ceaser mini thundercat engine height...

[supremesoundz]

as above just looking for advice on the best position of the center of the prop in relation to the hull

3m thundercat
yamaha 20hp 2 cylinder 2 stroke
powertech sra 12p


ive been running center of prop level with the bottom of hijackers,

not had a chance to test since adding the height and also have had the motor recently ports polished boyesen reeds 25hp exhaust and timing advanced 2deg at full throttle

read a few posts on the 4.2m thundercats saying the ideal position for the prop is to have it central with the center of the hijackers (50mm higher than my current setup) providing it sucks water,

just wondering if any one can confirm this, ive added another bit of 50x50 box on top of my transom so that i can raise the motor its currently bolted on later to be welded providing it all works out okay, ive added 2 lower holes so i can drop to -15mm from center and -30 from center taking me 20mm away from the old position with the saddle now sat on the box

[Roflhat]

Having the centre of the propshaft in line with the bottom of the hijackers is a good starting point. Most people go up 10-20mm from there depending on prop.

If you want more advice or good reading head here - Thundercat Inflatables Forum - Powered by vBulletin

loads of info

[Roflhat]

If you get some pics up of your setup would probably help too

[supremesoundz]

brilliant thanks so starting center of the hijackers will be a good start, thats wher im at now, previously had been running prop lining up with bottom of hijackers which was 50mm lower,

hopefully il find a noticeable improvement hoping i still pick up water for cooling that is

ive added a few pics

new position raised motor showing prop height
current prop powertech sra 12p
pic showing how its running flat out previous to height adjustment with above prop top speed 32knots

[wetbristol]

Just a bit of advice, I used to race Thundercats and to get the setup right you will need to employ the help of a tacho and GPS to see engine revs to see if the prop is working as it should be , 1mm up down or trim in out can make a massive difference, keep a log of what you change and the results. It will help you get the best setup. Looks good fun though, if you need help give me a message might have some trim plates lying about somewhere.

[supremesoundz]

ideal thanks hopefully going to brave the cols and get some testing done this weekend, i have a tiny tach in the post so the revs are going to be sorted

also just downloaded a decent gps speedo for the phone to log the speeds

when i had my old tiny tach and a worked solas 10p prop, and shaft setup with the bottom of the hijacker i was getting 6100 revs and 32mph it seems to run best on the second set of holes out from fully trimmed in

i then changed to the 12p power tech but the tiny tach went flat, but according to a mate who chased me im now getting 37 mph, but it must still be revving around 6000 from looking at a prop slip calculator,

hopefully log some data revs and speeds at the weekend as ive done a few things which are going to differ, porting ignition advance mixture setup and the jackin up

im not going to be able to go any higher than current middle of jackers but do have 15mm lower holes and 30mm lower holes

think the main thing is im gong to need to have a higher pitch prop to compensate from the extra hp and the engine being jacked up more.

do you have any prop recommendations cleaver or semi cleaver im limited to 10" diameter and 10 spline fitting

would you say from the pic id need some more stern lift, i can easily get more bow lift by trimming out but then loose cornering speeds and get slip

[wetbristol]

Where did you get porting done ??

You can have propeller worked to make a difference with stern lift but it would be better to work a pair of props so you know where you are going with prop working.
Looks as if there is a fair bit of boat in the water still from the pictures. So I would try getting some trim on the bottom and see what happens try 3mm first . Also it may be worth elongating the brackets mounting holes and getting an aluminium plate welded between the brackets . It will make raising or lowering the motor easier and you can do it by very small amounts.

[supremesoundz]

porting was done by a mate of mine that a marine engineer just cleaned the exhaust ports up and matched the liner to the casting it was rough as a badgers behind was looking at getting the head skimmed to raise the compression but the current squish is .50mm so would need that machining too, currently is 95 psi on each cylinder but it does have exhaust releif ports to aid pull starting, i cant seem to find the compression ratio any where for the motor either

when you say adding some trim on the bottom what do you mean?

there is some one fairly local-ish to me cornwall nick barsh who can work props, would it be worth sticking with the bunny eared power tech or moving to a semi or full cleaver

[wetbristol]

Just try a 3 mm piece of aluminium between the bracket and the engine at the bottom . A small amount of trim can give you a difference. I have a 25HP Yamaha dealer service manual that says the compression ratio is 7.20 you motor is the twin carb motor autolube ?.
We only raced with Batwing/Bunny eared props they are easy to get hold. powertech do some semi cleaver props I would stick to them. I personally wouldnt bother with full cleavers as you will probably only get a Ron Hill or similr that will probably need loads of work/Balancing to get right .
Am i right in saying that they race the 25 Yam in J25 it mat be worth seeing if you can find anyone into that as they may have some old props kicking about ?? . Its all trial and error im afraid with props, We were sometimes fast and slow on the same day racing with the same prop and setup its all about knowing what to do and recording what you have done before so you know where you can go back to. Black Magic is props and setup. Good luck with it !

