Stepped transoms

Tunnels,
I wonder if you could expand a bit on efficient tunnel shape and it's effects on speed and performance.

 

Efficency comes from the semi shrouding of the propellor so the deeper the tunnel shape the better it works .Like all things there are limitations and a point at which you draw the line !! it needs a long lead into the prop and the aft end needs a slight turn down to deflect the water to help with trim and to lift the stern slightly . Like all things under water stream lineing is very important to get away from turbulance and water disturbance before and after the propellor .
A few years ago Pro Boat had a lengthy artical in the magazine on this very subject .

 

I'll see if I can find it in their digital archives.

 

Tunnels, deep tunnels bring the fear Penn-Yans to mind. So I agree with your point on limitations. Hopefully research will dispel/avoid this.

Here are some articles I have been able to locate on the subject, in language that I can understand.

http://www.hydrocompinc.com/knowledge/whitepapers/HC141-PropExpertTunnels.pdf

http://www.yachtingmagazine.com/article.jsp?ID=1000071185

And one beyond my ability of understanding fully. Any insight appreciated.

http://www.banglajol.info/index.php/JNAME/article/viewFile/2025/1885

Any links to information appreciated.


Tolly, I thought about checking out Fox Island on my stopover in Port Orchard last week, maybe next time.

Tom

 

Where in AK are you?

 

Juneau

 

Hi that Penn yan Drive system has some good innovations about it , thinking has changed since 1978. The shape of the tunnel is critical and 50% of the prop dia is a better option and more efficent its the closness of the prop to the tunnel is where it gets its efficency from because it then gets close to being a jet unit ! ,sort of .
Some interesting stuff in those other sites , i must read and try and make sense of all the figures and graphs etc , thank you !!

 

IMO the use of a tunnel is just to cover a design that doesnt work.

Either you cant get a good shaft angle
The boat draft is top deep
Cant get the blade area you need for the torque you can produce
etc.

The planning ability will be worse as the tunnel will pull the transom down as it tries to run parallel with the direction of travel so it will fight the rest of the hull thats on another angle.
Turning is bad due to the added longitudinal surfaces
Must be loads more drag with the prop wash blowing very close to the hull
There is more wetted surface
Its as noisy as hell

Thats just my experience on both single and twin engined production boats that have these....

Its either a tug boat or a planning hull