Reply
 
Thread Tools Search this Thread
 
Old 03 September 2010, 05:43   #11
Member
 
Country: Australia
Town: Cairns
Boat name: KAHUNAS
Make: Zodiac
Length: under 3m
Engine: 30 Tohatsu
MMSI: wft is a dsc mmsi ?
Join Date: Jun 2010
Posts: 82
Props are worthwhile experimenting with if you have them readily available to you , or if you know the prop is tried and tested, and has proven results. Otherwise you could sink money into it with little or no gain, and in some cases be worse off. Just because it's SS and more expensive, does not guarantee it's going to perfom any better with your motor/setup.

But yes firstly i would look at bringing your motor up to factory 25 hp specs. IMO a 25% increase in power will outweigh any gains seen by a prop change over.

The prop's fitted from the factory are very well suited to the motors, the only time people usually change the prop is when they are under/over propped for their chosen application, or they are seeking more grip, more speed, which is where you see 4 blade props , cleavers ect being used.

Speak to your local Yamaha dealer if your not sure, and they will be able to give you the complete parts list that is the difference between the 20 and 25 hp models. Maybe jump onto some other fourms, and see if you can find some other mini Ceasar users, and find out what props they are using. I know of a few on a Thundercat forum i can ask them for you if you don't have any luck.

With a bit of time spent hunting around, you might even be able to find something used that is suitable. Also if what Pol said was correct about the 25 using a different prop to the 20, then you could even find a factory 25 prop that someone has removed and upgraded, shouldn't fetch much on the used market..
__________________

__________________
Bus_Boy is offline   Reply With Quote
Old 03 September 2010, 07:18   #12
Member
 
Country: UK - England
Town: kingsbridge
Boat name: felix
Make: cesar
Length: 3m +
Engine: yam 20hp
Join Date: Aug 2010
Posts: 31
thanks for the info bus if u cld ask about it would be great? thomas at prop revolutions is one of the top guys for the t-cats apparently and he says without a doubt the powertech sra is by far the only prop out there suitable for the application as its cupped good rake angle and was designed to be run semi submerged? he suggested from my figures getting the 12 which if needed whivh he stronglyh belives it wont can be up or down pitched by 1" but @ the mo im thinking hp's this 102 st/st prop im gettin of a mate was from a 30 so it seems the 20 25 and 30 have the same prop?

i've contacted a local yam dealer and explained my intentions and their going to look at the parts lists they even said from what they think the 30 runs the same powerhead
__________________

__________________
supremesoundz is offline   Reply With Quote
Old 03 September 2010, 07:37   #13
Member
 
Country: Australia
Town: Cairns
Boat name: KAHUNAS
Make: Zodiac
Length: under 3m
Engine: 30 Tohatsu
MMSI: wft is a dsc mmsi ?
Join Date: Jun 2010
Posts: 82
I'll shoot them a message and let you know what they say about their props.

30 D is a different capacity motor to the 20D and 25 N. Your Yammie dealer should have known that one......
__________________
Bus_Boy is offline   Reply With Quote
Old 03 September 2010, 08:28   #14
RIBnet admin team
 
Poly's Avatar
 
Country: UK - Scotland
Boat name: imposter
Make: FunYak
Length: 3m +
Engine: 2 stroke YAM 20 HP
MMSI: 235089819
Join Date: Sep 2005
Posts: 10,114
Quote:
Originally Posted by supremesoundz View Post
i've contacted a local yam dealer and explained my intentions and their going to look at the parts lists they even said from what they think the 30 runs the same powerhead
No your engine shares a lot in common with a 25 HP 2 pot 2 stroke that they did. I think the 30 HP also shares a lot in common with a 25 HP 2 stroke, but not the same 25HP as yours!

I'll be interested to hear what they come back with. I'm expecting that two carbs plus the exhaust are not going to be cheap. Perhaps 500 ish? And probably quite a bit of labour to get to the exhaust. I concluded it would be cheaper just to trade it in for a bigger engine. Now - if your guy digs deeper than I was able to and finds it only needs say new jets in the carbs rather than the whole assembly then that couid be interesting...
__________________
Poly is offline   Reply With Quote
Old 03 September 2010, 21:05   #15
Member
 
Country: Australia
Town: Cairns
Boat name: KAHUNAS
Make: Zodiac
Length: under 3m
Engine: 30 Tohatsu
MMSI: wft is a dsc mmsi ?
Join Date: Jun 2010
Posts: 82
Pol, are you sure they use diffrent carbs between the two motors? I would be inclined to think that the carbies themselves would be the same items. From a manafactuers point of view, it would be expensive to use the same disspacement, but completely redesign a pair of carb's to suit.

It's far easier, and cheaper, to restrict, or detune a motor, by simply using a different main jet, and in some cases a smaller gasket. You would probably find also there is about 5 degrees difference in the timing at WOT. I could be wrong about Yamaha though, but that would have been my first guess as to the differences between the 2 motors.

Either way look forward to seeing how you go Supreme, with you Yamaha dealer, and following your progress with your Ceasor, they are a great little boat.

BTW be sure to get some pictures up when out and about.
__________________
Bus_Boy is offline   Reply With Quote
Old 04 September 2010, 04:04   #16
RIBnet admin team
 
Poly's Avatar
 
Country: UK - Scotland
Boat name: imposter
Make: FunYak
Length: 3m +
Engine: 2 stroke YAM 20 HP
MMSI: 235089819
Join Date: Sep 2005
Posts: 10,114
Quote:
Originally Posted by Bus_Boy View Post
Pol, are you sure they use diffrent carbs between the two motors? I would be inclined to think that the carbies themselves would be the same items. From a manafactuers point of view, it would be expensive to use the same disspacement, but completely redesign a pair of carb's to suit.

