intake seacock on chine flat

Hello experts. Newbie here. Does the pressure on the hull at the chine flat near the transom (about 10% DWL forward of the transom) make this a poor location for the location of a thru hull for the gen-set's raw water intake? I ask because this is what I found on an aluminum boat that has the problem of forcing water up and over the vented loop whose purpose is to prevent our problem: flooding the exhaust hose/muffler and backflooding the engine through the exhaust when the gen-set's not running.

I saw this with my own eyes when we sea trialed the boat using clear hose before/after the vented loop. The muffler and and exhaust hose filled in about 5 minutes at top speed! We couldn't get water to go over the loop at anything less than about 25 kts despite high trim angle. Top speed was the only thing that forced water over the loop.

I think I learned once that the chine flat supports most of the weight of the boat when on plane. Is my thinking correct: On plane, the pressure of the water this aft portion of the chine flat is higher than anywhere else on the hull and therefore the worst place to a hole in the boat, especially one that is connected to a water intake.

Theoretically, could this problem of high pressure be solved by moving the thru hull off the chine flat and closer to, say, quarter buttock? As a band aid, we're going to put a solenoid-actuated valve on the seacock so it's only open when gen-set is running.

Sorry for long post. Thanks for any opinions.

 

It should have a scoop facing aft so it doesn't do that. Otherwise shut off the intake when not in use.

 

An electric waterpump is an aggravation but is more practical than a forced circulation system. The genset needs water only when it is running so tap off a bit of juice to run the pump. The pump will work when the boat is moored and also when moving astern. Scoops wont.

 

Thanks for replies.

Boat is fitted with an electric pump to supply water to gen-set. Not designed to forced circulation, which wouldn't work at slower speeds on this boat.

SamSam, I'm curious as to the pressures "downstream" of a clamshell scoop. I assume water flowing past at 30kts will eddy behind the scoop and be available for the water pump to draw instead of blowing past the impeller vanes.

 

Given how effective a self bailer on a dinghy is at 4 or 5 knots, I can not imagine that at 30 knots there would be any water available, more likely any water in the genset would be sucked out.

 

Yes, the chine area of a planning vessel at some points of performance can generate a slightly higher pressure than the bottom. It however does not support most of the weight of the boat.

Water intakes in the chine area can cause problems in cooling of engines as many times the water in this area is aerated or can get aerated based on speed and sea conditions. Move the intake to the bottom of the boat, within 20% of beam from the C/L or use a transom hung intake. There are many different types of strainers for engine water intake including some very unique flush mount designs. I would not mount the strainer backward if you ever plan to use the generator when the vessel is planning, you face the possibility of not getting enough water. I have had good lick with flush mounted thru hulls with a lip added just aft of the thru hull to generate a slight amount of pressure when underway, but not so much as to cause the problem you describe. Sometimes the only fix is a solenoid valve.

 

Yes, +25kts might create too much suction for the pump to overcome with a reversed clamshell, although a few holes drilled in the forward part will balance the pressures. Another problem is the extra drag and turbulence created doesn't help anything. A lip added forward of a flush mounted thru-hull could probably solve this particular problem. The genset should be thoroughly checked out before buying in any case, to assure no damage has been done previously.

 

Thanks, guys. Ultimately, we're going to move the thruhull toward the CL as drailton suggested and hope this reduces the pressure (and the risk of aeration) and see if we can draw water into the line without any protrusions around the thruhull. In the meantime, a solenoid seacock.