What tacho for 30hp twin cylinder mariner

win-or-bin · 21 July 2010, 20:56

Hi all, I am looking for a rev counter/rev counters for my mariner outboards as my boat doesnt have any! Just wondered what people would suggest as I have twin engines should I have two tacho's or could i get away with one? Also a lot I am looking at seem to suit 3/4/6 cylinder etc not a twin cylinder engine, anyone suggest a good brand etc.
Thanks
Seumus

Nos4r2 · 21 July 2010, 22:14

You need two, and the ones you want are settable for different pole alternators rather than number of cylinders.
Teleflex (now VeeThree) make one.

Poly · 21 July 2010, 22:18

Quote:

Originally Posted by Nos4r2

You need two, and the ones you want are settable for different pole alternators rather than number of cylinders.
Teleflex (now VeeThree) make one.

could you use one tacho with some sort of change over switch?

Nos4r2 · 21 July 2010, 22:33

Quote:

Originally Posted by Polwart

could you use one tacho with some sort of change over switch?

If you could get a reliable on-off-on switch then yes, just run the signal feeds through that.

9D280 · 22 July 2010, 08:28

Number of cyls should be irrelevant for a standard outboard tacho - they count alternator pulses and so don't care how many cyls are underneath spinning it. As Nos says, there are various numbers of poles on alternators, hence the commets above.

The other option is a comparator tacho- it takes the input from both engines and gives you a difference reading, thus ensuring you have them running at similar RPM. Teleflex do one, and I'm sure Ive seen one with an analogue comparator needle and a digi RPM, but stuffed if I can remember who makes it.

I guess it boils down to how much console space you have?

Oh, and it will be irrelvant whose tacho you use - I have used a Suzi one for my old Yam, my current Merc and, strangely enough the Suzi I got it with!

jwalker · 22 July 2010, 10:57

You could probably get away with one, you'll soon know if they're at different speeds, the engines will beat until they are in sync. If they're at widely different speeds you'll know cos they sound at widely different speeds. I did this on a Mariner 30hp a good number of years ago. I bought a general purpose alternator and triggered it with a few turns around one of the plug leads. I ran twins and then triples and found it ok with one Tacho.

win-or-bin · 22 July 2010, 19:55

Thanks for the replies guys, will look into maybe one tacho as space is a bit limited on the dash due to the size of the boat. Took her a run tonight for the first time, went fairly pathetic, pulling 24 knots with two 30hp outboards on the back of a 4.4m rib - not happy, not even close to what the guy said it would do who sold it....with just one engine it wouldnt even push the boat onto the plane. Its running 15" pirahana props so have ordered some 13" blades and will need to mess about with trim heights etc next week as am away this weekend. Wife not happy as will be stewing about the top speed all weekend!!!!!
Thanks for the help and replies

S.

Poly · 22 July 2010, 21:24

I just did a quick back of envelope calculation without knowing what revs you are getting, or the engine is rated for or even what the gear ratio is in your engine - but I'm guessing with 13" props you are still not going to break through 30 knots if that was your hope.

win-or-bin · 22 July 2010, 22:04

yeah, had a quick clac myself polwart and 30 odd knots is the best I can hope for. The guy I bought it off said it would do 40knots+ As I bought it as a project I didnt have the chance to try it out I stupidly took the guy on his word, he said he uprated the 30hp mariners to 35hp by changing the reed block and was highly dubious, bought it due to the good hull and immac hypalon tubes. If I cant get it to break 30kts I will sell the two 30hp mariners and quicksilver controls seperatly on ebay and get the largest single O/B I can squeeze on the back.
Lesson learned etc etc
(PS two pot mariner/yamaha o/bs by the way polwart 94 vintage I believe)

win-or-bin · 23 July 2010, 08:57

would this rev counter work on ebay?
http://cgi.ebay.co.uk/Suzuki-DF-Tach...item3caf20e39a

9D280 · 23 July 2010, 10:48

If you are an electical genius, you could potentially have lots of warning functions as well as a tacho. I guess one of those looms will be for the warning lamps and just lie disconnected in that application.

It should work, unless it's on some Suz data bus kind of system. I'll need to hand over to to someone who has one to confirm that, but if not, pop the guy a mail ask if there is a pole change switch (likely a hole for a small screwdriver) on the back, if so it'll be a pulse counter & fine.

As for 40 Knots form 60Hp, you'd need a vey light boat for that to happen, V unlikely with a deep V rib. I'd guess somewhere between 30-35 for that setup, not knowing any details of the hull or engines For what it's worth I get around 32 from mine when the engine isn't a pile of bits! (60 Hp, me+ boat about 520Kg)

What you could do if it's speed you're after, as you have the twins, go for smaller diameter bigger pitch props - that way the avaialble engine torque will be converted more to forward motion, as you have effectively double the prop blade area between the two, so can afford to loose a bit of diameter and stil lnot "loose grip". Problem might be finding a matched set to borrow to test.

win-or-bin · 23 July 2010, 15:54

Thanks for the reply, I have ordered 10x13 props for it should try on Monday,do you mean try 9 x 13s or 8 x 13s? as its pirahana props I have they only have 10 inch available? Sorry if my answer is a bit stupid, finding it hard to get my head round prop size and the difference two motors make, am a total speed freak so I will try any avenue possible for more speed! Just annoyed as sold my shallow v med style rib for this new boat thinking this would be better in rough seas and faster Ibut it is slower!