Prop to rudder

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Prop to rudder

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20 April 2009 at 14:59 #20337
Robs
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@robs46138
Hi, is there an optimum distance from the prop to the rudder? Is closer to the rudder better than say the distance equivalent to the pich of the prop away from the rudder? Or is it a case of if it looks right it is right and is not ctitical at all? The model that I am rebuilding is totally ad lib and there are no plans or any other information on it. Cheers, Robs
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20 April 2009 at 14:59 #1860
Robs
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@robs46138
20 April 2009 at 18:39 #20338
ashley needham
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@ashleyneedham69188
Robs, there was a thread on rudders a short while back with loads of useful and at times far too technical (for me) info. Colin may remember, being the FONT.

i say, in theory yes, in practice no. Stick it behind your prop, leaving space so you can change the prop ok, and if you cant withdraw the shaft from inside the boat, you could put the rudder slightly to one side to allow withdrawl from the rear. Over rudder your boat as its easier to cut down a brass rudder rather than buy or make a new one, or at least, as you are ad-libbing, make the rudder area generous rather than tight. Adjustments downwards can be made by attacking with tin snips if you have been REALLY generous. use a balanced rudder if a powerboat, that is, the rudder pivot is set just behind the leading edge, or simply buy one of those brass/plastic ones ready made…they are cheap enough and less hassle than making one. Ashley (who favours an empirical approach to the subject)

nb I read somwhere that theoretically the flow from a prop is straight back, and not spirally as one would think…any views on this?
20 April 2009 at 22:53 #20345
Robs
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@robs46138
Thanks Ashley, your advice is well noted. I did a search for rudders and I see what you mean about too technical, I try to to work to the KIS principle, “Keep it Simple”, so I will place my prop so that I can remove it easily without having to remove the rudder to do so. Hopefully when I have finished the model it will look similar to a Brede class lifeboat but not an exact replica. My next project will be another “off the top of my head” one, an airboat built the Scrapheap Challenge way from odds and ends that I have acquired over the years using soft drink cans for the hulls and bits salvaged from my wife’s old laptop that died and had too many useful bits to throw away. I will try to post a pic of it when I finish it but first I have got to finish the lifeboat. Thanks for your help, Robs.
23 April 2009 at 16:38 #20390
Mike Davidson
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@mikedavidson22772
Hi Robs I believe there is a distance between props and rudders where it is too big and the two do not work together. In the first instance, it is wise to know how each of the devices work, and then, it is easier to see how they work together. the rudder is a device that deflects a flow of water to one side. It cannot perform this function if there is no flow of water past itor, the boat is under way. The technical expression is that the boat has steerage way.If the vessel is standing still, and the motors are turning, there will be a column of water pushed astern by the propellor past the rudder the rudder will deflect the water to one side , and as we are told by physics teachers, every action has an equal and opposite reaction, so the action of pushing the water to one side will produce a reaction of equal force pushing the stern of your boat to the other side. at this point, I call your attention to the photo in the album Mikes Boats titled Tamar gear which is a photo of the hand made props and rudders for my model lifeboat a bit difficult to work out, as they are sitting snugly in a tunnel hull. From this , you can see the distance between rudders and props , and see how the props would push water past the rudders. The first thing to do is get the baby wet, chuck it in a pond and see what happens, does it go fast or slow ? does it steer left and right >? in any case, the answer to any wrong answers lies in judicious modifications,props can be moved for’d or aft with the addition or subtraction of thrust washers, and a rudder could be extended by soldering on a bit of sheet metal to extend it in any direction. You will see from my photo, that there is a bit of rudder ahead of the shaft centre linewhich I believe makes the servomotor’s job a little easier addin g a bit of rudder that helps the turning motion of the shaft. historically, rudders have had balancing tabs on them to reduce the amount of effort needed to turn them to see this at first hand, float your model in a bath and feel the push of the motors when they are running see the stern swing when you operate the steering servo, check out the speed control before your model goes out of range in the park pond. remember a quality product is right first time, and flight safety is no accident….Mike Davidson……….

Edited By Mike Davidson on 23/04/2009 16:40:43
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