Getting bigger motor, strengthen transom?

7

70johnson

Cadet
Joined
May 7, 2001
Messages
22
I sold my 70HP Johnson and will be putting on a 90HP Mercury from my Dad's boat. His transom is 3" thick. My transom is 2" thick. Do I need to strengthen the transom because of 20 more HP and additional weight? My transom is solid.
 
JasonJ

JasonJ

Rear Admiral
Joined
Aug 20, 2001
Messages
4,163
Re: Getting bigger motor, strengthen transom?

My old transom was 1 inch thick ply with 1/2 inch thick outer glass, and it had a 125 horse motor on it. I rebuilt it and the wood is now 1 3/8 inch thick (total outer measurement close to two inches), same motor, no problems. If you have a strong two inch transom, you'll be fine with a 90 horse Merc. Just make sure you are within the horsepower limit of the boat.
 
JB

JB

Honorary Moderator Emeritus
Joined
Mar 25, 2001
Messages
45,907
Re: Getting bigger motor, strengthen transom?

What is your boat rated for, 70Johnny?<br /><br />If there is no rating plate, check the USCG site for homebuilders to rate their boats. They will explain how to determine the max safe HP.<br /><br />My guess is that you will be okay with the 90Merc, but don't bet your life on my guess.<br /><br />Good luck. :)
 
7

70johnson

Cadet
Joined
May 7, 2001
Messages
22
Re: Getting bigger motor, strengthen transom?

Well...... according to USCG site, my boat is rated for 64 HP. Too funny. It performed OK with the 70HP Johnson. There is no rating sticker on the boat. As far as the 90HP Merc. I believe it is all in how responsible you drive it. I don't rip through turns in the St. Johns river or plow into waves of other boats at full throttle, but when the afternoon summer storms come out of nowhere, I like to outrun them back to the ramp. :eek:
 
7

70johnson

Cadet
Joined
May 7, 2001
Messages
22
Re: Getting bigger motor, strengthen transom?

Correction, I went out and measured. The formula says the boat is rated to 94.8 HP. :D :D :D
 
JasonJ

JasonJ

Rear Admiral
Joined
Aug 20, 2001
Messages
4,163
Re: Getting bigger motor, strengthen transom?

Slap 'er on, and post some pics...
 
graphicjunkie

graphicjunkie

Seaman
Joined
Jul 10, 2010
Messages
51
Re: Getting bigger motor, strengthen transom?

That link doesn't work. Can someone post the calculation mentioned so I can check out my boat?

Thanks!
 
steve riddell

steve riddell

Seaman
Joined
Sep 3, 2009
Messages
72
Re: Getting bigger motor, strengthen transom?

Welcome to iboats
I agree with the Rest, You will be fine!! If not You will have a fun little project to do. Pics would be cool.
 
You must log in or register to reply here.
Top