Prop slip question

[Tacklewasher]

I've just put an Evinrude 115 on the back of an 18' Starcraft SS.

Same motor and props that I tried to figure out in this thread a while ago.

http://forums.iboats.com/showthread.php?t=178243

Different boat though.

Saturday with the stainless. 13X19 OMC prop. WOT of ~4800 rpm and speed of 38 mph (gps). I figure 12% slip but not happy with the max RPM. Could trim quite a ways up without it ventilating, but didn't really try.

Sunday with the 13X15 aluminum. WOT of 5800 rpm but speed of 34 mph (gps). Same load (me and the wife), motor not moved and only the prop changed. This gives me about 17.5% slip. Now I really played with the trim on this run and found I had to be almost at the tilt before I heard any ventilating. As I was pretty close to max rpm, I trimmed back in at any noise.

So my question is, why would the two props give such different slip numbers on the same setup? Does the stainless to aluminum make that much of a difference?

My plan is to raise the motor one hole as I really could not get either prop to ventilate when being aggressive with the trim. I really want to see the motor spin at least 5500rpm. If I can do that with the 19, I figure I should see 45 mph. If not, I'll try to find a 17 pitch prop.

Does prop size matter? Someone has a 13 1/4 X 17 prop for sale locally.

 

[Dhadley]

Re: Prop slip question

You're on the right track by raising the motor and using the 19 ss. I think you'll end up higher than the one hole though but you have to start somewhere.

 

[Tacklewasher]

Re: Prop slip question

That's the plan for tonight. I'll let you know the results.

 

[Tacklewasher]

Re: Prop slip question

Well. That was anti-climactic.

Pretty much the same results as the last run with the 19p stainless. It was a little rough so I don't think I got everything out of the boat and maybe could have hit 39, but still only ran at 4800 rpm. Somewhere around 10% slip.

And I could not trim out as much before ventilating.

I think I'm looking for a 17p prop. So what would help get my rpm's up without losing too much speed. I really am not looking for a lot of speed

I figure a good stainless. I could use a little bow lift, but that's not a big concern. Just felt a bit bow down is all. But I was pretty light on fuel and I do intent to add a fuel tank in the stern.

Any help is appreciated.

 

[Tacklewasher]

Re: Prop slip question

Re-read my old thread so I'm thinking a 17p Stiletto is in the works. Now to find someone who will let me try it first.

 

[Tacklewasher]

Re: Prop slip question

Wow.

Never thought it would be this hard to get a price on a prop.

Pretty disappointed in the local shops right now. Went to three and called a fourth. Maybe it's the labor market, but sure did not get much help or input. The guy who lent me props a couple of years ago is gone from that store and the kid behind the counter was too busy talking to his buddies so I walked out. Only one shop did someone try to really help, but they are pretty much Merc exclusive and while they did have an OMC hub kit ($61), the guy seemed unsure about what I needed.

If this is mostly a fishing boat, should I just get an aluminum prop and save $400? I'm really keen on hitting 40mph, as I think this boat should do that with 115 hp, but am mostly interested in keeping the motor in the correct rpm range.

There is a 13X17 aluminum used for sale locally for $50, but I'd be worried about having to do the hub again.

Any advice on the Alum/stainless for a mostly fishing boat?

 

[Tacklewasher]

Re: Prop slip question

Update and question.

Motor has been moved up two holes.

Prop replaced with a new 13X17 aluminum.

Slightly heavier load as I just came back from a run with a friend and two kids, but this is probably closer to a normal load for me, as I will have a kicker, more fuel and downriggers most of the time.

5000 rpm at 37 mph. This figures to about 8% slip.

The only time the prop ventilated was fully trimmed up, and starting out at full throttle. Once the motor caught, the ventilation stopped. At WOT and at speed I could trim all the way up and ran the fastest with the trim there.

Do I raise the motor to the last hole?

I have a 13X15 Stiletto coming from ebay
http://cgi.ebay.com/ebaymotors/ws/e...76267455&ssPageName=ADME:B:EOIBSA:MOTORS:1123

so I figure that will move my rpm up.

Will this prop ventilate sooner?

And I looked at the prop I just tried (13X17) compared to my old aluminum and I swear there is twice the blade. I guess that other one had been trimmed down for something, but it just doesn't have the blade area any more.


So, do I try it one more hole up?

Thanks for any advice.

 

[Tail_Gunner]

Re: Prop slip question

You're on the right track by raising the motor and using the 19 ss. I think you'll end up higher than the one hole though but you have to start somewhere.

Pay close attention here.....Going to stilletto ss will be by far different than a cheap alum..