Is a bilge needed?

[Ride]

Say you are building a multi-hull with a shallow round-bottom or a flat-bottom and no through-hulls below the waterline. If the intention is to cruise the World then are there any pros/cons to not having a bilge? Of course there would be flooring but the space under it would not be deep enough for things like pumps or the like; think a few centimetres at most. Would the recommendation change if there was a below waterline through-hull?

Thanks.

[fxykty]

Do the hulls have any rocker? If so, your floors to remain level will necessarily change from hull level at the ends and however deep the hulls’ maximum depths are near the middle of each hull (fore and aft). Does it matter whether you call that space under the floor a bilge or something else?

As an example, our cat has no floor at either end but the middle 2/3 of each hull has space between the floor and hull from near zero to about 20cm near the centres (fore and aft) of each hull.

But you do want to have the floor high enough so that you can permanently fit a bilge pump into each hull (manual at a minimum) - that’s a safety requirement. And if a bunch of water enters your hull through a hatch say, you do want your floor to keep you above that water sloshing around.

And no through-hulls? Even if you have outboard engines, don’t you want salt water intake for your galley (saves lots of fresh water for cooking and washing)? How about a deck wash? Or a watermaker? How about removable transducers (speed especially)? Each of these are useful for long distance and long term cruising to make life a bit more comfortable.

Using a bucket on a string is possible but a PITA longer term.

[wtmf]

a man from Alaska told me "flat bottom flat water, deep V deep waves" not to mention side slip with flat bottom. I was on a flat bottom ferry in big waves. we got to the top and slid sideways, INSTANT seasick, vomit. my opinion for flat bottm NO THANKS!

I agree with no through hulls below waterline. why put a hole in a boat????

[Ride]

Originally Posted by wtmf

a man from Alaska told me "flat bottom flat water, deep V deep waves" not to mention side slip with flat bottom. I was on a flat bottom ferry in big waves. we got to the top and slid sideways, INSTANT seasick, vomit. my opinion for flat bottm NO THANKS!

I agree with no through hulls below waterline. why put a hole in a boat????

Surely dagger-boards will control the yaw.

[Ride]

Originally Posted by fxykty

Do the hulls have any rocker? If so, your floors to remain level will necessarily change from hull level at the ends and however deep the hulls’ maximum depths are near the middle of each hull (fore and aft).
...
But you do want to have the floor high enough so that you can permanently fit a bilge pump into each hull (manual at a minimum) - that’s a safety requirement. And if a bunch of water enters your hull through a hatch say, you do want your floor to keep you above that water sloshing around.

There would be very little rocker. I agree that a pump should be in the hulls. However, if that is the only call for a bilge then I could simply install a covered pump with large enough openings at the floor to pump water out from an errant wave.

Well, there is the case where their is enough water to drench the floors but not enough to engage the pump. I don't imagine wading around in toe deep water would be a welcoming experience.

Perhaps raised flooring of 8 to 10 cm would not be such a bad idea.

[Fore and Aft]

Plenty of cats I survey have no bilge. Often that space is used for a built in water tank. Other boats I survey you are walking right on the hull skin. Often these boats just have a portable bilge pump with a long hose. A Fountaine Pajot cat I surveyed last week had a manual bilge pump mounted in the cockpit and a very long hose that could reach into the hulls.
One 24 foot yacht I owned I removed the floor lining just so I could gain another 50mm in head height.
My own yacht is flat bottomed with rocker and has no bilge. The lowest part has a bilge pump and float switch mounted to the hull.
Cheers