Prop Alteration

Hello Forum,
My first post.

I have a Johnson 70 with a 13.25x17 aluminum prop
I bought a used 13.25x19 aluminum and threw it on.

The boat will plane eventually but it is slow. It does operate at a nice low RPM once up and planing.

I want to give it just a bit little more thrust out of the water.
What is the best way to alter the shape?
I can see it now... trade it in, buy a new one etc...
I am going to alter this prop and you can't stop me

It only cost $20.

I just want your best educated guess... grind down cup? reduce diameter a 1/4 inch? Reduce blade size somehow?
Thanks guys, I'm just havin fun with this and hoping to learn a thing or two.

 

williamsesno, welcome to the forum.
if just looking for better holeshot and have to do it with this prop, try venting the prop the right size hole at the baseof the hub will help out

 

Prop Ventilation

I have google searched the possibility of Venting a prop.

Venting and aluminum don't mix. I imagine it must weaken the structure too much with aluminum.

Say I did go ahead with venting.... I can't find any pictures on Google, or narrow my search well enough to find it on this forum.

Would a pair of Plastic fins, above the prop, help at all? I can't remember the names of them.

 

If you want to apply a bit of science rather than just take to it then you should learn a bit about foils and propellers.

You do not say why you took the 17" pitch off. Was the motor over reving. If it was not then the 17" is better than the 19". The speed is limited by the power and the boat design. Putting a higher pitch on might be making the gearing too tall and you will not get in the power band until you get on the plane. You could try reducing weight by taking all unnecessary items out of the boat. The angle of the outboard can also help you get on the plane faster. Tilting it so the prop is closer to the boat may improve initial lift but can make the boat skittish once on the plane. (Does the outboard have power trim?)

Reducing the diameter will reduce the prop efficiency on two counts but might enable you to get in the power band. This would be a last resort for me.

It is possible to change the pitch by relatively minor shaping of the blades. I have attached some sketches of typical blade sections toward the outside of the blades.

Also just filing out any bumps and dents in the leading and trailing edges will help efficiency.

The difference between the Original_Shape and the Modified_Shape does not seem much but there will be a significant change in pitch by reshaping the blades in this manner. The front of the blade is taken as the left hand side in the two images. If you print both profiles, cut out the Modified_Shape and overlay on the Original_Shape it gives you an idea of the filing or grinding you would need to do to reduce the blade camber.

You would need to get some idea of the profile you have to do calculations but you should be able to arrive at a result through trialing. If you want to take the time to do it properly you can get the profile by filing a metal strip, edgewise, to seat neatly on the front and back of the blade. Just one radial position gives a useful starting point.

The things to remember is to keep the leading edge of the blades slightly curved; the trailing edge pointed and the three blades identical (as best you can).

This site will help you with shaping the foil:
http://www.mh-aerotools.de/airfoils/jf_applet.htm
I can guide you in its use if you like.

You are right about things that weaken the blade. Any shaping should be done to keep smooth transitions. An abrupt change in section will cause stress raisers and could cause the blades to break off - not a good thing.

Rick W.

 

Holy Crap.

Thanks for the response... I think.

17 is pretty chewed up. The 19 was available and cheap.
no power trim.
I want it to be a fishing prop. Doesnt matter if it gets out of the water fairly slowly, if I get good gas milage while planing.
I only get up and plane a couple times a day when fishing.

Your right, It doesn't get into the power band. It is closer to what i need then the 17 I think. It is just a little too much. If I could, I think a 18-18.5 pitch seems ideal.

I read the cup edge on the prop usually adds about a half inch of pitch. What would be the result of flattening the edge?

 

Have you got a picture of the 17" prop. Why not try to clean it up and give it another go. Get your rpm at full speed and compare this with the rated rpm. Running at lower rpm for the same speed will not necessarily reduce fuel consumption if the motor is labouring.

If the prop is cupped then taking it back to a straight edge effectively reduces the pitch. That is similar to what I attempted to explain in the previous post. The prop blade is a curved wing and you are tring to shape it so the angle of attack is reduced for the same revs.

You could even adjust the pitch of the 17" a bit by shaping it.

If you do not have a power trim then there will be an adjustable stop that allows you to set the motor trim in four or five positions. What brand is the outboard?

Rick W.

 

You need a large lump of wood like an up turned tree trunk would be nice.

Altering a prop is frowned upon by many but you need only to tweak it.

Feel the blades with the flat of your hand, fell all of them and try to remember is shape.

Place the blade flat on the wood and hit it with a hammer where you want to bend it.

What you need to do is reduce the overhaul pitch of the prop. As this is vertually impossible you can reduce the angle of attack of the blade.

you will only need a few hits of the hammer per blade but you must only shape one blade at a time so you can feel each blade and make them all the same.

Try the prop. Its not advisable to try to return the shape to original, alluminium doesnt like being bent too many times.

If you need more adjustment the same procedure could be adopted for the trailing edge of the blade. it is unlikely you will be able to tweak more than the equivelent of 2 inches off the theoretical pitch.

Ok Im goping to get shot down with this post but what can I say . I have done this many many times.

