Spend money to change direction of my intake grates?

[Mathias]

On my HC33, the raw water intake grates on the hull exterior are facing forward. They really are meant to be facing aft. In theory, this prevents something from being stuck up against them due to water pressure, or minor dreck being pushed up into the opening.

The question is, how serious is it that they are facing forward? Has anybody had any experience with this? They have a low profile and are sloped, after all, so I can't imagine anything truly sticking to them.

I am not handy at fiberglass work, and they are pretty much set in multiple layers of who-knows-what (paint, embedded partly into the fiberglass? sealed with 5200?) So to turn them around, I would feel more comfortable with asking the yard to do it, and then re-farrow the bedding and paint and all that.

So, is this serious enough to spend money on? Have others been living with this situation?

-Mathias

[Russell]

I really wouldnt worry about it, the suction power of the engine is strong enough that if its going to suck something in and block it, it will do so reguardless of the direction they face. This is really a terribly minor thing IMO and not worth spending money on nor taking up the time it would take to change it.

[Tod M]

Mathias,

Is there a risk of starving the engine if they were facing aft? (think Bernoulli).

On the ferry pictured below, water to the main engines and other needs was drawn from a manifold that spanned the engine room. However, the manifold was small enough that with extra pumps drawing from it the velocity in the manifold was high enough that the main engines starved, melting the impellers. The problem was solved by "scooping" the flow rather than drawing out of the manifold at a right angle (or reversed scoop).

[img]http://www.htmills.com/images/LF-SouthBass.jpg[/img]

[Mathias]

You example got me to thinking. (It hurts when I do that.)

While I do think the Bernoulli affect might produce bubbles (what if I put golf ball dimples in the strainer? Ha!) I wonder if that would start at six or seven knots on such a shallow rise?

Then I got to wondering where I heard that the grates should be facing aft? I picked my Nigel Calder "Boatwoner's ... Manual" off the shelf and he doesn't address it.

So, does anyone know for sure? I am sure this can be resolved more definitively than the free-wheeling prop issue.

-Mathias

[Parfait's Provider]

I recall that hot rods have air intakes that face forward; I don't suppose they would work very well facing aft like the exhaust pipes, some of which are designed to take advantage of Daniel Bernoulli's insights.

Parfait's raw water intake is perpendicular to the hull and has a slightly bulbous bronze strainer over it. I don't recall, but I suppose the fingers run fore and aft. Trash gets washed away from it at least until the barnacles get too large.

If this is a concern, I'd simple add a strainer over the top of the intake scoop to ward off those large impurities in the water.

See: http://spartanmarine.com/image/28.jpg

[Tod M]

a self-bailer in a sailing dinghy. There are several designs (a throttle-body thing that lowers into the water, a wedge that swings down aft, etc) but they all work on the bernoulli principle.

There might not be a problem, but it might be an issue at higher speeds. Certainly not at low speeds. Maybe a stroll through a marina while the boats are on the hard (assuming northern climes) would be telling.

[Mathias]

I like the idea of the stroll through the boat yard. Though my mindset is still firmly winter here in Vermont. I worry what if that stroll might instill a terrible case of cabin fever.

By the way, just to be clear, here is a link to the type of strainer I am talking about.

http://www.reddenmarine.com/site/new-de ... KO65DP6PLB

-Mathias

[Tod M]

My guess is that it is installed correctly but to be sure...

http://www.perko.com/contact.html

[John Danicic]

To all:

I solved a parallel problem to this discussion.

When I first got her, Mariah's Perkins 4-108 tended toward "over heating" when I was running the engine over 2000 RPM for any length of time. After a discussion with our Diesel Mechanic, an expert who has attained mythic proportions on Lake Superior, we eliminated many of the usual suspects, but to no avail. I kept the revs down to solve the problem and kept the engine cool for that season.

After Fall take out, this same mechanic and I looked at the the raw water intake for the Perkins on the hull. It was the standard, round, multi-holed, (dare I say, dimpled), Spartan bronze strainer that had been on the boat since year one. He sighted down the hull and determined that at faster speeds, the strainer was being starved of water because of the faster flow over it. I replaced that strainer with a slotted Spartan Strainer with the slots a-lined to catch the water as it slipped past the hull. The next season and since, no trouble. I could run all day at 2800 RPM and never even approach the red zone on the temperature gage.

I compared the location of the standard strainer on a sister CD 36 who reported no such trouble with the location on Mariah's and found that they were visually, very different. Individual characteristics. Like people, our boat are full of them.

Sail on

John Danicic
CD 36 -Mariah- #124
Lake Superior

[jerryaxler]

John raised an interesting question. What is the optimum cruising rpm for the Perkins 4-108? I run mine at 2100 rpm and my fuel consuption is about 0.7 gal/hour.

[John Danicic]

I should have said that I don't normally run too much over 2000 RPM. I achieve about 6.5 knots at 2000 RPM in flat water. 2500RPM will barely push me over 7.2 knots or so which is not much difference in speed but a big difference in fuel consumption and noise. We did run last summer for 19 hours against a strong head wind and big seas at 2500 to 2800 RPM and you could almost watch the fuel gage drop. I figure we were burning at least a gallon an hour if not more and moving at a little over 5 knots. In our first two seasons we got about .40 gals per hour but never ran her over 1800 RPM because of the over heating issue. Now, if I figure on motoring any distance, I use about 3/4 gals per hour as a rule of thumb which would put us right with Jerry's .7gal/hr. One thing our diesel mechanic told me is that diesels and especially Perkins, like to be run hard occasionally. I would never say I am hard on my engine but I do follow his advice and, knock on wood, have had good luck so far.


Sail on.

John Danicic
CD36 Mariah #124
Lake Superior