Performance Boats Forum banner
1 - 6 of 6 Posts

·
Registered
Joined
·
4 Posts
Discussion Starter · #1 ·
I have a 1975 jetster it currently has a 70 hp evenrude outboard motor that needs more work then it’s worth. Can I put a evenrude 125 on it or would it be to much for the boat?
 

·
Registered
Joined
·
39 Posts
monohull outboard boats under 20 feet are required by law to be rated by the boat manufacturer for how much horsepower they can safely have, What is the boat rated for? Since it is a 16 foot boat it should have a capacity label with a HP rating on it. If it doesn't (it's been a long time since 1975) you can calculate the HP. The formula is:
center line length in feet (actual length) X maximum transom width in feet x 2 - 90. (use decimal feet such as 16.25)

Then round up or down to the nearest multiple of 5. For instance, if it comes out 82.5 round to 85. This boat is probably not rated for 135 HP. There are two issues here. One is the horsepower, the other is the weight. A 135 weighs a lot more than a 70hp. That will cause the stern to be much lower in the water. That can change the handling characteristics of the boat, making it unstable a speed. If the boat still has built in flotation, the amount, especially in the stern area, is based on the weight of the outboard it was rated for. Putting significantly more HP on the boat may not increase the speed much and as I said it will affect the handling. Plus it will suck a lot more gas. Not a good idea.
 

·
Registered
Joined
·
4 Posts
Discussion Starter · #6 ·
monohull outboard boats under 20 feet are required by law to be rated by the boat manufacturer for how much horsepower they can safely have, What is the boat rated for? Since it is a 16 foot boat it should have a capacity label with a HP rating on it. If it doesn't (it's been a long time since 1975) you can calculate the HP. The formula is:
center line length in feet (actual length) X maximum transom width in feet x 2 - 90. (use decimal feet such as 16.25)

Then round up or down to the nearest multiple of 5. For instance, if it comes out 82.5 round to 85. This boat is probably not rated for 135 HP. There are two issues here. One is the horsepower, the other is the weight. A 135 weighs a lot more than a 70hp. That will cause the stern to be much lower in the water. That can change the handling characteristics of the boat, making it unstable a speed. If the boat still has built in flotation, the amount, especially in the stern area, is based on the weight of the outboard it was rated for. Putting significantly more HP on the boat may not increase the speed much and as I said it will affect the handling. Plus it will suck a lot more gas. Not a good idea.
monohull outboard boats under 20 feet are required by law to be rated by the boat manufacturer for how much horsepower they can safely have, What is the boat rated for? Since it is a 16 foot boat it should have a capacity label with a HP rating on it. If it doesn't (it's been a long time since 1975) you can calculate the HP. The formula is:
center line length in feet (actual length) X maximum transom width in feet x 2 - 90. (use decimal feet such as 16.25)

Then round up or down to the nearest multiple of 5. For instance, if it comes out 82.5 round to 85. This boat is probably not rated for 135 HP. There are two issues here. One is the horsepower, the other is the weight. A 135 weighs a lot more than a 70hp. That will cause the stern to be much lower in the water. That can change the handling characteristics of the boat, making it unstable a speed. If the boat still has built in flotation, the amount, especially in the stern area, is based on the weight of the outboard it was rated for. Putting significantly more HP on the boat may not increase the speed much and as I said it will affect the handling. Plus it will suck a lot more gas. Not a good idea.
i appreciate your knowledge, I don’t want it to be unsafe at all. I just happened to have a 135hp motor available to me, maybe I might put it online and sell it or see about trading it for a 90 hp. The boat has a tag on it that says 70-90hp. The 135 is definitely significantly heavier then the 70 hp and it’s already a low profile vessel I wouldn’t want to risk it with the family or even my self. I really do appreciate your response. I will tread with caution on this one and just figure out how to get my hands on a 70-90 hp evinrude, do you know of any reputable sites that I can trade this 135 for a smallermotor or even if they have smaller motors that are semi affordable??
 
1 - 6 of 6 Posts
Top