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cavitation!!!!

Old 04-06-2008, 09:17 AM
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Default cavitation!!!!

houston we have a problem!!!!!

ok so i just finished building this 20inch hydro, 25 including strut and hardware, and anyway... i think its cavitating![:@]
my hardware is inline and its running a sv27 brushless motor on 12 cells, with just a 2 blade prop that a guy down the hobbie store reckomended, it had no tag or information, but honestly i think its a nitro hammer prop!!!

the prop sits about 2 inchs under the water from center shaft to water line when just sitting there, but as soon as i take off even slowly, it starts to catch air or cavitates or something like that, its more like its sucking the air in from above it. and its not a flex shaft so i cant just angle it more.

the prop is 31mm in diameter, plastic, and like i said its a 2 blade.
i dont no how to fix this problem, i think its cavitaion because its not peircing the surface but i know theres more to cavitation than that.
i was thinking maybe a 3 blade prop or a bigger 2blade or something??

ill pop a pic up alittle bit later if you need 2 hav alook at the hardware and where its positioned.

if anyone has any input at all, any questions about the boat that they need to know to help me out, just fire away!
any feed back at all would be muchly appreciated!

Cheers,

Corey
Old 04-06-2008, 10:45 AM
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Default RE: cavitation!!!!

I have a Hammer and it cavitated like crazy with the stock plastic prop. I switched to an Octura X430 metal prop and the problem completely went away.
Old 04-06-2008, 04:32 PM
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joccee
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Default RE: cavitation!!!!

the prop is too small...I think people are using 37-45mm props on their SV's
Old 04-07-2008, 12:22 AM
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Default RE: cavitation!!!!


ORIGINAL: joccee

the prop is too small...I think people are using 37-45mm props on their SV's



so would you recommend a sv27 3blade plastic prop? or steel??

if so what size mm??? i dont know if this is the info you need, but it weighs 1.6kg and is 20inches from tip 2 transom, 25inches including hardware.

thanks guys, i really want to get her running


Corey.
Old 04-07-2008, 07:19 AM
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Default RE: cavitation!!!!

Could you post a picture of the boat directly from behind, level with the bottom, with the prop in question installed?
Old 04-07-2008, 08:46 AM
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Default RE: cavitation!!!!

HI Corey
I think this can be easily fixed. What size of prop shaft do you have on it? 1/8" ,3/16" or 4mm threaded? Something else?
Nothing is wrong that a good prop can't fix. If you hydro is only 20inches long ,you will not need a large prop. You boat is rather small to use 12 cells though. Should go fast once you get it up though. If you have a 4mm threaded shaft you could use a large number of low cost Graupner CF props. Right now I guessing that you have a 3/16" prop shaft. The 3/16" props seem to start at about 38mm and go up from there. You only have the option of metal or CF props here as you have too much power for most ordinary plastic props.
Let us know more about what you have. n.h.schmidt
Old 04-07-2008, 08:53 AM
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Default RE: cavitation!!!!

ORIGINAL: SV27dayton

Could you post a picture of the boat directly from behind, level with the bottom, with the prop in question installed?


here is some pics. (i think) sorry 1st time for posting pics

i hope these can give you an idea, there not the best pics as there just from my fone.
the waterline (when sitting still) sits just above the gold water outlet tube you can see on the left of the transom.
oh and yes that is the prop in question that i was talking about.
thanks,
Corey
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Old 04-07-2008, 10:23 AM
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Default RE: cavitation!!!!

Your hull is set up wrong. On a hydro hull, the prop shaft must exit under the hull and the prop center line should be below the rear of the hull. Then a lifter type prop is used.

Ryan
Old 04-07-2008, 07:31 PM
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Default RE: cavitation!!!!

I've never seen a hydro set up like that either.
Old 04-08-2008, 12:24 AM
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Default RE: cavitation!!!!

