Kick-up propeller shaft for shallow draft

[wroc]

Hi all, I am designing a displacement motorsailing barge with a draft around 1 foot. I would like to use a lifeboat diesel around 30hp, which would have a prop diameter around 12-14 inches. One can see the problem already!
I don't want to increase the draft just for propeller immersion, because the shallow draft is part of the charm.

I've heard of the concept of a kick-up propeller shaft, involving a U-joint with some kind of skeg that moves up with it. Googling doesn't turn up much. Do you guys have photos, schematics, or ideas?

 

[Tomsboatshed]

This was the arrangement used by “Dispros” or disappearing propeller boats.

if you google disappearing propeller boat you’ll find a fair amount of information, and might find if someone is fashioning an updated version.

good luck!

 

[wroc]

View attachment 198480

This was the arrangement used by “Dispros” or disappearing propeller boats.

if you google disappearing propeller boat you’ll find a fair amount of information, and might find if someone is fashioning an updated version.

good luck!

This is actually a really good starting point, thanks! Doesn't seem so hard to whip up. There must be stainless steel universal joints available with PTFE bushings. I'm going to try to find a DIY version.

 

[gonzo]

A tunnel would be enough to get the propeller flush with the keel without so many complications. How did you calculate the diameter of the propeller?

 

[Tomsboatshed]

This is actually a really good starting point, thanks! Doesn't seem so hard to whip up. There must be stainless steel universal joints available with PTFE bushings. I'm going to try to find a DIY version.

I’ve seen it in action - though the use case then was for much smaller boats with less power.
Were it me, I’d consider all the other aspects of my use case/design as well before going too. deep in the weeds on a disappearing/retractable prop. A better option may come up as the design evolves.

Please keep us posted on your progress!

 

[comfisherman]

I'm trying to imagine what it would take to get any form of over the counter u joint to last submerged and lubed in salt water....

Wonder if a needle bearing less u joint using a bronze bushing would work, or at least have enough longevity.

 

[gonzo]

That drawing shows a bad design. To be able to lift the propeller, the space between the propeller hub and the bearing is excessive. Also, the tunnel is a poor design that will create too much turbulence. The hand drawing dates it and probably was published on the popular tabloid boating publications of the era.

 

[Tomsboatshed]

That drawing shows a bad design. To be able to lift the propeller, the space between the propeller hub and the bearing is excessive. Also, the tunnel is a poor design that will create too much turbulence. The hand drawing dates it and probably was published on the popular tabloid boating publications of the era.

Yep:

This design is over 100 yrs old (roughly 1915). There are some Dispros still out there being operated. No doubt modern methods and materials in design and manufacture could make a more sturdy and efficient piece of kit.

 

[wroc]

I'm trying to imagine what it would take to get any form of over the counter u joint to last submerged and lubed in salt water....

Wonder if a needle bearing less u joint using a bronze bushing would work, or at least have enough longevity.

I think I could find a self-lubricating stainless steel U-joint. Maybe with ptfe bushings?

 

[wroc]

I’ve seen it in action - though the use case then was for much smaller boats with less power.
Were it me, I’d consider all the other aspects of my use case/design as well before going too. deep in the weeds on a disappearing/retractable prop. A better option may come up as the design evolves.

Please keep us posted on your progress!

You're right, at this point I'm just trying to establish that it's possible. In the end I may just get a 25hp "high thrust" outboard, but I'd really rather have a nice inboard diesel.

 

[wroc]

A tunnel would be enough to get the propeller flush with the keel without so many complications. How did you calculate the diameter of the propeller?

Just going on what the Indian shipbreaker said! The engine comes with transmission, shaft and prop. Am I way off in the propeller dimensions?

 

[gonzo]

I'm sure they are quaint. However, a surface piercing propeller or a properly designed tunnel would be more efficient.

 

[gonzo]

You can't have propeller dimensions until you design a boat. Also, do you know the gear ratio? A barge may need a higher ratio than a lifeboat.

 

[wroc]

You can't have propeller dimensions until you design a boat. Also, do you know the gear ratio? A barge may need a higher ratio than a lifeboat.

I guess I have a lot to learn about propellers. I thought a craft with a similar speed would be fine, even if not optimal, with the same propeller and transmission, or at least a similar propeller (in diameter). The 10hp diesel on my 42 footer is a bit smaller than that, if I recall correctly.

 

[gonzo]

Boats are like land vehicles in many ways. The transmission on a 1 ton truck has a different ratio than in a sedan, even though the share the same engine.