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From: Arild Jensen (no email)
Date: Sat Sep 06 2003 - 14:31:40 EDT
Thanks to all who responded with suggestions.
The replies highlight the confusions that exist concerning how to cope with
this kind of problem.
The crux of the issue is how low the pump strainer cage can be positioned.
In addition most commercial strainers are made of bronze or Stainless steel
which is an absolute No No in aluminum hulls.
The flat bottom consisting of "relatively" thin aluminum plating further
exacerbates the situation.
You cannot screw fasten anything directly to the hull plate as you would with
wood.
Gluing or epoxying like you can in fiberglass is not always feasible.
Continued bonding is problematic.
In a relatively shallow bilge the amount of water adds up to a sizeable total
volume of water and thus weight.
Weight in most catamaran hulls is very detrimental to performance and boat
speed.
Strainer box design requires something better than what is currently being
offered in most marine stores.
A two step approach similar to fuel filtering sounds like it would be most
effective.
A coarse mesh or grille to keep out larger items such as wiping rags, chips
and other debris that ends up in the bilge.
Inside that containment area a finer mesh for a self priming positive
displacement pump that can lift water at least a foot while the fine strainer
keeps particles out that could clog the valves.
With such a pump, a check valve is likely not needed but a siphon loop is
still a good idea, just in case the pump valves do fail or get jammed in the
partially open condition.
In addition, a two step approach looks to be more effective.
For boats with small but constant drip leaks from shaft logs, pump glands or
simply wooden planking not being totally tight, a small volume but frequently
cycling pump may be more effective as long as it is backed up by a large volume
pump for situations when a major leak would overwhelm the little pump's
capacity.
Lastly, access to the lowest point of the hull is often problematic since
this often coincides with a point directly under the engines.
Power cat design often involves shoe horning a large engine into a tight
narrow space.
This pretty well precludes the normal approach of reaching under the engine
from the side as we do on most displacement hull trawlers.
With the increasing popularity of power cats instead of displacement hull
cruising boats, I wonder when more people will face these problems.
cheers
Arild
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