Reply
 
Thread Tools Search this Thread
 
Old 24 May 2013, 12:40   #1
Member
 
Country: UK - England
Length: 4m +
Engine: 50HP Tohatsu TLDI
Join Date: Apr 2012
Posts: 128
Tohatsu 50HP Prop advice needed...

Hi there,

I have a Tohatsu 50hp TLDI outboard on my 4.2m rib and with the stock prop, 11.1 x 13, according to GPS I can hit 39mph on my own in the boat. I have looked at the Tohatsu properller chart and it says the maximum I can fit is a 11 x 15 pitch prop. I am courious as to how much extra top speed this would give me? I appreciate it is very hard to say with all the variables etc, but what would your guestimate be? 2mph, 4mph increase or more? I have never changed/played with props before so have no idea and would like a rough guestimate before spending money to find out I have only gained 1mph! lol.

Thanks for your help,

James
__________________

__________________
J@mes is offline   Reply With Quote
Old 24 May 2013, 12:51   #2
Member
 
martini's Avatar
 
Country: UK - Channel Islands
Town: jersey
Boat name: Martini II
Make: Arctic 28/FC470
Length: 8m +
Engine: twin 225Opti/50hp 2t
MMSI: 235067688
Join Date: Jun 2005
Posts: 3,021
You'll lose speed if the motor hasn't got the power to spin a coarse pitch prop. If you want to try and improve performance, the first thing you need to do is hook up a tachometer, either installed on the dash or a test meter temporarily hooked up to the ignition system.

Comparing results with the max rpm rating of your motor might give you an indication as to wether another prop will help
__________________

__________________
martini is offline   Reply With Quote
Old 24 May 2013, 13:04   #3
Member
 
Country: UK - England
Length: 4m +
Engine: 50HP Tohatsu TLDI
Join Date: Apr 2012
Posts: 128
I already have a tachometer installed!
I appreciate what you say, but if the engine was not powerful enough to spin an 11 x 15 prop then surely Tohatsu wouldnt list it for my engine?!

Thanks.
__________________
J@mes is offline   Reply With Quote
Old 24 May 2013, 14:10   #4
Member
 
Country: UK - England
Town: Felmersham
Boat name: Kiss my bass
Length: 5m +
Engine: Mariner 40 2stroke,
Join Date: May 2013
Posts: 22
Quote:
Originally Posted by J@mes View Post
I already have a tachometer installed!
I appreciate what you say, but if the engine was not powerful enough to spin an 11 x 15 prop then surely Tohatsu wouldnt list it for my engine?!

Thanks.
It's powerfull enough to spin it, right up to the point you put the engine on a boat, tohatsu won't know what size weight every boat is, the size they list is what the engine will take, but what works on one boat may not work on yours.

The answer already given is good advice, you need to look at the rev figures you are getting now as to what to do next
__________________
Kiss my bass is offline   Reply With Quote
Old 24 May 2013, 14:30   #5
Member
 
Country: UK - England
Length: 4m +
Engine: 50HP Tohatsu TLDI
Join Date: Apr 2012
Posts: 128
Well I think WOT is 6k rpm from memory with just me in the boat = 39mph.

Also from memory with my dad in the boat (+ 85kg) the top speed is 35mph and I think the WOT is slightly less, maybe 5800 rpm.

What advice can you give from this?

Also just noticed on the Tohatsu chart is also states the stock prop 11.1 x 13 is for a boat with a moderate weight load (500-1500 lb.) but it says for a light weight load (100-500 lb.) then a 11.1 x 14 or 11 x 15 prop is recommended. Well according to this I only ever have a light weight load!! I appreciate your comments re every boat is different, but with their loading advice in mind and the fact my rib is not a heavy rib (116kg bare boat + 98kg engine + weight of battery/consol/steering/fuel etc I would conservatively guess 300kg max all in) I would have therefore thought an 11 x 15 prop would be ideal for my rib?!

Cheers,
James
__________________
J@mes is offline   Reply With Quote
Old 24 May 2013, 14:56   #6
Member
 
Country: UK - England
Town: Felmersham
Boat name: Kiss my bass
Length: 5m +
Engine: Mariner 40 2stroke,
Join Date: May 2013
Posts: 22
Provided its the same as my 50 tldi then the max revs should be 5850, if it hits 6k then the auto cut out should kick in, so on the basis of the revs given, I would say your under propped when alone, but about right 2up, if you go up a size then performance will probably suffer as it will top out before it reaches full revs.
__________________
Kiss my bass is offline   Reply With Quote
Old 24 May 2013, 15:13   #7
Member
 
Country: UK - England
Town: Bristol
Make: Ribcraft
Length: 5m +
Engine: Yamaha
Join Date: May 2006
Posts: 4,194
Just buy the prop n try it, its the best way.
__________________
matt h is offline   Reply With Quote
Old 25 May 2013, 10:57   #8
Member
 
Country: UK - England
Length: 4m +
Engine: 50HP Tohatsu TLDI
Join Date: Apr 2012
Posts: 128
Quote:
Originally Posted by Kiss my bass View Post
Provided its the same as my 50 tldi then the max revs should be 5850, if it hits 6k then the auto cut out should kick in, so on the basis of the revs given, I would say your under propped when alone, but about right 2up, if you go up a size then performance will probably suffer as it will top out before it reaches full revs.
My manual says WOT should be 5150 - 5850 rpm and from memory the rev limiter/auto cut out kicks in at 6150 rpm

Why do you think If I go up a size (I assume you mean to a 15" pitch prop?) performance will probably suffer? Surely with just me in the boat at WOT it will come down from 6000 to about 5800 rpm and with 2up to about 5600, either way it's inside the WOT 5150 - 5850 rpm range stated by Tohatsu!

Quote:
Originally Posted by matt h View Post
Just buy the prop n try it, its the best way.
If only money grew on trees!! Plus if it made my performance worse it would get removed from my outboard and would have been a waste of money and time, hence me asking here first!
__________________
J@mes is offline   Reply With Quote
Old 25 May 2013, 20:08   #9
Member
 
Locozodiac's Avatar
 
Country: Other
Town: Lima-Peru
Boat name: Nautile
Make: Sea Rider Boats
Length: 4m +
Engine: Tohatsu 18 /30 HP
Join Date: Sep 2007
Posts: 1,706
If already achieving 6K at wot with you alone, personally would stay there, it's normal that with more weight rpm will be slightly decreased. The top speed difference between standard delivered prop and next adyacent pitch is minimal, not cost effective.

Is 39 MPH not enough for you ? More top speed, lessens better hole shot and viceverse. Placing a 15" prop will decrease rpm by 200-300 rpm. Probably will benefit achieving better hole shot with slight top speed than with 13"

Happy Boating
__________________
Locozodiac is offline   Reply With Quote
Old 26 May 2013, 03:09   #10
Member
 
spartacus's Avatar
 
Country: UK - Scotland
Town: Aberdeenshire
Boat name: Sula
Make: Ribcraft 4.8m
Length: 4m +
Engine: Tohatsu 60hp + aux
MMSI: 235087213
Join Date: Jun 2007
Posts: 3,268
RIBase
If you were local - you could borrow my 15 pitch propeller, on the understanding if you damage it you replace it with a nice shiny stainless steel one!

Any Ribnet members near you - you could swap propellers with?
__________________

__________________
Is that with or without VAT?
spartacus 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





Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -5. The time now is 05:22.


Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.