stuffing box

25 BHM

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looking into a new shaft and box what are your thoughts on stuffing boxes styles and brands

25 BHM W/210cummins/2:1 velvet drive

Thanks
 

Keelboater

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I just installed the PSS dripless shaft seals in my Bertram. I'm in Clinton if you want to take a look. PM me for info.
 

mudhake

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is the engine forward of trhe main bulkhead or aft of the bulkhead, how much fuel and where is it located. The 25 wants all the weight aft to run like crazy. I know, I have built a FEW.
 

captainlarry84

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The PSS is an excellent stuffing box. Unlike conventional stuffing boxes they do not scare the shafting. However they do not support the shaft like the conventional ones do. if your shaft is long and marginal on size you may be better off with a conventional.
 

BillD

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The PSS is an excellent stuffing box. Unlike conventional stuffing boxes they do not scare the shafting. However they do not support the shaft like the conventional ones do. if your shaft is long and marginal on size you may be better off with a conventional.

Larry or others.
Is there a formula or "rule of thumb" on "how long" of what a certain diameter shaft can be supported just by the transmission coupler and stern bearing?

Say a 1.5" shaft ?? How long is safe with just a forward and aft support ??

20"/40" rule = 5 feet of length between front and rear bearings with 1.5" shaft

Or a 1.75" ???

Curious,

Bill D
 
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captainlarry84

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KristenFormer Charter Captain
Larry or others.
Is there a formula or "rule of thumb" on "how long" of what a certain diameter shaft can be supported just by the transmission coupler and stern bearing?

Say a 1.5" shaft ?? How long is safe with just a forward and aft support ??

20"/40" rule = 5 feet of length between front and rear bearings with 1.5" shaft

Or a 1.75" ???

Curious,

Bill D
Bill, I do not know the rule on shafting from the coupler to the stern tube. However based on what I have seen 5' for a 1.5" shaft sounds reasonable. Also keep in that things such a RPMs also come into play.
The non support from the PSS comes from the fact rubber bellow has a lot of free movement which allows the carbon & SS disk plates to mate perfectly & give you water tight seal. The flax packing in tight on the shaft & the re-enforced 6" rubber coupler has very little movement.

The one very key shaft measurement is the amount of unsupported shafting from the back of the stern tube to the rear of the propeller hub. I believe but I am not sure that that measurement should be no greater that 1 & 1/2 times the shaft diameter. Therefore a 1.5 shaft should have no more that 2.25 unsupported shafting. Top photo correct length bottom photo is too much unsupported shafting.
I could be off a little if anyone has more knowledge on this stuff.
Lastly as I have stated before. If you plan on using the PSS find a good water feed high in the system & not low & never remove a zinc pencil for a source of water to feed the seal.

kristen 009.jpg

178923[1].jpg
 
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captainlarry84

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looking into a new shaft and box what are your thoughts on stuffing boxes styles and brands

25 BHM W/210cummins/2:1 velvet drive

Thanks

Forgot about your question: 210 HP & 2:1 gear 1.5 shafting & either a flax or PSS would work fine.
I am assuming that your wheel will be anywhere from a 20" 22" diameter correct?
 

MASTERENEGADE

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Im NOT Mr. Educated on stuffing boxes, but since we are on the topic and this spring i plan on replacing my drip-less stuffing box in april. I was planning on going with a conventional stuffing box, and getting rid of the drip-less. The drip-less has served me well for 9 seasons. but is starting to get a lil tried. Im not sure how old it is, since it came with the boat.

I personally think the conventional stuffing box is less maintenance. I may be totally wrong. But this is my plan until i get convinced otherwise.

Good luck with your choice, and I love this DEBF, again great input on things that still learning 9yrs later.
 

Blitzen

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Bill, The rule of thum for unsupported shaft is 40x the shaft diameter

Is that right I thought it was 20x? Maybe I was thinking minimum recommended distance between the box and coupling.
 

Sailorgp

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my 2 cents

I'm a believer in the kiss principal (keep it simple, stupid). A traditional dripping flax packing with a good, strong, heavy wall stuffing box hose is a very proven, reliable, safe and simple way to get the job done. I have a PSS on my boat, it came that way when I bought it. The bellows is relatively thin neoprene, the hose fitting is plastic, the water hose that feeds the PSS is fuel line. Several points of failure possible. I like the PSS but if I had the traditional flax type stuffing box I would prefer it. I should also mention the I boat on the Great Lakes and don't mind a little lake water in my bilge.
 

BillD

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Is that right I thought it was 20x? Maybe I was thinking minimum recommended distance between the box and coupling.

