Help me re-engineer my Catamaran lifting drive leg

[NahanniV]

Hi,

I'm on a Wharam Tiki 46.

I am faced with a broken propulsion system that either needs to be fixed or replaced.

Currently the system is a single BetaMarine 28hp diesel in a central pod. It drives the prop through a 2:1 transmission and a very long (14') lifting drive shaft with a universal joint. Here are some pictures from the original build:





When I described the system to an engineer friend, he said that "it violates a good number of basic design principles".

The system has worked for about ten years and 30K miles with various problems fixed along the way, but recently the propeller shaft broke.
At this point all the drive leg components need to be replaced. The engine and transmission seem to be in very good condition.

The problems with the existing system are:
- noisy
- vibration at different RPM
It seems like the noise and vibration may be mostly due to the thrust from the prop being transmitted thru the 14' prop shaft and the single universal joint into a bearing that is not really meant to take that kind of thrust.

I looked at replacing the long drive shaft with a Sillete catamaran drive, but they are no longer being manufactured.

I'm thinking of adding a thrust bearing on the aft face of the engine pod angled to split the angle between the two shafts and with u-joints on either side at right angles to each other.

Any thoughts would be appreciated.

[Cheechako]

That is weird! It's like someone was trying to emulate the Thai stern drive apparatus or something. I can't help much other than to say get it modded to a normal type of outdrive of some sort if you can. Could you add a mid ship "pod" below the underhull to receive the shaft and transmit it to an outdrive leg?
A shaft that long and unsupported is going to start whipping as it wont stay straight, it deflects or bends (not permanent bending, just "springing")

Sonic Marine makes some deep outdrives I think.
Gemini Cats use an outdrive, what do they use?
In the end buying another engine and putting one in each hull may be just as cheap!

[NahanniV]

Originally Posted by Cheechako

Sonic Marine makes some deep outdrives I think.
Gemini Cats use an outdrive, what do they use?

Was all ready to install a sonic drive leg, but when I contacted the company to arrange payment and delivery they told me they are no longer producing them. Despite the fact that they still have them on their website.
Gemini used sonic.

Originally Posted by Cheechako

In the end buying another engine and putting one in each hull may be just as cheap!

Time is another factor.
I hope to replace all the worn out components, and make some improvements in a reasonable time and for a reasonable price.

[ggray]

Thing about U-joints is that if only one joint (and if two, they must be properly aligned)and there is an appreciable angle between the shafts, the driven shaft will rotate at a fluctuating speed.

If you can replace with a constant velocity joint, that could help.

Beyond that, I don't have any experience in setting up a prop shaft with thrust bearing. Perhaps this could help:

https://www.vetus.com/en/stern-gear-...er-shafts.html

I can appreciate the simplicity of Wharam's designs! That is a long unsupported shaft though. Perhaps with new parts and a CV joint, you can smooth it out enough.

[Cheechako]

Originally Posted by ggray

Thing about U-joints is that if only one joint (and if two, they must be properly aligned)and there is an appreciable angle between the shafts, the driven shaft will rotate at a fluctuating speed.

If you can replace with a constant velocity joint, that could help.

Beyond that, I don't have any experience in setting up a prop shaft with thrust bearing. Perhaps this could help:

https://www.vetus.com/en/stern-gear-...er-shafts.html

I can appreciate the simplicity of Wharam's designs! That is a long unsupported shaft though. Perhaps with new parts and a CV joint, you can smooth it out enough.

Actually an angle is best according to the makers of shafts and joint systems. They do not want it straight. 3-6 degrees IIRC. Had it spec'd for some military small boats we made when I was managing a boat company.

[jimbunyard]

Originally Posted by NahanniV

Hi,

I'm on a Wharam Tiki 46.

I am faced with a broken propulsion system that either needs to be fixed or replaced.

Currently the system is a single BetaMarine 28hp diesel in a central pod. It drives the prop through a 2:1 transmission and a very long (14') lifting drive shaft with a universal joint. Here are some pictures from the original build:


When I described the system to an engineer friend, he said that "it violates a good number of basic design principles".

The system has worked for about ten years and 30K miles with various problems fixed along the way, but recently the propeller shaft broke.
At this point all the drive leg components need to be replaced. The engine and transmission seem to be in very good condition.

The problems with the existing system are:
- noisy
- vibration at different RPM
It seems like the noise and vibration may be mostly due to the thrust from the prop being transmitted thru the 14' prop shaft and the single universal joint into a bearing that is not really meant to take that kind of thrust.

I looked at replacing the long drive shaft with a Sillete catamaran drive, but they are no longer being manufactured.

I'm thinking of adding a thrust bearing on the aft face of the engine pod angled to split the angle between the two shafts and with u-joints on either side at right angles to each other.

Any thoughts would be appreciated.

`

Would have been nice if your friend had specified some of those principles...

Unfortunately, I can't think of any simple solutions, though noise might be one, since it seems likely to me that the majority of it comes from the hard mounting of the pillow block to the square tube frame, which transmits noise and vibration to the rest of the enclosure and structure. Perhaps some sort of rubber isolation there might reduce the noise a bit...

As for the real problem, it is likely in the design of the lifting mechanism. Where did the shaft break?

A CV joint might help, though it would be just a band-aid; what is really required are two CV joints or a double cardan joint. Neither are designed for much thrust load, hence the thrust bearing would have to be before the joints, which with the current design is impossible.

To make the idea work, seems to me the only solution would be to eliminate the arced/side lifting arrangement and design an arrangement consisting of a gimbal mounted to the engine support/hull structure, to which the thrust bearing could be mounted, and then some sort of strictly vertical mounting arrangement for the propeller end.

Something like the very rudimentary sketch below...

