Outboard brakets

Hi,

I recently read some threads on some other boat forums that strongly recommend to only use 25 inches shaft outboards mounted to a transom braket.

Reason being 20 inch saft units are nearly positioned half under water considering the powerhead when motorind slowly/ not on plane.

Would this just indicate that the braket was positioned too low in the first place?

Is this a valid claim?

I have seen install instructions from braket manufacturers that have instructions/guidelines for 20 or 25 inch shafts.

For clarity i am talking more about large high hp outboard braket situations..

 

Bracket construction

So based on what I have read 25 inches is the minimum shaft length that should be used on a high horse power outboard bracket. Considering a boat that will likely see use on the great lakes.

I have got a 1973 21' starcraft holiday the boat was originally produced as IO powered.

I have decided I want to change it to OB powered. I want to build the bracket myself. Ill TIG weld it together. I purchased basically a 4X8 sheet of 1/4 inch aluminum yesterday. Basically because it was in two 4x4 sheets, which was nice for loading it into my truck.

I have started designing a bracket in solid works in my spare time.

Currently I have the bracket designed at a 30 inch setback distance. I know that i have seen setback "rule of thumb" being 1" per every 1' of boat length.
So, I can do that math. My concerns with a 21" setback bracket is room enough to tilt up to a 150 horse OB. I want to get a new 4 stroke. Thinking of anything from a 115 to a 150. Leaning more towards the 150 as the weights of the OBs in that range are similar if not identical per MFG web pages. Also have seen that mounting pad height should be raised 1" per every 1' of bracket set back. I have seen some threads where folks have used like a 26" setback bracket on similar projects.

Any thoughts or insights would be appreciated. It would be simple and quick for me to change the design. Looking to get this all ironed out before i start cutting on my AL.

attached is what i have so far. I calculated my transom angle to be 14deg.

 

How are you assessing how much setback you need to allow the engine to be fully tilted, with adequate clearance ? You should be able to access diagrams that cover that for the engines that interest you, but 30" seems too much. It also makes any issue with the weight back there, that little bit more important.

 

I am only assessing based on what I have read on forums, and threads like this one. I will look for those diagrams you speak of. I have not run across those yet. I agree 30 seems like a lot, but I saw that as a starting point based on what some bracket manufactures use as their default setback distance. I would like it to be shorter, because i do understand torque, and I would like to be as efficient as possible with my aluminum.

 

tilt clearence diagrams

If anyone knows where to find tilt clearance diagrams for Honda, Yamaha, and Merc. in the 115 to 150 HP I would really appreciate you passing that info along or pointing me in the correct direction.

Having a heck of a time finding any of that info on the websites...

Also called Honda, costumer support sent me dimensional diagram but does not included any usable dimensions for determining what I need for tilt clearance.

Talking clearance for tilting the OB completely as far as can go for "out of the water" position?

 

I hope this link works, and can assist you :
http://www.yamaha-motor.com.au/sites/yamaha-motor/files/2012_outboard_dimensions.pdf

 

Awesome! thanks for the data.

 

Based on that data I need 25 inches of setback to "overtilt" the yamaha 150.

 

26inch setback

My bracket with 26 inch set back and 60 inch platform almost complete with the design so I can make some component prints.


3D PDF attached

 

So did you ever complete your bracket? If you did, how did it perform for you?

 

Nope

I did not.

I supported the boat, took it off the trailer with the intention off deforming some rivets back to a sealing state. During that task I found some cracks in the bottom hull skin, at the ends of the transverse ribs on both sides. Like the middle 6 ribs on both sides.

So I currently have the boat upside down on the trailer, I welded all the cracks, and i am sanding it lightly to prep for paint while it's upside down.

So I have been side tracked...

Furthermore, given that the old straight 6 ran well that i took out, and i am no longer confident it is worth putting a shinny new OB on, i am leaning towards setting it back up IO.

 

Ok, good luck. Looking forward to seeing how it comes out.