John,

This article is where I referenced the the 20"/40" "rule"

Inboard Hardware: Propeller Shafts

From "Inboard Motor Installations". We hope this will provide enough information to make an informed selection from the prop shafts we offer in our Inboard Hardware section.
On a propeller driven boat of the straight shaft or v-drive configuration, it is the propeller shaft that transmits the power of the engine to the propeller. To do the job smoothly and quietly, the propeller shaft must be straight and true. A bent shaft not only causes noise, but vibrations that could ruin shaft bearings, couplings, and create other problems down the transmission line. A propeller shaft must be strong enough to take the "twist" or torque of the motor and transfer it to the propeller. While many boat owners consider only the torsional strength of the propeller shaft, some thought must be given to bending and impact strength on boats where the shaft is exposed. In today's littered waters, shaft damage from colliding with submerged or floating objects is all too common.
The size or diameter required for a propeller shaft depends on many variables, including the horsepower delivered through the shaft (called the "shaft horsepower", or SHP), the RPM of the propeller shaft, the propeller diameter, as well as the strength of the material to be used for the shaft. In the absence of any information other than propeller diameter, the propeller size as a general rule should not exceed about 14 times the diameter of the propeller shaft. This means that a 14" diameter propeller, for example, would use a 1" propeller shaft. A practical guide of shaft sizes for varying propellers may be obtained from the following chart. Note that no consideration has been given to the various materials that can be used for the shaft.
PROPELLER SHAFT SIZESDiameter of PropellerShaft Diameter10"3/4" to 7/8"12"7/8" to 1"14"1" to 1 1/8"16"1 1/8" to 1 1/4"18"1 1/4" to 1 3/8"20"1 3/8" to 1 1/2"
Another element to consider in shaft size is the support of the shaft along its length. A common installation in boats is for the shaft to be supported at the propeller and at the engine coupling with no more than one shaft diameter overhanging beyond the stern bearing (plus thread and taper). In this installation, which is considered as two rigid supports, a considerable length of shaft (sometimes exceeding 6') can be supported without an intermediate bearing. However, the placement of bearings depends on the shaft diameter, shaft speed, and strength of the material used for the shaft. While there are formulas and graphs that can be used to determine bearing spacing, a practical rule of thumb often advocated states that there should be bearings spaced not closer than about 20 times the shaft diameter, and no more than about 40 times the shaft apart. In other words, a 1" shaft should have an intermediate bearing if the distance between the engine flange and the strut or stern bearing is more than 40". This figure in practice, however, is frequently exceeded. In any case, if shaft whip occurs, an additional bearing should be installed.
 

Blitzen

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There you go Bill, good stuff.
 

Keelboater

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Larry or others.
Is there a formula or "rule of thumb" on "how long" of what a certain diameter shaft can be supported just by the transmission coupler and stern bearing?

Say a 1.5" shaft ?? How long is safe with just a forward and aft support ??

20"/40" rule = 5 feet of length between front and rear bearings with 1.5" shaft

Or a 1.75" ???

Curious,

Bill D


The 20/40 rule is not very accurate, and depending on the application, it can be way over rated. My method of solving this problem consists of three parts:

1) Solve for the strength requirements of the shaft. Calculating this will lead you to the shaft diameter that is required to transmit the torque at hand.

2) Solve for the torsional stiffness requirements of the shaft. This calculation can be solved for either the maximum distance between bearings if you use the diameter calculated in step 1, or provide a shaft diameter that is based on a known distance between bearings that you already have in your boat. However, a safety factor must be applied here, usually about 300%. This is related to the allowable angle of twist in the shaft. If solving for diameter here, and it is larger than in step 1, use this shaft diameter. If it is smaller than results from step 1, use the shaft diameter obtained in step 1 for strength.

3) Solve for shaft bending, which is another form of stiffness. The prop can create overhung loading on the free end of the shaft. This load can contribute to a bending moment along the axis of the shaft, and can contribute to shaft whipping. Also, the shaft is in compression from prop thrust as the boat is pushed through the water. This axial load on the shaft tends to want to bend the shaft, like a pole vaulters pole. When this happens, it contributes to the shaft whipping problem as well. The vaulters pole is an exagerated case of a free ended support system, and this is a fixed bearing system, so bending from compression will be reduced by the bearings in this case.

Once you know the above, you are in good shape to specify the shaft size and bearing distance for your application. I did not bother showing the calculations because this is not a math class, and people might begin to jump out of windows before they finish reading this.:lol: However, I did find a simplified one step formula by Dave Gerr in the Propeller Handbook. I have not checked it, but he claims that it provides a more accurate estimate than the 20/40 rule. The 1.5 in his formula is used for one piece shafts using ridgidly supported bearings at each end. See attached example. Dave does agree that the 20/40 rule leaves a bit to be desired, and this example compares the results. :confused:

bearing spacing.jpg
 

Toolate

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Keelboater. I am repowering and want to check the required diameter for my shaft based on my hp.

Can I use this formula to solve for shaft diameter if I know all of the other variables? If so, I will need to incorporate some factor of safety somewhere- can you elaborate on this at all? My shaft is 30 years old and was stock from Sisu so we would have to guess at the density of the SS.

What book from Gerr did this come from? Great post.
 

jerseysportfisher

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Keelboater. I am repowering and want to check the required diameter for my shaft based on my hp.

Can I use this formula to solve for shaft diameter if I know all of the other variables? If so, I will need to incorporate some factor of safety somewhere- can you elaborate on this at all?

What book from Gerr did this come from? Great post.

If you want to do it the long way , yes. If not, call your shaft manufactures (aquamet) tell them what your doing, they will advise you in the correct direction
 
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