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Author Topic: shovelnose hull  (Read 9042 times)

martno1fan

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Re: shovelnose hull
« Reply #25 on: March 17, 2007, 02:37:27 PM »

i wouldnt use the small prop theres a big difference between a 27 mm prop and a 50 you will probably blow the engine up by over revving it with that tiny prop ,id also suggest the 50 mm might also be too small , id say at least a 55 mm one .buy a larger one .your engine will be revving its *****off with the small prop and getting no where.the electric sounds promising keep us posted but my recomendation is buy a bigger prop for that .91.for a hydro prop prather recomended for 11 cc engines a 250 a 260 or a 270 prop for best performance and you will be looking at speeds up and over 50 mph easy!!also you cant just potter arround with a hydro to get her up and moving you need to give a few revs then launch it into the water a bit  forward is best but watch for that prop!! these boats are made for out and out speed not cruising around!!.hope this helps you out ,maybe you should pay a visit to RCU and ask the guys on there as there are more guys there running hydros than on here,go to the nitro forum and do a search for hydros or post a new thread asking for advice .ive seen a video somewhere of a similar boat ill try to find it. ;)http://http://www.rcuniverse.com/forum/default.asp?fh=1&forum=229
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martno1fan

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Re: shovelnose hull
« Reply #26 on: March 17, 2007, 02:46:37 PM »

i just found this too check it out gives you all the prop sizes for hydros for prather and octura.basicly your looking for a 50 mm prop minimum preferably a 60 mm.the last two numbers on the prop charts are the props diam in mm.a 27 is far too small.
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sheerline

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Re: shovelnose hull
« Reply #27 on: March 17, 2007, 07:10:15 PM »

Thanks M, good info and will check it out, the last thing we want to do is screw the motor in its early phase of break in. One thing has occurred to me even though we have'nt started yet, is how much rudder movement should we give this type of model. Any thoughts?
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omra85

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Re: shovelnose hull
« Reply #28 on: March 17, 2007, 08:26:04 PM »

When you set up an unknown boat, it's always best to err on the side of caution regarding steering.  If you set the rudderr up with about 30 deg each way, it may be too much so you may well find that you won't be pushing the stick so far over to turn.  This can then be modified by reducing the throw based on how much the boat turned relative to stick movement.  If, on the other hand, you set up too little throw, at best it will take huge circles and at worst will be up the bank :-
The 2227 prop you have (as it's ally) may well be a submerged drive prop!  The 22 refers to the diameter which is 2.2" and the 27 is the pitch.  If there is a thread in the boss (usually 5mm), it is for a solid propshaft and will not be suitable for surface drive which normally have a 5/16" straight hole with two notches at the front which engage into a 'dog drive'.  Another way to tell is, if the back of the blades are straight, it is surface piercing, if they are convex, it is submerged.  I suspect that it's a "Propshop" or "Mocom" which has just been "chucked in" with the sale.
To confuse you even further :-\  Octura props are available in plastic, beryllium copper or stainless steel.  The diameter is the SECOND pair of numbers and the pitch the first pair ie. 1447 where the 47 is the diam in millimetres and the 14 is the ratio of the diam to the pitch ??? ???
Prather don't make plastic (i think) so they are only BeCu or Stainless.  The sizing code is beyond me even though I actually use them :-[  They are numbered 220, 225, 230, 235, 240 etc.
To cut to the chase so to speak, your SC91 is a 'sports' engine which will probably not be tuned?? If so I would suggest one of the following, Prather 260 (2.52" x 3.9" pich) Octura X465 (2.56" x 3.58" pitch) or better still start with a couple of Octura plastic (GRP but not the hard GRP) which you can experiment with (cut down) until you get somewhere near.
As a point of interest, your tuned pipe is an early PMC pipe, with the water cooling jacket covering the front expansion cone.  It looks, from the pic, about a 40 size. If you can let me know the diameter at the widest part of the cone, I'll tell you what it's for.  Irrespective, if it will go over your header (manifold) you could still use it although it may be too small.  Using a smaller than optimum pipe will still give some improvement to RPM as long as it is set up to the right length.  Looking at the long pipe at the front, I suspect that this is how it's been used (and with the cost of a new pipe nearly 100, who can blame them).  Your most difficult task, I think, will be to get the manifold from the side of the engine, round to behind it, then through the tunnel at the rear.  You may end up having to make your own - but it's not that difficult, if you can silver solder.
Hope that helps and sorry for being so long winded ;D
Danny
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sheerline

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Re: shovelnose hull
« Reply #29 on: March 17, 2007, 09:21:13 PM »

