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Old 05 March 2013, 11:46   #1
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Country: UK - Wales
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Boat name: Happy Days
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1999 Mariner 3 cylinder 2 stroke

Hi Guys

Im new to this, I have a 5m Zodiac pro with a Mariner 75. What is the top speed i should expect from her? I`ve just come back from her maiden voyage where at full throttle she struggled to get just over 20 knots, is this about right?
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Old 05 March 2013, 15:14   #2
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What is the RPM you are achieving, you need 5500 tops. Of not drop prop size. Usual rules apply, make sure hull is empty of water and weight is evenly distributed, kit & people. Make sure engine is firing on all 3 cylinders by warming up on muffs, then fire up 1 pot at a time by removing two plug leads. Start with top & work down. All Ok. Expect 28knots ish. Maybe a little more.
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Old 05 March 2013, 15:24   #3
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It is only revving to a max of 4,000. Weight is distribution is fine.
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Old 05 March 2013, 15:35   #4
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i get 30kts out of mine. something isnt right there as ive used various sized props and still got way more than 4000rpm
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Old 05 March 2013, 15:46   #5
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Country: Ireland
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My 4.8m Lencraft with the same engine tops out at about 36 knots approx 5,500rpm.

4000 seems very low, if its not the wrong prop then it might be worth checking the compression on all 3 cylinders to see if they are even. My old DT55 was low on one cylinder and only made about 4000 rpm.
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Old 05 March 2013, 15:59   #6
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My money is on the wrong or poor prop


We have recently replaced on old 17" for a new 17" prop on our 75 and the revs have gone from 4200 to 5200.
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Old 06 March 2013, 04:59   #7
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Yep, generic +1
I get 31 knots out a 60Hp on a similar sized boat.

Other thing - if it hasn't been servioced recently, antother cheap thing is to change the spark plugs. That can make an incredible difference. (I've had small engines wouldn't even start, changed the plug(s) and roared into life first pull.

Prop -
Was that engine always on your rib, or have they only recenly been paired? I got a Yam 55 for mine - used to push a displacement thing along - way too low pitch prop on it meant I had to hold back at 2/3 throttle so I didn't destroy the engine (until I changed the prop). If your engine was on a wee fletcher or something similar it may be way too high pitched...

Have a look either on the back face or along the hub. There will be a number stamped on there to tell you waht size it is. I would estimate you'll need something in the 13 -15" pitch kind of ballpark.

(and check your hull isn't full of water!)
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Old 06 March 2013, 05:14   #8
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i get at full tilit on a good flat day 45MPH @ 5500rpm so about 40kn(on a good day)
17" prop mariner 75 2stroke on a 5.35 ribtec.
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Old 06 March 2013, 13:13   #9
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Thanks for all the help

I believe the engine and boat have always been together. The previous owner said he used to get 35+ knots out of it. This leads me to believe it is more likely to be engine related than the prop, although I will check to be sure.

The hull has no water in it

Ps where can I find service info online?
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Old 06 March 2013, 14:13   #10
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Turbodiesel on here (Blackdog marine) is an excellent source of info and very nice to deal with for all things Mariner, can highly recommend.
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