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Old 12 September 2007, 07:45   #1
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Questions about prop and electrics???

Finally got my late 70's Yamaha 40hp running properly and have just tried the electric starter hooked up to one of those mobile jump starting booster packs - starter works fine which I wish I had checked out sooner having spent 3 trips out pull starting it!

Could anyone tell me what sort of battery I could use with it? I dont want to spend a fortune on a liesure battery as it wont be getting used that often - would maybe a small car battery for a corsa or fiesta etc be of any use?

I checked the voltage going to the battery cables which was just over 12v and only slightly increasing when reving the engine so am sure that is working correctly and will charge the battery. Theres also another wire coming from the engine which I think may have been used for lights etc but this seems to be kicking out 30 to 40v - is this right or is my meter maybe a bit dodgy?

Just another question regarding the prop - its currently fitted with a 11 1/4 x 14 - g prop - the blades are quite a bit mishapen as someone has tried to round them off in the past so I reckon I'll need to get a new prop - was wondering what size of prop would be best for top speed and pulling power?

It was my first proper time out on the boat at the weekend and it did seem to run ok but felt as though it had more in it which i thought might be down to the mishapen prop - it struggled getting onto the plane but that could be because I had it too highly trimmed - once on the plane it seemed to go quite fast but nothing crazy - there were 5 of us on board though!
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Old 12 September 2007, 08:00   #2
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battery for 40.

a small car batter will do the job, they are SLA rated which is starting lighting and auxiliary, so as long as you dont expect discharge it heavily with accesories with the engine off it will be fine. the charging is another matter though, if the battery cables show 12 volts when connected to the battery and the engine is running as you suggest then its not charging, the voltage should be much higher, around 13.7-15 volts. It could well be that the wires you say have 40 volts are the charge wires(i assume you measured a DC voltage not AC), if the alternator is a low current one then i may not have a regulator, and will rely on a battery being connected to draw some current and keep the voltage down to a sensible amount. usually this type of alternator puts out 5 or so amps continuous, just to keep the battery topped up. You could just connect them to the battery observing the polarity and measure the charge current with a meter, if its around 2-5 amps thats fine if its higher you need a regulator. If however the voltage was an AC voltage you will need a rectifieras well.
try to get the wiring diagram if you can it will avoid burnt out stators and boiled batteries from overcharge!
hope this helps.
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Old 12 September 2007, 08:18   #3
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Thanks for that - yes I did check it using dc on the meter - I understand what you are saying regarding there not being a battery connected to stabilise the voltage although did leave the jump booster pack connected but maybe this is not the same - will get a small battery and check it out as you say.

Once connected and running properly would it be able to run a radio,nav lights and spot lights? - obviously the spot lights would draw the most current so would it be best to use smallest wattage car ones I can get?
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Old 12 September 2007, 09:21   #4
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yamaha charging.

I would say even if its only a low current charging circuit, it will keep the battery topped up enough to run a radio and some lights. it does not really matter if your battery runs down a bit anyway as you have manual start as an option. it really depends on what revs the engine is doing and what the power requirements of the lights are.
if you use spot lights thats a different matter, two spots at 55 watts each is 110 watts some of the small unregulated charge cuircuits are only 50 -100 watts max thats at max rpm so your battery will run down. if this is the case go for a 110ah leisure battery, that will take the cycling without damaging it and you can top it up at home before you go out at night, Really when you start running lights GPS radio fish finder etc you need a decent regulated charging cuircuit that canpush out 40 or so amps when the battery is down and bring it back up quickly. with a 100watt charge charging system i had on an old 4 stroke honda it would take days of daytime use to top up the flat 110 amp leisure battery, as half throttle charge current was oinly 2amp.
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Old 12 September 2007, 11:54   #5
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I used to use a lawn and garden tractor battery on my electric start Honda 40 when I had the 14' Achilles. I wasn't running much in the way of lights and electronics, though. The battery never suffered from insufficient current until it decided to die after a somewhat prolonged idle stretch (month and a half? Something like that.) It cost on the order of $35 US to replace it (with a $10 credit for the old battery which I never collected. Stupid, but there you go...)

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Old 12 September 2007, 15:05   #6
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I used a leisure battery on my sr4 with a 50 and it worked ok. It held charge far better when idle for a long time too-and fitted perfectly into the smaller sized (red top) battery boxes sold on ebay.

You probably need to buy a manual for the motor to figure out what the electrics should be-but you won't fry a battery by hooking it up for a minute or so and test running it to get a voltmeter reading.
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Old 13 September 2007, 06:49   #7
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Hi,

Glad to hear you got it running OK. I also have (Ok, had) a "bog standard" wee cheap car battery under the seat - worked fine keeping a VHF alive & cranking the engine. Only reason I don't have it any more is it was sold with the hull.......

VHF will draw next to nothing unless you're transmitting, but what portion of the day do you actually spend with your finger on the PTT at 25 watts? As your engine is of a similar technology vintage to mine, the net reasult of a dead battery is really just a case of having to go and pull a bit of string!

As for the spots, what about LED bulbs? - I can't remember where I saw them, but I'm sure I've seen LED versions of some "standard" 12V lamps. Might have been Halfords? Will post again when / if I remember ......
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Old 13 September 2007, 11:34   #8
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http://www.theledlight.com/12volt-led-bulb.html


Should be able to source them on your side of the pond.

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Old 13 September 2007, 15:09   #9
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cheers for all the advice - I think I'll get a small car battery over the weekend and see how I go with it - If the engine is kicking out abnormal voltage it wont be the end of the world if it fries the battery or it flattens as I always have the pull start for a backup.

I know normal car batteries dont take kindly to getting flattened then jump started too often as a mate of mine has an old 56' oval vw beetle - its been constantly getting flattened then jumped and as a result hes on his 3rd battery this year! luckily the shop he got it from has replaced it free each time as they come with a 2 year guarantee.

I was thinking about getting led nav lights but didnt think there was anything powerfull enough in the way of led spotlights? well reasonably priced anyway!

Still thinking about replacing my prop in the quest for the need for speed - anyone have any idea how much would the blades on a prop being mishapen effect top end speed? Also are those dole fins any good?
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Old 13 September 2007, 16:50   #10
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If I was in your position I wouldn't worry about doelfins for now-get the trim set up right and a decent prop on there and see what you think first.
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