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sledracr

making a bigger hole

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I'm fussing with the bilge-pumps in my boat, and as long as I'm doing it I figure I might as well fuss with the hoses, too.

Current setup is a centrifugal pump in the bilge, hose led through the structural grid to a thru-hull under the counter.

Pump outlet is 3/4", thru-hull is 3/4", but hose in between is reduced down to fit thru the holes in the grid.  Been that way for 30+ years.

I'd like to run real 3/4-ID" hose for this... which would mean opening up the ~3/4" holes in the structure to about 1-1/8"   Maybe 1-1/4" to be less headache to pull through.

Whats a good way to make those holes bigger?  if I had access to the back of the panel I could clamp/glue/screw a piece of wood to the back and use it to center a hole-saw.  But, no access.

Somewhere low on my list of options is to use a Dremel tool to hog out the existing holes.  But that seems like a really crude way to do it (not to mention messy)

Any good ways to open up a hole, ideally in a way that ends up, you know, round and clean?

Thx
_/)_

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They make grinding bits for a standard drill motor, much faster than a Dremel. Kind of messy, but a small sanding wheel bit will smooth things over.

 

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Can you re-route the hoses so they don't go through the grid. Since its pumped, a bit more pressure head and hose length won't hurt, especially as you're going to larger dia. hose..

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3 hours ago, Fleetwood said:

Can you re-route the hoses so they don't go through the grid. Since its pumped, a bit more pressure head and hose length won't hurt, especially as you're going to larger dia. hose..

This. Doubling the hose diameter will make up for a longer run and then some.

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Laminar flow; resistance goes up as the 4th power of diameter, 1st pwr of length. (Turbulent flow; more complex.)

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That double hole saw trick is a good one. I have had good luck crudely epoxy-ing a piece of wood or G-10 or whatever in the hole. Drilling it out the next day. You can move the hole sideways a bit this way too.

Bilge pumps shouldn't go to below waterline thru hulls, no?

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9 minutes ago, Fleetwood said:

Laminar flow; resistance goes up as the 4th power of diameter, 1st pwr of length. (Turbulent flow; more complex.)

Umm...resistance goes down...? 

Flow increases by diameter raised to power of 2.63.

So 3/4" to 1 1/8" gives 2.9 times the flow: (1.125/0.75)^2.63. If smoothness and length are unchanged.

Thru-hull fittings tend to have very small openings...that can be an issue, too.

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It's not that hard if you have a bit of space to work in.

Us a holesaw of the size you want.

Hold the edge of the holesaw against the edge of your existing hole and angle your drill so that only the edge is in contact, acting as a guide.  Probably 20 degrees or more but I've never measured it.

Hold your drill firmly and, at low speed, start drilling.  The holesaw will soon create a groove and you can start to reduce the angle if you want a round hole.

The new hole will be off-centre from the old, if that matters.

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If you have the room, 80 grit sanding drum chucked into a drill. I'm amazed how fast and controllable this is. You'll need a vacuum.

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3 hours ago, El Boracho said:

Umm...resistance goes down...? 

Flow increases by diameter raised to power of 2.63.

So 3/4" to 1 1/8" gives 2.9 times the flow: (1.125/0.75)^2.63. If smoothness and length are unchanged.

Thru-hull fittings tend to have very small openings...that can be an issue, too.

OK, how about " inversely proportional to"?

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9 hours ago, Moonduster said:

I wouldn't enlarge holes in the grid without consulting someone familiar with the structure.

Amen to that brother. The structural grid is what is keeping the keel in place or holding up the mast step. If you were up in the attic of a house and wanted to run a cable, would you cut a big hole in a rafter or top chord of a roof truss? Same idea.

 

However a useful rule of thumb - no holes in beams (grids) more than 1/3 the depth of the beam, centered in middle of the beam AND still check with the builder/designer. If no designer/builder available, local reinforcement around the hole of the same material of the beam and same approximate thickness (glass/metal) can be added (bond/weld to beam side). Diamond shaped holes with rounded corners are OK too. No square or rectangular hole without big radiuses. Stress concentrations in glass webs are a real thing, and much worse than a metal beam. So please be careful.

NEVER notch the bottom or top of a beam's flanges.

If the grid is really shallow and I've convinced you that cutting it is a bad idea, you could consider a portable pump, either electric or manual for the "oh shit" moment when you need a big pump. Keep the little pump for housekeeping duties and minor leaks.

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Just go get a tapered wooden dowl plug, lightly press it in and mark 1/4", then cut that off whatever extra there is and then whack it back in the hole. Take your new diameter hole saw and then drill it out. 

 

This method works well if there is interference on the edge of the current hole and it needs to be offset/moved. 

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On ‎11‎/‎19‎/‎2018 at 2:02 PM, Moonduster said:

I wouldn't enlarge holes in the grid without consulting someone familiar with the structure.

Thanks, all

I'm not the least bit worried about enlarging the hole in this grid.  It's a mid-80s Ericson with the fabulously over-engineered "tri-axial force grid", which is essentially a monocoque hull liner with laminated girder system that carries all the loads of rig and keel, plus providing foundation for all the furniture and tanks and engine and such.

It already has holes through it for routing hoses, wire-runs, etc.  I'm not contemplating making any new holes, just opening one up about 1/4".  The companion hole (for the 1-1/2" manual-pump hose) on the other side of the same structure is already much bigger than I plan to go.  (see the holes on the left side of the bilge structure in the photo below...)

 

 

 

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That's good. The grid on those is much deeper than the shallow ones found in the bilge of a Beneteau!

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Man those things must have been painful to laminate.

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Northern Tool has a variety of $2.50 flap wheels that fit a 1/4 once drill or die grinder.  

Wvery time I go in that place I pick up a handful. There are Soooooo many things they do well. 

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