 

The old calibrated hammer certainly works just does not have the precision of a file but much, much faster. Not something I would suggest for a novice but everyone has to start somewhere.

 

I get it, mostly

I can visualize what you guys are saying about reducing angle of attack to reduce pitch.

I can picture how hammering it work, and I might try that.

I would like to try to understand the aerofoil java tool.
I cant quite picture how the pic translates into a prop.

I do understand graph points. I hit create foil on the java default settings and I can see how the graph points translate in the picture, but I cant see from there to the actual prop. I have done a little drafting in the past, so I think I'm not too far off.

Johnson 70, It has 4 different locations for pin placement. I think I should raise it to the highest point because the boat planes easier if my passenger moves to the front of the boat.

 

I think the fins you spoke of in an earlier post are called a whale's tail.

Tim

 

I think you will find your logic on the outboard angle is reversed. You need to lower it so the thrust is angled down more. This will lift the stern of the boat and cause the bow to stay down more. This normally makes planing quicker because it increases the amount of boat in the water thereby increasing lift area and reducing the drag. So if you are getting on the plane with extra weight up front, angling the prop down should give you a good gain.

The JavaFoil applet only gives you one point of the prop. You need a series of these sections to create the twisted foil that forms a prop blade.

The blade is a continuous series of foils that are angled based on the flow vector. Think of the boat doing 10ft/s through the water while the blade at say 6" radius is doing 12ft/s in the radial direction. The flow vectory will be at 40 degrees to the direction of travel. So the blade would have an angle of around 40 degrees to the direction of travel at 6" position. Now at the 3" position the radial velocity will be 6ft/s but the boat speed is still 10ft/s. This means this part of the blade will be angled at 60 degrees. (These figures are arbitary and do not relate to your prop)

If you want to move from the foil to a prop then have a play with the JavaProp applet - it is similar to JavaFoil but used to design props. However it does not have a foil range of foil shapes inbuilt. It gives some very rough sketch of the twist. I have attached various images of a low power, high efficiency prop that gives an idea of the twist.

Rick W.

 

Oh I forgot to add that if you want to hit your propellor with a hammer its advisable to take a mould of it.

This is done by getting a bowl of cement that is big enough for the prop . Push the prop into the cement (dont bury it) just wriggle it in. when the cement has gone off ( not set) wriggle it out again and clean it. You will then have a mould with which you can see how far you have altered it or what you have to do to get it back again.

It is quite likey you will destroy the prop on your first go. I remember tweaking a mercruiser prop some years ago. I tweaked it every week untill I had 52MPH. I tweaked it again and got 49 . I never got it back to 52MPH.

 

Filing seems to be the choice

It seems the the best proposal is... file down the prop.


I can look at the prop and imagine what I need to do, I think...
It looks like if I take a little of the first few inches off the fin, starting at the hub, it would lessen the angle of attack.

What can i do to figure out the best shape, and then transfer that to the prop?

 

Filing will enable better control if you take the time to work out what to do.

The most important part of the blade is the chord around 75% of the radius. So for a diameter of 13.5" (radius 6.75") the 75% point is 5" from the centre of the hub. This is a good place to get right and use as a guide for the rest of the filing.

Cut out a piece of cardboard 10" in diameter and with an internal hole to match the hub so you have an annulus that fits neatly over the hub. Use this to mark an arc on both sides of one blade. (You might have to split it to get it on the hub if the end is flared).

Get two thin strips of aluminium (say 1/8" by 1/2"), curve each of them to match the arc you have drawn on either side of the blade and then file them so they sit neatly edgewise over the line of the arc. These will become accurate templates of the blade section at this point. Make sure you know which one matches the front and likewise for the back of the blade. Also mark the leading and trailing edges.

In addition to this it would be good to measure the angle of the blade at the arc you have drawn relative to the shaft. This can be done a number of ways and I am sure you can work out a good one. This will allow the pitch to be checked. I will give you the calculation for this.

If you can post the profile in some way by photographing your two template pieces placed together or using the "Modify" pane of JavaFoil to sketch it I can make some suggestions on reprofiling to reduce the pitch.

If nothing else, this will enable a good estimate of what can be achieved before you start removing metal. It may end up that you have no choice but to bend the blades if there is not much thickness to play with.

Rick W.

 

Pitch Check

For the 19" pitch prop the angle of the blade relative to the the shaft at the 5" point should be 59 degrees.

To calculate:
A point on the blade at a radius of 5" will travel 31.5" (2 * pi * 5) around its arc in one turn. The pitch of 19" means it advances that distance in one revolution if there is no slip. So the blade angle at that point will be Inv Tan(31.5/19) = 59 degrees. At 6" from the shaft it would be 64 degrees.

You might want to have a look at the 17" prop as well. At the 5" radial point the blade angle should be 62 degrees. An 18 " pitch would require 60 degrees.

JavaFoil indicates that a camber change of about 0.5 degrees will give a shift of around 2 degrees in AofA for a thin foil. This is not a very big change in shape but you need to know what is there now.

Rick W.