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Author Topic: shovelnose hull  (Read 9089 times)

martno1fan

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Re: shovelnose hull
« Reply #50 on: May 11, 2007, 03:37:07 PM »

id loosen the glow plug no matter what starter your using it will make life easier trust me.as for the prop sounds odd ive never heard of a prop having the hole drilled off centre!! ::)i dont understand what your meaning about balancing horizontal and vertical?i put my props horizontal on the balancer and the heavy blade falls to the bottom if neither does then its balanced?.
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sheerline

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Re: shovelnose hull
« Reply #51 on: May 11, 2007, 04:24:57 PM »

Hi Mart, here's how I checked the prop.. Firstly I found a piece of 4mm ground stainless shaft and checked it was true and in balance itself. Next, I inserted it into the prop (good close fit, no slop) and placed it on the balancer. Found one side a bit heavy so filed and polished until prop blades sat level, didn't have to do much really as it was'nt far out. The blades sit level and thats fine so I turned the blades upright, one above the other and they fell back to the horizontal position. I thought that was odd so I turned the blades  over so they were horizontal again but now rotated them through 180 deg and found they rolled completely back to the original side. If the prop were perfectly statically balanced, I should be able to place it in ANY position and it should remain there, not just horizontal.
So to sum up, the prop will only horizontally on one side only which would indicate the hole is offset in the casting. I can't take anything off the blades to improve it nor can I realistically take or add anything to the hub to correct it .... I think I have a naff prop!!
I find this sort of thing par for the course whever I buy anything.. you could have a hundred items on a shelf and sods law says I will always wind up with the 'friday afternoon' one!
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martno1fan

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Re: shovelnose hull
« Reply #52 on: May 11, 2007, 06:48:48 PM »

i still dont see how you know your getting it 100% vertical? surely its normal for the prop to roll over from vertical?youd have to get it exactly vertical for it not to.when you spin the prop on the shaft does it wobble? if not id say its fine!!also if one blade is slightly larger but same weight would that acount for it rolling over?.stick it on and start the motor up and let it spin or do as i have fasten the prop on the shaft and put the shaft in a drill and turn it on and see how it spins if its not balanced you will soon know. ;)
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omra85

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Re: shovelnose hull
« Reply #53 on: May 11, 2007, 08:44:02 PM »

Chris
Your prop is probably OK but just still out of balance.
Don't forget that a racing prop is heavily pitched and the blades go round the hub almost 360 degrees (poor explanation but will do).
If the BACK of both blades are horizontal, then the FRONT of the blade at the bottom is too heavy.  You will have to remove weight from the part of the blade that's at the lowest point. Don't try to remove your edges, just go in a bit at the lowest point and start filing!
Also don't forget to remove any casting flashes that are still there (although being so far from the edge doesn't make a HUGE difference to the balance).
The fact that it's dropping straight to the same position quickly would indicate that there's a LOT of weight at that point.
I've never come across a prop where the hole has been "off centre", so if it is you are EXTREMELY unlucky. ;D
Think yourself lucky it's not a THREE blade which I also use :o ;D ;D
Danny
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w3bby

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Re: shovelnose hull
« Reply #54 on: May 11, 2007, 09:40:02 PM »

What prop are you grinding on? If it's beryllium I hope you are taking the proper precautions, face mask, gloves, long sleeves etc. beryllium is not nice stuff. Do the work under running water..............

sheerline

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Re: shovelnose hull
« Reply #55 on: May 12, 2007, 11:53:12 AM »

Hi chaps, firstly, I think the prop is simply brass, I work a lot with brass and this item appears to exhibit all the qualities of the stuff. I think, given the amount of imbalance I would have to remove a considerable amount of material from the hub area of the blades to get this prop to statically balance but that would really screw up the hydrodynamic balance which may cause more problems than I already have. I am not nitpicking over this balance thing and am prepared to accept that nothing in this world is perfect but knowing how much importance you fellas place on balance given your own personal experiences, I would like to know I have got it as right as can be. I will try spinning it on the shaft as you described but I know it aint right.
Martn, just to elaborate on the balance theme, I liken the prop to a flywheel, it should be able to come to rest in any position throughout 360 degrees...that would be truly statically balanced. If each of the blades were perfectly symetrical and imparted exactly the same amount of thrust at a given rpm.. they would be exhibiting perfect hydrodynamic balance.
In the real world, I will never achieve either of these perfect goals and certainly have no means of checking the latter.
I will look at it again and see if it can be improved a little more. I suppose I have to take into consideration that this boat will not be receiving the hard graft and continual use your boats get so we will probably fit it regardless and use it for trials. We can always get a better quality prop (Prather) at a later date I suppose.
Thank fellas........... Chris
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martno1fan

