1988 Mariner 25HP vs. 2010 Mercury 20HP 4S

Hello -

I am wondering how much if any power I will lose switching froma 1988 Mariner 25HP 2 stroke to a 2010 Mercury 20HP 4 stroke? The weight seems about the same, will the newer motor perform better?

Thanks,
DewNut

Very little. The old motors were rated at the flywheel. In the early 1990's manufacturers started testing/rating outboards at the prop shaft. So that old Mariner might only be 20 HP at the prop and the Tohatsu (who makes all the smaller mercury 4 strokes) 20 is rated at the prop shaft from the factory.

I thought most manufacturers went from flywheel to prop rating around 1985?

I worked for a dealer that sold Mariner/Mercury up until 1995. Then they went Yamaha. Before the change over, there was an overlap period where there were some new 25 Mariners in stock and some new 25 Yamaha's.

I spoke with the Mariner rep after hours and he specifically mentioned that the rating for the Yamaha was at the propshaft and is why they were faster, and less reliable (according to him...he's trying to sell motors)...where the Mariners were still rated at the flywheel.

After the last Mariner was sold, they never looked back.

The prop shaft rating started in 1985-86 for OMC motors and I am pretty **** sure Merc was about the same. That is why the OMC 35hp stopped being made in 85 and it became a 30hp from there on out. There are more examples of this I am sure but that is one of them I know of. OMC and Merc were having a hard time competing with the Jap motors in the 80's and it was because they were doing prop HP and OMC and Merc were doing flywheel/crank ratings. Either way the manufacturers are allowed a 10% leeway in either direction from the tagged HP that is put on the motor I believe. So a 25 could put out 22.5 or 27.5hp and still be labeled a 25hp.

Most likely you will not be able to tell much difference between a 20 or 25. To many variables go into this like what prop is being used and age of motor and how well it was maintained..... and on and on...... But I would go with the newer motor myself unless you don't like the added weight of the 4 stroke. That Mariner is probably 125lbs as to where the 4 stroke probably 165lb or more?

I like the sound of 2 strokes on the back of my boat

The prop shaft rating started in 1985-86 for OMC motors and I am pretty **** sure Merc was about the same.

Click to expand...

+ 1 =D>