How to Predict Repower Performance

Lots of boaters are thinking about repowering after this summer and, of course their question is, “How will the new engine perform on my boat?” Our recommendation is that you look at our tests of the engines you want to buy on a boat that we have tested that has similar hull design and weight to your boat. A reader recently asked if a new 115-hp Mercury FourStroke he was considering for repower would perform on his old 21’ boat. We looked up a test we had done on 18’5” Tahoe 450 TS that was similar to his.

Find Out What Horsepower You Need

In looking over the boats we have tested with a 115-hp Mercury outboard we noted that most of them are in the 17’ to 19’ range. The only boat we found over 20’ was a 24’ pontoon boat. There is a message here. Since boatbuilders only provide boats for us to test that they think will perform well, they are not rigging boats for us to test over 20’ with 115-hp engines – even though they may offer a the engine as an option.

Remember, that different folks use their boat in different ways, so builders need to make several horsepower engines available. We would recommend 150-hp engines for most boats over 19’ to about 24’, depending on application and the top speed desired.

Following are our test numbers for the Tahoe 450 TS:

Tahoe 450 TS

Mercury 115 FourStroke

Tips on Finding out Repowering Performance

You can zero-in on what horsepower engine you need and even predict its performance on your boat. Here’s how--

First, go to the search engine on boat test. Then select the range in which your boat size falls, as well as selecting the type. Then, click “GO.”

Second, select the appropriate boat size and horsepower of the engine you are researching. Don’t mix one type of engine with another. For example, even though a sterndrive and outboard engine have the same horsepower, their test numbers are not comparable. Keep apples with apples.

Third, pay close attention to the weight and the beam of the boats you are researching. These are the two most important factors in determining a boat’s performance. Boats are very sensitive to weight and drag. Deadrise is also important. Boats with lower deadrise numbers will go faster.

While the performance you see is roughly what you can expect in your repower installation, it will not be exact. Rather, you will discover, generally, the horsepower engine you should be thinking about – and the type of engine – they all have their pros and cons.