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I Have Just Got An Old Mi 21 The Boat Has An 90 Hp 2 Stroke Engine It Runs Fine And The Boat Goes Well But Im Thinking About An New Engine Can Anyone Tell Me What Engines Tthat Are Fitted To There Mi 21 Boats Many Thanks
 

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Hi mate, welcome to the mad house,

we have a Souter Mi21,
when we bought her, she had twin 100hp 2 stroke mercurys fitted, but was a bit to heavy on fuel for us, so we changed to a Mariner 200hp Optimax,

the engineers got 42knots out of her when they tested her after fitting the new engine, 2 persons onboard and not a lot of kit,

we get a top speed of just over 30knots, fully loaded for a full days fishing with 4 persons onboard,
and she cruses at 23-25knots which burns around 1.25liters to the mile,

if i was going to look at a new engine now i think it would be between Honda 135hp and 150hp, the 200hp is a great engine but a bit of an over kill on a fishing boat of this size,

Andy
 

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I have the volvo penta AD31P 150hp turbodiesel with duoprop drive on mine - clean and empty she does 32 knots flat out, currently it's 28 with plenty of kit, 2 passengers and props with blistered paint - I'd expect 30+ with clean props and removal of the odd barnacle on the hull.
Guy moored near me has a new Pirate 21 (slightly deeper hull at the transom) with a honda 135 4-stroke and claims similar performance. He was thinking of the 175, but it's a bigger, heavier block. Weight in the stern of an MI really hurts performance, so I reckon he's got just about the perfect outboard, the only other option would be a similar E-tec if you want a two-stroke.
Edit - if you have a commercial fishing license on the boat then you may be restricted as to how much power you can have.
 
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