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Old 02 January 2004, 10:43   #11
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Country: UK - England
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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.
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Old 02 January 2004, 10:48   #12
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No probs

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.
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Old 02 January 2004, 10:52   #13
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That would be good. There are lots more good pics to go up there eventually as well. Happy New Year to yourself.
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Old 02 January 2004, 13:56   #14
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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
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Old 02 January 2004, 16:23   #15
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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
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Old 02 January 2004, 16:29   #16
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OOPS

Its not in Knots its in MPH,

Panic over, now I get 15% slip


Regards (Plonker) Gary
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Old 03 January 2004, 03:27   #17
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Quote:
Originally posted by gtflash
several experts that have been in my boat were amazed at the difference.

How is mr Rolla?
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Old 03 January 2004, 05:19   #18
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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,
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Old 03 January 2004, 06:53   #19
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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
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