Question of HP

[cdn1972]

I know HP is HP so 50 HP from 1975 is the same as 50HP today but with today's 4 strokes and heavier weights if a person was re powering a boat and wanted the same performance as a 75 Merc 50 what would we need to purchase?

 

[JimS123]

What's the rated HP for the hull?

 

[cdn1972]

What's the rated HP for the hull?

Lets just say 75HP but we want the performance we had with a 2 stroke 50.

 

[racerone]

??---A 50 HP in 1975 would not be PROPSHAFT power rated.-----Most motors today are rated at the prop !!----There is a difference.----Hurry to your local dealer or do some more reading I say.

 

[cdn1972]

??---A 50 HP in 1975 would not be PROPSHAFT power rated.-----Most motors today are rated at the prop !!----There is a difference.----Hurry to your local dealer or do some more reading I say.

Really?
I replaced a 1983 Merc 25 Hp this summer on a family member boat with an E Tech 30. Yes its a great smooth engine but the hole shot and quick rev of the older 25 was better. The top speed and for sure fuel economy is better with the e engine but the older 25 had more throttle response.

 

[ahicks]

It's not that easy. Part of the problem is weight. Darn few of those older 50's had power trim and tilt. You'll be hard pressed to find a 4 stoke that doesn't have T&T. So there's a weight issue with the increased weight of the 4 stroke.

Honda makes a 60 hp, but they're expensive. Pretty sure it would match up well with an older 50, or as close as you're gong to get when matching apples and oranges. One very noticeable difference will be noise levels. That alone has sold a lot of 4 strokes.

 

[roscoe]

50 hp motor in 1975 was rated at the powerhead.
New motors (after the mid 80's ) are rated at the prop.

So a new 50 will have approximately 5 more usable hp than the old one.
Plus whatever the old gal has lost over the years due to wear and tear, loss of compression, etc.

Some new 4 strokes are really heavy, and some old motors were real light compared to others. So you would need to know which motors you are comparing to get the exact weight difference.

In this hp range ( 50-60 ) you might see a difference of 75 - 100#.

If your boat turns into a dog if you add a 75# 10 year old kid, you may need more hp, or a different prop.
But if your boat can handle the extra 75# and still perform well, then the new 50 hp 4 stroke will perform just as well and its extra weight won't be an issue.

 

[racerone]

1972 it sounds to me that the new E-tec had the wrong prop ( too much pitch ).----And the old 25 might also have had the wrong prop ( too little pitch ) I say.!----Some concepts are difficult to understand if this is your first boat and motor !

 

[Captain Caveman]

I know HP is HP so 50 HP from 1975 is the same as 50HP today but with today's 4 strokes and heavier weights if a person was re powering a boat and wanted the same performance as a 75 Merc 50 what would we need to purchase?

Buy a rebuilt 1975 Mercury 50hp motor. It sounds like that's what you want.

 

[JimS123]

Really?
I replaced a 1983 Merc 25 Hp this summer on a family member boat with an E Tech 30. Yes its a great smooth engine but the hole shot and quick rev of the older 25 was better. The top speed and for sure fuel economy is better with the e engine but the older 25 had more throttle response.

That ETec is a powerful engine. It should run circles around the old Merc. Tach it out and put on the correct prop and it'll be alright.

To answer your original question, a new 50 HP 4-stroke would be an appropriate replacement. Be sure to install a tach on the boat, and then prop it so that at WOT it reaches the max spec rpm.

As far as the boat specifications, let's not just say 75 HP, let's see what the actual USCG plate says.

 

[ahicks]

As far as the boat specifications, let's not just say 75 HP, let's see what the actual USCG plate says.

Let's say the plate did say 75, would you mounted a 350-400 lb. 75hp 4 stroke because the plate said you could?

Point being, those old plates need to be taken with a grain of salt (or common sense).

 

[racerone]

Where I am the plates used to say ----" Recommended maximum HP "------That would be a rough guide for a first time buyer.

 

[ahicks]

Where I am the plates used to say ----" Recommended maximum HP "------That would be a rough guide for a first time buyer.

And a possible tough lesson for one that didn't take the time to look into it further....

 

[JimS123]

Where I am the plates used to say ----" Recommended maximum HP "------That would be a rough guide for a first time buyer.

That's a good one. I like it. That's going into my book of "Quotes of the Day".

Truth be told, the weight capacities are just guides as well. My buddy's wife insists that she has to go fishing with him ALL the time. And she's so fat she exceeds the recommendations for most every boat in town. So what is a guy to do?

 

[cdn1972]

To be fare I don't want to buy a rebuilt Merc 25 or I would have. I was spending my sister in laws money and thought the E Tech would be a nice easy running engine for her as a first time boat owner inheriting the craft from her father. I am no where near new to boating and engines but have never ventured into the modern 4 strokes so thought I would ask if perhaps 5 or 10 hp more then the old 2 stroke engine would be about the same given 4 strokes weight more. Honestly had not thought about the older engines HP rating at the crank and the newer at the prop. The prop is most likely the problem in this case, it is a heavy boat and we stayed well under rated as I said my sister in law is a first time owner. The E Tech max reved with me alone at 5400 mind you it has less then two hours on it as we got the boat in the water late in the season. Perhaps dropping one will wake it up as perhaps break in time next summer?
thanks

 

[JimS123]

The E Tech max reved with me alone at 5400

Since that's below the operating range for that motor it explains the poor hole shot. You can go up to 6000.

 

[Captain Caveman]

This is research that you can do, but I looked up two 50hp brand new motors. The Evinrude E-tec 50hp weighs 240lbs with remote or 250lbs with a tiller. The Mercury 4-stroke 50hp weighs 247lbs. They are virtually the same.

Tell us how much your 1975 motors weighs and we can tell you how much more horsepower you'll need to roughly be the same.

 

[Captain Caveman]

My buddy's wife insists that she has to go fishing with him ALL the time. And she's so fat she exceeds the recommendations for most every boat in town. So what is a guy to do?

Slap a new capacity plate on the wife?

 

[QBhoy]

Anyone’s guess these days. The new merc 4 strokes have totally changed my way of thinking the old way.
I know that a new mercury 115 2.1 4 stroke is way more powerful than a mercury 125 optimax for sure. Likely it’s around 130-140hp in reality. Perhaps an exception to most maybe. Not sure.