Why aren't all intake thru-hulls consolidated into one?

[NPCampbell]

Just curious about this ...

Through hull fittings are single points of failure. If there was a single (or double) seacock thru-hull that came into a reservoir that was bonded to the hull and all water intakes drew from valves in the reservoir then you would have two means of dealing with a failure. If a seacock failed you could cut off the reservoir valves and if a reservoir valve failed you could close the seacock.

I'm sure there is a T-ball "duh" reason out there why this isn't done that I haven't thought of ...

[SVTatia]

Manifolds are common on some boats but it involves a bunch of plumbing routes that unless planned form the design/build it is hard to implement. It really makes a bunch of sense though. I had one on my previous steel boat.

[Dsanduril]

It’s called a “sea chest”. Steve Dashew, for one, is a strong proponent. As with everything, there are pros and cons. Depending on boat configuration it can result in some long saltwater plumbing runs inside the boat. You’ve hit on the big pro, properly designed it provides a single point that is stronger and lower probability of failure than a seacock.

That configuration does exist and is found on some boats although not as common (in our size vessels) as multiple seacocks.

[NPCampbell]

Originally Posted by SVTatia

Manifolds are common on some boats but it involves a bunch of plumbing routes that unless planned form the design/build it is hard to implement. It really makes a bunch of sense though. I had one on my previous steel boat.

Was your manifold filled just by the hydrostatic pressure of a below the waterline opening + a purge valve or did a pump push water into it?

[coopec43]

I agree with the previous comments.


However I decided to have a dedicated input for the motor and another input for the toilet and desalinator.

[Benz]

I've gone a step further in my mind and designed my next boat (mostly in my head, unless I win the lotto) with the engine keel cooler inside the sea chest. No more raw water impellers! And the removable lid would be above the waterline, so it could be opened for inspection while in use.

[mvweebles]

Defever Trawlers almost always have a sea chest (attached Pic)

Groco makes a manifold to attach to sea strainer outlet. Read their description and you'll see they do not recommend it, but will sell it to you anyway. Lawyers talking I suppose. The sum of the outlets is less than the inlet.

https://www.groco.net/products/fitti...water-manifold

I am in the process of a major refit and reconfiguration. I went with a DIY oversized manifold. 2-inch through hull to 2-inch sea strainer that feeds my 4.236 75hp Perkins (3/4"), a salt water wash down pump (1/2") and an AC (5/8").

The risk is that if the raw water pickup or sea strainer becomes obstructed, the engine draw will suck air through the shared inlets. Which is why OEM engine manufacturers will void a warranty of the engine does not have a dedicated through hull inlet.

My objective in aggregating several through hulls into a single manifold was not really to minimize holes in the boat - well installed sea cocks (vs mushroom fittings with ball valves) are incredibly robust and reliable. My goal was to minimize sea strainers and to keep all through hulls in the engine room where I could more easily monitor and service. This is especially helpful for AC water intakes that often have small plastic sea strainers scattered in hard to reach places.

Im the end, I opted to put all my eggs in one basket, but watch the basket closely. It was not a cheap exercise - 2-inch fittings and strainers are expensive, and filling all the old holes tool time and money too.

Peter

[NPCampbell]

Originally Posted by mvweebles

The risk is that if the raw water pickup or sea strainer becomes obstructed, the engine draw will suck air through the shared inlets. Which is why OEM engine manufacturers will void a warranty of the engine does not have a dedicated through hull inlet.

This is a valid concern that I hadn't thought of. This would necessitate a check valve on the engine draw (or any draw that creates a siphon).

[tkeithlu]

I designed my boat with only one inlet, a big bronze Perko with a strainer. I moved the arm of the valve 90 degrees so that horizontal is "open," and then ran a wire and knob up to the salon above the engine room. In an emergency (flood) I could simply pull the wire to close the valve.

Yes, one inlet results in long hose runs, for example raw water to the genset and watermaker. IMHO that is far better than having seacocks all over the engine room, including under the engine where you would never get to them if they did pop.

Warranty? That's a reasonable concern that I avoided - a surplus engine that cost me $4,200. No warranty.

[noelex 77]

I share your concern about seacocks. A sea chest can be a good solution, but keep in mind that it generally results in longer hose runs than would be the case if individual seacocks were used.

If the hose is split or punctured as it travels below the waterline, in many cases this will flood the boat. The longer hose runs associated with a seachest increase this risk. The hose is arguably the most vulnerable and fragile component. Note that due to syphon action this often applies even if some part of the hose is raised above the waterline.

An alternative is installing each seacock on a standpipe so the seacock itself is above the waterline. Therefore if the seacock leaks it cannot flood the boat.

[Manateeman]

I have a seachests and 4 standpipes. I don’t understand how you feed an impeller pump if the valve is above the waterline.
M.

[thinwater]

All of the intakes in my PDQ are in bulkheaded compartments, one compartment in each hull.


I had a partial failure of one during my delivery trip. It been mucked with during the survey and not reassembled correctly. It flooded a small compartment but did not harm.


The boat also has crash tanks forward, aft (rudder post), and engine compartment bulkheads. Very hard to flood. Because it is a multihull with narrow hulls, the bulkheads are worked into the layout and invisible (they would have been there anyway).



It may not work in all cases, but it has merit. I suspect any boat could do it if the bulkheads were designed in BEFORE the furnishings went in. Just box the critical items.

[mvweebles]

Originally Posted by NPCampbell

This is a valid concern that I hadn't thought of. This would necessitate a check valve on the engine draw (or any draw that creates a siphon).

Correct. I forgot to mention in my original post. I placed groco check valves in the competing draws. They replace a close nipple. Not very expensive.

https://www.groco.net/products/valve...lves/cv-series

[SVTatia]

Originally Posted by NPCampbell

Was your manifold filled just by the hydrostatic pressure of a below the waterline opening + a purge valve or did a pump push water into it?

Initially it was a sea-chest, in essence a 4 inch pipe welded to hull with 4 threaded nipples and extending to about 12 inches above the waterline with a screw-on cap so that I could clean it - centralizing all valves. But it didn'work as it got fouled and blocked constantly no matter what I did and it was a pain to clean inside the nipples. So I closed it and installed a large seacock with a manifold containing the engine and the other 2 valves to service the galley and head - similar to the picture posted by mvweebles on post #7. Long hose runs, but it was preplanned and each had their anti-syphon valves.

[tkeithlu]

Good catch, Manateeman. Apparently hanging out with blubberbutts has not dulled your cognitive processes.

My point of view on the issue of hoses failing was to make every hose that comes directly off the central seacock/strainer, or branched from it before reaching it's destination pump, wire reinforced rated hose, with two good SS hose clamps at each end. That meant, for example, replacing the really sorry vinyl hose sent with the shaft seal (to wet the seal) with a reinforced alternative. Yes, they are heavy, and they are underfoot in the engine room. But, since failures are almost always at the ends, I doubt that the long runs are much more of a threat to rupture than would be short runs. After eight years, no aneurysms, no cracking, no bulges.

Oh, and "branches" are the bronze fitting at the end of the primary hose, not junctions in route. Single hoses, seacock to pump.