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Old 10-15-2016, 05:34 PM   #1
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Stretch my prop shaft

I've been keeping an eye online for various components for my build. I came across a prop shaft about 12" shorter than ideally need.

It could work but I would have to slide the engine back almost right to the bulkhead. It would make it tight in that area if something needed maintenance.

I'm talking about a couple grand in savings for this shaft which is in excellent condition so I would love to make it work.

Anybody have any ideas about how to gain some length to the shaft or am I out of luck?
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Old 10-15-2016, 06:03 PM   #2
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Use a "muff coupling", as it's in the engine room you
should be able to use steel shafting for the needed extension.

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Old 10-15-2016, 06:41 PM   #3
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Ted is that the kind that has two halves that are screwed together like a clamp w opposing socket head cap screws?
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Old 10-15-2016, 08:23 PM   #4
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Ted is that the kind that has two halves that are screwed together like a clamp w opposing socket head cap screws?
Yes, and keyed as well, although I have used one that wasn't keyed

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Old 10-15-2016, 11:04 PM   #5
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What about something like an Aqua drive CV shaft?
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Old 10-15-2016, 11:16 PM   #6
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What about something like an Aqua drive CV shaft?
To use that you will need a thrust bearing mounted in the boat to take the load
(push) from the prop. I don't think it would be an easy refit. I installed an Aqua-Drive once.

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Old 10-16-2016, 08:46 AM   #7
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It a brand new build so I can put anything anywhere, blank canvass so to speak.

The muff coupling may be perfect. What does the aqua drive do?
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Old 10-16-2016, 09:17 AM   #8
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The problems with a coupler as I see them are.

1. You have to get the short shaft segment properly aligned to the rest of the shaft. That should be done in a lathe.

2. Cost. I don't know how much couplers cost but I do know that ideally you will have to get the short shaft segment made up with a broached slot in it which will cost a few bucks.

3. Long term stability. You have to be certain that the coupler fits both the existing shaft and the short piece tightly and can be adequately tightened. Otherwise it will loosen up over time (lots of vibration in a prop shaft) and your alignment will go.

4. Is the shaft you have long enough to clear the packing and have room for the coupler?

I would look into having an extension welded onto the shaft. A good welder can make the extended shaft as strong the original. If you go this route do have the welded shaft inspected to make sure the welds are full penetration with no voids.
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Old 10-16-2016, 10:24 AM   #9
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I would be concerned with the dynamic balance after adding any type of splicing device
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Old 10-16-2016, 11:09 AM   #10
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I would do none of the splices, extensions, welds, etc. You have lots of time left before you need a shaft. Just keep your eyes open for one that a little longer than what you think you need. One will pop up somewhere.

You will not know actual length until hull is done and engine set. And you won't know actual engine location accurately until you layout the whole engine room. Sometimes it helps to scoot engine a little fore or aft for various reasons, you want to retain that flexibility.

What dia and what length are we talking about?

You don't want to do a new build and have obvious cobbled together bits on something as critical as the shaft line, even if it is harmless. Kind of like a zit on the nose of a pretty girl.
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Old 10-16-2016, 11:32 AM   #11
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scott,
Search McMaster Carr for shaft couplings. You will see that a 2" dia 2-piece coupling in steel is around $250 and is good for around 1000 ft-lbs of torque. Stainless ones are 2X the $$ and about 1/2 the load capacity because of lower yield point in SST.

There is a lot of torque-ripple on a propshaft, meaning you should have a safety factor of at least 4 vs. "normal" running torque, whatever that is on your boat, because this part is a mission-critical part like a propeller or the joint to the transmission.

Keep shopping for a single-piece and maybe put out a "wanted/bids" ad if you know the specs for the shaft. Have fun!
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Old 10-16-2016, 12:04 PM   #12
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I would do none of the splices, extensions, welds, etc. You have lots of time left before you need a shaft. Just keep your eyes open for one that a little longer than what you think you need. One will pop up somewhere.

You will not know actual length until hull is done and engine set. And you won't know actual engine location accurately until you layout the whole engine room. Sometimes it helps to scoot engine a little fore or aft for various reasons, you want to retain that flexibility.

What dia and what length are we talking about?

You don't want to do a new build and have obvious cobbled together bits on something as critical as the shaft line, even if it is harmless. Kind of like a zit on the nose of a pretty girl.

RIGHT ON!!

A 1 1/2" X 10' new shaft can be had for about $1K from Deep Blue Yacht Supply. A 2" shaft is twice that amount. All shaft machining are guaranteed to be true to within 0.0005."

