New stainless, excessive prop slip

[system-f]

1971 Merc 165 inline 6

I was running a 23 pitch aluminum 3 blade; at 2500 rpm I would do about 27mph with a prop slip of 18-20% using the calculator. WOT throttle rpm was 3900 rpm and around 45 mph with prop slip dropping to 12-15%.

I put a 4 blade 21 pitch stainless on and the prop slip has jumped way up. At 3000 rpm I could hardly get 25mph and would start to overrev around 35mph, around 4300rpm.

I am confused; I shouldn't be getting such high numbers with the stainless. The prop shop said they would let me take it back but I am starting to wonder if a 4 blade prop is going to work with my boat. My boat is a standard V-hull, 1971 Glastron V-174. When I go back to the prop shop should I try to swap this for a 3 blade? Right now my plan is to jump up to a 23 pitch 4 blade. I mainly use this boat for skiing and like the benefits of a 4 or 5 blade prop, but would at least like to get a comfortable 35mph for slalom skiers.

 

[hwsiii]

Re: New stainless, excessive prop slip

System, something is very wrong with what you are saying. Please tell me the manufacturer, model, diameter and pitch of each of these props. 99.9% of the time a 4 blade prop will always have less prop slip, especially if the 4 blade prop is stainless steel and the three blade is aluminum. And from the numbers you are giving me you are saying the SS 4 blade has over double the prop slip that the aluminum has, and I find that almost impossible to believe if there is not something wrong with one of the props, and I assume it has to be the SS one. Please take a picture of the two props side by side so I can see the differences in them. Also look at the numbers on the new SS and make sure it is a 21" pitch and look at the other prop and see if it has some stamped markings showing a change of pitch on it.

Prop Slip

H

 

[system-f]

Re: New stainless, excessive prop slip

I am 100% sure of those cruise numbers and that the Stainless is a 21 and the aluminum a 23 pitch. In fact the aluminum needs to be refinished, it is not in the best of shape, but servicable. The stainless prop is brand new and perfectly smooth..much smoother than the aluminum but the stainless seems to slip very easily. I have contacted the prop shop and I am taking the stainless in to swap it for a different model of 4 bladed prop with more cup and increased to a 23 pitch with a lost of 1/8" diameter over the current stainless I have.

I agree that it doesn't seem right, but it is true. When an increased load was put on the boat...hitting a wave or heading into the wind, I would see the engine rpms slightly increase as the speed decreased. Also, when pulling a skier I noticed that the rpms where 200-300 higher per a given speed than without a skier, but i thought this was normal.

 

[QC]

Re: New stainless, excessive prop slip

I put a 4 blade 21 pitch stainless on and the prop slip has jumped way up. At 3000 rpm I could hardly get 25mph and would start to overrev around 35mph, around 4300rpm.

Shouldn't your WOT RPM be way higher than that anyway? I am seeing recommended at 4800 - 5300 . . .

 

[system-f]

Re: New stainless, excessive prop slip

Duplicate post, deleted by original poster.

 

[system-f]

Re: New stainless, excessive prop slip

I am seeing recommended at 4800 - 5300

Nope, not for the 1971 straight 6 Chevy 250. The Merc. Manual says 3900 to 4300.

 

[QC]

Re: New stainless, excessive prop slip

You're right, sorry. Somehow I read 115 i.e.Tower of Power OB . . . Doh!!!! Carry on :redface:

 

[system-f]

Re: New stainless, excessive prop slip

Haha, no problem and it did make me double check which is always good because I needed to find dwell #s anyways

 

[system-f]

Re: New stainless, excessive prop slip

The prop shop called the engineers at PT props. They said that the type of prop I had was made for a higher horsepower application that would be spinning the prop faster. They recommended a prop with more 'bite' in the water. Of course this other prop is going to be $214 more than the one I have

In defense of the prop shop I do not think they knew what exact application I wanted the prop for since it was a gift.

 

[hwsiii]

Re: New stainless, excessive prop slip

System, by running my numbers I find you should be using a 21" pitch prop with your setup to reach maximum RPM. I believe the new SS prop has a slipping hub problem, does this new SS prop come with an interchangeable hub assembly, because that would explain the differences I see in prop slip and exactly what make and model of SS propeller is this, you never did say .
Otherwise it doesn't make any sense to me, at all.


H

 

[system-f]

Re: New stainless, excessive prop slip

ELE4 PT Prop, It uses the XHS hub kit and it is new. No signs of slipping on the plastic insert.

One thing that isn't really related, but maybe: I hate how my MR drive won't trim back in like an alpha. The furthest I trim it in is still straight up and down and there is no way to get it to go further in that I can see.