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Author Topic:   OUTRAGE 22 Hull: Thru-hull Fitting Length

lanlubber posted 08-01-2014 07:09 PM ET (US)
I'm adding a bilge pump on my 1986 Outrage 22. How long will the threaded portion of a thru-hull shaft need to be to go through the side of the boat just below the gunwale at the aft quarter?
Thanks
jimh posted 08-04-2014 10:10 AM ET (US)
On my Boston Whaler 22-foot hull, the location of the through-hull fitting for the exhaust of the sump pump is not close to the gunwales. It is just above the water line by perhaps six inches.
In my opinion, you don't want the exhaust of the sump pump to be so high as to be near the gunwales. A pump that lifts water out of a sump has two considerations: the lift of water into the pump intake, and the head of water the pump has to push out with its exhaust. In the case of the typical centrifugal pump used in Boston Whaler boats, the lift on the pump is zero; the centrifugal pump design cannot lift any water into itself. It has to be submerged in the water. The head of water on the pump should not be too great. The more head the less water flow. In the original equipment pump installation the head on the cockpit sump pump is perhaps only one foot.
jimh posted 08-04-2014 10:15 AM ET (US)
Lift is the height that the pump must raise its intake water. Head is the height the pump must raise its discharge water. As I mentioned, for centrifugal pumps, the rated lift is usually zero. Check the pump rating for flow versus head. You will find that flow decreases with increasing head.
lanlubber posted 08-06-2014 09:02 PM ET (US)
Thanks Jim. This is a Rule pump and not original. I've temporarily run the discharge hose over the gunwhale and the pump had no issues. I'm sure that I'm not getting optimal flow but in a dire situation I would rather have the discharge point as high as possible. Can you comment on how thick the side of the hull is where you mounted your thru hull?
jimh posted 08-07-2014 03:33 PM ET (US)
I don't know how thick the hull is. From the outer surface of the outboard fitting to the far end of its pipe, the through hull on my boat measures about 3.5-inch long.
jimh posted 08-08-2014 07:46 AM ET (US)
On my Boston Whaler 22-foot hull, the inboard end of the through hull fitting is set into a recess in the bulwarks. The hull thickness, if not cut away for the recess, is greater than the through-hull fitting's length. I recommend you check with Boston Whaler for details.
MattInSanDiego posted 08-09-2014 12:17 PM ET (US)
Is the transom open? If so, I can't think of a good reason to mount the discharge tube higher than the height of the transom.
stayinstrewn posted 08-10-2014 02:14 PM ET (US)
On my outrage 22' with a standard transom, the discharge lines from the bilge and starboard quarter both go over the transom...I certainly wouldn't mind going thru hull under the stern cleat, but haven't bothered to up to this point...if you really want to, place holes a few inches above the waterline and you'll be golden. Best guess on the length is 6".

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Author	Topic: OUTRAGE 22 Hull: Thru-hull Fitting Length
lanlubber	posted 08-01-2014 07:09 PM ET (US) I'm adding a bilge pump on my 1986 Outrage 22. How long will the threaded portion of a thru-hull shaft need to be to go through the side of the boat just below the gunwale at the aft quarter? Thanks
jimh	posted 08-04-2014 10:10 AM ET (US) On my Boston Whaler 22-foot hull, the location of the through-hull fitting for the exhaust of the sump pump is not close to the gunwales. It is just above the water line by perhaps six inches. In my opinion, you don't want the exhaust of the sump pump to be so high as to be near the gunwales. A pump that lifts water out of a sump has two considerations: the lift of water into the pump intake, and the head of water the pump has to push out with its exhaust. In the case of the typical centrifugal pump used in Boston Whaler boats, the lift on the pump is zero; the centrifugal pump design cannot lift any water into itself. It has to be submerged in the water. The head of water on the pump should not be too great. The more head the less water flow. In the original equipment pump installation the head on the cockpit sump pump is perhaps only one foot.
jimh	posted 08-04-2014 10:15 AM ET (US) Lift is the height that the pump must raise its intake water. Head is the height the pump must raise its discharge water. As I mentioned, for centrifugal pumps, the rated lift is usually zero. Check the pump rating for flow versus head. You will find that flow decreases with increasing head.
lanlubber	posted 08-06-2014 09:02 PM ET (US) Thanks Jim. This is a Rule pump and not original. I've temporarily run the discharge hose over the gunwhale and the pump had no issues. I'm sure that I'm not getting optimal flow but in a dire situation I would rather have the discharge point as high as possible. Can you comment on how thick the side of the hull is where you mounted your thru hull?
jimh	posted 08-07-2014 03:33 PM ET (US) I don't know how thick the hull is. From the outer surface of the outboard fitting to the far end of its pipe, the through hull on my boat measures about 3.5-inch long.
jimh	posted 08-08-2014 07:46 AM ET (US) On my Boston Whaler 22-foot hull, the inboard end of the through hull fitting is set into a recess in the bulwarks. The hull thickness, if not cut away for the recess, is greater than the through-hull fitting's length. I recommend you check with Boston Whaler for details.
MattInSanDiego	posted 08-09-2014 12:17 PM ET (US) Is the transom open? If so, I can't think of a good reason to mount the discharge tube higher than the height of the transom.
stayinstrewn	posted 08-10-2014 02:14 PM ET (US) On my outrage 22' with a standard transom, the discharge lines from the bilge and starboard quarter both go over the transom...I certainly wouldn't mind going thru hull under the stern cleat, but haven't bothered to up to this point...if you really want to, place holes a few inches above the waterline and you'll be golden. Best guess on the length is 6".