Prop Slip Confusion???? or lack of HP

Prop Slip #'s are pretty high

I'm thinking I don't have enough HP for the weight of the craft.

96 Pro Craft V180C 18' Fish and Ski rated up to 175 HP but original owner lived on a HP restricted Lake, dealer installed a 115 Merc/Mariner 2+2

NADA claims the boat to weight in at 1465, this # may be with a 175 instead of the 115 but that shouldn't be more than 100 difference.

3 batteries, 30 gallons of fuel, my fat @$$ 275, maybe 2 bags of ice and a 6'r of 16's, total weight right around 2100??????


115 Mariner Engine is tuned to a 'T', the 2.07 gearcase

Prop 1 is a Lazer II SS 3 blade 13 3/4 23P 38 mph gps @ 6100 slip 38% ?????

prop 2 OMC Shooter SS 4 blade 13 1/2 24P 42 mph gps @ 4800 slip 18% ????

Both props are vented, the holeshot with the Lazer is astounding, literally JUMPS out of the water @ 6500, noses over and bites, rpm drops to 5400 then rapidly accelerates the boat to 6100 @ 38 mph

Holeshot with the OMC Shooter is nothing to brag about, I am drilling out the vents step-by-step and making steady improvement.(I only run this prop to get these #'s then put the Lazer back on)

I'm sure the Shooter is too much prop and wonder just how much pitch the prop shop can take out of it to get WOT RPM up

and/or

should I have the prop shop ADD cup/pitch to the Lazer?

OR........

should I remount the 115 onto my Starcraft and find a 150/175 to hang on this ProCraft?

Re: Prop Slip Confusion???? or lack of HP

Ayuh,... A Good Prop Shop should be able to beat an inch outa the pitch on the 1 prop,...
And,...
They could also add some cupping to the other, to bring down the Slip factor...

Re: Prop Slip Confusion???? or lack of HP

Unless I'm missing something both those props seem like a lot for a 2.07 gearcasae.? Especially for the Lazer to "jump out of the water"
Unless that is the Lazers specialty.
Some times they remove a little cup to improve rpm with very good results.

Re: Prop Slip Confusion???? or lack of HP

Charlie, Spike is right as always, I just ran some preliminary numbers in my program on your boat and motor and you should be running a 19" pitch prop, just depending on what the blade geometry of the prop is, and NOT a 23" pitch.

You didn't say what year your motor was but I show your motor should have a maximum RPM range of 4,750 to 5,250 RPM at WOT, and I also show your gear ratio is 2.07:1, please verify this.

I haven't put any thought into this yet, but my first guess is you have a tachometer that is WAYYYYYY off, you need to get a mechanics tach and check your RPM. Because there is NO way you are turning that many RPM, unless these props are actually out of the water, or you have a problem upstream of the prop. Your Effective prop pitch is 13.6" and your prop slip is 40.8% with the Laser II, and Laser II's don't produce that much prop slip, UNLESS you have something underneath the boat creating MAJOR water turbulence in the prop stream, like Kymasabe has in his posting http://forums.iboats.com/showthread.php?t=386572.

Please take about 4 pictures of the ant vent plate showing the transom and using a straight edge as outlined in the following article at number 1. Also take some pictures of the bottom of the boat like Kymasabe did on his posting.

You might try reading this for a better understanding of changing props for better performance, and for the major differences between aluminum props and stainless steel.

http://forums.iboats.com/showthread.php?t=369057



H

Re: Prop Slip Confusion???? or lack of HP

I'll stick my inductive tach on it again next time out, could be a while, I don't get out much any more.

No probs with the hull, straight, so I' stuck wondering if the boat is too fat for a 115 to perform with acceptable slip #'s.

Re: Prop Slip Confusion???? or lack of HP

nada weights are the boat only.....not rigged, not w/ob and no fuel

Re: Prop Slip Confusion???? or lack of HP

So that figured 2100 lbs is shy the weight of the 115 and controls?

I already added batteries, fuel, ice, me,

oh, and need to add maybe another 50 (?) or 150 for the anchors, lines, bumpers, ski's, and spare tow lines, 2nd prop, extra gallon tcw-3.

1996 Mariner 115 ELPTO NADA claims 368 lbs dry add (?) 15 for gear & TCW-3

Thats 393, call it 450 with controls, plus the 150 add'l listed above

that makes total weight of the craft in the water in the area of ???????

