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Scupper Size

Posted: Sat Feb 08, 2020 5:13 pm
by rbecker08
So I got a pair of 1.5 in scuppers for my fs17 and holding them in my hand they feel huge.....

Are there guidelines for scupper size or better stated a disadvantage to having ginormous scuppers

Re: Scupper Size

Posted: Sat Feb 08, 2020 6:38 pm
by joe2700
rbecker08 wrote:
Sat Feb 08, 2020 5:13 pm
So I got a pair of 1.5 in scuppers for my fs17 and holding them in my hand they feel huge.....

Are there guidelines for scupper size or better stated a disadvantage to having ginormous scuppers
I'd say you want as big as possible assuming you have some way to stop water coming back in them. Even that would be an annoyance not a safety issue. I am also using two 1.5" ID scuppers for my FS17 with a little integrated duckbill. I wish I could fit bigger! I imagine how long it would take to drain say a foot of water out of those if you took a large wave, and I don't think they would feel huge. If you never intend to be in open water and only want to drain rain, then much smaller would probably be fine.

I'm also still building though, so no real world experience to share.

Re: Scupper Size

Posted: Sat Feb 08, 2020 9:53 pm
by Fuzz
Like Joe said, as long as you can plug them if needed, bigger is better. :wink:

Re: Scupper Size

Posted: Sat Feb 08, 2020 10:29 pm
by rbecker08
K I am there with you.....found plugs and these have those rubber flaps too. Anyone ever play around with an online check valve or not enough pressure to push out?

Re: Scupper Size

Posted: Sat Feb 08, 2020 10:33 pm
by rbecker08
K I am there with you.....found plugs and these have those rubber flaps too. Anyone ever play around with an online check valve or not enough pressure to push out?
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Re: Scupper Size

Posted: Mon Feb 10, 2020 4:03 am
by OneWayTraffic
My C17 has two 70mm (2.75") holes in the transom for scuppers. I'm going with elephant trunks (layflat hose over fibreglass tube). The trunks can be kept out of the water for calm days by pegging the ends to the transom.

The scuppers will be at deck level, but there will be a sump with pump under the motorwell to catch any backflow.

This concept works well on Arancia IRBs which are designed for use in breaking surf. Photos at the link.

https://www.fishing.net.nz/forum/irbs-f ... 77151.html

Re: Scupper Size

Posted: Mon Feb 10, 2020 4:21 am
by OneWayTraffic
With holes out the back you can estimate max drainage from a scupper by Area Scupper*forward speed of boat. There will be loss due to the flaps.

If those scuppers are a true 1.5" internal diameter then at about 5 knots (as fast as you would be able to go swamped).

Area =1.75 square inches.

Speed= 100" per second.

Drainage 175 cubic inches per second.

Or about 2700GPH per scupper.

There will be losses due to the flaps of course, but given all the other variables then this is as good a figure as any. If your inside diameter is not truely 1.5" then the actual drainage will be less of course.