Plastic Classic Forum

When motorsailing recently on starboard tack with only a slight heel, probably no more than 10 to 15 degrees, the engine went dry. I immediately shut it down and returned to my mooring where I restarted the engine and after a few seconds the water beagn flowing normally to the engine. On my last Triton I routinely motorsailed without a lot of thought given to the angle of heel, the only difference between the two boats in this regard is that the first boat had a scoop strainer and this boat only a thru hull. In my mind I can make a case for the water flowing over the thru hull causing a loss of pressure.
Has anyone had a similar experience or any thoughts on scoop strainers?
Thanks,
Barry

I think I would lean the opposite way: On a "slow" boat (i.e. typically 6 knots or less*), I would prefer not to have a strainer on the outside of the boat, because I would worry about it clogging with growth or being clogged by something splaying over the outside of it.

I would probably set it up with no strainer on the outside (just an open through-hull) and then a cleanable/reachable strainer on the inside of the boat.

Of course if the intake comes out of the water on a dramatic heel angle (or in waves) then it doesn't matter what's there. Some boats have two intakes for this reason, but I would think that would just be added complication for most boats if they have their one intake in an advantageous location.

(I did sail in company with a sailboat once wherein swells brought the intake completely out of the water on each "bronco" moment resulting in the engine beginning to overheat. But those were big/unusual/unruly swells and the conditions required one to take them, motoring, at the worst possible angle. The boat I was on did not have that problem.)

Rachel

*I think on faster boats the scoop-type strainers do help in getting water into the through-hull; but I don't think it is an issue for a through-hull alone on "our" type of boats.

Rachel, thanks for the opinion. I shared your view and the boat is set up without a scoop strainer, I keep a length of battery cable in the cockpit locker to snake down thru the seacock in order to clear any obstructions, it goes in through the raw water strainer and straight down and out.
I'm no longer sure this is the best arrangement since my engine ran dry while the thruhull was still submersed and appeared to be clear of any obstructions.
My concern is that as the thruhull gets closer to the surface with the boat moving forward at 5 knots is it possible to run dry and if so would a scoop strainer solve the problem?
I looked through the boatyard and every sailboat had a scoop strainer and I have to wonder if they are of no value why do all of the boats have them as opposed to no strainer or a plain round strainer. I have looked on the internet but have found no information on the theory behind their use.

Barry

I think the theories are as follows:

1) The scoop will direct water into the intake (which my understanding is that it will, but is either only effective or needed at speeds over 6 knots).

2) The scoop will keep crud out of the intake (which it will, but if it gets clogged while you are running it will also keep water out, with no easy way to clear it).

My impression is that the "scoop/strainers" on 6-knot sailboats is a custom that is changing, but at least for now is still "the way most people do it" and so people do it anew. I know that when my former A-30 was outfitted, the style was not for scoops but was for a static, "spaghetti colander" style grate. Which now that I think about it, might be some "evidence" for the no-scoop-needed idea. I say this because that engine had a "tiny" 1/2" intake and a somewhat-paint-clogged spaghetti-colander style intake strainer on it. And yet in the over 600 hours that the former owner meticulously logged on it, I never saw any reference to lack of water (and given his maintenance I'm sure he would have changed the set-up had there been a problem). This was an Atomic Four and I'm not sure if they for some reason need less water. I did change the through-hull/seacock to 3/4" when I overhauled it.

I'm hesitant to sound really strong-minded about this, as I'm not expert in this area, but I think if it were my boat and I had noticed a lack of water getting to the engine and the through-hull was wide open and not clogged, I would be wondering if there was some other reason for it (I'm not sure what... some kind of air lock or something?). I think that if your intake is located deep enough (i.e. not right near the waterline), and is clear of obstructions, you shouldn't need (or maybe even benefit from) a scoop at 6-knot type speeds.

A couple of questions come to mind:

1) How far below the waterline (and where) is your intake through-hull located?

2) What size is it?

I'm hoping that someone less tentative will chime in.

Rachel

Glenn,

Excellent, thanks. That discussion was well worth reading, I will heed the advice of both you and Rachel and not add a strainer. My problem may have been one of a temporary blockage. I will change the impeller if for no other reason than it ran dry for close to a minute.

Barry

Sorry, I read your post and looked at the strainers then read the discussion thread. By the time that I finished the discussion thread I had forgotten about the strainers.
The discussion changed my views about strainers and if I install one it will be one of the basket type with a hinge, not a scoop. Thanks again for the really through explanation.

Of the six Pearson I've worked on the biggest restriction of a water strainer has been bottom paint, some holes are completely blocked with paint and the rest are about 50 percent, the slotted ones are no better.