[supremesoundz]

I'm with you so by adding small shims to the lower of the bracket I can add a few mm of trim rather than the massive distance between the holes

Yeah my motor is a 2 cylinder autolube same block as the 25 395cc only part numbers that are different are the carbs and exhaust tube which I've already swapped

I was told the ideal compression ratio to try and get is 8:1 or 10:1 as to how I would achieve this based from 7:20 with my squish being .50mm I've no idea skimming and squish is all over my head

It does go a lot better with the power tech compared to my old Solas, I had read Ron hills require balancing and working from new and they seem to be the only cleaver or semi available and are all over prop exhaust rather than through,

So I really need to et out do some testing with the new setup and look at speed amd revs and trim see how it's sitting in the water and I I need some stern lift have the prop adjusted I'm guessing it will need some more pitch to lower the revs again after the adjustments from tuning and being higher

[Roflhat]

some good advice on here. get your tacho set up (don't wrap the wire around the ht lead too many times!)

judging from that pic looks like you need to trim out more, get the nose up.
You can get 10 x 18p merc choppers that work really well on these hulls, not quite sure if your 20hp will spin them. my mate got a 10 x 16 chopper for his surfcat with a tohatsu 30 and its really too little prop. Its a ron hill prop which we had to cut over an inch from to make it fit onto the splines! seems to be running alright now though.

Really you're only choice is ron hill for these small props, a 10 x 16 cleaver or chopper I reckon would work well on your hull. its a hit and miss with rons props, could be good straight out the box or may need some balancing.

[Roflhat]

If you're after proper performance I wouldn't bother with bunny props. cleavers or choppers are the only way. I've run 15p solas which i had cupped and pitched to 16p. Then a merc 18p chopper cupped. now running an 11 x 18p merc cleaver. the difference is night and day between the surface piercing props and the bunny ear.

Also if you haven't already you could try carbon reeds like Boyesen, increases compression a bit too. my slowhatsu 50 is at around 140psi.

If you go to a surface piercing prop you'll most likely have to raise the motor some more, probably at least another 10mm, these props like to be run higher, always having one blade out of the water.

[supremesoundz]

ideal cheers roflhat tacho should be with me soon (tiny tach)

id does look relatively flat in that pic thats with one hole out it does seem to quicken up the further i trim out but then when going very slow i get alot of prop slip and it sits up at some silly angle till it drops on to the plane maybe here is a better pic with the same settings and the same prop

https://www.facebook.com/photo.php?v...type=3&theater

https://www.facebook.com/photo.php?v...type=3&theater

i have some boyesen reeds on order from the states which are estimated for delivery on the 10th of feb, the compression is fairly low 95psi on each but it does have a exhaust relif port to aid turning over, the squish seems near as dam it perfect at .50mm

so out of the 3 props you've run the merc cleaver is the best then, it does seem that most of the racers run semi or full cleavers, am i right in thinking they give a bit of stern lift where as a chopper gives more bow lift, which i can get from my trim as its only one hole out.

the only 10 spline offering i have found is a ron hill which like above ive read mixed reviews on, and they don't offer a thru hub version for my fitment will that matter, the leg is drilled so hardly any exhaust come out of the prop any way.

would i have trouble picking up water if one blade is surfaced?

9.5 X 15 Signature Propeller for 20-30 HP Yamaha Outboard Racing | eBay
Yamaha 20-30 HP Outboard Racing Three Blade Cleaver Propeller 10 1/2 X 15 | eBay

[Roflhat]

yes cleavers are stern lift and choppers are bow lift. my cleaver and chopper are very evenly matched, its hard to distinguish between the two when set up correctly.

The chopper is over the hub (small hub) but the cleaver is through hub (big) doesn't make much difference if you've done the above prop exhaust mod (hole in the leg).

I've got standard water pickup, no lwp, and it still picks up water fine, you'd be surprised how high you can go and still get pressure.

Out of the two links you posted I'd go for the second one, although I reckon you may want 16p or 17p seen as your motor is modded.

[supremesoundz]

all good advice i think they do do a 16 and 17p too so may be worth looking at, guess it will all be down to the revs im pulling at the moment with my powertech 12

it just needs to stop raining

[Roflhat]

You can just lift the engine higher if you want more rpm, you can go from a 12 to an 18p cleaver and just lift the engine to suit. Obviously the danger is going so high you don't get enough water pressure.

If I use a 17p solas i can't hit the rev limiter at 5850, but if i use a 20p cleaver i can hit the rev limiter just because cleavers are run higher out of the water

[supremesoundz]

boyesen reeds came yesterday so today braved the weather and went out in the eastury 6knot limit today even got hailed on, it was a little choppy and quite windy,

i started off with the trim set to where i used to find it best 1 hole out, was getting loads and i mean loads of bow lift i went vert at one point, lots of prop slip and took ages to drop on to the place get grip and was off,

move it in to the first hole trimmed right in less prop slip and get on the plane quicker, but still lots of bow lift, more than before which is weird, maybe this is because with all the mods its pushing more power

i notice i can hear the prop breaking loose quite often (probably because its alot higher)

any way here are my readings wot was between 6800 and 7100 and top speed recorded was 46mph

[A1an]

7100!!


I bet that sounds scary!

[supremesoundz]

Yeah was ventilating a gooden now it's raised up and I've just checked on the power tech website my prop has 20degrees of rake which explains the bow lift from the extra power I've gained

The question now is do I get a full cleaver stern lift

Or

A semi cleaver which gives neutral lift

I'd like still to be able to use it on some swell I never use it in surf

[Roflhat]

7100 is high even for a yamaha! I reckon you need some more pitch on there

cleavers and choppers work almost exactly the same on this style of boat (despite one apparently giving bow lift and the other giving stern)
I wouldn't bother with a semi cleaver unless you're doing surf stuff.

I reckon a 16 or 17p cleaver would do you nicely. you'll probably have to get it worked either way so a Ron Hill should be alright