Propeller joints

[Dave Hudson]

I'm not that knowledgable on the actual props and how they were joined to the ship. I'm imagining the shaft going through the wing and ending at the propeller, but how did the hull and propeller join? Obviously the phisical propeller didn't attach to the hull, but that would mean that there was a gap (however small) between the two. Wouldn't water leak into the stern through the opening? How was that controlled?

David

 

[David G. Brown]

David -- Explained simply, the engine connects to the propeller shaft through a thrust bearing inside the engine room. It is on this bearing that the thrust of the propeller "pushes" to move the ship forward.

Aft of the bearing, the shaft goes through a "stern gland" or "stuffing box" (both terms in use). This is nearly watertight fitting that is designed to allow free rotation of the shaft. "Nearly watertight" is correct because it is desireable that some water leak through to lubricate the shaft against the packing material. (Some very new designs eliminate this leak.)

The stern gland is attached to the afterpeak bulkhead and beyond that the shaft rotates in a water-filled stern tube. There may be bearings in this tube to guide the shaft and prevent vibration. Water in the tube helps lubricate the bearings and a forced flow of water is sometimes used to prevent corrosion. At the tail end the shaft is tapered and a keyway is cut to allow attaching the propeller.

The stern gland or stuffing box is always an area of concern. Failure can result in rather rapid flooding.

-- David G. Brown

 

[Amanda McNeilly]

Could anyone please tell me how fast the propellors spun round.

 

[Mark Chirnside]

The wing propellers 83 r.p.m. at full speed; the central turbine-driven propeller 190 r.p.m. at full speed.

Normally, revolutions were about 75 r.p.m. for the wing propellers and 165 r.p.m. for the central propeller, delivering some 21 knots.

 

[Mark Chirnside]

David,

Regarding our earlier revolutions discussion, one of Olympic's pitch combinations was 14'-6" central propeller and 33'-0" wing propellers - central propellers with a roughly 125 sq. ft. surface and 165 sq. ft. wing propellers. This varied of course throughout her life, but the ratios of pitch remained roughly the same.

The central propeller is smaller, and its pitch is less than half the wing propellers. Might this account for the revolution differences on its own? I am no propeller expert but the pitch differences seem great just as the revolutions are.

Best regards,

Mark.

 

[David G. Brown]

Mark -- The pitch differences would be the result of different output (tail) shaft speeds. It is possible that the center screw revolved at some different speed than the outboard screws, although that does not sound like standard naval engineering practice. But, hell, I'm no naval architect.

It is possible that H&W chose to spin the center prop at an "off" RPM to reduce the inevitable vibration from propellers. The thought may have been to have the vibration of the center screw "fill in" between the pulses from the outboard props. Just a thought, but the Olympics were renowned for lack of vibration.

However, for the three props to work in unison they all have to push the ship forward the same distance in one revolution of the largest pitch propeller. So, if the center prop turns twice while the wing props turn once, there should be no problem. The center prop could even have turned 2.35 times (or some other odd number) in one revolution of the outboard props, if that produced less vibration.

The area of the blade has only an indirect influence on revolutions. Blade area provides the thrust. Slow-turning tugboats have blades with enormous area. However, the more area the harder it is on the engine to spin the prop. So, high-speed boats such as racing hydroplanes often have props with blades only an inch wide or so.

Getting the "right" combination of pitch and blade area has become easier in this computer-aided era. In 1912, however, it was a black art that involved a lot of "cut and try." This explains the somewhat regular changing of propellers.

Class dismissed before the teacher wanders any farther into his lack of knowledge.

-- David G. Brown

 

[Mark Chirnside]

David,

<FONT COLOR="ff0000">It is possible that H&W chose to spin the center prop at an "off" RPM to reduce the inevitable vibration from propellers. The thought may have been to have the vibration of the center screw "fill in" between the pulses from the outboard props. Just a thought, but the Olympics were renowned for lack of vibration.

That's interesting, certainly. We know various changes were made to the propellers early in Olympic's life. Also, I have some information on H&W engineers who were aboard Olympic in 1911. They had equipment to measure the number and strength(?) of propeller impulses - just for their records, I expect - and they had to take the engines up to over eighty or eighty-one revolutions to get any noticeable increase in vibration: even then, the ship was still lacking vibration and quite comfortable.

I'll be thinking this subject over.

Best regards,

Mark.

 

[David G. Brown]

Mark -- I divided the pitches and found no logical relationship. My thinking is that they intended the center prop to do 2X whatever the wing props were turning. Any other relationship would have required some quick calculations on the part of the engineers to match the speed of the turbine shaft to the wing props.

Never assume that the engineers intended a 14.5 foot pitch for the center wheel. My experience is that the "right" prop often translates to "the only one in the yard that fit the shaft." Or, somebody could have gotten the casting orders mucked up. Maybe the wing props were supposed to be 29' pitch, but turned out differently.

