What size engine should I buy?

[starcraftlegasea]

I am looking for some help on what size engine I should purchase for my boat. The boat was my fathers and I recently restored it. It is a 1978 21ft starcraft aluminum hull center console. The boat originally had a 70hp mercury on it but I think it might be a little underpowered. The manufacturer's plate on the boat says it can handle up to a 150, but I think that is way too much weight and power for a light aluminum hull. I want to use it for fishing in addition to water sports (water skiing especially) so I wanted to know what is the smallest engine I could use for these purposes. Thanks.

 

[Silvertip]

Re: What size engine should I buy?

70 HP is less than 50% of the maximum rated HP and at that power level represents a significantly underpowered boat. The general rule of thumb is to select an engine that is no less than 75% of rated maximum. This provides an acceptable level of performance. In your case, 75% is about 115 HP. If you normally carry an above average load, you might even up that a little -- say to 125 - 130 HP. The existing engine is having to work very hard and may eventually die because of it. If it's running well, sell it and put the bucks toward the bigger new/used engine.

 

[bds85466]

Re: What size engine should I buy?

It's sort of like when you went outside when you were a kid and your mom told you: "Dress warm, you can always take layers off". You can always slow down but you'll never be able to get to a certain speed/rpm/torque without a big enough motor. You'll never know when you might want that extra punch...maybe to pull two skiiers or tubes? I would say that if you're considering going larger you may as well get a 125hp. That should be adequate power for anything you will need to be doing.

 

[starcraftlegasea]

Re: What size engine should I buy?

Thanks for the help. I was just thinking though that the boat is rated for a 1978 150hp. I was once told that older engines don't have the same amount of power that newer ones do (for instance a newer 90 would have as much thrust as an older 115). Is this true??

 

[Chris1956]

Re: What size engine should I buy?

The difference in power is not that much, between the propshaft rate motors (newer) and the powerhead rated motors(older rating). The Lower unit gears rob some (say 5%) power. The difference you should be worried about is engine weight. An old inline 150HP weighed in at 300#. Some modern 150HP motors weigh 400+#.

I would look for a 90-115HP 4 cylinder motor. It will have about the correct weight and power for your needs.

 

[tashasdaddy]

Re: What size engine should I buy?

i have a 90 on an 18 foot aluminum center console. wot is 45MPH, plenty of out of the hole power. i think a V4 115 would do nicely on that 21 footer. i've been air born a couple of times by a rouge wave.