Rudder Size?

[damfurst]

I just have a couple of questions that I'm sure someone on here will have the answer to concerning rudders.
1. How do you determine what size of rudder to use according to the size of your hull or how fast you will be running?

2. Is there some kind of formula for determining the length and width of a rudder given your hull type or size? Or is it more of a square inch area thing where if you have a certain amount of rudder area per hull size it will work equally as well.\?

3. How important is the shape of the rudder? What I mean are in the pics below.

[ozzie-crawl]

there is a recommended length minimum, from memory its 1.5 times the diameter of your prop below the prop,so if you had a 40mm prop you would want 60mm below your prop blade at its lowest arc

[LuckyDuc]

I thought that it was 1.5 inches below the prop arc?

[ozzie-crawl]

could be luckyduc, would seem strange that a 16" hull would have 1.5" below and so would a 60" hull tho

[LuckyDuc]

I'm not positive either. Hopefully others will chime in.

[Fluid]

There is no general "rule of thumb" because there are too many variables. Some hulls turn better and need less rudder area. The rudder has to balance the turn fin on hydros - too little rudder and the boat hooks badly. Different size props on the same hull can need different rudders to stop prop walk - turning is not the only function of a rudder. Rough water requires longer rudders or you can unhook in turns.

How do you find the right size? Start with too much rudder and cut it off 1/4" at a time until it stops working right, then replace the blade with a slightly longer one.



.

[detox]

I once read somewhere that too much rudder will cause stern of boat to run wet or tight. IMO I would make sure water pickup port is deep enough to do its job then trim well below port. Grim Racer says to sharpen rudder blade opposit cooling port side for best water intake and less drag.

My Whiplash instructions say to trim 3/4" off length of the Speedmaster Mini rudder. I am sure this helps hull run faster (less drag).

[detox]

Here's my Speedmaster Mini rudder after I trimmed off 3/4" and sharpened opposite side for best water intake flow. I will use this style rudder in the Whiplash 20 and maybe Raptor Rigger. I performed a flat angle cut on bottom (works great on UL-1), but some people cut rudder off square and sharpen bottom to a knife edge.

[LuckyDuc]

There is no general "rule of thumb" because there are too many variables. Some hulls turn better and need less rudder area. The rudder has to balance the turn fin on hydros - too little rudder and the boat hooks badly. Different size props on the same hull can need different rudders to stop prop walk - turning is not the only function of a rudder. Rough water requires longer rudders or you can unhook in turns.

How do you find the right size? Start with too much rudder and cut it off 1/4" at a time until it stops working right, then replace the blade with a slightly longer one.



.

I'm going to have to order some spare rudders for each of my boats and do some testing.
A rough water rudder would be handy to have in the tool box for heat racing.

[LuckyDuc]

Here's my Speedmaster Mini rudder after I trimmed off 3/4" and sharpened opposite side for best water intake flow. I will use this style rudder in the Whiplash 20 and maybe Raptor Rigger. I performed a flat angle cut on bottom (works great on UL-1), but some cut off square and sharpen bottom to a knife edge.

Let me know when you find the minimum rudder length for the whip

[graill]

Fluid is correct in the no rule of thumb. It will depend entirely on your build, boat setup and driving habit. (fast rate, long throws, hard on the wheel, etc)

At about 30-37 deg a fullsize rudder depending on speed will stall, the boat loses total rudder control. On high speed boats and our little rockets this equates to your boat spinning out, one main reason anyway. Also about the same amount of degrees.

Keep in mind this can also happen in a straight line...........with enough speed and incorrect rudder and rudder setup.

like some have posted the more rudder in the water the more downforce on the front of the boat and the stern running like it was being sucked down. ideally you want your rudder providing just enough to keep the rig biting in well in turns and not drifting out from underneath you. Rudder travel and rudder speed will affect how much the stern drifts at running speed, keep this in mind when setting your rig up.

Stepped trailing edges and tapering chords but thicker material width wise is what alot of full size racing rigs use today, personally i havent seen any of that tech reach the hobby scene, not that anyone has done any testing of these at the hobby level.

As a side note personally i would not trim 1/4 inch at a time, a mm, or a 16/th at a time after you have your ballpark calculations in hand seems more reasonable, then manufacturer your finished rudder from your test piece specs.

[graill]

A last thought to throw at the op. Have you also considered factoring in the multi mix programs available with the newer radios such as speed/rate mixing, throttle/angle, and angle/throw speed? or any combination of the three? These, factored with a good rudder setup can really burn a person out having fun with the multitude of combos.

Once you find a two or three that work with your setup they are a hoot to use.

[damfurst]

Thanks for all the replies and thoughts and insights. Not sure I'm any wiser but do have more to think about and consider.