prop size (yamaha 80 on phantom 16) - Page 2

Jon Fuller · 07-07-2004, 08:20 AM

Working out the predicted speed of a boat is a tricky thing.
My experience says that most of us can make a pretty accurate guess on what the top end will be for a given setup, then use the oul equasion for working out what prop is required, based on the estimated top end, known ratio, and known max RPM.

If the boat is a known factor, ie, one is rigging a 26 extreme, or a 28 phantom, or a 21 phantom, then we have a very good history to work from, and as long as there are no major changes in the likely efficiency of the setup, ie, you are only really changing weight (CK) or power, then the 'Levi' formula seems to work pretty well.
This requires you to know the 'constant' No, which represents the basic efficiency/performance of the hull/drive.

Therefore, if your gonna take your Formula29, and add 200hp to the existing 600, you could calc quite accurately what your speed increase will be. Or work backwards, if you've increased your power, this will tell you how much more power, based on the speed increase.

I'm sure there are plenty of equasions for this, Matt could inform us there, but the crux is, to double the speed, you need to quadruple the power.

Jon Fuller · 07-07-2004, 08:22 AM

Quote:

Originally posted by Jono
Feckin' right Nobba! Give me toobs over horsepower anyday......me gran's ol' piles cushion wiv a seagull fitted is faster than that heap o'shite "Instant Fumble".... and it'll cope wiv an hurricane... and it'll carry 18 lardy blokes...and it'll cross the Norf Sea on a thimblefull of fuel..and it'll............ NURSE!NURSE!

Excellent!

whenever I hear Nurse,...Nurse, can't help thinking of Peter Cook/Dudly Moore...god bless em.

"We're not going to change your sheets again Mr Hastings!"...

L&N · 07-07-2004, 08:42 AM

Cheers all for the help but im even more confused now

can anyone tell me the best size prop i should buy then??

Jon Fuller · 07-07-2004, 08:52 AM

OK, if we assume the 5500rpm WOT to be correct, and that it is indeed a 2:1 gear box, and make a guess at about 50mph top end, or mebbe a tad more, then it calculates that you would need a 23" prop @ 5500rpm, assuming a 12% slip.

So, 22 or 23" pitch is my .02 pence worth.

It must be revving it's bollocks off with an 18p, as I reckon 41mph at the redline with this prop.

All this goes out the window if it weighs a load, or is an obscure inefficient hull (although I hear what your saying about it being a P16)

You really need to borrow some props to get a better idea, and BUY A TACHO!

L&N · 07-07-2004, 10:25 AM

Quote:

Originally posted by Johnson
OK, if we assume the 5500rpm WOT to be correct, and that it is indeed a 2:1 gear box, and make a guess at about 50mph top end, or mebbe a tad more, then it calculates that you would need a 23" prop @ 5500rpm, assuming a 12% slip.

So, 22 or 23" pitch is my .02 pence worth.

It must be revving it's bollocks off with an 18p, as I reckon 41mph at the redline with this prop.

All this goes out the window if it weighs a load, or is an obscure inefficient hull (although I hear what your saying about it being a P16)

You really need to borrow some props to get a better idea, and BUY A TACHO!

EXCELLENT!!!!!!!!

that is good to know what the maths work out at. i'll start trying at that size....

thanks very much...........

Phantom 19 · 09-07-2004, 07:23 PM

I used to run a 20" LaserII,

You can get that 80 up to a 90 by changing the exhaust and main jets, blank the oilpump and premix 50:1, or get even more Hp by fitting aftermarket exhaust.

My Yams always cooled fine jacked up to top hole on 16.

Cat · 17-07-2004, 09:51 PM

I used to run a 22x14 ss yam prop on a lightweight 16 sat in tandem with engine at 26Inches,I dont know what it weighed but it was very light,at the time it was the fastest in the northern series 1999
Cat

Ciao · 01-04-2005, 12:45 PM

Quote:

Originally posted by L&N
Cheers all for the help but im even more confused now

can anyone tell me the best size prop i should buy then??

The best prop ever used on a Phantom 16' with a Yam 90hp was made by Phil Rolla www.rolla-propellers.ch/ for Brian Eastham KIN Ell Fire (2 x 3b World Champion) 86 87

It was a four bladed cleaver.

It pushed the boat at 57 mph (timed on Windermere)

Not only was it quick, but kept the boat balanced, with no hooks, stuffs etc.

Wacky Racer · 01-04-2005, 03:09 PM

22 yam prop is the way to go, used to run one on my 16 Ring and Reiver years ago.

captain · 01-04-2005, 05:02 PM

Yamaha 20 prop, standard trim (unbutchered) is the one you want, I hold the O.C.R World 1.3lt speed record 60.43mph and two nat championships using a yam 90 on an Argo 17sorta like a Phantom, must prove something. Got a Phantom 16 with yam 90 and 20 yam prop now for fun and she really flies, no good if you want to pull a skier though.