Through Hull Fitting. How Important Is A Scoop?

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Dave100456

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I'm replacing some skin fittings on a sailboat (9 knts max speed - Wish!). Given that the 'hole' is more than 750mm below W/L even when heeled, do I need a fwd facing scoop or will std skin fitting do?
My thoughts are:
1, there's plenty of "head" and even 9 knts won't induce negative pressure.
2, the fittings are over size
3, the feed is to a pump that's also 500mm below W/L
4, and how effective is a fwd facing, grated scoop at 6knts?

Thoughts please and anything I've missed?

Thanks
Dave
 
It's probably there for the filter effect rather that any benefit from the scoop.

I've never had a yacht with one, but then I've had to suffer the occasional blocked intake when a guppy has decided it was a good place to hide from the big fish. On the other hand I suspect that they are difficult to keep clear in areas of barnacle and coral fouling. I doubt if it protects against plastic bags though as they will probably seal the intake anyway.
 
Thanks TradewindSailor - I'm not concerned about the strainer effect of a scoop but interested in the physics of whether a fwd facing scoop "forces" water up the fitting and conversely does a plain skin fitting suffer from negative pressure when underway (at 6knts)
Cheers
 
Scoops date from the days of weak pumps on engines. No need for one if you have the correct size plain skin fitting according to the manufacturers recommendations. Can't recall seeing a scoop on any modern sailing auxiliary with a conventional drive train (ie not a saildrive).
 
Thanks Tranona Do you think they are necessary for deck wash pumps, AC units or watermakers? Obviously as the hull speed increases there's more chance of a plain fitting sucking but does this happen at 6 knts?

And .....

I've just come across this from USA forum, no original source quoted.....

"Can't figure out how to post a graph, but the dynamic head (how far above the static waterline that water will rise in a forward facing scoop) is 1.6 ft at 6 knots, and 4.4 ft at 10 knots. This neglects any wave effects".

So if a scoop facing fwd causes 18" of extra head at 6 knts, the loss of head by a plain fitting can't be much hence I don't think a fwd facing scoop is necessary? However, AC and watermakers specify fwd facing scoops.
 
I would actually avoid a forward facing scoop, because I don't like the thought of water being forced into a system that may not want it. OK, in most cases a stopped pump should resist the inflow, but if it didn't you could fill up the exhaust and then the engine, or some equivalent nastiness on other machinery.

Pete
 
TradewindSailor said:
It's probably there for the filter effect rather that any benefit from the scoop.

I've never had a yacht with one, but then I've had to suffer the occasional blocked intake when a guppy has decided it was a good place to hide from the big fish. On the other hand I suspect that they are difficult to keep clear in areas of barnacle and coral fouling. I doubt if it protects against plastic bags though as they will probably seal the intake anyway.
Click to expand...

On my boat the stern gland is fed by a forward facing scoop and the engine is fed by a rearward facing one. It is difficult to get rid of all the barnacles from inside the grid's with a hacksaw blade and impossible to antifoul inside without completely immersing the fitting in it.
 
Thanks
I have found this from Nigel Calder..

##The raw water intake must be as low in the boat as possible (in order to ensure that the system receives a constant supply of water with no air entering the feed line). It must be located well away from toilet discharges, and sink, shower and bilge pump drains. Flat hull inlets should be avoided because they can cause a vacuum; a forward facing scoop is recommended. The through-hull inlet should be dedicated to the watermaker alone to avoid the risk of air entering from another system, and to ensure adequate flow to the watermaker.


So is it correct; just what speed do you have to do to cause a "vacuum"??? Or are fwd facing scoops on sailboats to be avoided as is the perceived thought in US??
 
I think the only thing "missing" is application. The quote from Nigel Calder applies to watermakers and presumably requires the assistance of a scoop as the pressure to be achieved for reverse osmosis is high. Experience suggests that most other applications with no high pressure requirement are easily supplied without a scoop, indeed it is common for the engine cooling to have a reversed scoop to prevent fouling with debris.

I have often read of owners arranging skin fittings such that the line can be broken above the water line, ideally directly above the skin fitting. This allows the pipe to be rodded through which should even dislodge a plastic bag stuck on the intake and can remove any growth within the hose. Of course a flexible hose may be similarly cleared with a flexible rod (think drain rods, here) but not so easy to smash mussels and barnacles out of the aperture. Once out of the water, a plain skin fitting can be antifouled internally with a bottle brush.

Rob.
 
prv said:
I would actually avoid a forward facing scoop, because I don't like the thought of water being forced into a system that may not want it. OK, in most cases a stopped pump should resist the inflow, but if it didn't you could fill up the exhaust and then the engine, or some equivalent nastiness on other machinery.

