Reply
 
Thread Tools Search this Thread
 
Old 01 January 2004, 12:10   #1
Member
 
Country: UK - England
Town: Margate / Ramsgate
Boat name: Bumbl
Make: Scorpion
Length: 8m +
Engine: Yanmar diesel
Join Date: Feb 2002
Posts: 2,837
Prop selection

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...
__________________

__________________
Daniel is offline   Reply With Quote
Old 01 January 2004, 13:49   #2
Member
 
Cookee's Avatar
 
Country: UK - England
Town: Salcombe, Devon, UK
Boat name: BananaShark
Make: BananaShark
Length: 10m +
Engine: 2xYanmar 260 diesels
Join Date: Mar 2003
Posts: 4,225
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 -
__________________

__________________
Cookee
Originally Posted by Zippy
When a boat looks that good who needs tubes!!!
Cookee is offline   Reply With Quote
Old 01 January 2004, 13:52   #3
Member
 
Country: UK - England
Town: Margate / Ramsgate
Boat name: Bumbl
Make: Scorpion
Length: 8m +
Engine: Yanmar diesel
Join Date: Feb 2002
Posts: 2,837
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".
__________________
Daniel is offline   Reply With Quote
Old 02 January 2004, 05:26   #4
Member
 
Cookee's Avatar
 
Country: UK - England
Town: Salcombe, Devon, UK
Boat name: BananaShark
Make: BananaShark
Length: 10m +
Engine: 2xYanmar 260 diesels
Join Date: Mar 2003
Posts: 4,225
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,
__________________
Cookee
Originally Posted by Zippy
When a boat looks that good who needs tubes!!!
Cookee is offline   Reply With Quote
Old 02 January 2004, 05:32   #5
Member
 
Country: UK - England
Town: scotland
Boat name: Leviathan
Make: Phantom
Length: 8m +
Engine: GM Diesels
Join Date: Jan 2003
Posts: 1,437
Quote:
Originally posted by Cookee
or a chopper if you need some stern lift -
__________________
Jonny Fuller is offline   Reply With Quote
Old 02 January 2004, 06:08   #6
Member
 
Cookee's Avatar
 
Country: UK - England
Town: Salcombe, Devon, UK
Boat name: BananaShark
Make: BananaShark
Length: 10m +
Engine: 2xYanmar 260 diesels
Join Date: Mar 2003
Posts: 4,225
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?
__________________
Cookee
Originally Posted by Zippy
When a boat looks that good who needs tubes!!!
Cookee is offline   Reply With Quote
Old 02 January 2004, 06:47   #7
Member
 
Country: UK - England
Town: Hamble
Length: 9m +
Join Date: Dec 2002
Posts: 2,317
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.
__________________
Dirk Diggler is offline   Reply With Quote
Old 02 January 2004, 07:57   #8
Member
 
Country: UK - Scotland
Make: HumberOceanOffshore
Length: 8m +
Engine: Volvo KAD300/DPX
Join Date: Oct 2002
Posts: 5,404
Prop swapping, what's that?

TQ rules.
__________________
JW.
jwalker is offline   Reply With Quote
Old 02 January 2004, 07:59   #9
Member
 
Country: UK - England
Town: Margate / Ramsgate
Boat name: Bumbl
Make: Scorpion
Length: 8m +
Engine: Yanmar diesel
Join Date: Feb 2002
Posts: 2,837
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
__________________
Daniel is offline   Reply With Quote
Old 02 January 2004, 10:35   #10
Member
 
Country: Sweden
Town: Stockholm
Boat name: Osprey & Ring
Make: Osprey & Ring
Length: 9m +
Engine: Plenty
Join Date: Mar 2003
Posts: 860
Mirage

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.
__________________
www.grandrib.se
Petrov is offline   Reply With Quote
Old 02 January 2004, 10:43   #11
Member
 
Country: UK - England
Town: Margate / Ramsgate
Boat name: Bumbl
Make: Scorpion
Length: 8m +
Engine: Yanmar diesel
Join Date: Feb 2002
Posts: 2,837
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.
__________________
Daniel is offline   Reply With Quote
Old 02 January 2004, 10:48   #12
Member
 
Country: Sweden
Town: Stockholm
Boat name: Osprey & Ring
Make: Osprey & Ring
Length: 9m +
Engine: Plenty
Join Date: Mar 2003
Posts: 860
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.
__________________
www.grandrib.se
Petrov is offline   Reply With Quote
Old 02 January 2004, 10:52   #13
Member
 
Country: UK - England
Town: Margate / Ramsgate
Boat name: Bumbl
Make: Scorpion
Length: 8m +
Engine: Yanmar diesel
Join Date: Feb 2002
Posts: 2,837
That would be good. There are lots more good pics to go up there eventually as well. Happy New Year to yourself.
__________________
Daniel is offline   Reply With Quote
Old 02 January 2004, 13:56   #14
Member
 
gtflash's Avatar
 
Country: UK - England
Town: southampton
Boat name: TOP CAT 2
Make: Scorpion 8.1
Length: 8m +
Engine: 250hp HO
Join Date: Aug 2003
Posts: 1,542
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
__________________
gtflash is offline   Reply With Quote
Old 02 January 2004, 16:23   #15
Member
 
Country: UK - England
Town: Great Harwood, Lancs
Boat name: Tigger II
Make: Bombardier Aerodeck
Length: 3m +
Engine: Tohatsu 25HP
Join Date: Jul 2001
Posts: 626
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 is offline   Reply With Quote
Old 02 January 2004, 16:29   #16
Member
 
Country: UK - England
Town: Great Harwood, Lancs
Boat name: Tigger II
Make: Bombardier Aerodeck
Length: 3m +
Engine: Tohatsu 25HP
Join Date: Jul 2001
Posts: 626
OOPS

Its not in Knots its in MPH,

Panic over, now I get 15% slip


Regards (Plonker) Gary
__________________
Garygee is offline   Reply With Quote
Old 03 January 2004, 03:27   #17
Member
 
Country: UK - England
Town: Hamble
Length: 9m +
Join Date: Dec 2002
Posts: 2,317
Quote:
Originally posted by gtflash
several experts that have been in my boat were amazed at the difference.

How is mr Rolla?
__________________
Dirk Diggler is offline   Reply With Quote
Old 03 January 2004, 05:19   #18
Member
 
Cookee's Avatar
 
Country: UK - England
Town: Salcombe, Devon, UK
Boat name: BananaShark
Make: BananaShark
Length: 10m +
Engine: 2xYanmar 260 diesels
Join Date: Mar 2003
Posts: 4,225
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,
__________________
Cookee
Originally Posted by Zippy
When a boat looks that good who needs tubes!!!
Cookee is offline   Reply With Quote
Old 03 January 2004, 06:53   #19
Member
 
Country: UK - England
Town: Margate / Ramsgate
Boat name: Bumbl
Make: Scorpion
Length: 8m +
Engine: Yanmar diesel
Join Date: Feb 2002
Posts: 2,837
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
__________________

__________________
Daniel is offline   Reply With Quote
Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off




All times are GMT -5. The time now is 17:29.


Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2020, vBulletin Solutions, Inc.