup. I was just hoping I could relocate the exhaust vents from the transom area, split them and run one to each side of the boat. So I'd have an intake vent facing forward and an exhaust vent facing aft two vents on each side if that makes sense. I guess I'm asking if the exhaust location is written in stone when it comes to the design of an IO powerboat. Thanks again hopefully someone had a similar experience. And sorry if this has been covered in another thread. I looked but couldn't find an answer.

 

In my State WV rule is as follows: (may differ in other states)

VENTILATION SYSTEMS

1.All motorboats, except open boats, which use gasoline or any other fuel which has a flash point of 110 deg F or less must have at least two (2) ventilator ducts fitted with cowls or their equiv. for the efficient removal of explosive or flammable gases from the bilges of every engine and fuel tank compartment.

2.At least one (1) exhaust duct must be installed that extends from the lower portion of the bilge to the open atmosphere.

3. At least one (1) intake duct must be installed that extends from the open atmosphere to a point that is either at least midway to the bilge or at least below the level of the carburetor air intake.

4. Cowls must be located and trimmed for a maximum effectiveness in preventing displaced fumes from being re - circulated.

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As you can see, rules can be vague and I suspect if you make an attempt and at the same time meet the above requirements it should be acceptable.

jango

 

A little common sense to add; you want fresh incoming air to dissipate any possible accumulation of fuel vapor. The likely causes of any vapor would be the fuel filter and fuel pump and associated fittings. Since the carb is normally the highest point on the engine and is already fitted with a spark arrestor it should be alright.

The single most important point is the starter motor (which should be of marine type, i.e. spark arresting) since it is usually quite low in the bilge where vapors are most likely to accumulate and since such high amperage is involved. I like to locate the intake side blower hose close to the starter in the bilge area.

The primary concerns for the exhaust vents is that they create good suction and that the they do not cause the evacuated air/fumes to come back into the cockpit.

In general I like to bring in fresh air high in the compartment to force any vapor down and place the exhausts low in the bilge to suck out any vapor.

 

Jango, Lt. Holden,
Thanks for your help. So am I right in saying that as long as I vent the compartment (with blower) prior to starting and I have all the correct number of vents in place and the cowls to each pointing in the correct directions to create a draft while running I can turn the blower (exhaust) off? In other words use the cross ventilation (unpowered - no blower motor) while under way?

 

JohnL
Yes. also you asked about the exhaust vent output locations being somewhere other than at the transom; I don't see any problem with it as long as there is not normally any backflow into the cockpit. Good Luck with your boat. Where are you in Connecticut? I am in Springfield, MA.

 

JohnL, Yes you are correct. By the way, the most critical time is during engine start-up. Make sure the blower is on - can be turned off a few minutes later .

 

Will your blower be in one of the exhaust vents? If so turning it off will effectively block the vent.

 

It would be wise to check with the manufacturer to ensure that you have at least the minimum required vent (in & out) area too. Failing that info being available, the calcs are pretty simple, and can be found in a number of places. Unfortunately I don;t have it here with me now, but Dave Gerr's Nature of Boats has it I think. And a couple of recent issues of Professional Boatbuilder magazine had articles by Dave giving the same info. Many are available for free download online ( www.proboat.com )

 

Thanks everyone!
I've been on several boats that turn the blowers off and some that keep them running so I wanted to be sure.
I ended up keeping the transom vents in place and I cut my deck yesterday and glassed everything in. I will have to move the side vents forward about 6 inches because they will run into the scupper drains that are already there.
Lt.H. I'm down near UCONN but I'm up at Mertons quite a bit for glass and other supplies. Thanks for the best wishes, this has been in the works since 2006.
Willallison, the vent holes are aprox. 16"x3" ans I have two on the transom and one intake each side. They are the original Wellcraft holes so I figured it would get me enough air. Even if I place a blower fan on one of he exhaust vents I still have the other that shoudl move quite a bit of air. I'll check out the link though, thanks.
CTMD, wouldn't some air move through a vent with a blower if it was turned off? I have the two in place but I did think about that, Jango that's why I wanted to know if it shoud be kept on or not. Hummm.
Thanks again everyone you all have been a big help. I'm going to be cutting the side vents today.

 

From the aforementioned Gerr's Nature of Boats, minimum vent area excluding blowers in square inches = hp / 3.3

This is the absolute minimum and Gerr recommends an increase of 10% over this figure. He also suggests the addition of continuous blowers with a vent area of about 1/3 of this figure.

Just because the builder put a certain size vent in, doesn't mean it's big enough.....

 

First read this page http://newboatbuilders.com/pages/vent.html, and the USCG page on ventilation. http://www.uscgboating.org/safety/boatbuilder/ventilation_landing.htm

Now, armed with that information you should be able to calculate the size of the vents and the blower capacity. A word of warning. Oversize it a bit. Those corrugated hoses cause a huge loss in air flow and any turns or restrictions will also reduce the air flow. The connection from the blower to the hose also causes a restriction, lowering the airflow, so go one size larger on the blower.

You can put the intacts and exhausts where you want to. Research done by USCG and ABYC have shown that for natural ventilation (without blower) the exhaust and intake are interchangeable and depend entirely on the way the wind is blowing. Scoops or louvers look nice and pointing them forward or back looks nice but only have an effect when the boat is moving.

Run the blower at least 4 minutes before starting. Always open the engine hatch and sniff. You nose is the best fume detector there is.

One caveat. If you put the intakes and exhaust on the side you will need to put a loop or a manifold in to keep water out of the boat. Water will splash in through side vents. Boats have been sunk because of this.