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Old 11-11-2005, 22:29   #1
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Yanmar 3GM30F Raw Water Intake Size

I'm replacing a scary old thru-hull and seacock for my sea water intake. The old one was a corroded gate valve that broke off in my hand when I remove it! It looks awefully small too. It seems to have worked since the engine didn't overheat, but I'd like to know what the recommended size is for that thru-hull.

What size should I replace it with?

Thanks,

Craig
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Old 12-11-2005, 06:52   #2
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I would go with a decent size feeding through a ball valve to a weed strainer. size of valve/fitting as required for the weed strainer. (IIRC Vetus make a suitable strainer)
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Old 12-11-2005, 06:52   #3
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You answered your own question: Engine has been running fine. Therefore replace size for size.
The cooling water flow rate for your engine at max. rpm is only about 3 gallons per minute - 1/2" to 3/4" dia. throughhull is fine for this flow.
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Old 12-11-2005, 11:47   #4
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h2o size

In my Yanmar install Manual for your engine the min. size is 1/2 inch. That is large enough to get your 396gph flow required by that engine.
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Old 13-11-2005, 03:18   #5
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Craig, were I you I would at least consider increasing the diameter of your thru-hull if it is 1/2" ID. A larger diameter would help avoid very small flotsam from fouling the thru-hull, and you may later find you want to consider adding a manifold to the thru-hull to provide for more raw water service (e.g. galley sea water faucet or anchor wash-down pump). You can use a simple reducer to step the larger thru-hull valve down to the raw water cooling line installed on your engine. To enlarge the hole in the hull using a hole saw, bed down a thin piece of scrap ply over the hole to take the pilot bit of the hole saw.

OTOH it doesn't sound like your engine needs a larger hole.

Jack
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Old 13-11-2005, 04:33   #6
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A small increase in pipe size results in a large increase in capacity.
The area of a circle varies as the square of itís diameter - hence a 50% increase in pipe size (from Ĺ" to 3/4" dia.) will result in a 225% increase in itís capacity
1.5 x 1.5 = 2.25
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Old 13-11-2005, 20:14   #7
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That would be valid if the boat had a centrifugal pump but a vane pump is a 'almost' a constant displacement pump and the little bit of pressure drop increase for the smaller size nozzle wont amount to very much at all .... unless you start pulling a vacuum down to the the vapor pressure of the intake water and thats not going to happen with a rubber vane pump.
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Old 14-11-2005, 06:30   #8
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I was making a general observation about the relative capacities of pipe sizes (or tubular tankage) - wherein a small increase in pipe diameter (ie: from Ĺ to 3/4" = 50% increase) results in a much larger (ratio squared) delivery (or storage) capacity. This is a basic principle of geometry, which is always valid, and has nothing to do with pump type.

A pipe or hose has a circular cross-sectional area.
The area of a circle can be found by multiplying pi (3.1416) by the square of the radius (Ĺ diameter).
ie:
Area (thus capacity) of Ĺ" diameter (1/4" radius) Pipe:
3.1416 x (0.25 x 0.25) = 0.19635 square inches
vs
Area (thus capacity) of 3/4" diameter (3/8" radius) Pipe:
3.1416 x (0.375 x 0.375) = 0.4418 square inches
and
0.4418 ų 0.19635 = 2.25 (1.5 squared)
Doubling a pipe size quadruples itís cross-sectional area.

Given the fairly low flow rates of engine cooling water systems, I have little doubt that a Ĺ" supply (thru-hull & hose) will be adequate for the purpose (as Richhh implied).
On the other hand:
- A 3/4" Thru-hull will deliver 125% more water than a Ĺ"; providing reserve capacity to overcome possible obstructions (fouling), and permitting manifolding for other purposes.
- A 3/4" thru-hull is more physically substantial, reducing the likelihood of accidental mechanical damage.
- A 3/4" thru-hull inlet will produce a more laminar flow of water than a 1/2".
- There is little cost premium associated with increasing to 3/4"

Positive Displacement Pumps:
There are two important factors that limit pump efficiency: frictional losses & fluid flow characteristics.
Frictional losses are directly proportional to: Length of pipe, Flow rate, Pipe Diameter, & Fluid Viscosity.
Each PD pump will have a specified NPSHr (net positive suction head requirement), requiring that restrictions and pressure drops in the suction lines be avoided.
Fluid flow losses in pipes occur in two principle modes: Laminar &Turbulent. Smaller inlet orifices (thru-hull) tend to cause more turbulent fluid flow, which can lead to pump cavitation.

Some basic Pumping system design criteria:
1.Restricting the inlet port size and the inlet pipe ID will cause cavitation and damage the pump.
2. It is best to have a straight run of pipe leading into the pump inlet.
3. The NPSHa must be greater than NPSHr of the system.

NPSHr (Net Positive Suction Head Required) determines the required suction head (maximum suction lift). It is inherent to the design of the pump and is measured in feet of water. NPSHa (Net Positive Suction Head Available) is determined by the pipe system on the suction (inlet) side of the pump.

You should configure your system so NPSHa > NPSHr (the head available from the system is greater than the head the pump requires). Failure to meet this requirement will cause reduced flow rate, cavitation, and vibration of the pump.

I donít disagree with Richhh, inasmuch as thru-hull & pipe size isnít really critical in this application - but a larger thru-hull and pipe certainly canít hurt.

FWIW,
Gord
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Old 15-11-2005, 00:31   #9
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I'm going with a 3/4" one

The existing one had an inside diameter of less than 1/2". I didn't actually have a measuring tape handy, and besides it was going in the trash, so I didn't measure it at the time. If they make such an animal, my guess is it was 3/16".

I like the idea of having the ability to withstand a bit more flotsam and having it physically stronger as well. I am going to "steal' a bit of water off that line to use with a foot pump for salt water at the galley sink. Of course that's a negligible amount and very intermitent use, but the extra capacity for future expansion won't hurt either.

Thanks for the replies,

Craig
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