Ugh, just learned one MORE thing you have to consider when finding a prop

[eavega]

So, I have a 19P stainless steel prop on my 70 HP Johnson, and it has been moving my boat at a barely satisfactory motor speed of 5000 RPM at WOT. It does move my 15' trihull at an acceptable 32 MPH. I have been wanting to try a 17P SS to see if I can get that extra ~400 RPM to my engine speed. Per prop slip calculations, I would be able to do this with no loss of speed. So, I go on e-bay, find a reasonably priced 17P ss prop, and buy it. Put it on the boat yesterday and find the engine LOST abot 300 RPM stepping down from the 19P to the 17P. WTH?! I start looking very carefully at the prop to see if I had missed some subtle design aspect of it, then I spot it; the prop is CUPPED! Definitely got the boat on plane a lot faster, but the loss of top end motor speed is unacceptable. So, just as a warning to all of those looking to find the sweet spot in motor speed by changing your prop, make sure you know if there is any cupping to the blades, as that will pull down your WOT RPMs.

Rgds

 

[jimmbo]

Re: Ugh, just learned one MORE thing you have to consider when finding a prop

Without know more about the props I can't but speculate. At first glance it looks like the19 is blowing out and the 17 has a better grip on the water, but... You say the 17in is cupped. What about the 19? I have never seen a SS prop for these outboards that did not have a cup. The rpm and speed with the 19 show approx 15% slippage. You say the engine lost 300rpm withthe 17 but you didn't say what the boat speed was. What conditon was the 19 in, dents, nicks, chunks missing, bent blade(s)? And the 17? Perhaps, at some time one or both of the props had be repaired or modified, perhaps poorly.

Edit: When you changed props, you did make sure the thrust washer was in place on the propshaft? if not, your rpm drop could be the result of some very expensive machining taking place

 

[steelespike]

Re: Ugh, just learned one MORE thing you have to consider when finding a prop

Is the motor a 76? If so the gear ratio is 2.42 I get 13% slip with the 19",
high normal.
A good prop shop can take some of the cup out of it.
I've seen a couple of posts here on iboats with excellent results.
With an increase in both rpm and speed.
Your wot rpm rating in 76 was 4500 to 5500 rpm
So while your at the low end of normal your not doing any damage.
It would probably be a little peppier with a 17 and may be easier to keep on plane at reduced throttle.
With 13% slip I get 31.3 mph at 5400 with a 17.
with 10% slip I get 32.3 @ 5400.

 

[eavega]

Re: Ugh, just learned one MORE thing you have to consider when finding a prop

Without know more about the props I can't but speculate. At first glance it looks like the19 is blowing out and the 17 has a better grip on the water, but... You say the 17in is cupped. What about the 19?

As far as I can see, the 19 does not have any cupping. The cupping on the 17 is fairly pronounced. I noticed it when I did a closer inspection after getting the boat out of the water on Saturday. I can definitely say the cupping on the 17 is a heck of a lot more pronounced than any cupping on the 19.

You say the engine lost 300rpm withthe 17 but you didn't say what the boat speed was.

Max speed on the GPS was 28 MPH with the 17.
Max speed on the same GPS is 32 with the 19 (verified several times, as I have been using the 19 for about a year now)

What conditon was the 19 in, dents, nicks, chunks missing, bent blade(s)? And the 17? Perhaps, at some time one or both of the props had be repaired or modified, perhaps poorly.

The 19 has one very small ding on one blade close to the hub. the 17 has no chips or dings, but some rough edges on two of the blades. I guess its a possibility that one or both of the props could've been repaired or modified. Perhaps my next stop should be a prop shop to have them both looked at.

Edit: When you changed props, you did make sure the thrust washer was in place on the propshaft? if not, your rpm drop could be the result of some very expensive machining taking place

Yes. I did switch from a brass thrust washer to a nylon one for this prop, though. I greased the splines when I changed out the props.

For now I am going to switch back to the 19, and maybe visit the one prop shop I have found in the general area. Thanks all for the insights and considerations.

Rgds

 

[Dhadley]

Re: Ugh, just learned one MORE thing you have to consider when finding a prop

Your thrust washer will always be brass or aluminum, never nylon.

As for your rpm / speed issues you should always start with set up. Most likely the prop you got will run at a higher X dimension which will improve your overall performance. Holeshot, speed, rpm etc.

 

[eavega]

Re: Ugh, just learned one MORE thing you have to consider when finding a prop

Your thrust washer will always be brass or aluminum, never nylon.

I verified it again last night. The thrust washer is plastic. Just to be safe I took that one off and put back the brass one I had on there before.

Rgds

 

[Dhadley]

Re: Ugh, just learned one MORE thing you have to consider when finding a prop

Are you talking about the thrust washer (between the prop and the gearcase) or the spacer washer (between the prop and the nut)?

 

[eavega]

Re: Ugh, just learned one MORE thing you have to consider when finding a prop

Spacer. Sorry. I went back and pulled the prop off again and verified there was a brass washer on the other end closest to the gear case. Sorry for the confusion.

-E