Things are often wonky in the real world. I once drove a ferry that had twin 4-blade props. The boat vibrated and we could never get the engines in sync. Not to mention it liked to yaw to port a lot. So, we had the props checked during our off season. The shop reported that we had 7 different diameters on the 8 blades with 4 different pitches. The owner balked at replacing the wheels, so the shop cut down all of the blades to the same diameter and re-pitched everything to one standard. If you ever look up the specs on that boat, you'll get numbers for the props that aren't even close to what's really down there doing the work.

-- David G. Brown

 

[Mark Chirnside]

David,

I wanted to post that table of impulses versus revolutions that we were talking about. Bear in mind that the figures for the turbine are sometimes on the high side — turbine revolutions were 175 to 180 revs with the wing propellers at 80 — and not that well correlated to the wing propeller revolution figures. I will only quote parts of the table, which was done a long time ago; thus, it assumes the turbine engine’s revolutions are the same as the central propeller, which we were discussing might not have been the case. However, even if these figures are wrong, it gives an idea of the build-up of revolutions/impulses. Bearing in mind

[ul][*]the ship’s smoothest speed was 78 revolutions; [*]and her ‘roughest’ about 81 revolutions, although even at that speed she was quite smooth.[/li][/list]

Wing propellers	Central propeller	Impulses
83 rpm	190 rpm	1258 impulses
82 rpm	187 rpm	1240 impulses
81 rpm	185 rpm	1226 impulses
80 rpm	182 rpm	1208 impulses
79 rpm	179 rpm	1190 impulses
78 rpm	176 rpm	1172 impulses
77 rpm	173 rpm	1154 impulses
76 rpm	170 rpm	1136 impulses
75 rpm	167 rpm	1118 impulses
70 rpm	152 rpm	1028 impulses
69 rpm	149 rpm	1010 impulses
68 rpm	146 rpm	992 impulses
50 rpm	92 rpm	668 impulses

Thoughts?

Best regards,

Mark.

 

[David G. Brown]

Mark -- I think that the "key" to what you are doing is harmonics. My suspicion is that some portion..or all..of Titanic had a resonant frequency at 1226 impulses and that 1172 impulses was a null point. However, that's the limit of my knowledge of this subject. Both of these numbers are so close to 20 cycles per second that I haven't a clue as to why one would be rougher than the other. However, I have seen things that tell me this is possible. Perhaps the vibration of the recip engines got "in synch" with the center propeller at 81 engine revolutions. The trouble is that we don't have any way of experimenting to find out.

-- David G. Brown

 

[Mark Chirnside]

Hi David!

<FONT COLOR="ff0000">The trouble is that we don't have any way of experimenting to find out.

I'll still be giving this a lot of thought though.

Two points with Mauretania and her impulses:

Propeller Revolutions	Total Impulses
<FONT COLOR="119911">180 rpm	<FONT COLOR="119911">2880 impulses
221.5 rpm	3384 impulses

[ul][*]<FONT COLOR="ff0000">N.B. The latter figure is taken as all-out 'full speed,' being the average number of revolutions for her fastest Eastbound voyage in 1929 which was 27.22 knots; the best run was at 27.65 knots.[/li][/list]

Thus, the first figure of 180 revolutions is the ship's normal full speed, equating to some twenty-five knots as I understand it, yet the other figure which gives a much greater number of impulses actually indicates 17.5 percent increase. Logically, it would be predictable that the lower number of impulses would be the smoothest speed, but in fact it is the higher number! For when the ship was full-out, a man's appendix was removed without slowing down!

Thus we have many instances that seem quite illogical and for which no explanation can be easily found, but overall I tend to go with the theory that Olympic's central propeller and its relationship with the outboard wing propellers gives the key to her smoothness.

Sorry if I've started being boring!

Best,

Mark.

 

[David G. Brown]

Mark -- You are seeking a multi-headed monster with a single net. For instance, the "roughness" at a specific revolution count may be something called "whip" in the propeller shaft. Or, it may be the result of a resonant frequency caused by the location of the piano in the 3rd class area. And, you can't compare two ships...not even two from the same blueprints. Each will be different.

The resonant frequency thing can be amusing. I once drove a relatively expensive car (I had gainful employment in my youth). One day I discovered a 5 pound block of steel bolted to the frame inside the left wheel well. My dealership at first said it wasn't there. However, after enough prodding they admitted it was a "harmonic dampener" put there by the factory to smooth out the ride. To my knowledge that block of metal is still there, rusting in some junk yard.

Unless you can find some H&W internal memos, you may never get closer to what you seek than you already are. Some of this was "cut and try," which may explain the many prop configurations over the years.

I was hoping somebody with real knowledge of props would jump in, but I think the subject drove him crazy.

-- David G. Brown

 

[Parks Stephenson]

I was hoping somebody with real knowledge of props would jump in, but I think the subject drove him crazy.

Dave,

I think he is just not in the mood right now to be "screwed."

Parks

 

[Michael H. Standart]

>>Groan<< Parks, you are bad!

LMAO!
Michael H. Standart

 

[Mark Chirnside]

I am beginning to think internal H&W memos may hold the answer. Doubt they've survived, though.

Best regards,

Mark.