Cutting down a prop ???

[phillnjack]

Here is another one of my strange questions thats probably been asked and answered many times.

if i got a 14 inch diameter prop of say 23 inch pitch and it was a bit too much for a particular engine
can say half inch be taken off the tips of each blade and still let the prop be ok ?

I see there is a few props out there today that have blunt ends to them instead of the normal round
ear or cleaver points etc.
is there a benefit to this or does this just destroy the prop ?

I have a prop that is 13 3/4 diameter stainless and have been told i realy need just a 13 diameter
maximum for the engine to spin this up ?
the prop is a omc silver streak/srx 13 spline prop in very good condition thats been worked on for improvements with a 90hp e-tec engine.
my engine is a evinrude 54ci 60hp triple but soon to be 70hp (bigger carb change etc).

now if this prop is too big in diameter, can i simply cut the ends off to take down the diamet of the prop.
I have a 12.75 x 23 to try but its just an ordinary alloy omc prop,but might give me an idea if i can swing
this size ok.
the boat weighs around 500lbs all up without driver so maybe the 13.75 is too much diameter ?
But 12.75 could be about right.
this would mean just cutting off 1/2 of each of the 3 blades ,can this be just squared off ?
or does it need to keep a certain contour to work right ?



phill...

[ProHydroRacer]

Here is another one of my strange questions thats probably been asked and answered many times.

if i got a 14 inch diameter prop of say 23 inch pitch and it was a bit too much for a particular engine
can say half inch be taken off the tips of each blade and still let the prop be ok ?

I see there is a few props out there today that have blunt ends to them instead of the normal round
ear or cleaver points etc.
is there a benefit to this or does this just destroy the prop ?

I have a prop that is 13 3/4 diameter stainless and have been told i realy need just a 13 diameter
maximum for the engine to spin this up ?
the prop is a omc silver streak/srx 13 spline prop in very good condition thats been worked on for improvements with a 90hp e-tec engine.
my engine is a evinrude 54ci 60hp triple but soon to be 70hp (bigger carb change etc).

now if this prop is too big in diameter, can i simply cut the ends off to take down the diamet of the prop.
I have a 12.75 x 23 to try but its just an ordinary alloy omc prop,but might give me an idea if i can swing
this size ok.
the boat weighs around 500lbs all up without driver so maybe the 13.75 is too much diameter ?
But 12.75 could be about right.
this would mean just cutting off 1/2 of each of the 3 blades ,can this be just squared off ?
or does it need to keep a certain contour to work right ?



phill...

I'm not an expert, but I have cut down racing stainless steel props. Make a pattern from one blade, trim the pattern the way you like it to be, and then trace the pattern to each blade. Cut the blade with a bench grinder. Worked for me.

[phillnjack]

Now that is the type of answer i have been hoping for.

Does a 1 inch total diameter change make a huge difference ?

what sort of amount of change did you make to your prop and what was the results for engine revs or performance ?

do i need to take of material from all around the leading edge or just the tips ? i am hoping for it to be just the tips if i need to cut it/grind it.

i will obviously re-balance the prop after wards to make sure it dont give bad vibration etc.

phill

[Yellowjacket]

What is it you want to accomplish and why do you want to cut it down?

Reducing the blade area will increase the blade loading, and increase slip, but if your blades are near their loading limit, then you could create a lot more slip and actually lose performance.

Have you tested the prop you are planning to cut down? What is the diameter of the props that you would typically use and what is the diameter of the prop you are planning to cut down?

If the blades aren't highly loaded at speed, reducing the diameter can improve performance, but it depends on where you are now. It isn't clear that what you are contemplating will improve speed, it could go either way.

[phillnjack]

ok
first of all ive not yet tried the prop but i will hopefully be trying it within the next week.

Now my engine is a omc 3 cylinder 60hp 13 spline with a 2.42 ratio gearbox.

do you think a 3 cyl omc 60hp or 70hp will swing a 13 3/4 diamter x 23 pitch prop.
prop shaft is about 2 inches below bottom of boat, total weight of boat engine fuel and driver is around 850 pounds .

the propeller on the engine at the moment that i have been using is a 13x19 sst stainless prop and the engine
gets straight to the limiter very quickly and i think over revving.(to be confirmed with new rev counter).
top speed around 45mph.

I think the engine will get to this point but have had many people tell me the prop is too big in diameter for the engine
to get to 6,000rpms and will not let the engine perform and get to its max hp rating.

Now before anyone just says get a smaller prop, they dont make the silver streak in any other size at 13 spline.
Also buying stainless props in the uk cheaply is very very rare, and not many choices available even with new props.
The postage from the states to uk is totally mental price of around 80 -100 dollars for a stainless prop!!!!.
This prop worked its magic on a 16ft phantom with a 90hp e-tec,so much so it held a speed record for 4 years
for a 90hp green engine on a stock 16ft phantom speedboat.

Now my engine has the right number of splines and the max diameter of prop recommended is 14 inch,but this would normally be of lower pitch to push heavy cruisers and fishing boats etc.

I have had a lot of people tell me that 12.5 x 23 would be about the best size with this type of prop,but like i say they did not make them and im not forking out a fortune for something that i might be able to do to this prop if it is too big.

