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Discussion Starter · #1 ·
I have an old 1985 Evinrude 25hp 2 stroke on my 16ft cuddy boat which is a bit like a Longliner but more substancial and with a planing hull.
It tries to go up on the plane but does not have quite enough power.

I've been told that a new or fairly new 25hp 4 stroke should give me around 50% more performance.

Could this be right?

Thanks for any info.
 
G

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Only in as much as your old 2 stroke is probably well down on the power it had when new.

...have you been told that by the engine dealer or by a friend?

25Hp is 25Hp no matter what way you look at it.
2 strokes and 4 strokes offer differet advantages and disadvantages which have been argued 'til we are blue in the face on here.

You will most likely find that the power from a 4 stroke will be delivered in a smoother line throughout the rev range of the engine and it will be a more pleasant experience to use, but two strokes may well be better suited to your boat in as much as they often offer much quick shots onto the plane which it sounds like you are struggling with.

It sounds to me like your engine is probably next to knackered and it isn't the lack the 25Hp that is the problem, more likely that it isn't getting anywhere close to 25Hp.

A simple and relatively cheap "fix" (it may not be the whole solution) would be to try fitting dolefins to the engine cavitation plate as they will help you on to the plane.

Tom
 

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You could check the compression of the engine, wouldnt this tell you if the engine was down on power.

Dosent poor compression indicate that the downward thrust acting upon the piston is in fact escaping down the sides of the rings, thus losing power.

I am not an engine expert, just what i think is the situation.
 
G

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You could check the compression of the engine, wouldnt this tell you if the engine was down on power.

Dosent poor compression indicate that the downward thrust acting upon the piston is in fact escaping down the sides of the rings, thus losing power.

I am not an engine expert, just what i think is the situation.
Potter

I was thinking along those lines, but I am not a mechanic so didn't know how to say it.....
 

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I have an old 1985 Evinrude 25hp 2 stroke on my 16ft cuddy boat which is a bit like a Longliner but more substancial and with a planing hull.
It tries to go up on the plane but does not have quite enough power.

I've been told that a new or fairly new 25hp 4 stroke should give me around 50% more performance.

Could this be right?

Thanks for any info.
You obviously need more power, but I would'nt put another 25hp on the back,in case you do not get the benefit you are expecting, what you need to find out is the max hp for the hull and aim for an outboard of that or near to that hp, hopefully then you wo'nt be disappointed.

IMHO

Cheers Ian
 

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I'd expect to only just be getting on the plane with 40hp on a 16' planing hull.

You'll know if the compression is shot-it'll try to die at tickover in gear.
 

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Are you confident you have the current outboard currently setup correctly? I had a 16foot fastliner and 25hp yam which didnt plane for years then I messed around with the height and angle on the transom and it worked a treat
 

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alsoa lot to do with weight you are caring mate as well.

my 14 ft coastliner will plabe a treat with just me on board but soon as i put my dad on board then its all lost.

i would say a 30 hp four stroke would do the job but alos prop pitch would need a play around to.
 

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alsoa lot to do with weight you are caring mate as well.

my 14 ft coastliner will plabe a treat with just me on board but soon as i put my dad on board then its all lost.

i would say a 30 hp four stroke would do the job but alos prop pitch would need a play around to.
Think you need to look at the prop pitch allthough 25 is possible the lower range
you may need to go up a few horses,think it has been mentioned 40 with correct pitch on prop should fix it
 

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Discussion Starter · #11 ·
Thank you all for your helpful replies.
Certainly given me things to think about.
One thing, apparently engines up to year 1999 had the horsepower measured at the power head whilst those from year 2000 were measured at the prop.
Been told the difference is around 12% on engines around 25hp.

Thanks again,

Owen
 

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Tom is right 25 hp is 25 hp. Except that!!! ...... It is where you measure it.
For years the USA taxed/licenced O/B engine use on their waterways by stated HP. Hence many 9.9 hp American engines, because the next tax break was 10 to 25hp inclusive. So if you measure an engines HP at the flywheel you could easily get 28 or 30 HP. BUT if you take it at the prop shaft you will get 25 HP.
Then along came the far East engines with a big enough home market to ignore USA local taxes and these guys were selling on notional HP, hence the situation of having a 28hp, a 35hp, and a 40hp engine all having exactly the same engine specification with slightly different carburation and ignition settings.

IMO your 25 is probably over prop'ed and not reaching full rpm. But having said that weight is a prominant factor. If you have a solid heavy GRP lay up (like the Orkney range for example) and a load of gear a 25 will never get you on the plane.

But do the maths before you change engine... Sell yours add £2500 for a modern 50 HP engine to go 10 mph faster????

Afishionado
 

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1985 Evinrude was measured at the prop. AFAIK it was only Mercury still measuring at the crank in '85.
 
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