Trying to decide on a pitch change.

[sutor623]

Hey fellas. I want to give ya'll an idea of what I have going on. Had a minor prop bashing the other day, so need to find out where Im going from here.

1986 16' Aluminum Starcraft

1984 Evinrude 75hp with TnT

Motor specs:
WOT Range: 5200-5800
Gear ratio: 2.42:1

Top speed is between 32-34MPH, via GPS. Current prop is 13 3/4" x 15P. Prop slip is between 1% and 7%, depending on current and wind. RPMs with one other on board is right at 5850.

Goals: Top Speed, Bring RPMs into safer zone, be able to carry myself and three passengers, and sometimes have three one of those passengers on a tube or water skis.

I need to buy a new prop. Do you guys think that I should stick with a 15P, or should I bump it up to a 17P? I guess going up in pitch will drop my rpms to around 54-5500 rpms, but then when I load the boat down with two other people I will be out of rpm range. :facepalm:

 

[Sea Rider]

Re: Trying to decide on a pitch change.

You're slightly over reving with 2, with just driver much more, assume you have not tested that way, right ? If going for same size prop but 17 higher pitch probably will be around middle min-max safe working parameters with 3. Some states there's a rpm decrease of 100-150 others 200-300 per pitch, who's right ?

Is there the possibility to test a 16 pitch before you go straight buying a 17 ? Anyway, it's good to have at least a spare prop for other boating applications as the one you're pretending going for.

Happy Boating

 

[steelespike]

Re: Trying to decide on a pitch change.

If it pulls a skier real strong now you could consider a 17" prop.It would surely slow your hole shot but may squeeze out a little more speed. If your 15 is older a simple change to a new 15 might improve things overall.
The Solas Amita or Rubex have a good reputation.Don't know if they have props for that motor.My 89 aftermarket manual indicates the 3 cylinder 75 is rated at 5200 so 5700 is probably the max rating.

 

[sutor623]

Re: Trying to decide on a pitch change.

Searider, I was considering a 16 p but they are few and far between. And ou are correct I have never run this setup alone.

Steele, just the guy I wanted to hear from. I have not pulled a skier or a tube from this setup yet. I have also not ever had more or less than 2 people on the boat. Everything I have read on this motor says 55-5800rpms, but this is just hear say. And yea Solas makes props for this motor for under $100, can't beat it. I plan mostly on cruising around with two people, but I do not want to be over revving this thing so I am kinda caught in a bind here. I don't want to lose a huge amount of hole shot going to 17p, but I a
Also concerned that a new 15 will also keep me up at the top of my rpm limit. Anyone follow up advice?

 

[steelespike]

Re: Trying to decide on a pitch change.

Aftermarket service manual says 75 @ 5500 with a sub note 78-85 75 hp is rated at 5200.
Typically rpm ranges are 500 below to 500 above.
I think your numbers aren't quite right.Slip is less than 1% at 34 mph.In still water I think the speed is probably about 32
and I think you would need every bit of that 5850 to do that. It does appear that a 17" prop at 5450 would make a little over 32
at a realistic slip.
You might consider a vented Turning Point Hustler prop,they have holes that let a little exhaust bleed over the prop allowing the motor to rev easier on hole shot. So good hole shot with no loss of top speed.More than a Solas but not unreasonable.
You can also vent your own prop.Read up on it I haven't read the details but basically I think a hole for each blade
carefully positioned then you fine tune the holes with various sized plugs.

 

[sutor623]

Re: Trying to decide on a pitch change.

Sounds good. And yeah 34mph was moving with current, 32 was against it. Is the the prop, it is a turning point hustler but I don't know if its vented. Turning Point Hustler Aluminum Boat Propeller #2143 1711 for Evinrude 75 HP Outboard 1975 - 1984 3-Cyl., Pleasure - iboats

Edit. Nevermind Steele it is vented. Thanks for the advice it is only $25 more than the solas. Ill let ya know how she rides after I get it on.

 

[jestor68]

Re: Trying to decide on a pitch change.

You're slightly over reving with 2, with just driver much more, assume you have not tested that way, right ? If going for same size prop but 17 higher pitch probably will be around middle min-max safe working parameters with 3. Some states there's a rpm decrease of 100-150 others 200-300 per pitch, who's right ?