Hello danny, thanks for taking time to explain that lot and yes, props can be a bit of a minefield as we have found out with one or two other projects. I have stuck some pics of the prop on here so you can see exactly what we,ve got and it is a surface runner made of ally and as you can see, it has a pair of notches at the rear to engage the drive dog on the shaft.
I have included some pics of the rescue boat which is a bit rough at the moment as all I have done so far is chuck a motor and shaft in it. It's about 34" long by 10 inch beam and although I only intended it to rescue the hydro, it looks as though it will be a bit brisk across the water with just a 12volt battery driving its FRS750 motor. This boat is made up of all the odd bits I have lying arund the place  and I think it will be a bit of fun on its own even before the hydro is ready.
Regards......Chris
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omra85

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Re: shovelnose hull
« Reply #30 on: March 17, 2007, 09:58:23 PM »

Well Chris
That prop should do for starters, at least it will give you a guide.
"Concentrate on your destiny, young propwalker" - I feel a disturbance in the force - you are in danger of being seduced by the Dark Side (fast electrics) ;D
Danny
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martno1fan

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Re: shovelnose hull
« Reply #31 on: March 17, 2007, 10:50:10 PM »

chris  buy the right prop from the start,you need a surface prop either stainless steel or ali, if you can find a plastic prop to fit your shaft which i doubt then give it a go but you need a 60 mm prop maybe even a 65 .the smallest i would even try is a 55 mm one but it needs to be a surface prop.surface props have a straight trailing edge to the blade.hers a pic of mine.mines a prather 280.under no circumstances would i try that 27 mm prop with any engine over a .16 my mate has a .16 engine and has a 35 mm prop just to give you an idea of how small that one is.a .91 is a beast of a motor and needs a big blade to spin or its going to over rev and blow up!!.heres a good place to get prather and octura props they deliver pretty fast and ive  orderd lots from them and theyre cheap too.this prop ive linked should be ideal for your engine.it will need sharpening and balancing ,i used a file and wet and dry paper and some metal poilsh.i balanced it over a pint glass on a steel shaft .they come with instructions on how to do it.half what you would pay in the uk too.
http://http://cgi.ebay.co.uk/100-NEW-PRATHER-PROPELLER-S-260-PROP-STAINLESS-STEEL_W0QQitemZ5992658181QQcategoryZ34058QQtcZphotoQQcmdZViewItem
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glennb2006

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Re: shovelnose hull
« Reply #32 on: March 17, 2007, 11:19:14 PM »


It will be good to get the smell of methanol and castor back up our noses again as we have'nt been flying for a few years.
Will keep you posted as things progress.
Regards... Chris   
[/quote]

I'm sitting here chuckling thinking that you may well be flying again before too long if the balance ain't right on that hull or the engine overpowers it!

Only kidding, you should have great fun with it.

Enjoy.

Glenn
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sheerline

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Re: shovelnose hull
« Reply #33 on: March 18, 2007, 06:54:42 PM »

Just a quick note re props, my lad has just informed me he has ordered a prop from Prestwich to fit the 91 ,along with the driveshaft so I reckon as they are specialists, they will probably source the correct prop. We'll see what turns up this week and hopefully, can finally make a start. Thanks to everyone for all the in depth and valuable information you have rendered up so far.
Regards. Chris 
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martno1fan

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Re: shovelnose hull
« Reply #34 on: March 18, 2007, 10:43:26 PM »

yep dave will sort you out hope you got a big wallet though,i payed 15 quid for mine deliverd from hong kong about 10 quid cheaper id say, than if id got one from uk.i was trying to save you some dosh but oh well ::).dont forget to sharpen and balance and polish the prop before you run it either.have fun!!
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sheerline

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Re: shovelnose hull
« Reply #35 on: March 19, 2007, 11:03:03 AM »

Hi 'M, I didn't realise he had ordered the prop so it was too late to do anything about it but I have had a look at the Prather site and if we need to try different props, then we'll give that a try. Nice looking props but as you say, I've no doubt Dave will come up with the best type suited for the job.
Waiting...impatiently!!
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martno1fan

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Re: shovelnose hull
« Reply #36 on: March 19, 2007, 04:17:14 PM »

just remember the prop will need balancing sharpening and polishing!!.do you know how?.if not i can go through it with you from some info i have on how its done .its very important to balance it and you sharpen it as you go but only on the one face of the blade,the convex side!!, the face looking at you from the drive dog side.dont take any off the concave side or you will alter the pitch.
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sheerline

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Re: shovelnose hull
« Reply #37 on: March 19, 2007, 09:30:19 PM »

Ok 'M', stuff still not here yet, when it arrives, I'll send you a pic of the prop and see what you make of it. I thought aircraft props were dangerous enough but his one sounds like a regular screaming scalpel!! God help any fish that are'nt quick off the mark.... or us for that matter!
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omra85

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Re: shovelnose hull
« Reply #38 on: March 19, 2007, 09:43:30 PM »

