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Old 06-07-2017, 09:05   #1
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Join Date: Jul 2017
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Boat: Lagoon450
Posts: 5
Air bubbles in generator intake

Hi all, this is my first post so hoping someone out there can help.
We just purchased a 2017 model Lagoon450F (built in 2016) we are its second owner.
On our maiden voyage we had issues with air bubbles getting into the generator intake and shutting down the generator. We were sailing in light to moderat seas, 12-18kn making about 7kn SOG.
After anchoring at our destination and swimming under the boat we see that there is no scoop/strainer over the intake, like there is on the intake for the air conditioning. We Reckon this may have been inadvertently left off, but would like to know if anyone with a similar model can tell us if they do or do not have a scoop/strainer over this intake.
That way we can ascertain if it is a warranty issue.
Thanks sailors.

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Old 06-07-2017, 10:11   #2
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Boat: 450S
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Re: Air bubbles in generator intake

I have a strainer 2016 450S, is the intake low in the water or it it possible for it to suck air in pitching conditions?

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Old 08-07-2017, 05:00   #3
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Re: Air bubbles in generator intake

It's in line with the head intakes/ pick up.
If a. No one else has a scoop over their intake and b. Lots of people have this problem, then we will consider lowering the intake to the level of the air condition intakes at our next haul out.
Seriously, we were in calm seas. We plan to cross the Pacific, so we will be in much bigger seas for a lot of miles.
We have time to consider this so will stand by for more input from owners out there.
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Old 08-07-2017, 05:16   #4
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Re: Air bubbles in generator intake

At those hull speeds a scoop type fitting over the thru hull may be an asset (one with protective strainer bars),

BUT being directly in line with another thru hull (if any protrusion especially) will always be an issue for any speeds probably over 8 knots.As you say in off shore conditions will only get worse, any way to re position lower in the hull?

Bad design i think, not so much of a worry in our slow mono hulls!!
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Old 08-07-2017, 05:35   #5
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Location: Saint Thomas, USVI
Boat: Fountaine Pajot Lipari 41
Posts: 303
Re: Air bubbles in generator intake

Directly from an Onan Manual

Section 5. Cooling System


Throughout this manual, flotation water drawn into the
boat for engine cooling is called sea water Water recirculated
through the closed system will be called captive
water. Thus, confusion is avoided with other generic
terms describing water origin and use.

The MDL-series gensets use heat exchanger cooling.
The heat exchanger is an integral part of the engine cooling
system and is covered on the following page.
System Plumbing
To adequately cool the genset under all conditions, the
plumbing system must be properly planned and installed.
Excess lengths of plumbing increases flow resistance
and results in reduced cooling. An air leak in the
sea water intake will reduce cooling, cause corrosion,
and can even destroy the neoprene impeller in the sea
water pump. The neoprene impeller must never be run
dry, and the pump should be primed before initial startup
of the genset.

The water line should have a minimum inside diameter of
1 .O inch (25.4 mm) for MDL3 and MDL4 models; 1.25
inch (31.7 mm) for the MDL6 model. For runs over 20
feet (5.2 m), increase the line one pipesize for each additional
10 feet (2.6 m) of length. Water lines can be either
copper tubing or flexible hose. Be sure a length of flexible
hose is used at the genset to allow for movement,
and for noise abatement.

Because sea water cannot be relied upon to always be
clean, OnanQ recommends a water strainer or filter to
protect the engine coding system. See Figure 5-1.
Onan has a hull strainer (furnished with some muffler
kits) that can be used with a flush through-hull fitting. The
strainer (Figure SI), installed with the slots parallel to
the keel, helps prevent pressure or vacuum when the
boat is underway. hays use aflush-type inlet with a hydrodynamic
marine muffier.

Caution Restriction in the sea wafer Inlet line
can cause engine overheating and shutdown. The
flush-type, through-hull wafer inlet must have an
opening at least as large as the wafer inlet line.
Stagger the genset water inlet so it is not directly in line
with other inlets. Not doing so can reduce the amount of
sea water available to the genset when underway and
cause overheating. Never use scoop type water inlet fittings
with a hydrodynamic muffler.


Fonvard faclng scoops can develop sufficient ram
pressure to force wafer past the generafor set sea
wafer pump. This can flood the exhaust system and
the engine cylinder. This happens when the generator
set is not running and the boat is underway. Rear faclng scoops develop vacuum which can Impede
cooling water flow.
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Old 08-07-2017, 06:49   #6
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Re: Air bubbles in generator intake

what captainjay is trying to point out: don't use a forward or aft facing strainer. :-)

is there a seawaterstrainer in between as well ? then don't use a strainer at all i'd suggest.

in any case, you just bought a new boat, talk to your dealer about it ! they'll want to know, and have possibly a better insight as to why and where, and what previous models have had issues with.
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Old 08-07-2017, 07:48   #7
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Location: NZ
Boat: Lagoon 450
Posts: 488
Re: Air bubbles in generator intake

I have been running our L450 Onan for 5 years. Sometimes in big seas and at up to 10 kts. I have not had a problem with that stopping the water flow and the generator. What has stopped the flow is firstly the non return valve just in from the seacock jamming with weed. I tried a different one and now have completely removed it as it actually serves very little point and never works. Secondly your impeller will have the vanes start to fall off at about 150 hours, as they start dry, they jam in the heat exchanger. There has been a lot of discussion in these forums about solutions. I maintain that the easiest which works well are the 'run dry' impellers. Cheer, Dave

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