Propeller sizing and clearances

[Jason Day - Sheffield]

15% 0f 16" is 2.4, or just under 2 1/2" - I'm fairly sure we have a lot more than that. (photo below)

 

As a rough guide though, the amount ahead of the stock in this photo is about 1/5 the amount behind, or 16.6% of the total chord. Upping that to 20% would be to increase the amount in front of the stock by only a very small amount. Probably not worth bothering with, but I will phone the dock in the morning to find out what they think.

 

OK... this is on my boat....oh - apparently I'm not allowed to use this image extension in this community...jpg not allowed.?

Edited November 2, 2016 by Jason Day - Sheffield

[Jrtm]

15% 0f 16" is 2.4, or just under 2 1/2" - I'm fairly sure we have a lot more than that. (photo below)

 

As a rough guide though, the amount ahead of the stock in this photo is about 1/5 the amount behind, or 16.6% of the total chord. Upping that to 20% would be to increase the amount in front of the stock by only a very small amount. Probably not worth bothering with, but I will phone the dock in the morning to find out what they think.

 

Upping it to 20% makes just over 3 inches if my off top head maths is right.

[Neil2]

Just a thought - has anyone here any opinion on extending the rudder blade, by welding a section on forward of the pivot point? I beleive (from sailing boat experience) that this could better balance the blade, and reduce turbulence against the rudder causing juddering. It also moves the rudder closer to the prop, and will reduce the force needed to hold it against the force of water.

 

But there's a good chance I'm completely wrong.

 

Any thoughts anyone?

 

You just have to be careful not to overdo it. There have been previous threads on this issue and a few have commented that a rudder with too much balance is a real liability.

[Jason Day - Sheffield]

BillyBobBooth - yes that's what I guesstimate, butt there is already about an inch and a half, so adding another inch and a half may not make much difference.

 

Good point about overbalance, you lose feel on the rudder and also block it when turning hard, but we already have a heavy rudder , so I'm tempted to try a little balancing. They have to get the welder out anyway.....

[Jrtm]

Do you have a weed hatch?

 

Reason i ask is you could make a bolt on 1-2inch plate that you could then try if your happyer you can leave it on till next at dock then weld on if your not happy you could access threw weed hatch and remove.

 

If you made it hook on at the bottom and bolt at the top you only need to reach the top one to lift it out? Only prob you may have is it ratteling /slight movement at the bottom unless you made it nice and tight

 

Just an idea saves having to have it put on then dock to have it removed. But i dont know how long you go beween docks

[Dalslandia]

2.5 of 15 is 16.6%

3 of 15.5 is 19,35%

2,75 of 15,25 is 18%

it is bloody sensitive

2,625 of 15,125 is 17,3%

 

so 3 mm longer will be perfect?

 

a rounded airfoil like a wrap back to 3-4/5 of the chord will reduce any vibes.

 

a 3 mm wrap around the LE, forming an airfoil, 20-25% thick at 20% of the total chord, and you can steer in circles big in diameter as boat is long.

 

 

A simple Schilling rudder airfoil, 20% thick at 20% of the chord (length) rudder stock at 17% but is possible to have it at 20% as well.

Nose radie is 5% of chord, fishtail is 6% wide of chord.

Edited November 4, 2016 by Dalslandia

[Dalslandia]

An even simpler high lift rudder, the "I scream cone" rudder.

here in 2 tickness, one 20% coning to 80% of the chord, and one 25% coning to 100%. Is said to steer good in reverse.

 

 

Edited November 5, 2016 by Dalslandia

[Jason Day - Sheffield]

Hmmmm thanks - slightly high tech for me .... I passed on the 20% in front and 80% behind the rudder stock to the fabricator who enthusiastically agreed, stuck 25% in front of the rudder stock, and put her back in the water. Bugger.

 

So the plan is to see how that goes, if necessary I can always grind it off the next time we are near a crane out point.

[Dalslandia]

Dalslandia had a flat plate rudder, 200 of 920 = 21,7% Was really heavy.

made a schilling 200/800= 25% but got 205/800=25.6% with the wrap around the LE. =over balanced

length the TE 40 mm, with a slight fish tail, so now 205/840= 24.4% still slightly over balanced at large angles and lot of power on.

