Rudder help

Working on a project and need advice from you guys that are more knowledgeable than me on rudder selection.

12 ft Aluminum planing hull with a 13hp air cooled pivot drive (mini version of Levi drive )

Estimated speeds 20 knots
Debating on traditional flat rudder or the Levi drive style rudder

which style would provide tighter more responsive steering?
Pros and cons of each?

thanks

 

Hello LTC - I am a bit baffled by your post.
What is the significance of the photo that you posted - it does not look like a 12' boat.....

I am thinking that the best engine for a 12' ally planing hull is a simple outboard motor.

 

The housing around the prop turns, not the prop. I like it. What I want to know is, why leave it open at the bottom? Fully encircling the prop would help direct trust and protect the prop from catching things, I should think. It would also provide greater strength from flexing and potential interference with the propeller.

-Will (Dragonfly)

 

The photos were just for reference In case someone did not know what a Levi drive was

 

Interested in which style has more maneuverability

 

Will, re leaving it open at the bottom, I think the issue is the extra resistance drag if you had the propeller completely enclosed, especially so at higher speeds.
Nozzles are good on vessels like tugs and fishing trawlers, but they are not high speed vessels.

LTC, I think that if you are going for a surface piercing propeller arrangement you need to have the 'half moon' shaped rudder above the propeller to help prevent ventilation.
Can you post a photo of the '12 ft Aluminum planing hull with the 13hp air cooled pivot drive' that you are working on please?

 

Why are you using a surface piercing propeller setup for such a low speed; is it for low draft?

 

Will, the flow around an annular foil has a very unfavorable L/D ratio. Advantageous in a tug, but definitely contra-indicated in a planing boat.

A rudder in the flow from the prop should give more responsive steering than a pair on either side of that flow.

 

What's L/D ratio refer to? Length/ Displacement doesn't make sense to me, in this context.

Also, while I believe you guys, you certainly have more education and exprience about this stuff than I do, I thought nozzle drives were very effective at high speed. That's what jet boats use.

-Will (Dragonfly)

 

Hi Will. Sorry to be unclear. Laziness on my part. Lift/Drag ratio. A nozzle increases low speed thrust, but dragging that nozzle through the water means high speed isn't a thing.
Using it for steering is worse, unless the tradeoff is worth it, like a tug with demanding steering requirements. An annular foil has an incredibly high stall angle, but flow around it is like a very low aspect ratio foil. You can get a lot of lift, but that comes with a lot of drag. And the entire point of using SPP is reducing appendage drag as much as possible.