How do you determine what pitch to get?

[Riverman]

All of the prop caculators I have seen contain 4 variables - RPM, pitch, speed and slip. As far as I can see, this is only good for finding what slip you have on an existing setup. How do you pick a prop pitch to start with if you have no idea what slip value to enter?

This is for a Viper/140 V4, small ear chopper.

Thanks, RM

[SUPERBASH]

10% is the # to use on performance boats. It gives good all around performance.
-Prop slip is the #1 Killer of speed. Performance suffers if prop is too big & top end speed can't be reached if too small.
(Bash has quite an extensive TrophyPlus collection.)http://img308.imageshack.us/img308/6416/bash5tk.gif (http://imageshack.us)

http://www.boatramp.com/prop_applet/PropAnalyzerAppletG.html

[chris_lacey]

RM, on an unknown set up you have to take an educated guess to start and that will give you a base line to make a final choice(s).
Luckily, a 140 on a Viper is not an unknown combination. You will need something in the 14X26P range. A merc small blade chopper, OMC hub and weedcutters removed will likely put a smile on your face. A 14X26P merc cleaver would also work, not alot of of bow lift required on a Viper.

[Riverman]

Chris, do you think a little motor like that could pull 26P?

Bash, thanks a lot for the link.

[chynewalkr]

you should be able to turn a 26p, i run a 25p mazco re3 which has a bigger ear than an inline chopper at 6200rpm on my vandal. it has a 115hp v4 with 140 heads and 1-3/8 carbs, the 2 boats are similar in weight.

theres a 23 srx that will bolt right up in the props for sale sections here, that would also be a good starter prop, better to start low and go up. The lower pitch props make good props for carrying weight

[dma251]

This might not be the right place to ask, but I am curious if my math is right.

My 2.5 260 is turning my 26 Lightning up to 8530 rpm, and running out at 106 mph. According to the calculator that comes to a slip % of around 5.8.

Can that be right? It seems awefully low to me.

What kind of slip can I expect when I get a 30 cleaver?

[150aintenuff]

light boats like STV's can have slip as low as 4-5% it isnt uncommon, But generally 10% is ROUGHLY normal

ALSO did ya add 1" for any cup in the prop... at 27P I get Right at 10% slip and at 26 I get 5.9% so it would be safe to say you had around 7-8 % if ya split the difference... but if ya know the ACTUAL (most people dont) pitch of the prop you very well could have slip as low as 5%.. and if ya do indeed only have 5% I would say yer set up prety good...

[hsbob]

had a 115 on a 15' checkmate. it turned the stock 21 omc SS [20"real pitch and 1"cup] at 6000rpm. with the 140 parts a 24-25 would not be out of the question, esp with setback and/or hoist.


did you add the tuned ehhaust bubble if not do. its worth atleast 2/3 the difference between the 115 and 140 [pre prop hp ~~~~ before 1984 era]

[Paul Vaillancourt]

Riverman,
My V-4 runs nice with a 26pitch x 13.75"dia mazco re3.
Take off is a little slow when carring a large passenger definately not a ski prop.

[Riverman]

Paul,

I thought 13.25 was the limit on V4s - Glad to hear 13.75 will work.

[pyro]

Slip numbers around 12% to 16% are fairly typical for a small-ear chopper. Keep in mind, a prop with any extra cupping added will act like it's 1" to 2" taller, so the slip may seem to calculate really low, but slip numbers less than 10% are VERY uncommon except for tunnels over 100 mph where hydrodynamic drag is not a major factor. For example, some props such as Trophy's tend to run taller than they're stamped. How tall of a pitch you can run depends on whether you can spin the prop up into your peak WOT RPM band. You can't calculate that, it's trial and error, ar ask someone that's run the same setup as you. Setup, setup, setup...

[Paul Vaillancourt]

jeff, you will have to remove the steering rudder only.

I have an extra SRX 23pitch available when you are ready.

I'll be honest I was real scared when I had my prop made cause it was custom, costed $750.00 and there was no returning it. I got luckey and all worked out, The owner of mazco had a Viper at some time and he kinda knew what I needed.

good luck

[chris_lacey]

Chris, do you think a little motor like that could pull 26P?

yeah, I do. I pulled a 24 to 6000rpm with a 120 on a V-King. The 120 looper probably makes the same or similar power as your crossflow 140.

I also had a 140 on a Valero and it would turn a 24 small ear to 6000.

The Viper should turn up a 14X26 no problem.
Try and find a V4 24P raker to try if you are worried about it. That will fit right on and you can resell when the pitch is too low. They are on here for $200 or less all the time.

[88 V-King]

My V-King & 115 tower of power (there's an oxy moron for you!) running a 21P 4 blade is running 55 to 58 mph. I din't take it any faster because I'm not very good at finessing it up on to the pad yet. If I were it would go a bit faster I'm sure, but it really needs more power to push it up on the pad. A 3 blade 26 will be slower out of the water, but should run a fair bit quicker than mine.

I went to a 4 blade to get the boat turning without losing the prop or cavitating too easily. It made all the difference in the world. The boat really handles now, it just needs more punch!