Rudder size for inboard Zip

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sproggy
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Rudder size for inboard Zip

Post by sproggy » Fri Mar 23, 2018 7:39 am

As posted elsewhere, I'm building a Zip with an inboard-mounted motor. The mounting position of that motor determines the location (and therefore the shape) of an extra lower frame I'm adding but the position of the motor is influenced by the position of the rudder. And the position of the rudder is determined by its size, so that it's as far back as possible without protruding beyond the transom. All of which means that I need to determine rudder size before I can proceed much further with the frames.

So after that waffle, my question is how do I decide/calculate what size rudder I should be looking for? I've heard suggestions that its area should be LWL x Draft/30. No units were given but I assume inches. That gives me a rudder area just under 20 square inches. Another (unverified) source suggested the rudder should be twice as deep as it is wide and that, using the above 20 square inch figure, means a rudder about 6 1/4" inches deep x 3 1/8" wide (long). However my gut feeling, common sense or whatever tells me that a little thing like that isn't going to turn even a boat as small as a Zip.

Does anyone have a view or experience on what size rudder would be appropriate? What does a Cracker Box, which is about as close a comparison as I can find, use for a rudder? Then of course there's a decision about the shape and section....

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Bill Edmundson
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Re: Rudder size for inboard Zip

Post by Bill Edmundson » Fri Mar 23, 2018 1:44 pm

Sproggy

The rudder needs to be almost as deep as the bottom of the prop. Wide/long, it needs to be as big as you can tolerate. In reverse the only thing that turns you is fiction on the rudder. There is no thrust off the prop.

Bill
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steveh41
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Re: Rudder size for inboard Zip

Post by steveh41 » Sun Mar 25, 2018 6:49 pm

Sproggy,

I've seen that formula used before... assuming about 14' length and 10" draft for the Zip (guessing typical load, at rest) I'm coming up with about 56 sq. in. for the calc but I think you'd be Ok north of that number. Or if you figure roughly 12" for prop diameter and 2 to 1 aspect for the rudder you're looking at around 72 sq. in. which still seems like a reasonable size. Agree with Bill, better to have some extra area for reverse/low speed maneuvering.

Here's a reference for a series of 3 articles on the subject by Dave Gerr from 2006 Professional Boatbuilder (Dec/Jan, Feb/Mar, Apr/May... no charge for archives) http://backissues.proboat.com. Maybe a little overkill but good info nonetheless.


Anyway, sounds like a very cool project!

Steve
The longest journey begins with a single step… then repeat as necessary!

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sproggy
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Re: Rudder size for inboard Zip

Post by sproggy » Mon Mar 26, 2018 12:43 am

Bill, Steve, thank you for your replies.

The Zip has an LWL of 12'4" and a draft, assuming it sits at the DWL, of only 4". Perhaps closer to 4.5" if you add the thickness of the bottom planking. Prop diameter will be something around or slightly over 11". I'm guessing at this stage that the bottom of the prop will be somewhere around 12-13" below the keel, protected slightly below that by the skeg. So a rudder length of around 13" would be about right.

I'm erring towards the 12/6-P from Eliche Radice (http://www.elicheradice.com/product-line/rudders/) - it's the right depth (could be shortened slightly if necessary so it doesn't extend lower than the prop/skeg) and 38.5 sq. in. area. This is a lot less than your calculation, Steve, but the Zip's draft is a lot less than you allowed for.

I'll get hold of those Professional Boatbuilder articles, though, before deciding.

Thanks again.

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sproggy
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Re: Rudder size for inboard Zip

Post by sproggy » Mon Mar 26, 2018 1:51 am

Well those Professional Boatbuilder articles were certainly useful - information that I'd spent hours looking for but hadn't previously found.

They reckon for a planing hull, rudder area is 0.018 x LWL x Draft which in the case of the Zip is 148 x 4.5 x 0.018 = 12 sq inches. But that's nowhere near enough to my mind, particularly as I'll be running at sub-planing speed a lot. 38.5 sq. in. will be plenty big enough.

They also say the leading edge of the rudder should be around 15% of the prop diameter aft of the prop blade (1.7" in my case - maybe slightly more than that to allow prop removal) and the aft edge of the rudder should be 10% of prop diameter forward of the transom lower edge (1 1/8"). So if I settle on the Eliche Radice rudder and decide on my transom curvature I know where everything needs to sit. Which is a MAJOR step forward.

Thanks again, guys, for your pointers.

Ian

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