Prop

[hans]

Hi all, I'm still trying to work out how this gear/pitch stuff works. What would the pro's and cons of the following be, given
( according to the calculator ) top end speed will be the same.
11.5 x 13 prop and 1.58 gears.
11.5 x 16 prop and 1.29 gears.
Hull is an Everingham shallow V (4 deg).
Engine is a blown 565 (~1000hp) Peak torque should be around 5000 - 5500, Max rpm wanted = 6500.
Don't know if you need any more info.
Thanks.
Paul

[fastvdriver]

11.5 X 16 and the 29's seem to be a good choice for most of the racers I turn this set up about 7300 rpm in the 1/4. you need to be able to spin the prop out of the hole for a smooth launch I spin the motor about 8500 out of the hole the boat sets very nice and doesn't do anything wierd. Guru would be a good one to get advice from. not sure about your hull never have heard of it? got any pics?

[Moneypitt]

Paul, the calculations work. The difference, or lack of difference, is based on changes. If you change gears, and prop pitch at the same time, as you've mentioned, one cancels the other. BUT, if you leave the 1.29 gear and change the prop, you will see drastic changes in the boats speed, based on the same RPM. OR if you leave the prop alone, and change gears, again you will see a difference.. What you have done is an equal percentage of change, SO, try this.... 1.29/13@5000=79.4 1.29/16@5000=97.7
1.58/13@5000=97.2 1.58/16@5000=119.6........As you can see, the speeds at 5000 can get quite serious, and you MUST factor out the slipage inherant with any boat hull design.. These numbers are only used as a base line to get in the "ballpark", and the only true speed indicator that I know of is a GPS....Hope this helps you........Moneypitt

[Rexone]

I would stick with the 16 prop and play with gear selection.
Just from experience. :)
Also you may need to trim some diameter. 11 1/2 may manhandle the boat some (too much bite). If it chine walks on the top where you can't settle it down, that is what I'd try all other conditions being what you are after as far as speed/rpm. Every prop is different though, you need to try it and change one thing at a time. :)

[hans]

Thanks guys. It's an Australian hull. Usually used for circuit racing. Here's a few pics:
http://members.austarmetro.com.au/~hans/0000.jpg http://members.austarmetro.com.au/~hans/pic_1001.jpg
I realise that they both work out to be about the same speed. What I'm after is an idea of how the boat would handle getting there. Would the 13 pitch get me on top of the water quicker, but have more slip ( ie. be slower ) getting to speed? And conversly, would the 16 lug the motor more getting out of the hole, but once on top of the water, accellerate quicker??
I'm interested in getting the best accelleration possible. I've seen a video of Guru and his boat sets beatifully.
Paul
edit: ok Mike, thanks. ( was writing when you posted ) I was originally going to go with the 16 but it has been suggested I go with 13 and was wondering exactly what difference it would make (given the change in gears to suit).

[Rexone]

A 15 or 16 pitch generally gives you a better range on the prop once you get the slip dialed in. My gut feeling tells me you'll end up trimming the prop diameter slightly (I would go 1/8" at a time, which isn't alot). When you do that it will likely settle the boat down (if you have a handling problem), and in turn may require you bump the gear up a bit to achieve what you're after.
This is all trial and error testing. Change one thing at a time so you know what did what and keep good records of the changes and the results.
Prop design in itself can make a huge difference too. It's an area I'm not an expert in though. The guy at the top of this page I'm sure can help there though (Tom Bently). Also if you know Phil Kall (Argo Engineering) in Australia, he makes some killer one piece CNC props that he's had great success with on his Racecraft hulls running the prop off the ass end.

[hans]

ok, thanks again Mike. Argo is the name of the gearboxes mostly used down here. It's a new boat, I have no running gear as yet. (and no idea what I'm doing ):confused: My usual way about deciding on a course of action though, is to keep asking questions untill I'm happy with what I'm going to do.
I'll try to get in touch with Phil. I'm sure he knows the hull and what works with it.
Tom has already suggested the 1.29 and 11.5x16. I don't doubt him, just trying to get a better understanding of how it all works. Seems to be a bit of a black art so far, excepting the calculator for max speed.
If you want to limit the speed ( for bracket racing ) how would you do that whilst maintaining max acceleration??
Paul

[coolchange]

Relationship of the prop to the strut is very important also. A nickel in or out of the shaft can fine tune a prop thats not quite there. But like Rex said" KEEP RECORDS!"

[fastvdriver]

I have forgotten the how to do the math for the speed factor. if any one out there can post the formula I say thanks. trying to see what amount of slip my set up has.:)

[coolchange]

Pitch x prop rpm divided by 12=feet per min. x60 == feet per hour
dived by 5280 = miles per hour.
Divide into actual mph= percent of slip or hull effeciency.(sp)

[fastvdriver]

Originally posted by coolchange
Pitch x prop rpm divided by 12=feet per min. x60 == feet per hour
dived by 5280 = miles per hour.
Divide into actual mph= percent of slip or hull effeciency.(sp)
thanks for the info I came up with 138 mph by the math and reality is 133 mph. I was shocked with the math hull and prop have a 4% slip Yes I'm very happy:p

[coolchange]

4 percent? are you sure your not using engine rpm?
i.e-7400rpm x 22percent over gears =9028 prop rpm.
ran a hydro a few years back and the best we could do was about 85 percent.:D

[coolchange]

by the way, nice ride.

[fastvdriver]

Originally posted by coolchange
4 percent? are you sure your not using engine rpm?
i.e-7400rpm x 22percent over gears =9028 prop rpm.
ran a hydro a few years back and the best we could do was about 85 percent.:D
7300 RPM's V-drive 1.29 % Prop 15.5
now maybe I'm going backarwds but wouldn't 100% slip not move? The 133 comes from time slips with CDBA running in the 8's
could you run the numbers and let me know.
thanks coolchange

[coolchange]

damn thats an effecient set up! got the same numbers. I need to look up our old numbers!
checked and we started out at about 85% runnin hi 120's eventuallywent hi 140's to low 160's and were in the deep 90% range. my bad:D

[fastvdriver]

just ran the numbers from the old motor and it came up with 8 % slip. this boat was a TAF back in the day (More Wheaties) Was layed up to be a Top Fuel Flat.
I guess I was very lucky to find such a sweet piece sitting in a garage for about 7 years:p