# Propulsive efficiency and propeller depth relationship

Discussion in 'Propulsion' started by Edolyi, Jul 25, 2015.

1. Joined: Mar 2014
Posts: 26
Likes: 0, Points: 0, Legacy Rep: 10
Location: Newcastle University

### EdolyiJunior Member

I'm now doing some studies about the effect of the depth immersion of the propeller to the propeller power. The study is all about the fully immersion and not including the partially immersion condition. I referred to some researches, there are some model tests results for some model ships, but I'm not quite sure whether the experiment outcomes is suitable for my vessel. Is anyone knows if there are some governing equations related to depth and propeller efficiency? Or can I just use the model test outcomes from other vessels? The time is limited and for computer simulation I just know Maxsurf, is there any method?
Thanks for your time!

2. Joined: Jun 2015
Posts: 541
Likes: 9, Points: 0, Legacy Rep: 117
Location: Saint John New Brunswick

### Jamie KennedySenior Member

The only thing I can think of is the effects of cavitation. If a propeller isn't cavitating I don't think there is any improvement to be had by increasing depth. On the other hand, if you increase depth you might be free to design the propeller to be more efficient without fear of cavitation. Power is another matter however. For a given diameter you can certainly increase power because you can generate more vacuum on the front of the blades without reducing pressure below the vapour pressure.

The tricky thing about efficiency is that it tends to favour a larger diameter, which brings the top of the blade nearer to the surface. So I think there is always a bit of a compromise in propeller design when trying to achieve power or efficiency while at the same time avoiding cavitation.

If you can I would just do the basic calculation for vapour pressure. For a standard propeller and a given hub depth you could trade off diameter versus depth. A larger prop diameter should give you more efficiency but less maximum power at the point of cavitation, which would be temperature dependant. You could also look at multiple props. The other thing is you could look at the tradeoff of increasing hub depth but having to increase the angle of the propeller shaft if it is an inboard. Keep it simple, but not too simple, and have fun and learn something along the way.

3. Joined: Oct 2008
Posts: 6,201
Likes: 415, Points: 83, Legacy Rep: 2488
Location: Japan

### Ad HocNaval Architect

All you need is to read:

"Propeller design and model experiments" by Emerson A & Sinclair L., RINA Trans 1978.

All your questions are answered in that excellent paper.

4. Joined: Mar 2014
Posts: 26
Likes: 0, Points: 0, Legacy Rep: 10
Location: Newcastle University

### EdolyiJunior Member

Thanks a lot Jamie that's very comprehensive! But for the cavitation effect it is currently not into a count, I think I will do some near water surface study and may focus on the near surface effect to the propeller thrust.

5. Joined: Mar 2014
Posts: 26
Likes: 0, Points: 0, Legacy Rep: 10
Location: Newcastle University

### EdolyiJunior Member

Thanks for advice AD HOC! I'm very willing to read this paper but where can I find it? I've tried online and searched the school library system still couldn't find it.

6. Joined: Oct 2008
Posts: 6,201
Likes: 415, Points: 83, Legacy Rep: 2488
Location: Japan

### Ad HocNaval Architect

If the university library does not have it, i'll eat my hat!!!

7. Joined: Mar 2014
Posts: 26
Likes: 0, Points: 0, Legacy Rep: 10
Location: Newcastle University

### EdolyiJunior Member

Hahahaha. Not in our library bro I'm sure. But I will try to find it somewhere. Enjoy your hat bro!

8. Joined: Oct 2009
Posts: 4,444
Likes: 219, Points: 63, Legacy Rep: 1485
Location: Midcoast Maine

### DCockeySenior Member

9. Joined: Oct 2008
Posts: 6,201
Likes: 415, Points: 83, Legacy Rep: 2488
Location: Japan

### Ad HocNaval Architect

10. Joined: Mar 2014
Posts: 26
Likes: 0, Points: 0, Legacy Rep: 10
Location: Newcastle University

### EdolyiJunior Member

Thanks guys! I'll eat my hat!

Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.