Or, if your fast enough and he hasn't sent it yet, and you don't want the hassle for a one-off, ring Dave and get him to do it.  Up to 60mm for 3.50.  Given that it takes me a couple of hours (and mine aren't perfect) AND it's hardly the most exhilarating bit of boat building, what's your time worth ??? ;D
Danny
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martno1fan

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Re: shovelnose hull
« Reply #39 on: March 19, 2007, 09:47:45 PM »

it may come with info on how to sharpen and balance it all i did was put the prop on a small length of shaft suspended it over a pint glass see which blade falls thats the heavy one you sharpen the edge of it on the one face with a file and polish the face too either with a dremmel or wet and dry (hard work).do it enough so the blade is lighter than the other and is sharp.then work on the other one untill they balance!!.thats it its easy enough but hard work using wet and dry and a file, then get some ali polish from halfords and polish it till its gleaming .
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martno1fan

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Re: shovelnose hull
« Reply #40 on: March 19, 2007, 09:51:05 PM »

for 3.50 id get dave to do it but hes usually quick at posting and i usually get his stuff the next day or at the most 3 days later. ;)
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omra85

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Re: shovelnose hull
« Reply #41 on: March 19, 2007, 10:02:03 PM »

Martin
When I started doing my own props, I used a couple of old fashioned razor blades in a piece of wood to support the shaft.  I thought I was doing a good job on them but after getting through three rear skeg bearings in a season and the prop waggling round like a dolphins tail, I bought a magnetic balancer.  When I put my "balanced" props on it - they were MILES out, so I had to do them all again.  Now the bearing lasts at least a season or two.
Danny
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martno1fan

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Re: shovelnose hull
« Reply #42 on: March 20, 2007, 08:13:52 AM »

Yea ive seen the razorblade trick too in fact ive seen balancers for sale using a similar setup.If your prop was waggling around a lot why did you run the boat? ,surely if its waggling around it isnt balanced or something isnt lined up right. ;D
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BJ

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Re: Props and Balancing
« Reply #43 on: March 20, 2007, 12:01:59 PM »

Martin
When I started doing my own props, I used a couple of old fashioned razor blades in a piece of wood to support the shaft.  I thought I was doing a good job on them but after getting through three rear skeg bearings in a season and the prop waggling round like a dolphins tail, I bought a magnetic balancer.  When I put my "balanced" props on it - they were MILES out, so I had to do them all again.  Now the bearing lasts at least a season or two.
Danny

It is not hard work to balance a prop BUT a good balancer makes a lot of difference. Mine is a Top Flight Magnetic
Balancer. It is impressive to see a prop rotating rock steady with only one end of the balance shaft touching an end magnet but do not make that gap too large - just a few "thous". I broke a skeg due to an improperly balanced prop once and that was once too often!
If you search on "Octura Prop Charts" you will find many sites offering the information (Remember that PRATHER ceased trading at the end of January).
You can also get Octura props from Tony Ellis at http://www.modelmarinesupplies.co.uk/shop/ either balanced or unbalanced. His turnround from order is fast.
I do not know where you can ABC props form in this country - did find http://www.niagaramarinehobbies.com/ABC%20Props_1.htm

Here is a "complete" props chart - follow the link - http://martysrcboats.com/props.htm
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martno1fan

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Re: shovelnose hull
« Reply #44 on: May 08, 2007, 09:58:49 AM »

Ok 'M', stuff still not here yet, when it arrives, I'll send you a pic of the prop and see what you make of it. I thought aircraft props were dangerous enough but his one sounds like a regular screaming scalpel!! God help any fish that are'nt quick off the mark.... or us for that matter!

whats happened with the boat? is it running? did it work ok or was it a disaster? let us know but the silence maybe says it all?.hope alls worked out ok for yaa!! ;)
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sheerline

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Re: shovelnose hull
« Reply #45 on: May 10, 2007, 08:54:21 PM »