Hi N.H.Schmidt

im at work at the moment, but im almost 100% sure that my shaft is 1/8 is this ok??

that is just the setup that the bloke at the boat hobby store showed me to use when i took the hull in 2 the shop.
he drew a quick sketch and gave me that shaft, prop, strut and rudder assembly, he just said i will need 2 make something 2 hold the strut in place, which is the 2 L shaped bits aluminum you can see holding everything together.

will this setup still work?? or will i need 2 try and make a new 1?
problem is theres no room left in the hull!!
it doesnt help the fact thats its a straight shaft not a flex[:'(]

thanks,

Corey
Old 04-08-2008, 09:28 AM
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Default RE: cavitation!!!!

Well your setup is unusual but I think it can at least be made to work . This is after-all a learning experience.
A 1/8" shaft can be dealt with,even with this much power. I have run several 12 cell and 700 powered hydros using 1/8" shafts adapted up to 3/16". The real problem for you is your scources for props. As you now know not just anything will do. I have a Octura Y534 that will fit right on a 1/8" shaft and may work. Its still plastic but a good performer and very cheap. The shaft can be adapted up for 3/16" props . That will give you a good selection of props to try. If you local shop does not have many props suitable then you may have to send for them. This is something most of us have to do. n.h.schmidt
Old 04-08-2008, 09:35 AM
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Default RE: cavitation!!!!

so what props and prop sizes would you recomend? as i have no idea!
and yes i think ill have to order 1 as my hobby shops dont seem to be the greatest around hear.

cheers,

Corey
Old 04-11-2008, 01:17 AM
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Default RE: cavitation!!!!

come on guys i need some help with my props, what would you recommend for my boat??
Old 04-11-2008, 06:27 AM
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Default RE: cavitation!!!!

I think you are seeing hesitation from people telling you how to spend your money b/c they honestly don't know. That's an unusual setup. Nobody can say what will work for shure form experience, certainly myself included.
Old 04-11-2008, 07:21 AM
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Default RE: cavitation!!!!

X640

Ryan
Old 04-14-2008, 03:04 AM
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Simon.O.
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Default RE: cavitation!!!!

That is an interesting build you have there. Not a bad comment as I have two home-brew hydros undergoing development at the moment.
Your thrust angle is a quite aggressive therefore will lift the rear regardless of the type of prop, however a lifting prop will increase the effect.
A 1/8" shaft is fine to work with for a wide variety of props. You can use 3/16"octuras with a sleeve of 7/32" K&S tubing, I do !!
I do not know what rpm you are developing on the bench, but I get close to 25,000 rpm and use a octura 534 on my 20" prototype hydro with a solid shaft similar to yours.
If you have wads of rpm on hand go for a small prop and work steadily upwards. Keep a diary of changes.
I run for 30 - 60 seconds on a prop then swap to monitor changes. Given the cells I use I get 2 or 3 swaps per charge, once they are off the peak then i just run the boat for a nother min or two before breaking out the CAT and bigger mono.
Summary. Small prop to begin with, I suggest a 534 ( plastic ) and go from there.

Old 04-14-2008, 02:23 PM
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Default RE: cavitation!!!!


I can get real specific if you like. I don''t want to get you spending a Bunch of cash to get something to work. You can''t go wrong with the Octura Y534. This was mentioned twice now. It should work and cost little. You can also give a Graupner CF K42 a try . Get the one that uses a 3/16" shaft. All the metal props could cost you $30 USD''s or more each. You are starting from nothing so you need everything. The problem for you is where to order from? n.h.schmidt
Old 04-16-2008, 12:03 AM
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Default RE: cavitation!!!!

thanks for the feed back guys i havent bought any props as yet but i know i shouldnt have a problem buying a few from the US, i think ill look into the octura Y534 Plastic and give that a wirl. might not receive it for a while but ill keep you updated with my progress.

simon .o.

have you posted a forum for your home made hydros??
id love to see them.

cheers guys.
Old 04-16-2008, 03:33 AM
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Default RE: cavitation!!!!

Perth !! Dam that''s a long way from anywhere let alone the states. For online buying may I suggest High Performance Models. They are here in Auckland and shipping to WA is probably quicker and cheaper than from the US.
For my hydros the first one is over on the Red Board, but the development of the newer one may be shared here too.
There is a lot of knowledge out there to tap into for running boats, I learn from here, ose, and rrr, and taking the boats to the lake.

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