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Old 04-11-2019, 12:12   #1
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double universal joint to connect the drive shaft to the engine

I'm thinking of up-sizing the engine on my sailboat. Problem is, installing the larger engine will require chopping the top off the current GRP mounting cradle, and probably eliminating the oil well underneath it; this in order to get the larger engine in line with the prop shaft, and within the inclination angle tolerances specified by the manufacturer.

I'm reluctant to do any of those things for a number of reasons.

Has anyone used a double universal joint to resolve this conundrum? Not an engineer, but intuitively, that's what universal joints were invented for; i.e. transmit power at different angles, and it works in a zillion different scenarios and applications.

This said, I never seen this solution on a small sailing boat (which does not, of course, means it's not used), and wonder why. Any insight much appreciated!

Cheers
Rafa
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Old 04-11-2019, 12:32   #2
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Re: double universal joint to connect the drive shaft to the engine

U-joints cause vibrations. The sharper angle, the bigger the vibration and the faster they wear out. Unless you're careful, u-joints fling grease.
What will you gain with a bigger engine if you're making hull speed now?
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Old 04-11-2019, 12:36   #3
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Re: double universal joint to connect the drive shaft to the engine

Quote:
Originally Posted by seadago View Post
I'm thinking of up-sizing the engine on my sailboat. Problem is, installing the larger engine will require chopping the top off the current GRP mounting cradle, and probably eliminating the oil well underneath it; this in order to get the larger engine in line with the prop shaft, and within the inclination angle tolerances specified by the manufacturer.

I'm reluctant to do any of those things for a number of reasons.

Has anyone used a double universal joint to resolve this conundrum? Not an engineer, but intuitively, that's what universal joints were invented for; i.e. transmit power at different angles, and it works in a zillion different scenarios and applications.

This said, I never seen this solution on a small sailing boat (which does not, of course, means it's not used), and wonder why. Any insight much appreciated!

Cheers
Rafa

There are a lot of issues with what you propose. Let's start with "chopping the top off the current GRP mounting cradle." What you'll find id it is not (probably) solid GRP, but is actually a structural wood stringer. That stringer transfers all the load of the drive shaft to the hull. By "chopping" it up you will reduce it's stiffness, and strength.

Do NOT do this without having a marine architect review the modifications to be sure they will be strong enough after you are done!!! This isn't something you can fool around with in ignorance and just hope it comes out right.
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Old 04-11-2019, 12:40   #4
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Re: double universal joint to connect the drive shaft to the engine

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Originally Posted by Lepke View Post
U-joints cause vibrations. The sharper angle, the bigger the vibration and the faster they wear out. Unless you're careful, u-joints fling grease.
What will you gain with a bigger engine if you're making hull speed now?
Hi Lepke
Thanks. That's exactly the point. I'm not making hull speed now!
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Old 04-11-2019, 12:47   #5
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Re: double universal joint to connect the drive shaft to the engine

What you are looking for is a marine rated CV coupling. As you may know, these permit some really bad angles as your front wheel drive runs your bouncing front suspension. Beware however that they like a Z type offset. This means that a plus angle is offset by a minus offset angle.

Google search. marine cv joint drive shafts

These can handle some tough angles with minimum vibration. I have an Aquadrive in my drive line. These make it possible for idiots and amateurs to adequately line up a drive.

A universal joint is different than a CV. A universal joint generates a sinusoid also output, causing vibration. Only an exactly perfect installation of a double U joint will be without vibration. A CV joint more easily handles error and it removes thrust from the engine shaft entirely.

Another option is an offset gear box with or without reduction.

Is your real problem an improperly pitched prop? Too steep a pitch will prevent the motor from reaching the top of its rpm-torque curve. Too shallow and itís just spinning in place.
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Old 04-11-2019, 13:00   #6
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Re: double universal joint to connect the drive shaft to the engine

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Originally Posted by billknny View Post
There are a lot of issues with what you propose. Let's start with "chopping the top off the current GRP mounting cradle." What you'll find id it is not (probably) solid GRP, but is actually a structural wood stringer. That stringer transfers all the load of the drive shaft to the hull. By "chopping" it up you will reduce it's stiffness, and strength.

