Is bigger BETTER?

[modelnut]

OK.

I am currently sidetracked on construction of my DeepSea Designs NAUTILUS. I have a complex warp in the aft section. Very annoying.

In the meantime...

I am building my sub in 72nd scale. So I have the blueprints sized to match. The diameter of the propeller in this scale is 4 cm which gives a scale diameter of 2.9 meters.

But Verne gave his NAUTILUS a propeller with a diameter of 6 meters. And the SKIPKACK, a real sub of comparable size - with a four-bladed propeller! - had a propeller diameter of 6 meters also.

Should I change my plans and go for propeller that is 8cm instead of the 4cm propeller that I have in hand???

This would definitely alter the aft section and rudder assembly. Would it be necessary for an R/C boat of this size?

-Leelan

http://groups.msn.com/ModelersAndHob...ar.msnw?Page=1

[bob the builder]

I'm definitely not an expert,

I'm definitely not an expert, but I think either arrangement would work. A smaller prop would most likely mean higher rpms for comparable speed. A larger prop would probably need more torque for the same speed (someone please correct me if I'm wrong).

Go with what looks best!

[Guest]

Larger propellers will generally give

Larger propellers will generally give greater efficiency (depending upon design). However there is one important factor to consider when choosing your prop size, and that is torque roll.

A larger diameter propeller will produce a larger torque moment than a small one, thus increasing the tendency for the sub to want to spin in the opposing direction.

How bad this affects the boat is depndant on the static stability of the boats design and the RPM of the prop.

This is why a lot of boats use counter-rotating props, to balance the torque effect.

Also bear in mind that a bigger prop will need lower revs, which means either sourcing a low revving/high torque motor or using gear/belt reduction with a higher revving model.

Personally, I'd go with the smaller prop- it'll give you a lot less hassle, and any efficiency lost will be insignificant, IMHO.

Regards

Andy

[modelnut]

OK!

Smaller prop equals fewer hassles.

OK!

Smaller prop equals fewer hassles. Got it.

I have a 72nd scale SKIPJACK in hand (or rather in indefinite drydock) and the coincidence of the six meter diameter with Verne's description made me wonder if I shouldn't just go whole hog and have done with it.

This will be something to think about while I fix that blasted warp!

-Leelan



Edited By modelnut on 1061483174

[tk-7642]

As mentioned above a larger

As mentioned above a larger prop is more efficient because it moves a larger amount of water but at a slower speed, as opposed to a small amount of water at a high speed (small prop) . About the torque roll you could put a compensating tab on the sub to stop the roll. I would go for the more authentic but larger prop despite the roll difficulty and need for the adjustable tab.

[modelnut]

OK. I give.

To whom do

OK. I give.

To whom do I write to get a four-blade brass propeller that is 8 to 8.3cm in diameter???

The biggest I have found is 7cm from http://www.shipsnthings.com/ . (I haven't contacted them yet...)

And does anyone have a guess as to how much such a propeller might cost?

-Leelan

[modelnut]

OK. I have my propeller

OK. I have my propeller in hand and the redesign is proceeding slowly.

In fact this 8.5cm propeller alters the old plans so much that I have cut a new silhoette out of plexi and simply started over.

In addition to the more "authentic" propeller I have decided to include the keel. It should present a +90cm by 1cm flat plane against any tendency to roll. It is by no means the same sort of keel as found on a sailboat! No upside-down shark fin.

Thinking ahead, would it also help to concentrate the balast along the centerline just above the keel? Just thinking out loud here. My sub building skills are mostly imaginary right now. I am just thinking things out logically in a sort of thought experiment. Trying to anticipate trouble.

Thanks!
-Leelan

[bob the builder]

Leelan,


Make your keel hollow and

Leelan,


Make your keel hollow and fill it with your ballast. The lower you can keep this ballast, the more effect it will have to counter torque induced roll and keep your model stable.

I actually considered hollowing out my keel to accept ballast, but the thought of punching through to the outside scared me enough to simply add the weight in the bottom of the hull...

[captain nemo12]

I agree Bob. Leelan, if

I agree Bob. Leelan, if your sub is "rolling" in the water instead of going forward . Adding ballast on the bottom of the hull is........ yeah, nescessary.

My Nautilus's prop is quite big (approx. 8.5 inches) I wonder if the torque will be high....... plus, my Nautilus is shark shaped, I wonder if the dorsal fin will prevent this from happening once submerged.

[captain nemo12]

Correction! It's 8.5 CENTIMETERS http://www.subcommittee.com/forum/icon_smile.gif

Correction! It's 8.5 CENTIMETERS not inches.

[bob the builder]

CaptainNemo12,


If I'm not mistaken, your

CaptainNemo12,


If I'm not mistaken, your large dorsal fin will cause roll in your model when you turn. Large sails on actual submarines such as the US Skipjack cause the submarine to "bank" into the turns. It looks really cool, but can cause control problems as your dive planes start acting like rudders and your rudders as dive planes...

[captain nemo12]

I will try to limit

I will try to limit the fin size as much small as possible, plus I needed a place to store the antenna, what more better than the dorsal fin itself? I tried storing it inside the hull but I find it quite fragile and not very bendable.........

I'll see what happens when it comes to Testing Day.

[carcharadon]

None of my subs do

None of my subs do that, but then there are no moving parts, no prop.

http://home.comcast.net/~tyourk/4foot.html

[modelnut]

Ahoy Nemo!

My sub isn't rolling

Ahoy Nemo!

My sub isn't rolling YET. It isn't really off the drawing board yet. I haven't even cut the first bulkhead or foam sheet.

-Leelan

[bob the builder]

Carcharadon,


Do either of your jet

Carcharadon,


Do either of your jet nautili experience a tendency to want to dive under full speed? Your hull form is close enough to Disney's that I was wondering if you had the same problem that I do with her wanting to dive under power...