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Old 25 January 2014, 12:21   #1
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Country: UK - England
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ceaser mini thundercat engine height...

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
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Old 29 January 2014, 14:16   #2
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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
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Old 29 January 2014, 14:16   #3
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If you get some pics up of your setup would probably help too
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Old 29 January 2014, 17:13   #4
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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
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Old 29 January 2014, 17:23   #5
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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.
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Old 29 January 2014, 17:41   #6
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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
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Old 29 January 2014, 17:51   #7
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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.
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Old 29 January 2014, 18:08   #8
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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
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Old 29 January 2014, 18:49   #9
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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 !
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Old 29 January 2014, 19:16   #10
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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
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