New prop..Perfect setup?

[Toonedup]

Ok, guys. Ive been trying to dial in the prop on my 08 Riennel 230 with 5.0 gxi (max wot 5000 rpm) with 1.6 sx-a outdrive ( changed from 1.79 to 1.6..hence the reason behind this post) and Nautilus smart tabs set at 4. results are as follows:
Prop 1 = 14 1/2 x 19 oem 3 blade alum. (came on 1.6 outdrive) 4200 rpm wot, 38 mph, 9.4 sec to plane @ 3300 rpm and 19 mph, cruising at 3500 rpm 21 mph
Prop 2 = 14 3/4 x 17 3 blade ss g3..4500 rpm wot, 41 mph, 7.3 sec to plane @ 3100 rpm and 21.4 mph, cruising at 3500 rpm 23.6 mph
NOW FOR THE BEST PART!!!
Prop 3 = 15 1/2 x 15 3 blade ss( Power Tech OFS3R15 )..4850 wot, 49.6 mph, 4.6 sec to plane @ 3000 rpm and 25 mph, cruising at 3500 rpm at 30.3 mph.

If my calculations are correct thats -4% prop slip!!! Also getting about 1.5 feet of stern rise which got the bow up to handle the rough water on Lake Texoma, is it normal to have negative prop slip, am i calculating wrong, or is it the perfect setup?
Thanks.

 

[Faztbullet]

Numbers wont lie... lowest I ever got from my Bullet was a 2% and that was a labbed prop($2K)

 

[WesNewell]

It's virtually impossible to have a negative prop slip. A 15" pitch prop can't go farther than 15" in one rotation in calm water. Winds and currents could cause variations in readings. Other causes for - slip reading include incorrect tach reading or setting and incorrect speedo. Max theoretical speed for a 1.6 GR drive and 15P prop is 43 mph. Anything over that and you have some wrong readings.

 

[Toonedup]

It's virtually impossible to have a negative prop slip. A 15" pitch prop can't go farther than 15" in one rotation in calm water. Winds and currents could cause variations in readings. Other causes for - slip reading include incorrect tach reading or setting and incorrect speedo. Max theoretical speed for a 1.6 GR drive and 15P prop is 43 mph. Anything over that and you have some wrong readings.

MPH was measured at speedo on boat, gps on Lowrance Elite 5sonar, and gps app on phone..phone and sonar showed 49.6, speedo showed about 48 1/2. i guess tach could be off, but that doesnt account for the extra 6 mph...5.0 builds the best torque around 4800 rpm, you can feel it wanting to fall off around 4900 thats why i assume tach reading is correct.

 

[WesNewell]

What about water current? River, Ocean, or lake with no current. Don't know what's wrong, but something is as you can't exceed the laws of physics. 49.6 mph with 1.6gr, 15P prop at 4850 rpm is actually a -15% slip. So yes, your calcs are way off.You need ~5800 rpm to get 49 mph with a 15P prop with 3% slip. Now that's doable if your engine can do it.

 

[Toonedup]

What about water current? River, Ocean, or lake with no current. Don't know what's wrong, but something is as you can't exceed the laws of physics. 49.6 mph with 1.6gr, 15P prop at 4850 rpm is actually a -15% slip. So yes, your calcs are way off.You need ~5800 rpm to get 49 mph with a 15P prop with 3% slip. Now that's doable if your engine can do it.

In a larger cove on Lake Texoma..pretty smooth on the water that day. I know its theoretically impossible, but i know that motor wont wind up much past 5000 rpm without noticable change in performance, i also had a friend verify tach on previous test runs with scanner, which is why im stumped. I appreciate the input but i cant find anywhere in the data i gathered that says otherwise. i havent verified gears on outdrive, just going off tag..it was used when i purchased it off ebay when the 1.79 died on me, so maybe thats the missing link. Thanks again.

 

[QBhoy]

Tachometer and speedo are telling you fairy tales on all accounts, I’d think.
best slip my boat has even had was 2% once and a few 3%. That is from a serious performance hull with apex steps and a pad.

 

[Toonedup]

Tachometer and speedo are telling you fairy tales on all accounts, I’d think.
best slip my boat has even had was 2% once and a few 3%. That is from a serious performance hull with apex steps and a pad. [/Q

How bout the two gps readings?

 

[WesNewell]

It's real simple. you need a GR of under 1.4 to achieve what you say you get. So one of the 3 things is not correct. Or combination of 2 or more of the 3 are wrong.

 

[Critter getter]

Ok I have a 1448 Jon boat I have only used it at 3500ft elevation it is a 2004 boat and Mercury big foot 25hp the prop that is on it is a 10.75 x 11pitch with me 180lbs and my wife 1ice chest a80 lb dog and some fish gear and 6 gal of gas it at wot.went 30mph according to the fish finder I now live near Havasu az elevation 500ft prop no good here so I am gonna put a Michigan wheel vortex 10 x 16 on it I'll let you all know how that works

 

[QBhoy]

Tachometer and speedo are telling you fairy tales on all accounts, I’d think.
best slip my boat has even had was 2% once and a few 3%. That is from a serious performance hull with apex steps and a pad. [/Q

How bout the two gps readings?

Well it’s possible that one of your GPS readings might be close (although I apologise for not noticing that you said they were gps)....but for almost certain...your speed of 49.6 mph at 4800 odd rpm...is without doubt not a possibility. Especially with that prop, ratio, engine rpm and hull. I don’t doubt she will be a boat that will go well once you get her dialled in...but I’d perhaps suggest that should you get a prop totally dialled in and with a low slip...you will be almost certainly doing so well to see anything like 50 mph.
although many many skippers will claim 50 and even 60 mph....in reality it takes a special boat and with special power to see anything over 50 mph in gps terms.
60 mph is another level of performance that 90% of boats will never see. Anything over 60....is incredible.
ive been trying to touch the magic 70 now for 5 years. I’ve eventually given in at 67.3 mph GPs. With an estimated 310/320ish (at most) HP...in an 18ft apex and stepped hull with a pad....I think I need at least another 50-100hp to gain just 3/4mph to see over 70. That’s how hard it is to see marginal gains over 50 !

Anyway. I’m ranting on. Have some experiments with good 3 blade props...and I’d be pretty sure you’d need to be starting the process with at least a 17”. Good luck.