Do I need battery fuses?

Benny17441 · Apr 27, 2015

Yes, it may help to prevent a fire. On the other hand boats, both inboard and outboard have been delivered for decades without these fuses. The incidence of fires caused in a boat by shorts or arcing in the high load wires has been statistically nil. Frequent inspection and proper maintenance might be just as effective. As the boat's owner it will be your decision to make.

JK_Boston_Catalina310 · Apr 27, 2015

If it were my boat I would fuse it. Probably with multiple fuses.

You fuse wires based on the size. You size wires based on the amps and distance. When you measure distance for 12volt system it's round trip distance. So if your engine is 6 feet from your batteries you have 6 feet for the positive side and 6 feet for the negative side for a total of 12 feet.

I use this chart from Blue Sea Systems for sizing wire and fuses. (warning large pdf) It has all of the information in one place so it is very easy to use.

Good luck,

Jesse

Ward H · Apr 27, 2015

I fused my 9.9 w/ electric start using a Blue Sea Battery Terminal Block for a Marine Rated Battery Fuse. (MRBF). About $17 for the block and $11 for the fuse. I figured it was cheap protection and while updating the boats wiring why not do it right.

Bill Roosa · Apr 27, 2015

You are not "required" to fuse certain engine charging and starting power cables IF they are of limited length. The thought being that short high power cables are probably not going to be getting something shifting onto them while underway. Basically limits the cabling to the engine compartment.

JK_Boston_Catalina310 · Apr 27, 2015

Bill Roosa said:
You are not "required" to fuse certain engine charging and starting power cables IF they are of limited length. The thought being that short high power cables are probably not going to be getting something shifting onto them while underway. Basically limits the cabling to the engine compartment.

I don't think this is correct. IIRC the exemption for starting circuits has to do with large engines that may require starting amps beyond what fuses can deal with. I think Maine Sail has said that this is largely overstated and that most engine starting loads can now be fused. I know my small diesel is fused with no problems.

Also, I presume the OP is using the batteries for house loads too. The house loads should definitely be protected by a fuse(s).

Ron20324 · Apr 27, 2015

Wake up, guys... he's talking about an outboard motor, not an "engine"

JK_Boston_Catalina310 · Apr 27, 2015

Ron20324 said:
Wake up, guys... he's talking about an outboard motor, not an "engine"

Meaning?

Benny17441 · Apr 28, 2015

I have never blindly accepted generalized statements. Have learned that terms like "Never" and "Always " don't apply well to boating. Some might consider it cheap protection while others may decide it is an unnecessary waste of money and the thing is that both may be right. If I had no life insurance and could purchase a policy against the eventuality of a meteor falling on my head at a premium of $10 a year would I get it? I wouldn't but that is just me. Can not purchase peace of mind on something I do not loose any sleep over.

Johnb · Apr 28, 2015

cb32863 said:
I have been reading the thread on battery fuses and doing some looking around and I am wondering if in my situation if I need to fuse my battery. Everything I have read refers to inboard diesels. I have a 9.9HP electric start outboard. I have one Group 24 (or 27, can't recall of the top of my head). Do I need to fuse my battery? If so, I am not sure how to figure out the right size.

What you need to protect against is not your motor charger supplying excess current (that is not going to happen with an outboard) but a short that draws a huge current from the battery, with the possibility to melt wires and start a fire.

Therefore the fuses needs to be placed between the source of the big rush of current (right at the battery) and everything else.

The fuse needs to be sized big enough to provide for heavy loads (starter or invertor for example) and may not provide protection to smaller size wires that serve lights or electronics for example. That is why distribution panels have breakers or fuses sized to protect the wiring downstream of them. The big fuse at the battery protects the wire from battery to panel/starter and the panel protects the wires to individual loads.

The size of fuse at the battery must be commensurate with the size of wires connected to it. That way it can blow without melting them. Start by ascertaining the size of wires connected directly to the battery.

Maine Sail · Apr 28, 2015

cb32863 said:
I have been reading the thread on battery fuses and doing some looking around and I am wondering if in my situation if I need to fuse my battery. Everything I have read refers to inboard diesels. I have a 9.9HP electric start outboard. I have one Group 24 (or 27, can't recall of the top of my head). Do I need to fuse my battery? If so, I am not sure how to figure out the right size.

You don't "need them" nor do we "need" boats. Are they a good idea? You bet...

