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Author Topic: Propshaft angle for submerged ic power  (Read 7916 times)

Brigadair

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Propshaft angle for submerged ic power
« on: January 19, 2015, 08:35:17 PM »

Hi all


Just pondering on an issue about submerged propshaft angles.


Always wanted to keep angle as small as possible, the only limiting factors being able to screw on a decent size prop with out the blade tips catching the hull, and to leave a sufficient gap between the hull and flywheel to get starter belt under flywheel and away once engine fires up.


Question I have is as follows.


I have seen some boats achieving the above objectives even to the extent that the hull needs to have a slice / cup dremeled into it to allow the prop tips to turn without catching the hull.


My question is, will this compromise performance or enhance it? , or is it better to have a slightly larger prop angle with a minimum gap between prop tips and hull??


Hope the above makes sense?


Any comments appreciated.




Garry Dickson



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w3bby

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Re: Propshaft angle for submerged ic power
« Reply #1 on: January 20, 2015, 08:20:51 AM »

FSR-V boats normally are as close as possible. Many have a nick in the hull to get that extra millimetre. I don't believe it compromises performance.

Brigadair

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Re: Propshaft angle for submerged ic power
« Reply #2 on: January 20, 2015, 06:04:22 PM »

Thanks Ian for a prompt reply.
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martno1fan

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Re: Propshaft angle for submerged ic power
« Reply #3 on: January 21, 2015, 06:41:03 AM »

Run surface drive problem solved  :-))
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Danny

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Re: Propshaft angle for submerged ic power
« Reply #4 on: January 22, 2015, 08:48:56 AM »

Hi Garry
The drive force from a submerged shaft transmits to the boat at the point where it attaches to the hull.  So, you have forward and slight upward thrust.  The shallower the angle, the more forward thrust.  If you have too steep an angle, the lift makes the boat lighter to the point where it will start to "flop" from chine to chine (and is 'wasted' power).
The angle can only be reduced so much with a solid shaft, then you would have to use a submerged flexi drive which can be parallel to the bottom of the hull (with no lift at all)
The thrust from a propellor takes the form of an expanding spiral. This adds lift to the back of the boat and, in conjunction with the C of G, (and the hull shape) determines the ride of the boat.
From this you will see that the shaft angle and position plays a vital role in determining the 'ride' of the boat and thus, its handling characteristics.  Working out these factors can be done by maths (but I've no idea how) or by experience - trial and error :)
The two major reasons that submerged drive can be an advantage over surface drive is -
1.  Sharper cornering.  As the rudder is further forward towards the CLR (centre of lateral resistance) the force available to swing the transom is greater.
2.  Engine wear.  On choppy water, the prop (being lower) remains in contact with the water more, so the resistance is more even.  Skipping from wave to wave, especially with surface drive, loads and unloads the prop which in turn loads the engine unevenly, causing excess stress on all moving parts.
The trade off with submerged drive is lower speed due to increased drag.


Basically, working all this out has so many variables that there has to be an element of "luck" (coupled with experience) in getting it all right.


Good luck
Danny


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craig dickson

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Re: Propshaft angle for submerged ic power
« Reply #5 on: January 24, 2015, 08:09:25 PM »

Hi Guys


Fascinating input from Danny :-))  Excellent stuff to think about  :-)


In respect of your original question Garry


A small "Nick" in the hull to get that minimal prop clearance I agree should be no problem. However, I would be uncomfortable about grinding out a big section to give clearance for a big prop.


Of course, assuming that the engine is sat as low as possible in the hull, there are two ways that you can further reduce shaft angle(assuming new build):


1) Move the engine further towards the bow of the boat.
2) Have the end of the solid prop sfaft further towards the transom of the boat. And if you can not do that on an existing installation wanting to try a prop that is a tadge too wide in diameter, try some spacers to move the propeller a little further towards the transom.


Craig





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martno1fan

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Re: Propshaft angle for submerged ic power
« Reply #6 on: January 25, 2015, 11:18:01 PM »

Unless its a multi boat go with surface drive simple  %%
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craig dickson

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Re: Propshaft angle for submerged ic power
« Reply #7 on: January 26, 2015, 06:44:29 PM »

Hi Mart


Garry may have been referring to an existing boat kitted out with submerged drive, and just interested in experimenting with prop/engine combinations with that set up.