24% might have been perfect, 205/850 mm

 

 

20% is 1/5, one part in front of and 4 parts behind the stock, maybe the welder was thinking 4 parts total.

 

A friend made a new rudder for his cabin boat, made it 33% in front, I told him if he let go of the wheel it will go fully to one side. at one time he forgot what I said, and let go in a short moment at high speed, his wife that is a lot of ballast flew off the boat in that sharp turn that was the result, now rudder is 20% balanced, she is still his wife.

Edited November 6, 2016 by Dalslandia

[Jason Day - Sheffield]

Yes, I think the our fabricator either forgot what he was told, or thought he knew better. We wont be doing high speed, or tight turns on the english canals, but I like the idea of maybe balancing it up by adding to the trailling edge. Our boat was originally built as a hire boat, so I can inagine that it was designed to under perform and reduce the twitchiness of amateurs on the tiller.

 

Sadly our plan is to sell her after next summer's cruising, as we are having a career break and going off to Europe in a camper truck to climb mountains for a couple of years, so I won't get to continue refining things, but if I can deal with a couple of improvements in advance of that I will. I like the idea ofthe fishtail rudder!

 

thanks for all the suggestions, Jas

 

p.s. just re-read what I wrote, so before I get flamed by everyone reminding me that a: there are tight turns and b: maneuvrability is important - what I meant was

a: usually and

b:I've done my time dodging gravel barges on the Trent, and better plan ahead now!

Edited November 6, 2016 by Jason Day - Sheffield

[Jason Day - Sheffield]

Boating yesterday and today proved that the tiller is more responsive now, but has gone through neutral and on into twitchy. I spoke to the guy who added the plate and he hadn't even measured it, he heard what my wife told him about 20% of total chord, then ignored that and just eyeballed it 'because he's experienced'. Then charged us £75 for it. Aaaaaaaaarrgh

[Dalslandia]

Boating yesterday and today proved that the tiller is more responsive now, but has gone through neutral and on into twitchy. I spoke to the guy who added the plate and he hadn't even measured it, he heard what my wife told him about 20% of total chord, then ignored that and just eyeballed it 'because he's experienced'. Then charged us £75 for it. Aaaaaaaaarrgh

Then he will re-do it for free, with a hack saw under the water. tell him 20% is 1/5 of total length (chord) or 1/4 of the length behind the center of the stock.

[Dalslandia]

Back to O.T.

 

The numbers of clearance have been said before, but the reason behind it might be unclear.

 

the propeller is the only item on the boat that propel it, not the engine.

 

The vertical clearance,

We know the bigger the propeller diameter the better efficiency (if engine and gearing can make it) The reason for big clearance is to minimize vibrations, but must be in contrast to propeller efficiency.

The absolute minimum on top is said to be 10% of prop diameter, but can lead to vibrations, others minimum is 0.23 - (0.02*Z) = 17% on 3 blade, 15% on 4 blade. 13% on 5 blades, but always minimum 2"

 

At the bottom 4% is minimum to skeg. or at least one inch, better with two.

 

Horizontal clearance.

To the rudder, minimum 10% of the prop diameter to the leading edge of rudder. small distance is better for propulsion, larger give less vibrations.

 

Distance to the swim or propeller post, should be minimum 0,33 - (0.02*Z) or 035 - (0.02*Z) (Norske Veritas) so 27%-29% (3 blade) of propeller diameter at LE at 70% radii.

This distance is only positive, the longer the better, if we see the propeller as the fixed point, a longer distance to the propeller post will make this part of the hull shorter and increase hull drag, but the larger distance reduce the suction on the hull and make the fluid less turbulent into the propeller disk, this effect is seen up to 2 propeller diameters of distance between end of swim/propeller post and most forward part of propeller, but purely possible during the design phase.

 

The angle of the swim should be less then 15 degree each side, or less then 30 degree each side on a slow boat, less is more here.

[Jrtm]

Intresting I spoke to prop makers today after give all the details I could give ive been told my prop is bang on, but as the size hight of the prop is causing a problem they do make a smaller prop that will give the same results power and flow but just on a smaller prop size but bigger blades. This was after talking to them about making mine smaller and adjusting the pitch a bit to help compensate.