Ratio for hp of outboard to thickness of stainless steel transom plate.

I am converting a Hunton 43 with triple Volvo D260s to twin Yamaha XTO450s. This means putting an outboard platform/support system behind the existing swim platform of the boat. To take a lot of power I plan to have a thick stainless steel transom plate added to the boat, to which I will attach the outboard platform/support system with thick steel tubes as struts.

Are there tables or rules of thumb for calculating the thickness of steel per hp of outboard power, or something similar? For example does a 50hp outboard need 5mm but a 200hp need 20mm (these are just random numbers to illustrate). It's been easy for me to calculate the size and placement of this plate, because it follows the lines of the boat, but calculating the thickness is proving harder to pin down.

 

The only rule of thumb I know of on this is
make it thicker/stronger than you think it needs to be.

Pictures really help

Good luck.

 

The stiffness of a plate or beam does not increase in a linear fashion. In fact the stiffness of a plate increases as the third power of thickness. You need to provide more details. A metal plate is not the only consideration for this project. You also need to provide more detailed information about the structure of the boat, particularly the transom area.

No one here is likely to tell you exactly what to do about the engineering of the conversion. You need some professional advice from a local engineer or certified naval architect.

After all is said and done, do follow Blue Bells advice and make it stronger than that which seems adequate.

 

What’s the maximum thickness that the mounting for the outboards will take……I’d use that and then figure out how to tie the transom into the rest of the hull structure (that’s the hard part…..) and I don’t think I’d use a stainless steel plate but that’s just me……very hard to tie into the hull structure

 

Yah, so many questions.

Pictures are your best route here.

Good pictures with good descriptions please.

 

Thanks everyone, I will upload photos and more of a description - I didn't want to overwhelm people with information and tried to keep it simple.

 

You may need to calculate what the extra HP and shifting the center of gravity will do to the behaviour of the boat. Are there any boat like yours with a similar modification? One of the common problems with the modification is a boat that sits low at the stern at rest, and porpoises when running.

 

See attached file. It may help in this matter.