15' Boston Whaler, single group 24 battery, outboard motor - electrical short:

3 minutes prior to this fire four 8 year olds in the junior sailing program had been on board this "chase boat" They and the coach stepped off onto the dock, the coach turned around and a fire was erupting from under the bench seat where the excess length of battery cable was in a coil. Apparently it may have been stepped on or chafed just a bit too much and the single group 24 battery drove enough current into the short to start a fire and total the boat.

I quote Nigel Calder here:

"The net result is that nowadays, electrical shorts are probably the number-one cause of fires on boats. There is simply no excuse for not protecting all high-current circuits, including the cranking circuit." (From the Nigel Calder Cruising Handbook)

Benny17441 said:
The incidence of fires caused in a boat by shorts or arcing in the high load wires has been statistically nil.

Where did that stat come from....?

55% Of All Boat Fires Are Electrical
AC Shore power 54%
DC Shorts / Wiring 25%
DC Engine Wiring 12%
AC Appliances 4%
AC Wiring / Panel 2%
DC Battery Charger 2%
AC Power Surge or Lightning 1%

Source: ABYC & USCG with Boat US

pk104 · Apr 29, 2015

Ok this hit home. I chair our Jr Sailing program every summer and have kids in my 13' Whaler all the time. What fuse and what size do you recommend and how would I install it with a battery box? I have inline fuses on the accessories but none on the main motor cable.

berner73 · Apr 29, 2015

pk104 said:
Ok this hit home. I chair our Jr Sailing program every summer and have kids in my 13' Whaler all the time. What fuse and what size do you recommend and how would I install it with a battery box? I have inline fuses on the accessories but none on the main motor cable.

Did you see Maine's article?

http://www.pbase.com/mainecruising/battery_fusing

Maine Sail · Apr 29, 2015

pk104 said:
Ok this hit home. I chair our Jr Sailing program every summer and have kids in my 13' Whaler all the time. What fuse and what size do you recommend and how would I install it with a battery box? I have inline fuses on the accessories but none on the main motor cable.

Our Jr. sailing fleet boats are now fused with MRBF fuses sized to protect the wire. What size fuse you use depends upon your starting cable wiring.

pk104 · Apr 30, 2015

I did go to Maine's site and read the article about battery fusing. They should fit in a battery box on a runabout. I will be buying the right size for my boat and the club's runabout and looking at any other runabouts we use for safety boats this summer.

I worry constantly about keeping the kids safe. We have written procedures for the safety boats and all kinds of safety rules but until I saw the picture of the burnt Whaler and Maine's comments I had not considered a boat fire. Thanks for making me think of it.

Gary

Maine Sail · Apr 30, 2015

pk104 said:
I did go to Maine's site and read the article about battery fusing. They should fit in a battery box on a runabout. I will be buying the right size for my boat and the club's runabout and looking at any other runabouts we use for safety boats this summer.

I worry constantly about keeping the kids safe. We have written procedures for the safety boats and all kinds of safety rules but until I saw the picture of the burnt Whaler and Maine's comments I had not considered a boat fire. Thanks for making me think of it.

Gary

My daughter races in this program so it hit very close to home. I was there when the fire happened and was able to snap some photos. Scary stuff!! It happened extremely quickly. From the time the smoke was noticed under the seat to the fire/inferno erupting full bore was less than 20-30 seconds, according to the coach. They rushed the kids off the dock, sails still up on the Opti's, and eventually pushed the burning Whaler off the dock to save the other boats. There was no time to reach for an extinguisher when trying to focus on getting he kids off the dock. Remember these boats are "not required" to have battery switches or fuses.

:cussing: Not required does not mean it is not a good idea it just means it is not required. If you have a battery on your boat it is a very good idea to have it fused or to have the cables extremely well protected.

Batteries have IMMENSE CURRENT CAPABILITY!!

pk104 · Apr 30, 2015

It could easily be our dock in the picture and we often have up to 15 Sunfish on the docks being loaded of unloaded with kids and a couple of our safety boats tied to the same docks. Thanks for the pictures. I plan to show them at my next committee meeting and I will advise others to fuse their outboards. I certainly will fuse ours.

pk104 · May 1, 2015

I have #6 cable 6 feet between motor and battery. Is a 70amp MRBF fuse the correct size?

Stu Jackson · May 1, 2015

Gary, go to Blue Sea website and find the chart for fuse sizing. You can download it and save it.