Revd Up

Hi Steve,
Yes we are Freezing up here! Was wondering if you were following this thread, and yes Janaky's got my Mirage dialed in. I have to agree on the factory rigging. How else can you explain the huge difference in speeds? BTW, those billet battery boxes worked great.
Steve

HPJunkie

Is that steve take it easy snider? whats he know about freezing weather? Greg

tcelano

I agree. From my research on the subject of supercavitating propellers, the blade loading that most single engine powerboats encounter put it on what I call the "low side" of the propeller's "sweet spot". By this I mean that even your best propeller, on its best day, can't exceed about 76% efficiency, and there are combinations of advance ratio (another form of representing propeller slip), speed and loading which result in the prop working well. You can stay at or near this 76% efficiency number by running most props between about 15% slip and 6% slip. Any more slip, and you are throwing water around; any less, and you're dragging too much blade through the water. As you fall off the prop's design point, sometimes you get horrible performance and efficiency. Look at the slip ratios of some of the normal high-performance rigs while they are running slow, at say 40 mph. Some of them see 40% or more slip at those speeds, and efficiencies down in the sub 50% range. That's the answer to the question of why many boats get as good or better fuel economy cruising near WOT than they do poking around. Single engine boats, and some really heavily laden twins run down at more than 15% slip (some as high as 20% or more), while the well set up twins hook up and run at about 8%. The unfortunate reality of all of this is, especially if you are dealing with a Bravo drive, there just aren't a wide variety of prop diameters available, so you are really using the "parts bin" approach to setting a boat up. SSM folks have more options to get the "right" prop for the application.

There are cases, mostly on big ships where the "wake fraction", or water entrained by the ship causes the apparent advance speed of the propeller to be lower than the surrounding free stream. These cases cause the prop slip to look very low, but a careful wake survey will reveal that the prop is still just doing what it is supposed to, and there is no magic or free lunch. High-speed powerboats don't have enough hull in the water to encounter any significant wake fraction.

That's a lot of words to say, if someone reports a top speed of a single engine boat that results in less than about 14% propeller slip based on the engine RPM and drive gearing at WOT, I would immediately raise the BS flag and start looking for hard, cold GPS numbers.

502SCS

Hello,
Wow this is an education and I mean that! This is why I recently joined this group. So much of the internet is junk and no fact comments that is nice to see real debate and after that last post ... Well over my head for sure so if it is BS you got me because it sounds good :-)
Anyway with that complementary commentary out of the way ... Thanks for the education guys.
In trying to explain why I saw my results and to in my own mind prove that I had not lost my mind on my resent GPS run of 76 in my 97 23' Scarab SCS w/502-415 turning just over 5000 RPM with a Bravo 1.5 I assume and 25P Mirage Plus I did do is look at these tools and start using them ( http://www.rbbi.com/folders/prop/propcalc.htm ) .
The results I got were 5200+1.5+25+.14=70.6 So I see where this makes little sense.
I just purchased a Mirage Plus 29P for a future upgrade ( part of the Procharger + 29P has got to be great upgrade ) so I ran the same calculator with that prop and came up with 81.9. How does this sound guys?
Thanks again for the great input.
Wayne

boatn70

wow, this is a killer thread!! we've even got good local coverage here!!

steve, you gonna procharge that thing or what?? i think thatd be a blast!!

good info here for you though.....see ya

kevin

Revd Up

I recently saw a couple of 22 scarabs for sale on another site. One had a procharger on it and ran upper 80's gps, the other had a whipple on it and ran low 80's gps. BTW 502SCS the calculations don't work out right for my boat either! And yes, I'm considering a procharger.
Steve

502SCS

If you get to the project before me share the wealth of info and I will do the same in return. When you said the calculator did not work for you ?? Does yours run faster on the GPS than the calculator says it should? Do you have a 502/415hp?
Wayne

Mudball

There is a good prop slip calculator at Bam Marine, at http://www.go-fast.com/Prop_Slip_Calculator.htm
===
Sorry, but some numbers from this thread are not physically possible.

502SCS, if you are turning around 5,200 rpm, then your 76 mph speed is around a 7% slip! That is VERY good. I am impressed!

I have never seen below 10% slip with a Bravo 4 blade prop. My old Mirage Plus was at about 13%. The Donzi does not like the 3 blade though, as it also has a relatively high 6" X dimension, and is also aerated by the step. I could hardly get on plane with the Mirage.

Take care,
Kent

robyw1

modded 454-MPI 1.36:1 Bravo (why 1.36 I do not know) Houston Prop Accelerator 25-P 73.5 MPH @ 4400 RPM on the GPS

Roby

302SR1

Robyw1's numbers look very accurate.

I have a close friend with a 22. He's got a Flagship 510ci (basically a stock rebuild 502) with Gil Exhaust.

Hes got a huge stereo ( 4-15's under the bow ) so balance is way off.

He see's about 75 most of the time and 76is on a good day