I still remember an old Ford I owned whose driveshaft had a tiny, tiny bend in it. The car could not go over 40mph due to vibration. What will the OP do if he goes ahead with a shaft extension only to find vibration that will kill the cutless bearing?
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Old 10-16-2016, 12:26 PM   #13
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There is an assortment of shafts over on Ebay for less than $1K
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Old 10-16-2016, 01:04 PM   #14
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Out of all the professional advice I have had relayed to me or have read....this one is my favorite.....

"For heaven' sake ... why do these guys keep overcomplicating stuff so friggin much?

Have a machine shop make up a 12" intermediate shaft with flanges to match the existing items. It just bolts in place between the gearbox and the existing shaft flange."
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Old 10-16-2016, 03:07 PM   #15
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What am I missing? In order to save $1K on the correct length shaft, you would consider spending $1K to add a reasonable extension per suggestions.
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Old 10-16-2016, 03:15 PM   #16
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Out of all the professional advice I have had relayed to me or have read....this one is my favorite.....

"For heaven' sake ... why do these guys keep overcomplicating stuff so friggin much?

Have a machine shop make up a 12" intermediate shaft with flanges to match the existing items. It just bolts in place between the gearbox and the existing shaft flange."
I've made those in my shop. I doubt I would do it for under $1k. The accuracy on flange face concentricity and parallelism has to be dead nuts or you will get a shake. Not a trivial fab job.
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Old 10-16-2016, 03:33 PM   #17
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Scott,
To expand on what Ski wrote ..
Go to places where they make up prop shafts like boat shops and machine shops that regularly make up boat shafts. Soon you will probably find someone w a shaft a bit longer than you want that can be made to fit your boat easily. They will probably take your shaft in trade. Some shafts are made out of better, more expensive metal.

The Aqua Drive is the best way to drive a boat IMO. Look it up w a web search.

But a good coupling is excellent. And be aware that there is no such thing as a straight shaft. If you have several new shafts all can be tested for varying degrees of straightness. But none will ever be perfectly straight. .002" is considered straight .. as I recall. And a different degree of "straightness" will be required for various applications, lengths and dia of shaft ect.

Like Ted I installed an Aqua Drive and consider it a challanging undertaking. One needs to install a sort-of bulkhead to mount the AD to and this needs to be done very accurately. Not easy but often shims or washers can be used to fix mistakes. I know about that.
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Old 10-16-2016, 03:55 PM   #18
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Scott,
The Aqua Drive is a thrust bearing. The installer builds a bearing holder, mounting platform or whatever you want to call it but it needs to take the thrust of the engine w gear and prop. Also it meeds to hold the shaft in place laterally and vertically. The AD bearing needs to be mounted so as not to transfer much shaft vibration to the hull of the boat or the much of reasons for installing the AD would be compromised.
The shaft aft of the AD bearing will be permantly mounted in the boat and perfectly aligned to the cutlass bearing. Unless the hull gets tweeked, bent or otherwise deformed the prop shaft will never need to be aligned again. In theory anyway.
Then ahead of the AD thrust bearing and between the gear output flange and the input flange of the AD bearing the AD provides a CV joint. Just like on the front end of your car. This allows the engine to be able to move (within limits) any way it wants from vibration. Also all the thrust of the prop is taken by the AD thrust bearing so this relives the trans thrust bearing from that duty. Not that it matters but aft trans seals could last linger ??

Since the engine is no longer shaking the prop shaft the prop shaft no longer shakes the boat. And more importantly the engine can (and usually is) mounted on much softer engine mounts. And the engine mounts no longer need to absorb the thrust of the prop. So they can be designed to move in ways regular engine mounts are extremely limited.

More money but clearly a better way to go ... IMO ..
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Old 10-16-2016, 03:59 PM   #19
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Quote:
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I've made those in my shop. I doubt I would do it for under $1k. The accuracy on flange face concentricity and parallelism has to be dead nuts or you will get a shake. Not a trivial fab job.
Good to know, thanks Ski...but like a lot of projects...

What is handy...materials or professional services, etc....and what can it be done for.....

In the end...to me at least...what is the most practical, cheapest, easiest way for me to accomplish a project. Those 3 become a balancing act.

If the short shaft and flanges are that much...it probably would bump me into the right sized shaft depending on all costs of course.

Then again...like they say....friends in low places ....who might make one up for a deal.....just throwing out an idea that was passed along for the typical DIYer.
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Old 10-16-2016, 04:30 PM   #20
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...The Aqua Drive is the best way to drive a boat IMO. Look it up w a web search...
Sounds impressive but the price..? Your right more money. From their web site it looks like $1500 to $2300 plus the installation costs for Hobo. As far as increasing transmission life. We're rebuilding ours now but only for PM at 8400 hours. Call me old fashion and one who likes simple but I'll pass.
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