It's far easier, and cheaper, to restrict, or detune a motor, by simply using a different main jet, and in some cases a smaller gasket. You would probably find also there is about 5 degrees difference in the timing at WOT. I could be wrong about Yamaha though, but that would have been my first guess as to the differences between the 2 motors.
I agree, that's what I found when I did some digging. Here's the "parts list": http://sakuraru.ru/documentation/Tohatsu/20D_25N.pdf

The apparent differences are:

Carb Assembly (top)
20D 6L3-14301-00
25N 6L2-14301-01

Carb Assembly (bottom)
20D 6L3-14302-00
25N 6L2-14302-01

The jets etc are listed as the same parts, so I assumed that the carb body itself must have a difference (now it may just be a whole / plug or something?

Exhaust manifold extension
20D 6L3-41131-00-5B
25N 6L2-41131-00-5B

As I said earlier a dealer might be able to provide more info on the actual differences between the part numbers.
__________________
Poly is offline   Reply With Quote
Old 09 September 2010, 23:08   #17
Member
 
Country: Australia
Town: Cairns
Boat name: KAHUNAS
Make: Zodiac
Length: under 3m
Engine: 30 Tohatsu
MMSI: wft is a dsc mmsi ?
Join Date: Jun 2010
Posts: 82
I had a chat with a friend of mine who runs a 3.2 Ceasor with a 30 3 cyl. Basically the advice he gave is along the same lines as that in which have given you.

In keeping your existing motor, there's 4 options.
1- Upgrade the motor to factory 25 hp specs,
2- Go for a more race orientated prop, such as a semi cleaver, even full cleaver.
3- Modify the existing prop
4- Modify a slightly larger factory prop, i.e default prop of a larger hp motor


Now i'd still put my money on the table to say that a 25 hp motor spinning a factory prop, will outperform a 20 hp running an expensive racing prop.

5 hp increase might not sound like much, but you have to remember that you only have 20 hp to begin with. So a 25% increase in power, is quite a jump in a motor that size.

Modifying the existing prop-
You could add a bit more cup to your current prop. When you cup a prop, you can reach the same given speeds, at a lower RPM. However, since your motor is allready spinning at it's WOT range, to effectively gain any more speed, you would have to remove the rev limitier from the motor.

This can be countered sometimes by using a slightly larger pitch prop, and then modifying it in the same way.

So IMO, to gain the most while still spending the least, start with the factory power upgrade. Once that is done, look at getting your hands on a larger factory prop, and speak to some stores about having it modified to suit your needs.

I really wouldn't go laying down any large sums on something like a cleaver, for a 20-25 HP motor, i just couldn't see the gains being worth what you would spend.

What's your fuel like over there? Do know it's octane rating? Fuel is very important, and i hear from a few people running fuels around the 88-90 ron rating. If you don't have accses to decent fuel, (95 is preferable) then i would look at using an octane booster, and calculating how much to use the get the optimum octane.

Finally, you tubes. How much air are you putting in them? Over here in Australia, given our climate, we have to monitor the pressure of our tubes a little more closely. Given your air and sea tempratures however, i would suspect that you could slightly over inflate the tubes beyond the manafactuer's reccomendations. We do it in our winter seasons, especially early in the mornings, and there's possbily a couple of mph to be gained in doing that alone.
__________________
Bus_Boy is offline   Reply With Quote
Old 10 September 2010, 07:10   #18
Member
 
Country: UK - England
Town: kingsbridge
Boat name: felix
Make: cesar
Length: 3m +
Engine: yam 20hp
Join Date: Aug 2010
Posts: 31
hi thanks for having a look round for me im looking into doing the factory upgrade at the mo i think from the parts list the carb bodies are different thought the jets are the same? so maybe a bigger bore? and also the exhaust? other than tht id expect the timing to be slightly different too

as for prop mods ive layed my hands on a very very cheap cheap stainless solas prop like for like as my standard one 10" pitch just cupped? so i could get that worked to match new performance once i hace sorted out the bits i need for 25horses

if not i will then invest in a powertech 12" semi cleaver its not gonna break the bank as its only 175gbp

as for preasures i have no idea what they are i have one of those plastic cylindrical hand pumps i just pump it up till the hose pops off the pump

how can i gauge the pressures?
what should they be

just out of interest what sort of speeds is ur guy reaching

once again thanks for the info and time
__________________
supremesoundz is offline   Reply With Quote
Old 10 September 2010, 07:28   #19
Member
 
Country: UK - England
Town: Southampton
Boat name: Ginger
Make: Avon SR4
Length: 4m +
Engine: 30hp yamaha
Join Date: Jun 2010
Posts: 207
usually 3.3 ish psi main tubes, about 8psi highjackers. you could buy a bravo HP pump - its electric and you just dial up a pressure and off it goes. or if you look around for a kitesurfing pump they often do double action hand pumps with integrated pressure gauges (its what I used to use on my avon sportsboat)
__________________
rik_elliott is offline   Reply With Quote
Old 10 September 2010, 08:05   #20
Member
 
Country: UK - England
Town: kingsbridge
Boat name: felix
Make: cesar
Length: 3m +
Engine: yam 20hp
Join Date: Aug 2010
Posts: 31
thanks for the info rik il have a look and get one ordered
__________________

__________________
supremesoundz 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 20:45.


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