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Old 12 March 2012, 18:51   #1
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Battery Location, Pitot tube?

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.
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Old 13 March 2012, 11:30   #2
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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
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Old 13 March 2012, 11:31   #3
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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.
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Old 13 March 2012, 14:52   #4
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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...
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Old 13 March 2012, 17:12   #5
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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.
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Old 14 March 2012, 05:18   #6
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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
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Old 17 March 2012, 17:26   #7
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Quote:
Originally Posted by 9D280 View Post
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.


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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.

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Old 17 March 2012, 19:54   #8
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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?
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Old 18 March 2012, 06:26   #9
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That would be less pitch, assuming your 5500 was at WOT, and trimmed for max rpm.

Can't help on the motor spec, sorry.

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Old 18 March 2012, 14:50   #10
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Quote:
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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...
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