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Re: shovelnose hull
« Reply #56 on: May 12, 2007, 12:57:08 PM »

chris the prop will not balance at any position through 360 degrees if its balanced it will allways come to rest in a balancer with both blades horizontal to each other its not like a flywheel.also can i ask what prop your using who makes it? is it octura? if so it will almost ceratainly be stainless steel or berrylium .
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sheerline

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Re: shovelnose hull
« Reply #57 on: May 12, 2007, 02:44:17 PM »

Martin, the prop came from Prestwich, there was nothing with it describing it or its material type, all I know is it appears to be brass. It looks, files and polishes like brass and thats about all I can say. How would I be able to tell if it was beryllium ?
I have now fitted it to the shaft, also, the motor along with its rubber mounting and silencer assy are now ready to bolt down into the hull  as I have fitted blocks  to accomodate them.
I reckon I will get back to it after next week and finally fit the top deck out with its manifold.
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martno1fan

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Re: shovelnose hull
« Reply #58 on: May 12, 2007, 05:59:25 PM »

ask dave but what prop did you buy it as? what size and pitch etc?.he sells mocom props (submerged) and octura and thats it i believe.it should say what they are made of on his site.
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martno1fan

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Re: shovelnose hull
« Reply #59 on: May 12, 2007, 06:01:43 PM »

yup octura are berylium copper(bronze) hazardous to breath in.
http://www.prestwich.ndirect.co.uk/octurapropellers.htm
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sheerline

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Re: shovelnose hull
« Reply #60 on: May 12, 2007, 06:32:38 PM »

Ok Mart, Berylium it is, but I don't think I have done that much work to the prop to render me any more knackered than I already am... hopefully. I will have another look at it when I get time. Thanks for that.
Regards... Chris
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glennb2006

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Re: shovelnose hull
« Reply #61 on: October 16, 2007, 12:12:24 PM »

http://www.zippkits.com/rivslow.wmv

Sheerline, all seems to have gone quiet, hope the build is progressing. See the link above for some video of a shovel nose hydro running, this one is pinched from another forum but I thought you might like to see what the boat performance looks like, I believe the boat in the video has a 26cc petrol engine, so similar or maybe a little less performance than the .90 methanol burner you have.

Shuld be fun.

Glenn
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sheerline

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Re: shovelnose hull
« Reply #62 on: December 09, 2007, 03:58:21 PM »

Glen, sorry I haven't been on this thread for ages and although I have trolled through and contributed towards other threads, I obviously didn't look in here. The shovelnose is virtually complete although work ground to a halt a while back, I will post some pics when I get a moment and you can all have a good larf at my attempts and tell me where I've gone wrong. The last hang up was over an engine starter, my lad reckoned he would have one man enough for the job languishing in his loft somewhere but as he had moved and stuff was still in boxes, I have grown even older waiting for it to appear. :(
 I have run the motor in on an aircraft prop for about three hours at third throttle and set rich. It starts with one flick and runs faultlessly so I have high hopes for it providing I am able to turn it over and start it in the boat. I have taken note of the advice given, regarding slackening the plug to lose a bit of compression when set up for boat use. Needless to say but with the aircraft prop fitted it was a delight to start and no plug fiddling of course. Shame it has been fitted to a boat really as I would love to stick it in the nose of an aircraft!
I will come back with some pics soon.... promise!
Regards to all ........ Chris
   
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glennb2006

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Re: shovelnose hull
« Reply #63 on: December 10, 2007, 02:20:33 AM »

Nive to hear that the model is near completion Chris. Look forward to seeing pictures and some updates as it goes on.

If the motor is run in, which it sounds like it is, it may well strat easy enough on a more standard starter. Here's hoping. My SC61 now has around 8 hours on teh cylinder and piston, but not the con rod - long story! It starts really well now. Although I did replace the carb with a replacemetn form Just Engines. Well worth 20 quid. Easy starts and great low speed performance. Idles at 3k!!

All the best.

Glenn

Happy Christmas.
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