To "Scoop" Or Not To "Scoop"

[lexluthor]

Hello fellow boating enthusiasts,


Normally, on sailing boats the raw water strainer is connected via
pipe or flex to a seacock and then a water scoop facing stern.

Althoug it is common practice, some sources tell that

a. it isn't needed to put on a scoop for running the engine normally
b. with a flawed impeller blade in het water pump the scooping effect could flood the engine.
c. it takes a hell of a time when a plastic bag gets sucked against te scoop to clear it off.

I would like to hear about your thougts about this subject,

with kindest regards,

tom

[S/V Illusion]

Originally Posted by lexluthor

Hello fellow boating enthusiasts,


Normally, on sailing boats the raw water strainer is connected via
pipe or flex to a seacock and then a water scoop facing stern.

This is simply a matter of opinion - there is no definitively "best" approach.

Was your omission of a strainer in this description intentional? Kind of makes the question moot.

[GordMay]

Greetings and welcome aboard the CF, Tom.

Don’t fit an exterior scoop, unless you have an overheating problem caused by the position of the thru-hull fitting on the hull.
When installing a scoop on a slower vessel (below 11 knots) install a slotted scoop facing aft. This helps reduce the possibility of a leaf, stray plastic bag, etc., blocking the intake, and wont force water into the engine while sailing.
From "Yanmar Help" ➥ RAW WATER SYSTEM

[lexluthor]

Hi S/V,

the raw water strainer helps to prevent small items getting sucked up trough the engine, english is not my native language so my description has no double meaning whatsoever and I do want to apologise if I made some error in description.

Hi Gord,

Thx for your answer, i'm refitting a 32 foot steel hull, the engine installed is a Bukh DV20 rawwater cooled engine, since the boatdesign is rather classic (small fine longkeeled hull) my engine is installed largely above the waterline (only the oilsump an 1/4 th of the crankcase is below, my first worry is that when motoring or sailing, bubbles of vacuum through the speed of the vessel would hinder the johnson impeller pump to suck enough water since the motor is installed above the waterline, the installation now has a slotted scoop facing forward. Since i was reading up on marine diesel installation and repair the author wrote that a scoop is best omited because of the risc flooding the engine, this made me kind of worry (since i'm repositioning the raw water inlet (now barely accessible, the stainles seacock is over 20 years old and will be replaced by bronze grocco seacocks) if i should stick to the scoop. Would installation without a scoop be better close to the keel or rather with some reasonable distance?

kindest regards,

tom

[Stumble]

I absolutely recommend against scoops, regardless of the location. Unless you have a boat that travels above 40kn there is no need for them, and the screens reduce the surface area of the through hull. This then requires installing a larger hole in the boat to get the same flow area.

Unless you don't have a sea strainer which almost all engines have, they are just not needed and add another thing that can clog.

[rtbates]

scoop or not there should be a slotted cover of some sort over the opening . You haven't lived until you've sucked a piece of plastic bag several feet up the plumbing...disclaimer, it was a charter boat!

[lexluthor]

thanks for the varied replys, and advice,, i'll leave the scoop out and follow the advice,
you really made my day, one peace of hardware less to worry about,


i also have a scoop installed for the waterlubricated sterngland, should i ommit this one too or is this a total different matter?

[SkiprJohn]

Originally Posted by rtbates

scoop or not there should be a slotted cover of some sort over the opening . You haven't lived until you've sucked a piece of plastic bag several feet up the plumbing...disclaimer, it was a charter boat!

With more and more plastic in the water I think this would be a major concern but what I just read from the other responders is that they have no slotted cover?
kind regards,

[Stumble]

Slotted or holes in a water intake massively reduce the amount of water that can get through. Slots generally reduce the amount of surface area by a little less than half, and holes while better are about 35%. If you want them, you need to size the through hulls appropriately, which normally means going up at least one size, and adding a reducer between the seacock and the hose.

Alternatively you can add a basket strainer to the system. They have a negligible effect on water flow, and can be cleaned from inside the boat.

http://www.defender.com/product.jsp?...4243&id=142507 and Swivel Nut Strainer do the same job. But one has to be installed through the boat and is much more subject to clogging from marine growth.

[SkiprJohn]

Stumble,
Thanks for that. I've got the strainer just was concerned about the slots.
kind regards,

[barnakiel]

IMHO

A scoop facing aft will not flood the engine because it creates suction.

In a scoop, the grid protects the system from most all large debris, seaweed, etc..

In our ship, which is a slow boat (max 7 knots) the scoop faces aft and there is no strainer. No issues over last 10 years whatsoever.

I did read something about reversing the scoop in some applications, be it waterakers (?) or (something else?).

b.

[barnakiel]

Originally Posted by rtbates

scoop or not there should be a slotted cover of some sort over the opening . You haven't lived until you've sucked a piece of plastic bag several feet up the plumbing...disclaimer, it was a charter boat!

+1!

b.

[malbert73]

Read this with interest. My boat rarely sails above 7.5 knots and motors at 6
The boat has had a forward facing slotted scoop since I have owned it, and looks like it has been there for a while prior (10 yrs +)

I guess I am hearing that the risk is of sailing at decent speeds, and flooding the engine if a vane breaks off of my impeller. Anyone actually ever have or hear of this happening or is it just a theoretical concern?

[Stumble]

Malbert,

Flooding from scoops is so common it has a name. Hydro-lock.

It is rare, but it does happen. And it doesn't necessarily take loosing a vane for it to happen. As the boat moves through the water the scoop forces water into the system which pressurized it. Since the engine isn't running this preassure can drive water into the engine through the exhaust, get into the internals of the engine, and destroy it.

Just take a look at the installation recommendations from Westerbeak, Cat, Detroit, ect on the use of scoops. They basically all say not to use them unless you have a boat traveling Aton excess of 40kn, where the scoop helps to ensure enough water is being pulled into the engine for cooling.

[David_Old_Jersey]

+1 on the strainer.

FWIW, also useful if the strainer is "gently" fixed on (i.e. not through bolted or with 3 inch screws) so that if push comes to shove (i.e. if a plastic bag is sucked up against it / it has become blocked by weed or growth etc) then can be cleared from the inside with a long stick - and some brute force!.....of course probably depends on how the intake is set up inside.