Replacing a thru hull with a larger one - hole enlargement

[Captain Larry-DH]

I want to replace the puny 750gph bilge pump in my engine compartment with a 4000 gph model. That requires using a 1.25” hose vs. the 3/4” one I have now.
The current thru-hull is Marelon, so I was thinking an easy way to do this is pop a tapered wood plug into the current hole, and drill from the outside with a hole saw. It probably wouldn’t even require removing the old fitting, as I don’t think the Marelon mushroom would impair a good hole saw. I could pop off any remaining head material later.
Am I under thinking this? Please let me know if this is foolish.

 

[chuckwayne]

That should work - just make sure you've got a good center on the plug. There is also a hole saw mandrel made just for this- it's got two threads for saws. you scree on the larger saw blade on the outside thread, then a smaller saw on the inner; this would be a 3/4" saw or whatever the size of the current hole. I'd remove the marelon thru hull first whatever method you use, it might cause the saw to jump, damaging the 'glass

 

[Charlie Jones s/v Tehani]

That should work - just make sure you've got a good center on the plug. There is also a hole saw mandrel made just for this- it's got two threads for saws. you scree on the larger saw blade on the outside thread, then a smaller saw on the inner; this would be a 3/4" saw or whatever the size of the current hole. I'd remove the marelon thru hull first whatever method you use, it might cause the saw to jump, damaging the 'glass

bingo- I've used that often

 

[Benny17441]

I would not call a 750 GPH bilge pump puny. Boat designers understand that a bilge pump is a convenience item designed to discharge incidental water that collects in the bilge so that it doe not have to be manually scooped up and that anywhere from a 500 GPH to a 1200 GPH should be adequate for the task depending on the installation parameters. I'm sure you know that bilge pumps are bench rated under ideal conditions with no lift loads and that in actual life installations their delivery capacity is significantly reduced from their rated capacity. If what you are considering is a pump for an emergency situation that is a different design criteria in objective and function. I would consider a diaphragm pump for that function and would recommend a separate dedicate discharge port. I sincerely doubt that a 4000 GPH pump will outperform the 750 GPH pump in removing nuisance water. Don't know your electrical power capabilities but the installation of a couple of 120V powered sump pumps may serve well in emergency situations. Good luck.

 

[Captain Larry-DH]

Benny, your post has me thinking I will add it on, rather than replace. Thanks everyone for your advice.