Thursday, December 30, 2010

Fuel tank venting

Several years ago Phil de l'Etoile, Brave Duck #67, discovered his fuel vent lines were clogged with fuel. This week it dawned on me that every LNVT with fuel vents on top of the tank will suffer the same problem. That's because after leaving the tank, the 3/4" vent hose must dip down to get under the floor joist. It's only a matter of time until the dip fills with fuel. Fuel-filled vent lines also explain why there have been so many Forum discussions concerning filler port fuel eruptions during refueling operations.

Phil's solution, which is fix-and-forget, is pictured below. The vent on the port forward tank was relocated to a new location about 2" down the tank's side. This required drilling into the tank and installing a 3/4" fitting. The abandoned fitting on the top of the tank was plugged. Then the two forward tanks were connected together with 3/4" hose. A 'T' was put in the connecting hose and leads to the cabin-sides vent port. Care was taken to make sure the vent hose slopes only upwards from the 'T' to the cabin-sides vent port. Now, any fuel that enters the vent hose will automatically be routed back to the port forward fuel tank. The aft tanks were vented in a similar fashion except the starboard tank had the new fitting installed. To work properly, the tanks with the new taps can be filled no higher than those taps. This results in a reduction of fuel carrying capacity of approximately 30 gallons ( 2" x 2 tanks x 7.5 gal/" = 30 gal).

Another solution, which admittedly isn't fix-and-forget, is to pretty much leave the venting system as-is but install a trap to give the fuel caught in the vent hose someplace to go. From the aft tank the vent hose runs forward and constantly downward until it arrives abeam and about 2" below the forward tanks vent port. Here the forward tanks' vent 'T's in. Another 'T' just forward leads the hose ever-upwards to the cabin-sides' vent port. Attached to the downward facing leg of the second 'T' is a piece of clear 3/4" hose, about 2' long, which is capped on the bitter end. Since the clear trap line is lower than the vent, it will collect any fuel. Only when the trap is full does it need to be drained.

Starboard tank shows vent on top while port tank has vent on side
Here the starboard vent has been relocated to the side of the tank while the port vent is on top.

Tuesday, December 28, 2010

How Loud is it in the Pilothouse?

The following noise level readings were taken while underway using an Ipod Touch 4G running Sound Level(TM)—a free app. I was standing in the middle of the pilothouse, facing forward. The Touch was held upright and was about 5'4" high . Hmm, looks like Nellie has a resonance problem at 800 rpm ;-)

RPMNellie D. (63)Knock Off (66)

Saturday, December 25, 2010

The Ceiling Lights—An Easy Fix to an Old Problem

Pretty but not always functional

The ceiling lights are finicky—to put it nicely. Invariably either the red, white, or both lights don't turn on. It's not that the bulbs are burned out either; it's simply a bad connection between the bulb and socket. As the close-up picture of the bulb shows, there's a circular deformation in each of the bulb's contacts. Heating, due to a poor electrical connection, allowed the socket's steel pins to sink into the bulb. To fix the problem the contact between bulb and socket needs to be kept clean and oxidation free. Something easier said than done in a salt water environment. Fortunately the fix is easy; before installing the bulb simply apply marine electronics grease to its contacts. This will keep the environment at bay and help dissipate heat too.
Replace the old bulbs or grind-out the indents
Liberally grease the bulb's contacts before reinstallation

By removing only two screws and loosening the third, the lens easily slides out
Note the heat-caused indents in the bulb's contacts

Monday, December 20, 2010

Gel Coat Rub Rail Cracking

What causes those consistent and persistent LNVT rub rail cracks and how can they be eliminated once and for all? The cracks are consistent in that they almost always appear on the upper (i.e. the skyward) section of the rub rail and are usually nearly vertical. They're persistent in that many owners have repaired the cracks only to have new ones appear. When Nellie D. was repainted last year her many rub rail cracks were ground out, filled and faired. These cracks were only in the gel coat and did not penetrate the underlying fiberglass. So, what's causing these cracks? I had an experience yesterday that got me thinking. But first I must digress.

An opening in the stern bulwark clearly shows the rubrail's curved indent

I've preformed some exceptionally ugly landings lately. Yea, it's been 10 months since I've had Nellie out—but still. Anyway, during the last 'crash' I managed to drive Nellie's hull, just below the stateroom's port light , rather abruptly into the corner of the floating dock. Fortunately the dock was well padded and left only a rubbery raspberry on the hull. After counting my blessings, and spending 30 minutes removing the offending blemish, I noticed a big, new rub rail crack (see photo) almost directly above the point of impact. Hard to believe my little bump didn't contribute to this new battle scar. On the other hand, some strong winds in the past few days had pushed us rather forcefully against the floating dock. Either event would cause the hull to flex and I've suspected that movement of the fiberglass substrate is the root cause. Flexing can be caused by many types of dynamic loading: a rough passage; bumping a dock, a piling, or even from a fender; and certainly from the straps during a haul-out. But why does this otherwise normal fiberglass flexing manifest itself as a crack only in the rub rail?

The new crack …

So here's the theory, the hull mold was upside down, i.e. keel facing up, when it was sprayed with gel coat. Remember, the hull mold includes not only the rub rail but the bulwark too. Imagine a guy standing inside the hull with a spray-gun trying to put an even, thin layer of gel coat on all the surfaces surrounding him. It's fairly easy to do on the large flat areas of the turn-of-the-bilge, hull sides, and bulwark, but the rub rail, which is the nexus between the hull sides and bulwark, is a narrow, 6", concave indent (see first photo). If the spray was too thick in the indented area, the gel coat would run and pool on the indent's lower back and bottom surfaces. The exact surfaces, in fact, where today's rub rail cracks appear. Thus it's this too thick gel coat that's the problem; unlike the thinner gel coat everywhere else, the too thick gel coat can't handle the nominal deformations of the fiberglass it's attached to. The only practical way to eliminate rub rail cracking then is to get out the sander and remove the too thick gel coat.

Tuesday, December 7, 2010

Lani Hart's Lord Nelson History

Received an email from Lani Hart in which she documents the Lord Nelson Corporate history. Very interesting.

Saturday, December 4, 2010

Cummins 4BT3.9M Fuel Consumption Measurements

So how much fuel does a 150hp Cummins 4BT-3.9M use? The following data was compiled in calm conditions, aboard Nellie D. #63 on the Caloosahatchee Canal west of Lake Okeechobee, Florida. RPM readings came from the helm's VDO tachometer, ground speed came from a Garmin GPS and fuel flow rates came from a Flowscan.


Thursday, December 2, 2010

Why have a Waterlift Muffler?

One reason to have a waterlift muffler, according to John Mackie, John William #68, is to prevent water, which is aft of the muffler but has yet to be discharged overboard, from backing up into the engine. This could occur in large seas when the bow is down. Interestingly, Knock Off #66, has no muffler at all.