Do NOT do this without having a marine architect review the modifications to be sure they will be strong enough after you are done!!! This isn't something you can fool around with in ignorance and just hope it comes out right.
Thanks for your input. Unfortunately, my cradle is solid moulded GRP and it is glassed onto the hull. Also, as said, I will have to eliminate the oil well to accommodate the larger engine sump. That's major structural work.

I agree, it's not an amateur mechanic's job! I am surprised, however, that of the two engine installers I've consulted, both suggested modifying the cradle, and none of them suggested a double universal joint.

Now, u joints (double Cardan joints) are used from go-carts to tractors to combine harvesters to goodness knows what other static applications, with far more powerful engines and loads.

Sometimes things are not done in a particular way, just because they have never been done in that particular way. No other good reason. I wonder if this is the case in this instance.
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Old 04-11-2019, 13:14   #7
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Re: double universal joint to connect the drive shaft to the engine

A cardan joint is just another name for U-joint. Read my post above. If you do this, only a CV joint can do this. It is really important that the prop thrust is carried by a thrust bearing in combination with the CV shaft. Thrust in a double U joint is a mechanical train wreck. Please go to the Aqua drive site and learn about these devices. They are made to separate and contain the thrust at the prop shaft’s shaft log and transmit pure torque so your crank bearings are not ruined. Without a thrust bearing, the Z offset shaft is like pushing on an S shaped noodle.

BTW, I am the chief engineer, PE, retired from a custom machine builder with over 40 years in the business. I have designed and built these things up to 5000 HP with 30 inch shafts.
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Old 04-11-2019, 13:31   #8
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Re: double universal joint to connect the drive shaft to the engine

Quote:
Originally Posted by seadago View Post
Hi Lepke

Thanks. That's exactly the point. I'm not making hull speed now!

A general engine guideline for yachts with a displacement hull is the weight in tonne multiplied by 4 hp.
Before replacing your engine and spending big money , I would personally considering to check if your propeller have the wrong pitch, size or design. The lower pitch makes the engine reach maximum rpm at slower speeds. Conversely, a higher pitch will deliver greater top speeds, but slower acceleration.
A quick test you can easily do is,after warming up the engine, try to know what is the maximum RPM you can actually reach by doing a short test. If according your engine specs, you cannot reach the maximum RPM, your prop pitch is too high. On the opposite, if you can go over the maximum RPM, it is too low.
The number of blades and prop diameter should also match the weight and underwater drag characteristics.

Here is good links on the subject:

https://www.pbo.co.uk/gear/choose-ri...ler-boat-62058

http://www.oceannavigator.com/Januar...-over-propped/
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Old 04-11-2019, 13:35   #9
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Re: double universal joint to connect the drive shaft to the engine

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Originally Posted by seadago View Post
Thanks for your input. Unfortunately, my cradle is solid moulded GRP and it is glassed onto the hull. Also, as said, I will have to eliminate the oil well to accommodate the larger engine sump. That's major structural work.

I agree, it's not an amateur mechanic's job! I am surprised, however, that of the two engine installers I've consulted, both suggested modifying the cradle, and none of them suggested a double universal joint.

Now, u joints (double Cardan joints) are used from go-carts to tractors to combine harvesters to goodness knows what other static applications, with far more powerful engines and loads.

Sometimes things are not done in a particular way, just because they have never been done in that particular way. No other good reason. I wonder if this is the case in this instance.
Don't forget, the shaft on a boat is fundamentally different that a drive shave on a car. The driveshaft on a boat transmits all of the drive load of the prop, both push and pull, to the engine mounts. If you have a flexible, or other than dead straight, drive shaft you need to have something else take that load. It can be done, and has been done. but is non-trivial.

