Battery Location, Pitot tube?

Redknight · 12 March 2012, 23:51

Hi all,
My last boat had the battery mounted up inside the centre console, when I filled it up with water there was no problem. My new RIB has the battery in the standard location, sitting against the transom, should I move it to a safer drier location? or is there some kind of waterproof battery housing that can be used?

How does the speedo work? is it some kind of pitot tube thing in the leg of the motor? Mine seems a bit erratic. Its a 1992 Mercury Motor.

jyasaki · 13 March 2012, 16:30

As far as I know there is no waterproof battery box, as batteries off-gas hydrogen (so either need to breathe, or are an explosion hazard.) FWIW, I completely submerged the battery on my small boat (water coming over the bow while on the hook and diving), still cranked like a champ.

Speedo works as follows: On the front of the leg, there is a tiny hole. That connects to a plastic or rubber tube that runs up to the speedometer in the console (or wherever it happens to be located.) As you move forward, water is forced into the hole, which pressurizes the air the tube. The speedo converts the increased air pressure to speed (roughly.)

Problems occur when: you get a hole in the tube, or the tube falls off (small jet of water spraying all over); The tube gets pinched off (no reading); pitot hole in the leg clogs up (sand, shell, kelp, little bits of the whale you just hit.) Normally, a paper clip or appropriately sized drill bit will push the blockage through to where it doesn't affect the operation anymore.

jky

9D280 · 13 March 2012, 16:31

If your console has a shelf built in it's probably worth fitting it there. I had one battery went swimming i na previous boat. it seemed to survive rematrkably well, unlike the main isolator switch which had died within 12 hrs of the submergance.....

As for the speedo, yes, likely a pitot. My Merc had the same hose fitted. I removed it & blocked the hole as I had a Garmin handheld to tell me speed. Those gauges are purely mechanical so it might be the mechanism is sticky or you got a salt blockage in the tube.

Redknight · 13 March 2012, 19:52

Thanks for your help, I was tidying up some dodgy wireing inside the console, removing the CD player and other old leftover wires and connections etc. I found the wire that comes from the battery and it's quite beefy so I'm going to build a shelf and install battery high in the console. I might also stick a voltmeter on the dash so I can see that it's charging.
I found the speedo tube, and a few likely looking holes the leg, my guess is a blockage or air leak, I've had a poke around with a wire and squirted water up the holes, so fingers crossed, it might work a bit better now...

willk · 13 March 2012, 22:12

If the tails are long enough, you might pare back some insulation on those battery cables and check if the copper is blackened. If it is, you'll have to rip them out.

9D280 · 14 March 2012, 10:18

The other thing with a pitot speedo is that the pressure induced in thto the pipe by the leg is not the same relative to your speed compared to the transom mount ones. Might be worth once you get it going borow a handheld GPS & check your gauge is compatible with your engine (i.e telling the truth) .

The hole for the pitot is tiny, and should be below the cav plate height. Mine is (was) on the port side about 3-4mm back from the "point" of the gearbox

jyasaki · 17 March 2012, 22:26

Quote:

Originally Posted by 9D280

Might be worth once you get it going borow a handheld GPS & check your gauge is compatible with your engine (i.e telling the truth) .

Or get a GPS and use that for speed. The pitot ones are fine when they work, but that isn't saying much. About a quarter of the time, mine reads 4 to 6 mph while tied to the dock, another quarter of the time reads 10 mph low, and about an eights of the time reads zero at speed.

Quote:

The hole for the pitot is tiny, and should be below the cav plate height. Mine is (was) on the port side about 3-4mm back from the "point" of the gearbox

On my F115 it's right at the point of the LU, below the cav plate. Nearly impossible to see, but easy to find by running a thumbnail down the leading edge of the leg.

jky

Redknight · 18 March 2012, 00:54

Took the boat out yesterday, the speedo works a bit better, but still not very accurate. At 5500rpm I'm doing just on 27knots measured with gps.
The motor can rev up over 6000, so I think I might need a prop with a bit more pitch. Though the one on there might be set up for when the boat is loaded up I guess...
Does anyone know what the maximum revs should be for a 2stroke 4cylinder mercury?

jyasaki · 18 March 2012, 11:26

That would be less pitch, assuming your 5500 was at WOT, and trimmed for max rpm.

Can't help on the motor spec, sorry.

jky

Redknight · 18 March 2012, 19:50

Quote:

Originally Posted by jyasaki

That would be less pitch, assuming your 5500 was at WOT, and trimmed for max rpm.

Can't help on the motor spec, sorry.

jky

Oh yes, less pitch! I just need to find out what the max revs should be as I can easily exceed 6500...

jyasaki · 19 March 2012, 03:22

Year and model? Should be fairly easy to find online.

jky

Redknight · 19 March 2012, 03:44

Quote:

Originally Posted by jyasaki

Year and model? Should be fairly easy to find online.

jky

It's a 1992 40hp 4cyl mercury doesn't seem to have a model number...
I've had a good look online and haven't found anything conclusive.
Most outboards about that size seem to have max revs of 5500 ish?
It's probably quite conservative, I'm used to running 10,000+ revs on my paramotor, but that's what it's designed to do!

jyasaki · 19 March 2012, 17:11

Lots of models to choose from, apparently (these are all listed as 1992 4-stroke 4-cyl models for US consumption):

40 1992 ELHPTO D082000 - D181999
40 1992 EO D082000 - D181999
40 1992 ELPTO BEL 9682745 - ??
40 1992 ELO D082000 - D181999
40 1992 MH D082000 - D181999
40 1992 ELPTO D082000 - D181999
40 1992 MLH D082000 - D181999

Still poking around to see what info I can find.

2-stroke 40 spec, 1992:
Manufacturer: Mercury
Year: 1992
Horsepower: 40.0
Cylinders: 4
Type: 2 stroke
Gear Ratio: 2.00:1
Min WOT RPM: 5000
Max WOT RPM: 5500
Spline Count: 13
Drive Type: Outboard

Re: 4-stroke: saw a post where someone said to prop it to 5800 lightly loaded, so it sounds to me like it may be 5500 max. Lots of people lauding the motor.

About all I can find right now.

jky