Hello all, I have not finished the hydro yet but in between work have gradually fiddled with it and have now carried out the following: hull assy now joined and sanded, driveshaft assy in place (with free running driveshaft), rudder servo installed and rudder capable of around 30 -40 degrees of swing. Now I know you will all say "lazy sod, it should have been run and crashed by now" but I have not been able to dedicate a lot of time to it.
I have run the sc91 in on an 18 x 6 aero prop and a gentle drip feed of cooling water to allow around 60deg C head temp. The engine was run at around 3000 to 5000 rpm at a guess as I didn't want to cane it from new, the large prop gave it a nice load and that big flywheel action meant she ran as steady as a rock for around two hours. It has freed up beautifully and starts with one good swing.
I have spent a fair amount of time making a silencer system in an effort to keep the whole thing quiet and although it's not a tuned system by any stretch of the imagination, I think we are going to have our hands full with the setup as this is a large and powerful motor which, in a boat is untrodden ground for us.
The idea is to arrange an exhaust manifold in the removable top deck/ engine cowling and have twelve working exhaust stubs sticking out (6 each side). I hope to arrange it so the manifold will engage with the silencer (which is fixed in the lower hull) so that when the top deck is placed on the boat, we should have exhaust emanating from all twelve stubs.
I have also arranged a small venturi in the main exhaust outlet so engine cooling water will be expelled with the exhaust gasses instead of squirting out of the side of the boat. It is hoped the water injection into the exhaust will not only keep it cool but will also contribute to quietening the exhaust note. It should also create quite a mist of vapour when the engine is opened up.
As you can see, we are hoping to make this boat look quite good rather than going for all out racing performance. If we want to make it perform better, we can always fit a tuned pipe at a later date.
I have'nt got any pics to put on here for you as yet but I will post some soon, so you can all have a good laugh at my feeble attempts. I think the motor is going to be a real cow to start when installed in the boat as the flywheel has no mass and is really quite small for such a large motor. I reckon the rpm will go through the roof even if I shut the carb barrel right down so I think a bit more running in at some elevated throttle settings will have to be done before I finally bolt it into the boat.
By the way can anyone tell me if these engines should be mounted on flexible rubber mounts when installed in boats? We had such a mount in the kit of parts which came with the hull and I have fitted it but obviously could'nt use it with the aircraft prop setup. Should I fit it to the boat?
Best regards everyone........ Chris


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martno1fan

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Re: shovelnose hull
« Reply #46 on: May 11, 2007, 10:28:09 AM »

good luck with it mate and yes fit the rubber mounts  ;) also use nyloc nuts on everything,a tip for starting using the flywheel loosen the plug when engines primed ready to start then attach glow plug starter and start it as normal .loosening the plug lessens the compression allowing it to spin easier.once shes running tighten the plug back up and toss her in!! what prop are you running in the boat did you get a good sized prop? dont use that small one the engine will over rev and blow up!! :o.
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retro boats

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Re: shovelnose hull
« Reply #47 on: May 11, 2007, 11:23:10 AM »

hi
 nice to here it is still going on
 as mart says use rubbrr mounts some boats without them make more noise through solid mounts than there exhaust
 also the water exit will be hard to see if it exits to the rear. that is why most exit to the side just to make sure water is flowing
                            good luck steve
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glennb2006

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Re: shovelnose hull
« Reply #48 on: May 11, 2007, 12:41:00 PM »

Hi Sheerline.

Notice in your last post you mention the engine running fast with the carb shut down. This is not uncommon on the SC engines, my .61 did it. I cured it on mine by fitting a bit of silicon fuel tubing over the fuel needle screw. It seems there was an air leak there.

It also helps to stop the screw undoing with all the vibration.

I have heard of people changing carbs., apparently the OS carb that just engines sell works well on the SC. I have no direct experience though.

As to starting, I do have direct experience!! I concur with Martin regarding loosening the glow plug, I found with mine that I had to buy a Sullivan high torque starter, the standard one I had previously just would not touch it. Maybe it was a bad one.

Good luck with it, look forward to seeing pictures as it goes on. The exhaust system sounds interesting, like retroboats, I personally like to have the cooling water flow discharging out the side as a "tell tale" so's I can tell the engine is being cooled when running.

Regards,

Glenn
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sheerline

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Re: shovelnose hull
« Reply #49 on: May 11, 2007, 02:40:28 PM »

Hi again, and yes you have a point about the water outlet, we thought we would try it as an experiment first and were hoping the resultant and water vapour trail would be sufficient indication of water flow through the motor. If it does'nt work, it is all reversible so we'll just stick a pipe out the side. I won't be able to do anything to it next week but I think with the work I have done so far, the bits will be installed and it will go together quickly in the end. We want it to look the part so a decent spray job will have to be done.
One last thing, the prop Prestwich sent me will ballance in the horizontal plane but if the blades are rotated to the vertical, it will return to the horizontal on one side only. This indicates to me that the prop has had its hole bored off centre and I don't think there is much we can do about it. I have already sharpened it and polished it as I have gone along and tried to eliminate this bit of imbalance, but to no avail. It may have to go back to Prestwich as it would appear incorrectly made.

We have got a Gold series starter which we are going to try on 24Volts, as the aircraft starter we had is no match for the compression of the 91.keeping fingers crossed on that working. We will try your tip of loosening the plug if it won't play ball.
If this had been an aero engine, I would have been tempted to machine  the cylinder head and fit a compression relief valve in there but there's no easy way to do it with a marine motor
I will come back with pics when I have the boat in some semblance of order.

Regards........ Chris   
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