starting from scratch, v- bottom bowrider prop selsction? pitch and diamiter?

acecv31

Cadet
Joined
Aug 1, 2011
Messages
11
starting from scratch, v-bottom bowrider prop selection, pitch and diamiter?

1995 starcraft v-bottom bowrider
120hp force outboard
17'2" lenght
beam 82"
max depth 30"
dry weight 1363 ( i think the boat is on the heavier side, i believe it to be some wet )
fuel cap. 24 gal.
gear ratio 2.70:1
full rpm 5250

#1 i bought used with a 20" quicksilver, it did approx 42mph at 4700 rpmsand sluggish out hole.
i tried getting repitched to a 18" with a cup.
that was terable, felt like bushing spun, rebushined and same thing happened, as soon as it starts on step, slipping, rpm tacks out. if i play with trim, i can get on top with very very light load, top speed aprox 33mph at 5000rpm

very confusing

# 2 while quicksilver in shop i borrower a power tech 17" pitch, much better, but only 37mph top end at 5150rpms and jumps out the hole.

i repitched to get my rpm up to maxium 5250 inorder to get a faster cruise speed.

im lost on what happened to my quicksilver.
So what would be a good idea for a new prop, make, pitch and diamiter? i need a spare anyway.

any help or guidence would be greatly appreciated.
 

craze1cars

Lieutenant Commander
Joined
Dec 26, 2004
Messages
1,822
Re: starting from scratch, v- bottom bowrider prop selsction? pitch and diamiter?

The prop shop ruined it. Sounds to me like they simply did a poor job of repitching and adding cup. In their attempt they wrecked the geometry of the blades and introduced a whole bunch of slip. I have no idea what kind of "Quicksilver" prop this is that your baseline figures came from....Aluminum? Stainless?

I feel repitching is a fine-tuning tool. But it should not, in my opinion, be used to completely reshape and re-engineer a prop as drastically as it sounds like they attempted to do.

You needed to increase your rpms by 550. Removing 2" of pitch might get 350 to 400 of that accomplished. But adding cup? That takes a bit of of those rpms away. So I'm not at all surprised you ended up at about 5,000 rpms. And I have no idea what you were trying to do when you asked them to do two things that counter-act each other in terms of RPMs, and therefore had no prayer of getting you to your 5,250 target...or why they didn't try to talk you out of it when you made your request....

If 5,250 is your target, it sounds to me like you should be trying to purchase approximately a 16" pitch prop for your boat, but this will vary a bit by design/brand/material, and still not sure exactly what you started with. There isn't a prop shop on this earth that could redesign a 20 pitch prop to make it act like the 16 your boat probably needs...you simply need to buy a different prop.
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: starting from scratch, v- bottom bowrider prop selsction? pitch and diamiter?

You typed your ratio at 2.70 it is 2.07.I did some very basic math and it seems to me that your 17" prop at 37 mph is about eight.
A 2" repitch is a lot and it appears they messed it up probably either forcing it too far or cupped wrong.
Your antivent plate(just above the prop) should be about even with the bottom or up to about an inch above.In the extreme this could net up to a couple of mph.
Be sure the motor is in excellent tune and the throttle opening all the way.
If you think your hauling some waterweight that could slow your top end and hole shot.
Your Force is rated at the power head so hp at the prop could be up to 10% less.
You sometimes see an over propped motor make a little more speed.
 

SparkieBoat

Captain
Joined
Aug 17, 2009
Messages
3,643
Re: starting from scratch, v- bottom bowrider prop selsction? pitch and diamiter?

have a 110 rude on a 17.5 bass boat, 88' model on boat and motor. it will spin a 17P SS prop close to 6K, but my motor is jacked up as high as I could get it, and my motor is in tip top condition.
 

acecv31

Cadet
Joined
Aug 1, 2011
Messages
11
Re: starting from scratch, v- bottom bowrider prop selsction? pitch and diamiter?

thanks craze, i feel they ruined my prop also. a local boat mechanic told me thats what my prop needed. im unfortunatly just now understandin a little more about props.( expencive lesson )both props are stainless.

i had no idea they were countwer reacting each other.

another question that may determine my prop mans credintials.
when i gave the same information to him, he told me that power tech 17" was equivilant to quick silver 19" pitch, is this fact?
 

craze1cars

Lieutenant Commander
Joined
Dec 26, 2004
Messages
1,822
Re: starting from scratch, v- bottom bowrider prop selsction? pitch and diamiter?

I don't personally know the exact differences between those two props. However your own testing results reveals that they would not be equivalent...you compared a 20 QS to a 17 PT...pretty darn close to comparing a 19 QS to a 17 PT.

If he says a 19 pitch Quicksilver is same as 17 pitch Power tech, then it should be expected that your 20 Pitch Quicksilver rpms should only be about 150 to 200 rpms slower than the 17 pitch Power Tech, because in theory they would act like a 1 inch pitch difference when compared to each other. However this was not the case. Your unworked 20 pitch Quicksilver ran 4,700, while the 17 pitch Powertech ran 5150. That's a 450 rpm increase...not quite as much as I would normally expect from 3 inches of pitch reduction (typically I'd expect about 175 rpm per pitch = 525 predicted reduction from equivalent props...but 450 you experienced is pretty darn close to that.

So based on those results, I would say indeed he is wrong in claiming that a 17 Powertech is equivalent to 19 Quicksilver.

To illustrate specifically why I stated that his prop work negatively impacted your propeller, use a prop slip calculator. http://go-fast.com/Prop_Slip_Calculator.htm
When I enter your figures, assuming they are accurate, your 20 pitch prop originally calculated at 2% slip. After he reworked it to an 18 pitch, it now calculates to 20% slip. This mathematically proves he introduced substantial inefficiencies to your propeller. And again, this is only true if your figures provided in this post are 100% accurate from both your before and after runs.
 
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