New donk question..

[vermin]

Just had a new Merc 75 2 stroke fitted to my 4.9m Hooker. Goes like a rocket but I'm a little concerned with max revs. I had a half full 85 lt esky, half full 80lt fuel tank and 3 people (around 240kg) and the motor rev'd out to around 5700rpm. Max revs for this motor is 5250 ... I have a 16pitch vengeance ss prop. Should I consider another prop while we are early days. The dealer won't take the prop back if it has discolouration ...

Geoff.

[peterbo3]

Geoff,
I had a 75 Mariner on the Lightning before I upgraded to to a 90 4St.
I ran a 17 alloy prop & revved out to about 5300. My boat was 5.3M & I carried 160L of fuel plus heaps of other crap. You might need to go to the 17 cos there only seems to be room for another 40L of fuel & a few stubbies. ;) ;)Speak to the dealer or Solas.

[Spaniard_King]

Vermin,

I have 2 friends running 75hp 2 strokes, both use 18 " vengance props and both are happy with the performance.

yo may have a slightly heavier boat so the 17 may be ok, it better to be at the top of the rpm range than at the bottom..loading the engine up

Garry

[vermin]

17 might be the go. The dealer reckons the next step up is 18??? Might be telling a furfy. I'll check this out.

Thanks guys.

Geoff.

[vermin]

mmm ... rang the dealer again, he says the 17 won't make that much difference and reving out to 5700 isn't going to hurt it. Also said that it is good to get out of the hole quickly in shallow creeks etc with the 16.

Not sure what to do.

[mark221263]

I generally don't like ot lug motors and always prop my boats to run at the recommended max wot rpm or 100~200 more.

This has the benifit of unloading the motor a lot as it does not have to lug,
has better out of the hole
is good in rough weather when you need to slow down a bit as the motor will generally hold beter low speed planing

In saying that though 500 rpm over is a lot and it may even increase slightly when the engine runs in further.

Normally 1" of pitch will reduce WOT rpm by 200rpm so I would increase pitch by 1" definatly and try 2" to start with. Your fuel ecconomy should improve a tad.

As far as not hurting anything, try and get the dealer to put that in writting with a counter sign from Merc Australia!

Cheers Mark

[rajawolf]

I am having the same problem with my 85 HP merc but in reverse ..my WOT is 4800 and should be 5500 ....my prop pitch is 17..but i dont know know if the tacho is reading right.

With 2 up and a full load (only carry 60 litrs of gas) the boat moves along at 50 KPH ...at WOT... runabout glass boat 5.2 metres.

The adjustment on the back of tacho was set up wrong when I bought the boat and was reading off the scale...It has 4 position settings and i have now set it to 6p which is for a 12 pole alt.....all other settings were off the scale.

Is ther someway to to hook up a external tacho to check that the one on the boat is reading right??

Cheers,

Tony

[Y-Not]

I run a 90 hp merc 2 stroke with a 19" ss vengance prop and am smack on 5'500 rpm wot with a 3 man crew and 3/4 fuel load (approx 100L) and all fishing gear . Am happy at this .
Would try lifting your 16" to a 18 " should pull wot down by 400-500 rpm. far better fuel economy and not that big a differance in holeshot unless you are loaded to the gunnels.

As for rajawolf your 85HP merc pulling high 4800 with a 17" prop, you should get your regulators and tacho checked. (leaking regulator diodes can cause inaccurate tacho performance)

[Spaniard_King]

I am having the same problem with my 85 HP merc but in reverse ..my WOT is 4800 and should be 5500 ....my prop pitch is 17..but i dont know know if the tacho is reading right.

Tony

:o :o :o you get 4800 in reverse... crikeys ;D

Yeah Tony, you could hook up an external tacho, you would need to get one that clips to one of the spark plug leads. I don'tm have one to lend either :P

Garry

[rajawolf]

Y-Not..thanks mate will have a check of that...


Gary lol....I should write my post better.....it made complete sense till you pointed it out..and I almost cried laughing.... :D

[vermin]

Thanks guys. I'm going to have a test run with the 18 pitch prop. You can't put (or can't get) a 17 pitch for this motor any more ... will let you know how it goes.

[vermin]

Update ....

Just done a test with the 18" pitch prop. Still got out of the hole quite effortlessly. Only difference I noticed was it took a bit longer to reach top speed. The 16" got me to about 40 mph (by the Merc guage) very quickly and then held me there above 40 mph on about 5800rpm. The 18" sat on about 37 knots and eventually got to around 40 mph... trimmed out a bit on around 5500rpm.

To summarize, the 16 is more responsive but rev's out more ... which the guys say is fine. Does this mean a lot more fuel on those days when I can open her up on a lake?
The 18 is ok out of the hole but is being a bit less responsive up top going to be a problem?

Any thoughts? I'm struggling to make a decision ..

[vermin]

Think I'll go the 16" ... will be better in the swell at low speeds and getting on the plane quickly (good for shallow creeks). Boat will only get heavier with a planned second fuel tank up the front, canopy (drag) and the thousands of fish I'm going to put in the esky ;D!!!

I won't swap it over until this afternoon so if anyone has some last minute advice ... let me know.

Thanks.

GG.