2700, getting pretty fat for a 115 to push around efficiently, I'm pretty sure my WOT #'s are good.

Re: Prop Slip Confusion???? or lack of HP

2,700 lbs is not excessive.With a healthy motor and a well setup boat you should be capable of of over 30 mph with a cruise of about 26 at about 4 mpg.
If your boat has an extremely deep V it might be a little less but still in excess of 30 mph.

Re: Prop Slip Confusion???? or lack of HP

Fish & Ski Pro Craft is a Bass Boat with a couple more seats, flat pad bottom hull.

I've already raised the motor mounting 2 bolt holes above dealer mount so the cav plate is almost 3 inchs above even with the pad. There is a just enough set-back in the hull/transom that there is no blow out problems until your very hard over in a turn.

Keep a sharp eye out for logs and should not have to turn that hard.

I'm not really concerned with top speed, 35 - 40 is OK, 40-45 could be a bit better.

The boat from the manufacturer was spec'd for a 150 - 175, I'm wondering if the 115 , less power, is the cause of the slip #''s?

Most the prop calculators found on-line simply figure pitch, rpm, speed, without considering the weight/displacement of the water.

HWSII has one of the best programs around that uses weights, which is why I posted here, to gather all of your guys experience before I make any further changes.

Re: Prop Slip Confusion???? or lack of HP

Charlie, I thank you very much for the kind words.

The boat from the manufacturer was spec'd for a 150 - 175, I'm wondering if the 115 , less power, is the cause of the slip #''s?

Click to expand...

Theoretically you are right, that not enough horse power for a heavy load can create higher prop slip, this is proved everyday by Heavy cruisers, like the big Sea Rays with a single 260 HP engine. But your boat is not that heavy and has a low power to weight ratio, it also has a pad bottom which really helps attain maximum speeds for minimum HP.

Your boat only has a power to weight ratio of 22# per HP, whereas heavy cruisers have power to weights of 30 to 40 #. So, in my opinion, you should not be having that much prop slip.

To the best of my knowledge they never made a Shooter prop in a 24" pitch. If I recall right the 23" pitch had a 13.5" diameter and the 25" pitch had a 13 1/4" diameter and they were only offered in odd pitches, so to have a 24" pitch shooter should mean that it had to be repitched, please check this, but Of course I am NEVER wrong UNLESS I am breathing.

I ran some new numbers on prop pitch and if you will look at them I think you might have a better understanding of what I am trying to say. Even with the Shooter 4 blade you still have a prop slip over 20% and if you will check other bass boats even with low HP you will see that they don't have that kind of prop slip even with 3 blade props.

My numbers show that you can get 40 MPH with a 19" prop and 12% prop slip running at 5,250 RPM, which is what I show your motor should be turning, and that is a more reasonable prop slip.

Prop Slip


If you will look at the chart above you will see that the Laser is only producing an effective prop pitch of 13.6" and the Shooter is producing an effective prop pitch of 19.1", but by using a 19" pitch that is turning at 5,250 RPM and 40 MPH it is producing a 16.6" pitch, but only has a prop slip of 12%. Effective prop pitch, combined with the correct RPM, is what really matters in props for maximum speed. This is a little known perspective in the world of props and is very misunderstood and not utilized the way it should be and not near as much as it should.

I know that is a lot of information to digest, but in my opinion if you will move your motor down 1 notch you will see what I am talking about. I know that is counter intuitive to the norm, but in my opinion that is why your prop slip is so high, when you move the motor down you will find that your prop slip will drop dramatically.



H

Re: Prop Slip Confusion???? or lack of HP

OK, I won't necessarily say that my memory is FAULTY, but it IS selective.

I went out and looked

The Quicksilver Laser II is NOT 23P it is 22P

The OMC Shooter is NOT 24, it is 23P

The a/v plate measures 2 & 1/2 inches above the pad bottom

Gearcase setback/leading edge is 16 inches from deadrise, rear of the pad.

This may reduce the slip #'s slightly, but still nowhere near 10 - 15% range.

Which is why I am wondering if I am flogging a dead horse until I bolt on a motor with half again more HP.

There IS a method to my madness, if I tell my other half that I NEED more HP she'll think that I need another hole in my head, whereas, if I can SHOW HER scientific #'s that PROVE I need more HP, she'll just claim we can't afford it.

Re: Prop Slip Confusion???? or lack of HP

If you are doing this to get another motor, just let me know and I will make you up a set of numbers to prove your point charlie.




H