"T" Type Fuse

Jun 15, 2012
Hunter 50 AC St. Petersburg
Got to my 2007 Hunter 41DS 2 days ago. Tried to turn on the TV (which is fed 120 volts AC thru a dedicated 12 volt DC inverter), but TV would not turn on. Then I noticed that the 12 volt meter on DC panel was acting strangely. When I turned on a DC load such as the engine compartment blower, the meter needle would jump all around the gauge, sometimes pinning to the high or low extreme before settling to an average reading of around 12.7 volts. With the help of UncleDom( an esteemed member of this forum !!!), he determined the problem to be caused by the main battery fuse, a JLLN300. Upon replacing with new fuse, all issue were solved.

When I subjected the defective fuse to a continuity check (at 200K ohms), the reading was randomly continuity and then no continuity. Anyone have an answer as to why this change of continuity occured? Do these fuses just go bad over time? I was surprised that the local West Marine had 4 in stock, so I am guessing I am not the first with this issue.


Feb 6, 1998
Canadian Sailcraft 36T Casco Bay, ME
You'll want to find out why you blew a 300A fuse, if in fact you did?

I have never seen a Class T fuse fail sort of closed/open so be sure to clean any corrosion off that may give a false reading.

What model inverter is it?

Is this fuse shared with any other loads such as a bow thruster?

Is there any melting on the fuse holder? High resistance and heat can damage the fuse element if it gets hot enough. Hard to tell from the image if that fuse has been hot or that is just surface corrosion.
Jun 15, 2012
Hunter 50 AC St. Petersburg
Inverter is a Xantrax Freedom 20

"T" type fuse shared with Lewmar 140TT thruster installed 2013.

No melting or signs of overheating on fuse holders. Also fuse for bow thruster NOT blown.

Maine Sail, after reading your reply I cleaned the old fuse with sandpaper (see photo below) and re checked continuity. Same issue as before. I checked many times and am 99.9% sure fuse is "sort of closed/open".

One more mystery to add to this issue, last week boat was hauled to paint bottom. The captain who brought the boat to the yard reported that the thruster worked perfectly on the way to the yard, but when leaving the yard the thruster operated, but created no thrust. Before I changed out the fuse, the thruster made only a wimpy sound when operated. After installing the new fuse, it sounds strong, but has no thrust. I am waiting for a replacement shear pin, which I hope solves the lack of thrust issue. The yard advises to have never operated the thruster, only to have turned off the master power switch before painting the thruster tunnel. If the yard jammed up the thruster to cause the shear pin to break and/or the thruster to damage the "T" fuse, why didn't the thruster fuse blow first? If memory serves me correctly, the thruster fuse is 200 amps and came with the Lewmar thruster.
IMG_20190315_164750956.jpg IMG_20190315_172148482.jpg
Jun 11, 2011
Hunter 41 Lewes
One thing to mention, this is a Hunter 41 with the standard alternator which means that there was never a connection to the house bank due to the fuse being blown. Every thing was trying to run off the inverter, hence the shaky needle. Every time the engine was started there is a Solenoid that energizes and bridges the house and start battery circuits. This gave all loads on the house side power from the start battery and made every thing seem normal, except for the thruster which didn't have enough power available from the lone low amp battery plus the stock 70A Hitachi alternator.
Jun 15, 2012
Hunter 50 AC St. Petersburg
Just took apart bow thruster. Drive pin fell out of hole in motor drive shaft. Replaced drive pin which was tight fit needing to be tapped back into place. Reassembled with Locktite hoping it would help retain pin. Lewmar supplied Tywrap which was in place on shaft failed to retain pin.