I think you will find that boat engine mechanics might know engines, but they do NOT know hull design. DON'T let them cut up your boat.
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Old 04-11-2019, 14:59   #10
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Re: double universal joint to connect the drive shaft to the engine

Hi, if you need to up the horsepower and can only achieve it with an engine that requires the modifications you have described I would go down the path of an offset drive gearbox, allowing the new engine to be mounted without cutting up the bed and not requiring flexible drive devices. Just my 2c worth.
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Old 04-11-2019, 18:39   #11
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double universal joint to connect the drive shaft to the engine

I owned a Sabre 38 mkII. It had an aqua drive. Offset shaft. Two cv joints in a single boot. And a thrust bearing as well. The thrust bearing had a compliant mount.
Once (and I was fixing a non-factory repair) I dug out the shaft strut and lined it up properly and realigned the thrust bearing and brought the engine from way out to more or less ok and had the shaft prop and flange straightened and fit to each other the vibration went to super smooth.
Oh yes and I had to replace the aqua drive early on (delivery home) because the PO (or his yard) had not realized that the grease all over was a thing. The boot was leaking.
Aqua drives have specified offset and angular misalignment tolerances. Stick with them and all is well. Go outside of them and itís a problem.
Iím not complaining here. Once the system was brought into spec it was great. Nil maintenance. Just be aware there are limits you need to stay within.

Btw there was nothin flexible in the shaft part of that drive train. Flexible mounts yes.
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Old 04-11-2019, 18:50   #12
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Re: double universal joint to connect the drive shaft to the engine

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Originally Posted by Uncle Bob View Post
Hi, if you need to up the horsepower and can only achieve it with an engine that requires the modifications you have described I would go down the path of an offset drive gearbox, allowing the new engine to be mounted without cutting up the bed and not requiring flexible drive devices. Just my 2c worth.
A further comment, a more powerful motor will likely require a stronger gearbox, so an offset output model to suit the new motor would likely add very little to the cost.
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Old 04-11-2019, 18:50   #13
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double universal joint to connect the drive shaft to the engine

Yes you will need to CV joints, not U joints and a thrust bearing
You could probably just have a Sigma Drive and not have a seperate thrust bearing as they are a Rzeppa joint and that can accept thrust.

However you would have to drive me kicking and screaming down that road just to gain a knot or two. If fact Iíd advise you to sell the boat and buy one that will make the speed you want, youíll come out ahead doing that.
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Old 04-11-2019, 19:21   #14
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Re: double universal joint to connect the drive shaft to the engine

Stupid question here. Maybe the prop is not the right pitch and diameter? Re-prop is waaay cheaper and simpler.
Iíd triple check that.
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Old 04-11-2019, 20:17   #15
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Re: double universal joint to connect the drive shaft to the engine

Quote:
Originally Posted by seadago View Post
I'm thinking of up-sizing the engine on my sailboat. Problem is, installing the larger engine will require chopping the top off the current GRP mounting cradle, and probably eliminating the oil well underneath it; this in order to get the larger engine in line with the prop shaft, and within the inclination angle tolerances specified by the manufacturer.



I'm reluctant to do any of those things for a number of reasons.



Has anyone used a double universal joint to resolve this conundrum? Not an engineer, but intuitively, that's what universal joints were invented for; i.e. transmit power at different angles, and it works in a zillion different scenarios and applications.



This said, I never seen this solution on a small sailing boat (which does not, of course, means it's not used), and wonder why. Any insight much appreciated!



Cheers

Rafa


According to your personal information, your boat is a Freedom 30 cat ketch and originally, the boat was fitted with a Japanese Yanmar 2GM20 diesel engine of 18 hp with a standard two bladed propeller that most owner replaced with a three bladed propeller.

18 HP should be plenty enough for a boat of 3 T displacement.

Following is a good article about your boat:

https://www.practical-sailor.com/iss...30_4860-1.html
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