Sea Strainer vs Thru Hull

[SkiprJohn]

I have a Sea Strainer (Raritan Plastic) that has a 1 1/2" incoming port. I don't relish drilling a 1 1/2" hole for a thru hull in my boat. Can I use a 1" thru hull instead? Does it matter? Do you think there are any consequences that I should be aware of.
The Sea Strainer will have a manifold at the outlet which will send seawater to engine cooling system, galley sink, saltwater washdown pump and possibly the head including sink.
Engine is 40hp diesel.
Comments please.
Thanks,
JohnL

[Pelagic]

John if you have an existing 1” thru hull then I would say just use it and plumb an adapter to the 1 ½” Strainer.

But if you are having to dill a hole anyway and buy a thru hull…. then staying 1 ½” all the way makes more sense as you can maximize your cooling flow when the strainer gets dirty and you have anticipated additional demands from washdown/ head/ maybe something else in the future?

Don’t unnecessarily restrict yourself unless you see a fitting problem. Safety and structure should not be an issue.

[David M]

The size of the hole depends on how much water you need to pull thorough it. Have you done that calculation? What sized raw water opening does your engine manufacturer require? Its going to be the same sized opening that's on the inlet side of your water pump">raw water pump.

With one hole supplying a number of devices that need water, there is always the potential of one of those devices introducing air into the system. That of course would be a bad thing if your engine cooling pump starts sucking air....air that is being provided by another device that is open and providing that air.

[HorizonMarine]

IMHO the 1" is probably the recommended size for the engine alone. I would not draw off water to other fittings through that fitting. If you read engine manuals, they will usually tell you to use a dedicated thru hull. I assume the new rules say you should not use those plastic strainers below the water line. I notice ob all the boats being sold that I have been hired to fix the hit list on the survey says replace with the bronze type or other non plastic ones. All the surveyors are adding that comment so I assume the rules have been changed.

So.. my suggestion is to drill another hole or if using a true manifold, you need bigger than 1" hole to feed them all.

[Evan]

Unless you use a dedicated sea chest, (a large volume of water inside the hull), using a single sea strainer to feed all those consumers is not a great idea. The engine pump will be working against the suction of the other pumps, causing reduced or poor performance of the each pump. We had a SW foot pump tee'd off the engine suction and it was useless when the engine was running.

[GordMay]

The capacity of a hole (pipe, hose, etc) varies as the square of the diameter, hence:
A 1.5 inch diameter hole has 225 % the capacity of a 1" dia. Hole.
Conversely, a 1" hole has 44.4% the capacity of a hole 1.5" in diameter.

(Thanks to opsailor for technical editing advice.)

[Pelagic]

Originally Posted by David M

With one hole supplying a number of devices that need water, there is always the potential of one of those devices introducing air into the system. That of course would be a bad thing if your engine cooling pump starts sucking air....air that is being provided by another device that is open and providing that air.

David is absolutely correct. When I bought Stargazer the M.E. Intake had smaller T’s for the RO Watermaker and a large aircon pump. They would starve for water if the Main engine was running at cruising speed so I added another sea chest for them and plumbed it also as an emergency backup to the M.E.

Point is, don’t restrict yourself!

[scrumble]

Good Day All,
Please humour me with my continuing this post and hitching a ride on SkiprJohn's question.
I am building two hulls and have been aiming to keep the holes in each hull down to a manageable two.
That is four 1.5 inch marelon seacocks two exits and two raw water intakes
The intakes seacocks will each have strainers fitted that have 1.5 inch outlets.
These strainer outlets will each go to a manifold with 1 x 1 inch, 1 x 3/4 inch outlets.
The 1 inch outlets will each feed an engine, the 3/4 inch in one hull will feed (on occasion) a Blakes Lavac in the head.
The other 3/4 inch will feed a water maker in the future.
Please comment, is my logic and math alright or do I really need independent seacocks for the engines.

Regards, Tom

[James S]

Remember that when you're using your washdown (if for cleaning the anchor) it will be while the engine is running.
I have two thru-hull and strainers of the same size 1-1/4"
They are about two ft apart and are connected together with a valve that is normaly closed...engine is on one the wash down on the other.
We have in the passed sucked a plastic bag against (not inside) the engines strainer while going through a canal.....now if it happens I'll have an option that may not include shutting the engine down at a very bad time.

[SkiprJohn]

Thank you all for your comments. I already have most of the components to do a 1 1/2" thruhull so will go that direction. I do, however, want to stay with just one big hole in the bottom for the seawater cooling and other amenities if I can. Has anyone ever discovered the reason for surveyors recommending bronze vs Raritan other than it really looks shippy? My old Perkins in a different boat had a plastic sea strainer for 20 years and it never seemed to be a problem.
Thanks again.
Kind regards,
JohnL

[SkiprJohn]

P. S. It appears I really need to start diagraming these things out before I end up purchasing a bunch of stuff I don't need. Lots of the things I've put together have been surplus other peoples projects and boats and yard sale items. Then I try to make them fit my project. Not the way to go.
A caution to other folks putting together boats. Don't buy things just because its a great price. Buying frenzies can be costly in the end and make you put things together that might not be what you wanted in the first place.
Kind regards,
JohnL