In respect of surface drive versus submerged drive, I don't think it is quite as cut and dry as to which set up is best. Arguably a hull designed for surface drive and set up properly should have significant advantages on oval type courses where sharp turns are not the key requirement.


However consider the BMPRS Championship tables for 2013 and 2014. In the smaller AA and A classes, what boats won the championships for two years on the trot? They were boats designed for and fitted with submerge drive prop shafts. So that perhaps puts a different angle on things. ;)


Danny suggested a couple of potential advantages with submerged drive set ups and I would like to suggest a couple more:
1) For beginners, the lack of the big rooster tail of prop wash makes the boat easier to see when developing one's driving skills.
2) Durability and maintenance: The solid prop shaft in my submerged drive Crusader 3, has never been removed from its tube in two years of regular racing. All I do is apply a little bit of grease through the ball race housing hole, prior to each new run. It doesn't get more maintenance free than that.


All in all though I accept that it is down to personal preference and of course the choice of propulsion may well be dictated to, by what the hull is designed for.


Finally I am interested to know what Garry is really doing with a potential modification....is he trying to up the speed of his B-Class challenger with a bigger engine?? O0 O0 O0


Craig :-))









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martno1fan

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Re: Propshaft angle for submerged ic power
« Reply #8 on: January 27, 2015, 10:11:15 PM »

How many boats are running surface drive in the classes you mentioned ?,at the end of the day on an oval course a surface drive will or should out perform a submerged drive setup.
Mart
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Brigadair

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Re: Propshaft angle for submerged ic power
« Reply #9 on: January 28, 2015, 08:58:49 PM »

Thanks Craig, Danny, Martin, and all so far with you info on this thread.


Have to be honest, my boat in question is a challenger 48 with a submerged drive.


It needs to be quicker to get from 2nd to first place using nitro power not gas.


I am experimenting with engines. Several colleagues have told me to switch to surface drive to add 20%!!


Hence I read with interest and value all contributions.




Thanks all for replying so far.


Garry





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craig dickson

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Re: Propshaft angle for submerged ic power
« Reply #10 on: January 29, 2015, 06:56:44 PM »

How many boats are running surface drive in the classes you mentioned ?,at the end of the day on an oval course a surface drive will or should out perform a submerged drive setup.
Mart


Hi Mart


The answer is very few in the AA and A-Classes. You would need to look at our website and individual race results tables to see which boats and engine combos were raced. The majority of members run hulls designed to take submerged drive propulsion in these smaller classes.
Is that due to personal preference or due to limited choice of hulls available for these smaller engines? I don't know the answer to that. {:-{ [size=78%] [/size]
http://www.bmprs.co.uk/index.html



I do take on board the general point you made.


Cheers
Craig
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martno1fan

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Re: Propshaft angle for submerged ic power
« Reply #11 on: January 29, 2015, 10:14:08 PM »

I think theres a few hulls available for the classes you mentioned to take surface drive,i think people just need to go out and buy them instead of just sticking with the old tried and tested.Can you give me some dimensions of your AA class and A class hulls.I might need to come up with something suitable if i dont allready have it available.
Mart
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craig dickson

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Re: Propshaft angle for submerged ic power
« Reply #12 on: January 30, 2015, 06:56:02 PM »

Hi Mart


It is difficult for me to offer dimensions representative of the classes because I don't know the dimensions of boats other than my own. However as an example:


Sea Spirit (popular in the AA-Class) is approximately 34 inches long and 10.5 inches wide.
Crusader 3 (popular in the A-Class) is approximately 41 inches long and 12 inches wide.
Of course these are designed for  submerged drive.


Then there is the Challenger hull mainly seen with surface drive, which has proved popular and I believe these come in lengths of 31, 43 and 48 inch lengths.