Pete
Click to expand...

"Additionally some thru-hulls are designed with a scoop and strainer. These types of thru-hulls are generally used for items that do not have self priming pumps, for example marine air conditioners. Do not use a scoop thru-hull unless specifically suggested by the manufacturer. Scoop style thru-hulls are not recommended for most generators and if used will damage the unit."

Installing a Thru Hull
http://www.boatinghowto.com/content/installing-thru-hull-207/
 
I know it's pedantic of me, but I hate the current trend for people to set themselves up as pedagogs (sp?) and then use inexact language and half baked truths...

The article in the link appears to be advertising "Marelon" rather than talking generally about plastics, yet doesn't touch on the minefield of what can be served up as "bronze" nor the possible consequences of adjoining dissimilar metals. But for inexactitude, I would suggest ...and if used may damage the unit.

Rob.
 
rob2 said:
I know it's pedantic of me, but I hate the current trend for people to set themselves up as pedagogs (sp?) and then use inexact language and half baked truths...
Rob.
Click to expand...

I believe I put the link on this thread that you are referring to. Like you I have contempt for people who set themselves up as experts and pass on their superior knowledge to the unsuspecting readers. We must always remember that no-one checks the accuracy of people posting on the Internet.

However the author referred to "Marelon" which has been approved for underwater use. I really doubt that just any plastic would be approved.

Also the author in a number of places in the article cites the AYBC regulations on through hull fittings. To expect the author to go into the consequences of adjoining dissimilar metals in such a short article on "Installing a Thru Hull" is expecting a bit much!!
 
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I fitted a water maker with a standard through hull fitting well below the water line on a Beneteau Oceanis 373, the hull is fairly flat bottomed as are many modern boats and when under sail can get small air bubbles running along the length of the hull and sucked into the water maker intake. I am planing on fitting a rear facing scoop in an effort to reduce the chances of air getting in the water maker. My thoughts are that the shape of the scoop will deflect the small air bubbles, typically 1-2mm. Does anyone think this would help?
 
pvb said:
Grated scoops on engine cooling inlets are supposed to face aft, not forwards.
Click to expand...

Yes but you fail to say why. I have just installed a generator and the instructions specifically say the gratings should face aft to avoid the water being pressurised which at high speeds could force water up the intake and flood the exhaust causing the engine to suck in water when next started. Frankly I don't think this is a very likely scenario unless the complete installation is below the waterline but have always fitted engine intakes this way as that was what I was taught to do. Watermakers and AC units go the other way as there is no detriment to adding to the pressure here and it helps maintain the priming of the pump. Considering the original question, not having a grating at all does indeed create a vacuum. Passing water across the hole at 90 degrees to it will actually suck water down the pipe and spoil pump prime. Most pumps will be able to overcome the negative pressure but it's good practice to avoid dry starts. In fact most vane failures are caused by running dry so it's good practice to keep them permanently primed. Theoretically the slots in gratings minimise the vacuum effect with the slots facing aft and the negative pressure is negligable. The other theoretical reason (that I don't know actually makes any difference in practice) is that aft facing gratings are less likely to get blocked by weed and such. I can see this as an argument but can't say if it's true or not.......
 
Networks

In a word No. Most watermakers have either piston type pumps (AC) or diaphragm pumps (DC) Either will deal with any air that is in the water due to turbulance and there is no detriment to the running of the membranes anyway. Don't worry about it!
 
coopec said:
Also the author in a number of places in the article cites the AYBC regulations on through hull fittings. To expect the author to go into the consequences of adjoining dissimilar metals in such a short article on "Installing a Thru Hull" is expecting a bit much!!
Click to expand...

But he also states, presumably also referring to ABYC, that ball valves should not be attached to skin fittings. In fact I doubt if there is a European manufacturer who does not do this. ABYC is renowned for being highly conservative. I'm sure that what they say would be perfect in an ideal world but most of us have to take economics into account.
 
vyv_cox said:
But he also states, presumably also referring to ABYC, that ball valves should not be attached to skin fittings. In fact I doubt if there is a European manufacturer who does not do this. ABYC is renowned for being highly conservative. I'm sure that what they say would be perfect in an ideal world but most of us have to take economics into account.
Click to expand...

Actually I didn't realise until I read his article there was a difference between a "ball valve" and a "sea cock". I must admit I have looked at a very solid looking marleon ball valve I have installed and thought it looked pretty vulnerable.

You suggest the AYBC is conservative but I wonder what our yachts would look like if they were all built to survey!!
 
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