If its too big then its no good to me so i might as well try to make it the right size, i just wanted to know if it can
simply be done or if the whole blade area would need to be changed etc.

I have a 12 3/4x 23 alloy prop to try as well to give an idea if it will work better with smaller diameter.

so wht i might (if its no good) be wanting to do is just trim the diameter down to a usable size for my engine and
keep it right in the powerband.

untill i test the prop on the boat this is just an idea, but i wanted to ask if it was ok to do it and could it be the answer
to keeping the revs up to where they should be.

im not after 100mph, but do need to get to the mag 50mph plus with the boat and engine as it is now.
i have hit around 45mph with the 19 pitch screaming its nuts off and i dont think a 21 will get it down low enough.
i dont want the engine over revving and not producing its max power.

or would just a 1 inch extra diameter be of such little difference, that its not worth bovvering with ?


phill

[ProHydroRacer]

Now that is the type of answer i have been hoping for.

Does a 1 inch total diameter change make a huge difference ? YES, I would go slowly....take about 1/4" off at a time.

what sort of amount of change did you make to your prop and what was the results for engine revs or performance ? I was looking for a little less diameter and blade area...made improvement in rpm and speed by cutting and a little cupping.

do i need to take of material from all around the leading edge or just the tips ? i am hoping for it to be just the tips if i need to cut it/grind it.

i will obviously re-balance the prop after wards to make sure it dont give bad vibration etc.

phill

Hope this helps.

[rumleyfips]

Walking around the Valleyfield pits in the 60's, I saw someone reducing the diameter of a prop with an angle grinder . He didn't have to take it off the shaft. I can't remember how that boat ran

John McManus

[phillnjack]

Thats probably ok if you realy know what your doing or very desperate ha ha


I have seen some very wierd looking props with the tips squared off and wondered how they perform ?

And why the tip shape is so different to the normal props.

Here is a couple of pics of a yamaha ballistic for example of what i mean by cut off the tips





is this just a gimic ?
or is it something that has a real use ?

the first time i actualy saw one of these for real, i thought it had hit the ramp or something.




phill...

[zul8tr]

Prop mods are a black art to some degree and only by trim a little and run will you determine the effect. If you trim the diameter near the tip you will be left with a blunt edge. Theoretically if all trimming is identical for each blade the original prop balance will remain unchanged (not saying the prop was initially balanced but what ever it was will remain the same). If you remove the blunt edge only grind from the front facing face (travel direction) to produce the edge. No need to be super sharp (you are not a race boat) but finish with a very slight round edge. This trimming on the front face is where the material is removed to obtain a balance. Do you have a balancer?

As far as what diameter reduction will do for you "rules of thumb" in Dave Gerr's Propellor Handbook states that 1" in diameter absorbs the torque of 2 to 3" of pitch as a rough guide. Every 2" pitch increase will decrease rpm about 450 and vice versa. Or the other way to twist it is about a 2" pitch decrease or 1" diameter decrease will increase rpm by about 450. The same propellor can't deliver both high speed and max power. High speed propellors are small diameter and large pitch and large diameter and less pitch is a power or thrust propellor.

Note there is nothing here about blade shape, rake, cup, etc.

As with all rules of thumb they are sort of ballpark and all changes are subject to systematically testing over and over and over.

In the end you have to be satisfied with what you want out of the rig.

As for expected speed use this to estimate:

http://www.rbbi.com/folders/prop/propcalc.htm

If interested in the factors of propellor dynamics and sizing here is the Gerr book. It does not completly detail high speed props but offers some insight.

http://www.amazon.com/Propeller-Hand.../dp/0713657510

For more detail into props here is a more definitive guide:

http://www.aeromarineresearch.com/about.html

Ron Hill from this site makes and knows alot about props and could set you up right.

[phillnjack]

for balance i will make my own balancer, i have made them in the past so thats not a problem.
balancing the balancer is the hardest part !!
it will only get staic balnced, but thats good enough for my use.

sharpness of blade is not going to be razor sharp,but sharp enough to cut if its handled wrongly !!!!!

my ideal prop is probably around 12.5 x 23 pitch... this prop is 13.75 x 23 pitch and cupped.

if it dont let the engine spin up i was planning on taking of a total of 1 inch diameter to bring it down to
12.75 inch diameter and see what happens.
Maybe if i just cut it down to 13 inch and try it if it dont work as is, then it might be ok for me.

Now this prop does lift the stearn of the boat and my engine at the moment is running with prop shaft about
3 inches under the bottom of the boat.

By raising up higher does this allow more diameter of prop ?

i have seen a few pics of the 3 cylinder omc engines running these very same props and they dont look like they have
been cut or altered in any way.
But these engines were all running with jack plates and the prop shaft about level with bottom of the boat.

at the moment its all theory incase it dont run right with this prop, stainless props are very expensive in the uk and
its not often these come up at a decent affordable price.

to see a prop the size i would idealy like is impossible in the uk, and from the states the postage is more than the props
cost.

so if this is too big in diameter then i propose to cut it down to fit, but wondered what the pro's and cons were.

if i do cut it down i will make a templete to clamp against the prop to make sure each blade is exactly the
same, then re-balance once its finnished.

phill