Engine Air

[nic]

Almost ready to start the repower to diesel on our 1963 fbc. We have hull side air vents a little over half the size/volume of those on 'Buddy Boy" so we are planning to double up - then I noticed that a lot of boats on this site and on Yachtworld have NO hull side vents at all. My experience is engine manufacturers call for unrealistic amounts of air volume but just the same, performance diesels like air. How do those boats get their air?

Thanks for any guidance in advance.

Nic

[CaptPatrick]

Nic,

The long standing rule of thumb for air capacity to a diesel engine is 1/2 sq in of free air flow per 1 hp. (300hp x .5 sq in = 150 sq in) Free air being defined as air that can freely flow without restriction.

That's the formula that I used for both of my side induction styles. The formula was derived decades ago when almost all diesel engines were low RPM high torque engines such as Detroit, Cat, Cummins.

The new higher RPM engines like Yanmar & Steyr aspirate more like gas engines and don't require quite as much air for complete combustion.

The design of the usual B31 is such that there is quite a bit of free air that can be drawn in through the interior of the cockpit. Loose fitting hatch covers, spaces around the side panels & engine boxes, air being drawn from the bridge down through the pilasters, etc. I'm guessing that a stock B31 has about 75 - 100 sq in of free air flow available to the engines even without gunnel vents.

So for gas engines & higher RPM diesels below the 320 hp range, there is little need for additional air supply.

While engine air is best drawn from the outside of a boat and as far forward from the exhaust as possible, additional air, when needed, can also be drawn from the cockpit area. The problem with drawing from the cockpit is noise. Where air can enter, engine noise will also exit...

Br,

Patrick

[nic]

Thanks Capt. Patrick, we're going to investigate some extra air down the pilasters from a f/b vent as well, maybe directed for combustion air, as we are focused on minimising noise.

It's getting very close & and it's looking like the Yanmar 315s!

Nic

[CaptPatrick]

Nic,

If you go with the Yanmars, you probably won't need to create any more air access than you already have.

Br,

Patrick

[nic]

Thanks Capt. that's exactly what we're thinking/hoping & I always wanted some clean combustion air from the bridge.

Nic

[Brewster Minton]

I have the 315s yanmars and they are the best thing I ever did. They turned my 31 into a new boat.