What size motor will my boat carry?

[iggyw1]

I have a 16 ft., 1965 Geneva fiberglass boat that is solid as solid can be. I currently have a 1965 Eivinrude motor 75 HP on it. This motor weighs 232 pounds. I am considering buying a newer 75 horse Mercury motor for it, and that motor weighs in at 305 pounds. there is no plate on the boat anywhere that I can see that tells the max size or wt capacity for the motor. How can I get the info that I need if the Geneva boat company is out of business??

The boat handles the existing 75 horse motor great that is on it now. It has been on the boat since 1965. Motor, boat and trailer all look great, but the motor is old and tired!! I am in need of a newer model. Can anyone help me find the info that I need?? Thanks!

 

[gddavid]

In adding a heavier motor you have two issues to consider. The first and most straightforward issue the trim of the boat with the heavier motor both at rest and underway. This can be tested pretty easily by putting some extra weight in the back of the boat with the existing motor and testing it out. You may likely be able to counter balance this by finding items to move further forward. The second and more challenging issue is how the transom will respond to the extra weight hanging off of it and bouncing around when running in chop. This is much harder to analyze and would take an engineer to get a definite answer. If you existing structure is sound you could build it up a little with knees between the transom and stringers for some added insurance. You may consider looking at the torque specifications on the newer vs. old motors. You may be able to get similar torque out of a lower horsepower newer tech motor saving weight. I assume by newer you are not talking brand new as the mercury's in that size are all well over 300# now.

It is funny to consider how automobile engines have dramatically improved in power to weight ratio from 1965 to now, but outboard motors have gone in the opposite direction. I do understand why and the fuel efficiency is vastly improved but it is still kind of funny to me.

 

[iggyw1]

In adding a heavier motor you have two issues to consider. The first and most straightforward issue the trim of the boat with the heavier motor both at rest and underway. This can be tested pretty easily by putting some extra weight in the back of the boat with the existing motor and testing it out. You may likely be able to counter balance this by finding items to move further forward. The second and more challenging issue is how the transom will respond to the extra weight hanging off of it and bouncing around when running in chop. This is much harder to analyze and would take an engineer to get a definite answer. If you existing structure is sound you could build it up a little with knees between the transom and stringers for some added insurance. You may consider looking at the torque specifications on the newer vs. old motors. You may be able to get similar torque out of a lower horsepower newer tech motor saving weight. I assume by newer you are not talking brand new as the mercury's in that size are all well over 300# now.

It is funny to consider how automobile engines have dramatically improved in power to weight ratio from 1965 to now, but outboard motors have gone in the opposite direction. I do understand why and the fuel efficiency is vastly improved but it is still kind of funny to me.

I appreciate your input. I also worry about the extra weight while the boat is on the trailer being toted around from home to launch ramp. I may be better off just getting less horse power , lighter motor that would give me enough speed to get out fishing. I do not necessarily require it to be 75 Horse power. With the 75 I have now I can hardly ever go wide open throttle anyway as I am on Lk. St. Clair in Mi. and it is usually too choppy for me to go WOT. A smaller motor would work, say a 55 or 60 horse even. Thanks!

 

[Pony]

IMHO if the transom can't take the extra 70lbs of a motor of the same HP, then I wouldn't want to be on that boat. If the transom is sound, I don't think the heavier motor is going to be a safety issue.

That being said, you may notice some handling differences with the weight difference....but that would be easily fixed with a little weight distribution change.

My first question is why the change? The HP is the same. The only way I would swap the motor is if the one on the boat currently is having issues.

 

[Chris1956]

Iggy, I think you have bad info on that Merc weight. Depending on the year of the motor, it will be an inline 4 or 3 cylinder and weigh much less than the Johnny V4. For example, a Merc inline six weighs only 300#.

 

[airshot]

Keep in mind that much newer outboards will perform much better than the old motors. A much newer 65 or 70 hp will outperform that old heavy 75hp.
The 3 cylinder 70 hp jonnyrudes of the late 70's and early 80's were power houses compared to that old 75 and at much less weight. You may be able to use less hp of a more modern motor and actually get better performance.