prop tunnels? 33 sport fisher going from 12 to 8 degrees

Hi all - just finally got my two yanmars in the shop. my 69 owens had a 12 degree shafts 1-1/4 and now i have an 8 degree and 1-1/2 shafts 21/21 prob from a 20/16 iirc. i think i need to put one motor in on mounts loose and start measuring etc-

I have about 2" clearance currently so its going to be a close call if i can get it all to line up-

I'm going to consider prop tunnels if necessary- for a 21p prop whats an approx lead length of the tunnel?

 

Congrats on your first post Fstarocka.

I am sure that it makes sense to you, as you are intimately familiar with your boat and engines, but for the rest of us I think that we are a bit baffled (I certainly am).

Can you tell us a bit more about the boat and the engines, and ideally post some photos as well?

 

You need to do your drivetrain calculations first, to determine what the propeller diameter needs to be and start planning from there.
I like to draw the components out full size side and transom section views on the shop floor.
Then you can lay in a stick representing the hull bottom and adjust it to find best fit.

 

Did you swap a 12 degree gear for 8 degree gears? Or did you swap physical shaft alignments?

 

45 inches, use only small part of a circle so tunnel height 4 to 5 inch max, about 12 percent prop dia.

 

• Maintain at least 15% of propeller diameter clearance from prop tip to hull where possible.
  • For a 23-inch propeller, that's about 3½ inches clearance.
• The tunnel should be long and fair, blending gradually into the hull rather than having abrupt sides.
• The propeller should not be buried deep inside the tunnel; ideally the prop disc is partly below the original hull line so it can draw clean water.
• Excessively deep tunnels can cause aeration, vibration, and loss of efficiency.

For the boat you've described previously, a 23–24 inch propeller is probably in the right range. The tunnel depth would typically be only enough to gain the clearance needed, often around 2–4 inches at the deepest point, depending on the exact shaft angle and hull geometry.

 

this is 23 inch prop

 

You may have to make compromises in prop diameter, unless you are considering some pretty radical tunnels.
A tunnel for prop/hull clearance is a whole different breed of cat from a real propeller tunnel, which IMO requires serious engineering effort to get it right.
Maybe consider a hull extension, or moving the motors further forward?

 

@fstarocka appears to have lost interest in this thread - after his original post he has only logged in once, and that was before KapnD's first post 4 days ago.

However I hope that he will tell us a bit more about the boat in due course, and ideally post some photos as well.

 

I’m always interested in this type of challenge, as I have crammed 6 cylinder diesel inboards into boats as small as 24’!
It worked, but was necessarily full of compromises, steep angles, short shafts, tunnels, prop/rudder clearance, etc.
With the practical experience I’ve gained over the years, I’d say that anything under 30’ gets vee drives or outboards.