Turns per knott.

[mylo]

Does anyone have any data on what the operating diesel rpms were at certain telegraph settings ? Eg., what rpm was the engine running at when the telegraph is set at slow, or 1/3, full, flank, etc.

Further to that, was there a reduction drive ? Was the prop rpm the same as the engine ? Given these prop rpms, what was the type VIIc speed relative to those rpms (provided calm seas, no wind, etc.) The turns per knott ?

Lots of questions. I suspect SOMEBODY knows.

Thanks in advance,

Mylo

[JWLaRue]

I've seen numbers for the

I've seen numbers for the U.S. fleetboats like:

120 RPM - 6 knots
140 RPM - 7 knots

As for the German Type VII, the rule of thumb based on the testing done with the captured Type VII HMS Graf was a straight 37-1/2 RPM per knot.

-Jeff

[mylo]

I'm assuming there was NO

I'm assuming there was NO reduction drive ? I've never seen one in any schematic or heard of there being one. Direct drive to the electrics, and direct out of those to the prop shafts. Correct me if I'm wrong.

Based on that, they type VII is "right around" 38 turns per knot.

If my math is correct, "flank" rpm (20 knots) was around 750 ? That doesn't sound too terribly fast to me. I was expecting closer to twice that.

[JWLaRue]

Here's some more information, this

Here's some more information, this from an OP-16-Z report. There's always some variation in these wartime reports....also note that not all Type VII's had the Mann diesels, so the numbers would be different between power plants.

Telegraph Speed R.P.M.

K.F. (dead slow) 170
L.F. (slow) 240
H.F. (slow) 300
2 x H.F. (3/5) 390
G.F. (3/4) 415
2 x G.F. (4/5) 445
A/K (full) 470

I can't say that I've ever heard of reduction gears being used on the Type VII....

-Jeff

[mylo]

JWL,

Given those rpms, it

JWL,

Given those rpms, it doesn't "jive" with the turns per knot, unless there was some sort of drive to INCREASE the shaft speed, which I have serious doubts on.

Hmmmm.....ok, well, I'll work with some of these numbers "kind of". I want my type VIIc/41 to operate realistically to scale, which includes shaft speeds producing relative scale speeds. That's why I'm asking these questions.

Thanks for the input.

Mylo

[davietait]

What your fogetting is that

What your fogetting is that the MAN diesels were NOT capable of reving much over 780-800rpm. We had a 500hp Mirlees Blackstone turbo charged diesel engine in our last trawler that was at full power and revs at 720rpm.

Slow rpm diesels produce a LOT of Torque low down in the rev range so you can fit a propellor with a greater pitch so you can get decent speed thru the water. Our trawler had a 2.5:1 reduction gear in the gearbox so at full speed our prop was only turning at 284rpm and giving us 10.5knots. This was a large propellor ( 6ft 4 blade ) but the theory still holds for a smaller propellor so long as the pitch is adjusted to give it more grip of the water column.

Davie

[JWLaRue]

Mylo,

I'm not sure if it

Mylo,

I'm not sure if it makes sense to look to create model shaft speeds with an eye to producing relative scale speeds........

It would seem to make more sense (certainly easier) to adjust the real speed range of the model to represent prototypical looking speeds.

-Jeff

[mylo]

JWL,

Definately, which is

JWL,

Definately, which is the basic "plan". I was just trying to put some research into the kind of reduction gear drive I'm going to have to build in order to bring, say, a 3000-5000 rpm electric r/c motor into a more "realistic" range of say around 600 or so. I'm not hung up on getting the numbers exact. I was just hoping to get my r/c prop turning at "around" the same rate at the real deal, and not pulling rpms like a blender on full speed.

Davie, as for your comments, I certainly understand the lower rpm running ranges of the larger marine diesels. What I was trying to figure out is how the rpm ranges coincide with prop speeds, which in turn coincide with turns per knot, ....which...relate to the subs speed given a certain engine rpm. My brain was having trouble figuring out how an engine/prop speed of 480 rpm (flank) equated to 20 knots (or so) of speed, given the figure of 38 turns per knot. My calculator spit out numbers (480/38=12.6 kts) that didn't work out. I appreciate the first hand info on your experience. This question has gotten more complicated than I intended. I've got what I need and have an idea of the reduction drive I'm going to build, which of course, will be dependant on the rpm range of the motors I intend to use................which is a discussion for another day.

Thanks.