Cheers
Craig

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martno1fan

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Re: Propshaft angle for submerged ic power
« Reply #13 on: January 31, 2015, 06:38:16 AM »

Scoop was going to race one of my Cigarret hulls in bmprs but felt the cowl needed bringing forward for nitro use,not `100% sure thats necessary but ill take scoops word for it god bless him,i might need to change it and see how that works out,its def a nice boat though,shes 41" long 10"wide if memroy serves lol.Heres a few pics,one showing an air cooled sc.61 motor for an idea of room etc.Also posted a pic of the first hull from the mould alongside the deck plug been made.So far most people i know of have built her with electric power and im told she runs very well in the chop.
Mart
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black magic racing

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Re: Propshaft angle for submerged ic power
« Reply #14 on: January 31, 2015, 03:55:43 PM »

Garry
run it semi submerged mate for the best result all round,plus you already have the hole in the bottom of the hull.. :-))
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craig dickson

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Re: Propshaft angle for submerged ic power
« Reply #15 on: January 31, 2015, 04:08:44 PM »

Hi Kurt


Having seen Garry's Challenger, I think it would be quicker and easier to build a new one rather than attempt to rip out the old prop tube {-) {-)


Mart, thank you for sharing the photos - that is a good looking boat. I do think that Scoop was right though. Putting the ruler on the Sea Spirit and the Crusader gives about 22 inches from transom to the opening. And once the fuel tank (if mounted mid ships) and radio box is installed the space is soon eaten up.


I realise however that this again is down to personal preference. For me I like big openings for best access to all components as I hate trying to fiddle about trying to access things under the deck etc.


Craig
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craig dickson

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Re: Propshaft angle for submerged ic power
« Reply #16 on: January 31, 2015, 04:22:08 PM »

Hi Mart


Another consideration is the length of the tuned pipe. With my SC46 engine to keep the pipe from protruding out of the transom, the distance from glow plug to transom is 21 inches. So to fit it in your hull I would want to move the engine about 8 inches further forward than shown in the photo.


Craig
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martno1fan

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Re: Propshaft angle for submerged ic power
« Reply #17 on: January 31, 2015, 04:44:43 PM »

What header are you using for that mate,21" is a lot as i only put my gas motors at 17 1/2" from transom measured to plug and get the pipe inside just fine.
Maybe ill build a mini arrow deck for this hull that way the hatch will be plenty long enough.
Mart
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craig dickson

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Re: Propshaft angle for submerged ic power
« Reply #18 on: January 31, 2015, 05:03:48 PM »

Hi Mart, this photo was taken dry fit, shows the set up. Manifold was subsequently shortened:





Craig
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martno1fan

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Re: Propshaft angle for submerged ic power
« Reply #19 on: January 31, 2015, 05:24:59 PM »

Wow thats a loooong pipe,im sure theres much shorter pipes than that available.Thats longer than my pipes on the sparkies lol.
Mart
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craig dickson

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Re: Propshaft angle for submerged ic power
« Reply #20 on: January 31, 2015, 05:33:20 PM »

That is the Prestwich Competition pipe.
Beautifully made, strong, quiet,  and even with this low powered sports engine, it proved really good.
Craig :-)
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Brigadair

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Re: Propshaft angle for submerged ic power
« Reply #21 on: January 31, 2015, 06:47:05 PM »

Hi Mart, Craig and all.


Thanks for the advice.


Keep the topic rolling as it is interesting  even though it has deviated from original post headline.


My Challenger 48 I think is a good hull. I'm also aware that my set up with submerged drive, wanting to keep a low shaft angle and wanting to keep pipe (again the superb pmc competition pipe) inside of the hull to comply with Bmprs regs, does mean compromises on build.


On my installation, ideally I should have cut a big slot into the front deck of the challenger 48 to allow the engine to sit further forward and give better access and a better overall setup. As it happened I didn't want to spoil the lovely finish of the mould by cutting bits out of it and possibly this was a mistake.


Alternatively I should have used semi submerged drive or surface drive which Kurt correctly suggests.


I do agree with Craig, that there seems to be a gap in the market in terms of hatches used with great access especially with the smaller nitro boats. Even some of the bigger gas boats suffer!


Cheers


Garry



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Brigadair

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Re: Propshaft angle for submerged ic power
« Reply #22 on: January 31, 2015, 06:58:49 PM »

Forgot to add,


Bernard holder hulls (whilst they have been around for ages) don't have any cockpit access problems.


And many of our members still build, run, race and win races with them!


Garry
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black magic racing

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Re: Propshaft angle for submerged ic power
« Reply #23 on: January 31, 2015, 07:33:16 PM »

hope pics work
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Re: Propshaft angle for submerged ic power
« Reply #24 on: January 31, 2015, 07:35:17 PM »

this is my challenger running semi submerged drive,handles very very well,and really is one very fast boat :-))
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