Prop selection

Daniel · 01 January 2004, 16:10

Having now had a chance and the conditions to do a V-Max test i can confirm my maximum speed as:

46knts
5200rpm
21" prop
1.87:1 gearbox ratio

Triming out past 30% of the range gave no advantage...just instability.

Using this http://go-fast.com/Prop_Slip_Calculator.htm and the above figures it seams i have a 20% slip - this seams quite high.

Assuming the 20% slip is true, and will be constant with a prop change, it also seams droping to a 19", at 5750rpm will give me... 46knts, and a much bigger fuel bill.

Am i overlooking anything? Comments appreciated!

I am quite willing to accept that this is as fast as i can go, and i'm very happy with the set-up but if there's more in her 'capin...

Cookee · 01 January 2004, 17:49

Daniel if it's an ally prop try and borrow a stainless 20" - that will give you less slip and more speed, try a 19" if it's available for peace of mind as well and you'll be sure then

Happy New Year -

Daniel · 01 January 2004, 17:52

It's a Laser II stainless 21". I did not think you could get even pitch for my engine, or is that just in the Laser II range? I've arranged to borrow a 19" Laser II but am concerned the test will not be that realistic because it's not as new and shiney as my 21".

Cookee · 02 January 2004, 09:26

Try the 19 anyway - it will still give you an idea. If you can get hold of something like a Raker, they give more bow lift if the boat is a bit nose heavy or a chopper if you need some stern lift - sadly everything I have is a bit big for you!

If you get a chance Tempest are a good neutral prop and work well on a well balanced boat.

Hope the above helps a bit,

Jonny Fuller · 02 January 2004, 09:32

Quote:

Originally posted by Cookee
or a chopper if you need some stern lift -

Cookee · 02 January 2004, 10:08

That's what they seem to do to my boat Jon? Maybe my brain is a bit rusty from all of the celebrations - Which does your alchohol free brain say is the best stern lifting prop Jon?

Dirk Diggler · 02 January 2004, 10:47

Quote:

Originally posted by Cookee
or a chopper if you need some stern lift

Steady on Cookee, your starting to go all "Manos" on us.

I've always found the chopper to be an excellent prop, but it certainly don't give stern lift...

Mind you, I've never understood the obsession with propellor swapping, why not just accept that the rib does 46 knots (?) and be happy with it. As you should be aware through racing, it's almost impossible to find identical conditions to compare props, even with back to back testing the fuel load, tide, wind etc changes, and so you can never be 100% sure that a different prop is an improvement, unless of course your original choice was totally wrong.

jwalker · 02 January 2004, 11:57

Prop swapping, what's that?

TQ rules.

Daniel · 02 January 2004, 11:59

Quote:

Originally posted by Dirk Diggler
why not just accept that the rib does 46 knots (?)

Quote:

Originally posted by Daniel
I am quite willing to accept that this is as fast as i can go, and i'm very happy with the set-up

Petrov · 02 January 2004, 14:35

Hi Daniel we had a Laser II picth 19 on a Viper 6m 150opti.
We changed to a Mirage 19 and that one made us go 1 1/2 knots faster, i don`t know about fuelconsumption tough.

Daniel · 02 January 2004, 14:43

Thanks Janis, a £500 prop for 1 1/2 knts might not be that good value for money! I've just finished fitting the 19" Laser II, just go to find time to get out there now.

Interesting how you selected a 19" initially and myself and my engine dealer selected a 21". I guess the 19" would be needed with a heavy boat (divers...) where as i am interested in 3-up long distance fuel economy realy - the speed thing is a side line!

Thanks also for info by e-mail the other day.

Petrov · 02 January 2004, 14:48

You have some nice pics on your site, if you want i can put up a link on my site, like Osprey related sites or similiar.
Good 2004.

Daniel · 02 January 2004, 14:52

That would be good. There are lots more good pics to go up there eventually as well. Happy New Year to yourself.

gtflash · 02 January 2004, 17:56

i must respond to your comments dirk. Im not a prop expert and even the so called prop experts do not know everything and will usually admit that prop swapping is not an exact science very much trial and error. However changing the prop on a boat can effect more than just speed as i am sure you know.

My boat is a completely different boat for fitting a new prop, she accelerates faster (my opinion only), gets on the plane like a scalded cat now, handles around corners like a different boat (on rails now!) and even cuts through chop better, ive lost negligable top speed and no noticable change in economy, several experts that have been in my boat were amazed at the difference.

I love the difference and am glad of what knowledge i have gained about props

ps. i went from a merc 3 blade ally to a hifive of same pitch

Garygee · 02 January 2004, 20:23

PANIC !!!

Just tried the speed/slip calculator

I get actual 34Kts at 3600 RPM with 24" stainless props on my RIB with a 1.81 gearbox.

The calculator shows I should get a theretical 45Kts and at 34Kts I am getting 34% slip

I know the Ribcraft is a heavy RIB but this seams very very high.
Would it make any difference on the Brave 3 being contra rotating.

What do other big RIB owners get ?. Is 34% reasonable ?.

Thanks Gary

Garygee · 02 January 2004, 20:29

OOPS

Its not in Knots its in MPH,

Panic over, now I get 15% slip

Regards (Plonker) Gary

Dirk Diggler · 03 January 2004, 07:27

Quote:

Originally posted by gtflash
several experts that have been in my boat were amazed at the difference.

How is mr Rolla?

Cookee · 03 January 2004, 09:19

Quote:

Originally posted by Dirk Diggler
Steady on Cookee, your starting to go all "Manos" on us.

You really know how to hurt a bloke Dirk!

As Daniel said he is just having a go to see if it will go any faster - I also think as GT Flash has already found out, that a different prop can make a huge differance in hanling, and whilst I havn't had a Hi 5 on a boat of mine I have heard they are very nice props to drive a boat with.

I certainly think that spending over £300 to get a couple of mph gain is silly on a leisure boat, but if you can borrow a couple of props to try it would be an interesting excersise with everything reversible.

Good luck Daniel,

Daniel · 03 January 2004, 10:53

Thanks mr Cooke!

Also a HUGE thanks to Gary who has pointed out that i am a superbly large plonker since the prop. calc website is in MPH not knts.

Re-visiting my initial figures, converting 46knts to 53mph (x1.15 right?) gives me a now unbeleivably small slip of 4%???

Of course it does not affect the maths that a 19" at the max RPM (5750) will acheive the same speed, in theory. I have thought the route to go might be to get Steel Developments to alter my prop to be a 20"... i have heard that is possiable. Either way i think i'll just borrow and try as many props as i can untill June when there will be even more to borrow at RIBex