Since the 150-200 rpm change comes from the prop manufacturers, I'd stick with that one.

 

[sutor623]

Re: Trying to decide on a pitch change.

Man guys I am bummed!! I bought the hustler turning point in a 17P and it is vented. Took her out for a spin and at WOT with another person onboard she was turning at 5050 Rpms, at 33 mph. I am clearly over propped. I lost 800rpms by moving from a 15pitch to a vented 17pitch. That is just nuts. Honestly I am afraid to even get a 15p turning point so I can reuse the hub. I think even the 15 hustler will drop my Rpms down too low. Now I'm stuck with a $140 prop that I can't use.

 

[steelespike]

Re: Trying to decide on a pitch change.

I think your tach is not accurate Your low slip numbers should have made us suspicious.
Again with the hustler your indicated slip is very low.
If we use a more reasonable 12% slip at 32 mph your rpm iS 5450.
Perhaps you can borrow a shop tach to confirm your rpm.

 

[sutor623]

Re: Trying to decide on a pitch change.

Hmmm there are no lenders around here. All the shops are small town and don't lend out anything. I hear ya though. It sounds like I am revving a little higher than my tach is reading. Do the tinytachs seem to read accurately? should I just buy an analogue tach and hook it into my harness?

 

[sutor623]

Re: Trying to decide on a pitch change.

Other than that Steele, it is a comfortable ride, nice handling and great grip. Cruising speed feels awesome also at 23-24 mph. I used 5gallons of gas and fished all day. Put two and a half hours on the motor, some of that was 3-7 mph while I was running side imaging to find structure.

 

[steelespike]

Re: Trying to decide on a pitch change.

a lot of folks have had good luck with the TinyTach.Be sure to tell them its going on an outboard.
A new analog should be accurate.

 

[sutor623]

Re: Trying to decide on a pitch change.

The tinytach is actually what I have right now. What do you think about adding some pitch to the prop calculator for cupping? I have heard both ways on this matter. I agree that my slip numbers sound low. The boat was moving precisely at 33.4 MPH according to GPS, in calm waters. At 12% slip that would mean the boat is revving at 5700 RPMs which would be perfect. I guess the other scenario could be that the lower unit could be a different gear ratio if the P.O. changed it. I guess I'll have to do the spin/count test when I get home and see what it is.

 

[sutor623]

Re: Trying to decide on a pitch change.

Steele I checked on a few things tonight. First thing that I did was confirm that my gear ratio is in fact 2.42:1, and it sure is. I turned the flywheel 2 and almost 1/2 spins and the prop made one full revolution. Next thing I did was check my ground wire for the tach, and it was a bit loose. I'm glad I talked to you because you are def correct, there is no way that my slip is 3%. I know my gear ratio is correct, pitch is def 17, and my speed is calculated with my brand new humminbird GPS unit. I am going to tighten up the tax ground and see what type of numbers I run.

On a side note, what would one expect a typical WOT prop slip be on a 16' aluminum boat?

 

[steelespike]

Re: Trying to decide on a pitch change.

Slip will vary based on prop design hull type,weight and construction.
Some al boats are hard to push, probably lots of seams,rivets and convolutions.
I think typical slip would be something like 8 to 12%. Below 8 while possible I think we need to begin to be suspicious.
I have begun to use 12% when I have to guess to fill a empty slot on the prop calculator.
Seems to bring up reasonable numbers.

 

[sutor623]

Re: Trying to decide on a pitch change.

So Steele, I did some more runs a day or so ago, after I attached my ground from my tach a bit better. Tach readings down low seem very accurate. I was cruising with 2 other people on board, and I was at 22mph and 3700 rpms, which clocks in at 11% slip at cruising speed. Boat felt great at this speed, and handled way nicer than it did with the stock 15P prop. Nice grip, great turns, and I did a few WOT runs (which was tough because of some chop) and I managed to get 30mph and tach was reading 4850rpms, which gives me 7% slip. Seems relatively reasonable I guess. I have read that the aggressive cupping and rake of the hustlers have been known to drop RPMs down way more than the usual 200rpms per pitch change. What do you think, should I invest in another tach ($75-100 for a good analog) or spend that money on a new 15P Solas?

 

[sutor623]

Re: Trying to decide on a pitch change.

Steele have you had a chance to read my last post yet?