Upped the pitch and Baffled by the results

rusty McCaw · 30 December 2014, 09:19

So I've got a small rib/tender 3.2mtr alloy hull, with a 25hp rude....it goes pretty good 26knots.... But the the prop was pretty shaggy so I tidied it up with I metal file then wet and dry sand but thought I would get another prop to have as a spare

The original prop was oem 10/13p the prop I purchased cheap second hand but good condition is 10/15p also oem

My expected results were to lose some bottom end but gain up top so I held off using this prop for a couple of weeks while I've been pulling the kids around on toys and playing in the surf. I have now changed to the 15p,prop and am super surprised that I've gained a nice pace at idle, a better smother speed at cruze, faster pop out of the hole and way more usable power low and mid range, it seems I've actually lost a bit of top end but I am not at all fased, but totally confused.. The 15p prop is now staying put intill I can figure which the best setup for me will be???

I am thinking a new 13p with some cup to have as a spare and surf prop?? Any advice would be appreciated

Hightower · 30 December 2014, 10:38

By using a more aggressive pitch the outboard will be revving less at WOT (wide open throttle) and thus you probably wont be in the ideal rev range for the outboard. The result of that would mean that there's not enough grunt from the outboard to push you to a higher top speed.

With regard the better hole shot from the 15" prop. Well all I can say is that your older prop which has obviously sustained some damage in the past is past it's best. This probably explains why your 15" prop is outperforming it.

A 2" pitch change on a 25hp outboard is a massive change, I would recommend looking for another 13" prop which should claw back your top speed and reveal a better hole shot to boot.

rusty McCaw · 31 December 2014, 04:21

Yea I totally agree with the advice thankyou, I will be getting another 13 in the near future and will continue to try and the ultimate setup so wether it's a slightly cupped 12 or 13 I'm not yet sure. Hopefully I will be able to track down a outboard wreaker with the stash a stash of cheap props. Stoked on the 15p and its performance for now

jyasaki · 31 December 2014, 06:11

Agree with Hightower. You really need a tach to see what's happening. Even a temporary clip-on auto tuning type would do, as you only need readings at wide open throttle.

jky

rusty McCaw · 31 December 2014, 07:42

Tiny tachometer ordered and payed for....will be interesting to see how well my ear is tuned to my motor

tuskar · 31 December 2014, 13:11

Hi Guys , What are the indications of a too low pitched prop . My prop pitch is 9x11 on a 12 ft with 15hp johnson . My rib seems to take of rapid and revs out a roughly half throttle !
Thanks

Nos4r2 · 31 December 2014, 16:45

Quote:

Originally Posted by tuskar

Hi Guys , What are the indications of a too low pitched prop . My prop pitch is 9x11 on a 12 ft with 15hp johnson . My rib seems to take of rapid and revs out a roughly half throttle !
Thanks

That sounds like you might have a spun hub. How fast is it going?

tuskar · 31 December 2014, 17:21

Hi
Dont think so, can go 18mph with two large adults 80kg each . just feels like I could pitch it up 1 degree and maybe reduce noise level and revs ? Any advice welcome.
Thanks bud.

rusty McCaw · 31 December 2014, 19:04

Tuskar, you might be rite the only way to know is properly read you rev range with a thacho.. I know with my setup I was at over rev with my old prop still having a bit of throttle left on the twist grip and sound of the engine reaching a point where I would back off. My new prop is much quieter and smoother thru the rev range and now am reaching full throttle on the wrist without the engine screaming...

tuskar · 31 December 2014, 20:39

Pete7 · 04 January 2015, 20:27

Quote:

Originally Posted by rusty McCaw

I am thinking a new 13p with some cup to have as a spare and surf prop?? Any advice would be appreciated

Don't over look the fact that you were using a shagged 13" prop before, so a new 13" should really show an improvement.

rusty McCaw · 05 January 2015, 03:02

Agreed, might possibly have to go to a 12sst with cup, all will be reviewed when the tacho arrives. Anybody know what the surf boats lifeguards use?? Usually 30hp mercs here in NZ but no stamps on their props

rusty McCaw · 24 January 2015, 03:52

Prop geniuses please chime in, sorry I know it's been awhile since I last posted, but the tacho took longer than expected and well the surfs been pumping....any hoo..... Out on the water today with a little ruffle and a lightish load...top revs achieved were 5280rpm while flying over little lumps and normal top revs in the flats were around the 5180....

I don't know the exact top revs required for my donkey but am sure it's around 5500/5600rpm so would down pitching back to the 10/13p be on the money?? And would a sst option mean I would have to drop another " in pitch?? Or not due to cupping?

Evinrude 25hp
1998
2cyc single carb
Omc prop 10/15p

jyasaki · 24 January 2015, 17:20

You'll lose a few revs (100 or so) just by going stainless (added weight to turn.) That assumes the props are otherwise identical (which they won't be.) Figure on losing more revs if there's cupping on the SS prop.

jky

C-NUMB · 24 January 2015, 19:00

[QUOTE That assumes the props are otherwise identical (which they won't be.)

jky[/QUOTE]

So right, its sometimes frustrating when testing props and the results is not logical, one of the challenges is that the pitch can be pretty much off compared to what is reported... And even if the pitch is right, still different diameter, cupping, thickness of blades can make unexpected results. So what looks good on paper might not work so well in reality. So real word testing is only way to make sure.

Roflhat · 27 January 2015, 14:00

What's the gear ratio in that engine? In the surf there's not really any facts or figures you can go by when prop tuning, it's just how it feels.
If you're into top speed prop tuning you can use speed, revs, engine height, trim etc. Have you got a GPS you can use for speed readings? you might end up going faster with the 13p
You should check out boatracingfacts, they know a lot about tuning. If you're not on the rev limiter (if there is one) then going down in pitch is a good idea for the surf, better acceleration and not much loss in top end if you're still revving. Definitely go stainless though, better performance all round

rusty McCaw · 27 January 2015, 22:27

Thanx guys, yea I'm chewing things over and I'm leaning towards a SS prop due to my uses with the boat.., I'm not to worried about losing a tad up top but definitely can't afford to struggle down low when heading out to spots in bigger swell, playing in the surf , or pulling kids on toys....

My mech told me that my max Rpm,s for this engine are 5200-5500 so I'm not far off the mark, I think 13p SST with cup will be the ducks nuts...

I'm toying up between the Solas Saturn and the Michigan ballistic 3blade,,,,what way would you lean??

Sorry I don't know the gear ratio for this engine? It's short shaft

rusty McCaw · 27 January 2015, 22:37

2.15 gear ratio??

Roflhat · 29 January 2015, 16:24

Really depends on your boat and what you use it for, I only really know about thundercats.
SOLAS make good allround props, not so many people have tried ballistic but apparently they're pretty decent. Maybe even better in the surf due to the cut on them, more like a semi-cleaver. Mercury props are just about the best out there if they fit your engine. They make a prop called a vengeance which is great in the surf.
the order I'd put them in
1: Mercury vengeance 10.5 x 13p
2: SOLAS 13p
3: Ballistic 13p

Whichever you get cup is usually a good idea for more grip, but try it out the box first. this is all just my opinion