[Sand crab]

You could look at a hydraulic drive like the one by Vetus. This would do away with the longtail entirely and you could have a prop in each hull. Or do away with the entire system and install an outboard in the pod. If the Beta is in good shape then you can get some decent bucks for it to offset the cost and it would be lighter.

[Mr B]

A shaft that long unsupported will flex and thats where your vibration will be coming from,
Your front joint with the four bolts has too be balanced or it will cause vibrations too,

You can still get parts for the Sillette Drive leg,
There is a complete Sillete drive leg for sale on Gemini Catamarans on Facebook,

[Dave_S]

IMHO... I don't think it looks too bad, the 14' drive shaft may flex, you could go to a larger diameter tube type shaft and spend a bit of xtra time shiming the alignment of the engine/gearbox to the self centering bearing to minimize the work of the flex joint at the front.

You should reduce the distance from the uni joint to the self centering bearing (down to 2-3mm and a shorter uni joint if there is one) but I don't think the thrust is going to over work the joint. It will be putting strain on the shaft from the joint to the bearing which will amplify any noise from the prop shaft flex or speed fluctuation through the SHS steel frame the bearing is mounted on.

I think it is a good example of the KISS system. They look like easy to find, non expensive components that are quick and easy to replace. Not perfect but does the job. You won't get rid of the noise and vibration but hopefully have less.

Which shaft failed and where did the shaft fail ?

[Dave_S]

Another thought, is the engine mounted directly to the SHS or through elastic mounts ? it looks like a thin wall 50 x 50 tube, it may flex under torque and create some bind.

[NahanniV]

Originally Posted by Dave_S

Which shaft failed and where did the shaft fail ?

The shaft failed at the top of what I would call the stern tube.
That's an aluminum tube that the prop shaft runs thru with water lubricated bearings at the ends and at intervals inside. It is what the swinging supports are connected to.
The shaft broke just inside the upper bearing.
The plastic bearing was quite worn, which probably worsened the situation.

[captlloyd]

Whatever you do you need a thrust bearing solidly mounted to the hull before any of the other hardware. The prop shaft needs to push against something solid is step one. I would redesign the whole system.

[RaymondR]

Originally Posted by ggray

If you can replace with a constant velocity joint, that could help.

This is the first thing to try and I suspect that a lot of the noise will disappear.

You have a long flexible shaft between the prop and the U joint which is axially loaded. Therefor it is working as a column whilst rotating and consequently prone to buckling. The U joint is also putting out-of-column inducing vibrations which will exacerbate the buckling.

The way they solved this problem in motor vehicle with solid axle rear wheel drives is they put a large diameter tube between the two U joints of the drive shaft.

I am getting on a bit age wise and a time will come where I won't want to continue to sail around in a largish sail boat. In preparation I have designed a house boat with a similar concept drive such as you have. However I intend to use a shaft drive without any U or CV joints and pivot the whole engine enclosure.

[NahanniV]

Originally Posted by Dave_S

Another thought, is the engine mounted directly to the SHS or through elastic mounts ? it looks like a thin wall 50 x 50 tube, it may flex under torque and create some bind.

There are flexible engine mounts.

[TrentePieds]

Could you talk to your engineer friend again - mebbe lay some beer on 'im ;-)?

1) Yes a Constant Velocity joint (Hardy Spicer Joint) will help. The HS type found in Austin Minis and such are MUCH better the CV joints found in cheap Chryslers. The Chrysler joints are just hotted up spider joints not worthy of the "CV" appellation. NO "universal" joint is intended to take axial thrust. That is NOT how you transmit thrust in a motorcar, and that is why one yoke of a "U-joint" is splined so it can slide axially.

2) You have a "stern tube" and you say that within it are bearings. I assume that they are "cutless" bearings lubricated by water in the tube, and that to achieve that, the bearing on the after end of the tube is a cutless in order to permit water to get in. These cutless bearings will shorten the column of the shaft and if there are enuff of them they will raise the harmonics of the shaft's vibration way above the engine RPM and any vibration set up by propeller cavitation. But cutlesses wear. Beyond a certain, in your case immeasurable, amount of wear, they will no longer serve as vibration dampers. So they must be replaced now and then. Probably at intervals of only a few years, and if any of them run dry, they will wear really quickly. Because you cannot guarantee that there will be no water ingress to the tube you cannot replace them with roller bearings.

3) The inboard (forward) beaaring in the tube NEEDS to be a thrust bearing. A tapered roller bearing is a standard solution to providing a thrust surface in this sort of situation, but mind what I just said about water ingress. Front o' the toob, and nowhere else, is the place to put the thrust bearing. The tube itself needs to be hinged forward on a revolving crossmember resting in solid bearings in both hulls so that the prop thrust is taken by the shaft and transmitted to the thrustbearing, thence to the tube walls, thence to the crossmember, thence to the hulls. This hinging of the crossmember has to be so designed that the centre of rotation of the crossmember is EXACTLY coincident with the centre of deflection of the CV joint. Or vice versa :-)

4) ALL CV joints (that I have ever seen, at least) are encased in a rubber boot filled with grease. Replacement boots are available for the cars for which a given CV joint is specified. The repair boots are split and the parts are held together with "hose clamps" (or bespoke strapping requiring a bespoke tool to apply) in order to hold them in place over the joint and contain the grease within them. The grease (and possibly the boots) should be replaced at intervals.


Hope that helps some with the concepts. This whole rig is typical Wharram. Cheap and nasty from an engineering point of view, but be not mistaken - there are some situations in life where "cheap and nasty" is just what the doctor ordered. In these days of conspicuous consumption and "mine is bigger than yours" (and more expensive!), I am increasingly in favour of "cheap and nasty", particularly if it means that you can divorce yourself from "perfessionals" :-)!

TrentePieds