ceasar mini cat 20hp 2stroke yam prop help

[supremesoundz]

hi iv got a 3.2 mini cat with a yam 20hp on the back

std prop is a 9 7/8 x 10.5 pitch

2 up w.o.t = 27 mph @ 6100 rpm using a tiny tach and gps in flat water

engine is sat about 50mm off the 2" box section as its meant to be so clamps bite on the box i did try it lower no differance

its trimmed out on the second hole out? cant trim out any more as i get loads of prop slip on hole shot and cornering.

steel developments recomender a 13p solas thinned edges bit of extra cup and a progressive pitch from 12 to 13

any advice welcome

[Poly]

can't really offer any advice but if you decide to get rid of the current prop send me a PM/email with some photos and the price you want for it...

[supremesoundz]

it s only a std ally yam prop ?

[Poly]

Originally Posted by supremesoundz

it s only a std ally yam prop ?

Aye I know, its the right prop for my engine / boat, and I've got a couple of slightly bent ones sitting here. If you don't need it / its gathering dust / your chucking it out then I'd be happy to give you some beer vouchers for it. But if it would be useful as a spare then hang on to it and I'll send these two off for repair when the bad weather comes.

[supremesoundz]

i have just got a solas cupped stainless prop same as std pitch im going to give it a try but im thinking il need a bigger pitch as ive been recomended a 12 cupped if ur interested in it if its no good on mine

[Bus_Boy]

What are you trying to achieve from a prop change? I've got a bit of exp with mini t-cats, but never with a motor as small as a 20, particulary a 4 stroke.

At WOT, your spinning at 6100 rpm, which is just about ideal i'd say, what is the WOT range for a 4 stroke 20 yammy?

Your talking about prop slip, well your going to get that with your motor trimmed out regarless or prop choice. Trimming your motor out sacrifices hole shot, and cornering, for top end speed.

With less prop in the water, you get less drag, more speed in a straight line, but the prop has less bite when cornering. If your trimmed out and want to corner, you will need to sit the bow right up and then corner, otherwise with the bow down, it will just keep slipping, and if you keep and eye on the cav plate, you'll probably see it sits up above water line when your getting prop slip.



With an SS prop, you might get a small gain, bit i'd be inclined to think that the gains of an SS prop, would be more towards having motor fully trimmed out, and your only running 2 out of 3 blades in the water.

[supremesoundz]

hi thanks 4 the reply its a 2 stroke yam auto lube? yh it revs at 6100 2 up 6595 solo?
the wot rev range 4 the yam is 5500 so i was told by yamaha

what size engine have u had experience with? i thought mine was big as the mini cat is rated 4 a 18hp

my top speed is pretty low @ 26 mph ive seen on another forum tht one with a 18hp was doing 56kmph and one with a 30hp doing 76kmph

[Bus_Boy]

These days they rate the mini's to 18hp, previously they were rated to 25hp. 18hp is a very conservative rating for a boat commonly seen with a 30hp.

The 2 i have driven were powered by a 3 cyl 30 Yamaha, and a 2 cyl 40 Tohatsu.

If your talking top speed, then you have to sacrifice on everything else. You need to run as much trim as possible, and also look at raising the motor further up off the transom. It will take forever to plane and get going, and will prop slip and cavitate like crazy until you finally get the hull in a position that allow's minuimum drag, and just enough prop in the water (usually 2 out of 3 blades) which is where an SS prop also comes of an advantage, as they don't flex like a standard prop.

If your bored and got time, check out the likes of Youtube ect, at some video's of T-cats doing speed trials, and it will give you a bit of an indication, just listen to everytime the prop hub (exhaust) comes up above water level.



Ceasor rate the mini with a 18hp to reach speeds estimated at around 35 mph, which is about 56 km/h.

So before you go spending a small fortune on a new prop without any guarantee that it will bring any improvments, i'd suggest doing as much testing as you can with your current setup.

Also on a sidenote, if you want a bit more top end, have a flick through your Yammie's part list, and compare the parts list against the 25 hp model.

20 and 25 are the same cu in dissplacement, and without saying exactly which parts are different (haven't owned a Yammie in a long time) i can almost say for certain that upgrading your motor as per factorty specs to a 25 hp will offer you greater value for money vs new prop.

[Poly]

Originally Posted by Bus_Boy

Also on a sidenote, if you want a bit more top end, have a flick through your Yammie's part list, and compare the parts list against the 25 hp model.

20 and 25 are the same cu in dissplacement, and without saying exactly which parts are different (haven't owned a Yammie in a long time) i can almost say for certain that upgrading your motor as per factorty specs to a 25 hp will offer you greater value for money vs new prop.

It sounds to me like you are using the same engine as me. My 20HP shares the same block as a 25HP 2 pot. The physical differences are: new carbs x 2, and new exhaust manifold. There may be changes to timing etc too - but those are the parts changes. Obviously to get the best out that extra power you would still need a new prop anyway.

[supremesoundz]

ideal bus boy i have had a bit of an experiment with the engine height as when i got it it was jacked up so the clamps sat on the box section so i lowered it didnt really change much lost a bit of top end which from what u sed is right and allowed me to trim out one more notch, but after some playing i found it better to be raised on the transom and trimmed in one notch? ive got a like 4 like st/st prop on its wat which a mate has so il try tht b4 forking out?

so i could just change some parts for other oem parts then? i know on the engines we use in ocr that the 115 is the same as the 130 just bigger carbs different exhaust tube timing and power plate?

so like pol just said the carbs and exhaust manifold are the only different part no's and probablt the timing adjusted would be a instant 5hp gain over stock

[Bus_Boy]

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..

[supremesoundz]

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

[Bus_Boy]

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......

[Poly]

Originally Posted by supremesoundz

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...

[Bus_Boy]

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.

[Poly]

Originally Posted by Bus_Boy

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.

[Bus_Boy]

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.

[supremesoundz]

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

[rik_elliott]

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)

[supremesoundz]

thanks for the info rik il have a look and get one ordered