Help me re-engineer my Catamaran lifting drive leg

[Dave_S]

I like the idea of Raymonds of making it all rigid and pivot everything - engine, gearbox and prop as one unit, not hard to do. You will need a two way thrust bearing (assuming the gearbox has reverse). I think I would go this way, pretty fool proof, and complies with KISS, it seems a neat solution. Part of the framework could carry bearings for the prop shaft.

[jimbunyard]

If it were made rigid, depending on the construction of the gearbox, he probably wouldn't need a thrust bearing at all. I think some are underestimating the difficulty of engineering a pivoting mount robust enough to withstand the rigors of ocean travel, especially for use in the not-necessarily-particularly rigid space between the hulls of a catamaran. Not to mention if the engine can be run for a sustained time at that inclination, or the method of securing a 400 lb chunk of iron in a fixed position (down or up) in a violent seaway.

Not saying it couldn't be done, but it's a long way from being simple...

[Mr B]

The Sillete drive leg has a Uni joint at the top of the drive leg that runs in the oil from the drive leg, Its contained in a rubber grommet, or cover, That contains the oil, With a hose clamp on each end,
But it could have an oil seal on each end that would make it independant of the drive legs oil
So only the Uni joint is running in oil, Not grease, It could also contain the thrust bearing as part of the assembly,
Im also part of the KISS brigade,

Hope the piccys help, It may give you an idea or two to help sort your problems out,

Cheers, Brian,

[atoll]

i would scrap the single engine and fit twin long shaft high thrust outboards on lifting brackets inboard of each hull .

a single screw non steerable set up as you have is absolutley useless for any thing but moving in a straight line forward untill you have some boat speed so the rudders have water passing over them to be able to steer.
probably ok if you only ever anchor but forget any close quarters manouvers in marinas or getting on a dock without a long direct approach.

if you want to keep the single engine and can't find a sillete steerable drive leg,look at fitting a mercruiser or volvo sterndrive on a lifting bracket that way you will be able to steer with the drive without needing boatspeed to steer

such as in the photo;

[Mr B]

Originally Posted by atoll

if you want to keep the single engine and can't find a sillete steerable drive leg,look at fitting a mercruiser or volvo sterndrive on a lifting bracket that way you will be able to steer with the drive without needing boatspeed to steer

such as in the photo;

Thanks Atoll,
I wasnt aware of the mecruiser or Volvo stern drive,
These would be very easily converted in case my Sillette drive leg ever packs it in,
I do prefer Diesel over petrol on my Boat,

[NahanniV]

Originally Posted by Mr B

There is a complete Sillete drive leg for sale on Gemini Catamarans on Facebook,

Can you give me a link to it ?

[Mr B]

Originally Posted by NahanniV

Can you give me a link to it ?

Unfortunately, I dont know how to do that, PC Dummy,
Join Gemini Catamarans on Face book, Its free to join,
Then search for the drive leg,
Its only a couple of weeks since I seen it for sale on there,
Its in the USA as well,

Cheers, Brian,

[RaymondR]

A hydraulic drive would give you the most design flexibility and allow you to retain the existing engine.

[atoll]

Originally Posted by Mr B

Thanks Atoll,
I wasnt aware of the mecruiser or Volvo stern drive,
These would be very easily converted in case my Sillette drive leg ever packs it in,
I do prefer Diesel over petrol on my Boat,

just bear in mind the volvo and mercruiser stern drives have the gearbox built into the drive,and the leg is only 60cm from input to output at the propellor.

your sillette drive the leg is either 80cm or 100cm from input to output and requires a gearbox,which would have to be removed from your engine and a direct drive from the flywheel of the engine fitted somehow to the sterndrive which would be mounted close to the waterline to avoid cavitation.

[NahanniV]

Originally Posted by RaymondR

A hydraulic drive would give you the most design flexibility and allow you to retain the existing engine.

OK, pump, motors, reservoir, cooler, controls,... all standard.

Other than switching to prop-shafts with stuffing boxes and motors in the hulls, are there off the shelf components to create a lifting hydraulic drive leg ?

[NahanniV]

I am getting ready to go ahead with replacing the drive leg.

I have decided to just replace and make slight improvements to what is there.

Here is a rough sketch of what I have:

[NahanniV]

This is what I am thinking of changing to:

[NahanniV]

I would welcome comments from engineering types.

Here's my parts list so far:

Prop-shaft and stern tube section:

For the most part just duplicating what was there with some small
improvements.

1* A19 Propshaft 14' From Deep Blue yacht supply
https://www.deepblueyachtsupply.com

7* ???? Cutless bearings from Deep Blue Yacht Supply
https://www.deepblueyachtsupply.com/...s-babe-bearing

-7 is every 2' in the 12' stern tube is that more than necessary ?

1* 6061 aluminum stern tube 2"dia. .375wall 12'L from Online Metals
https://www.onlinemetals.com

-the spec sheet says: extruded, I.D.1.25" do you think I will be able to
push 1.25" non metallic cutless bearings into it ?
-.375wall is thicker than what is there now(.25"), hopefully significantly
stiffer.
- the current setup has plastic exhaust and water intake tubes
lashed to the stern tube.
I am also considering replacing those with aluminum tubes either
clamped or welded to the stern tube to further increase stiffness.
They would extend to just below the waterline.
Do you think that would be worthwhile ?

5* Clamps for Stern Tube From McMasterCarr
https://www.mcmaster.com/2218t43

-using these instead of welded tabs to attach supports and control lines
to stern tube.
This avoids welding, most of the welded tabs have broken off the
original stern tube.

Drive Shaft, U-Joints:

Trying to improve this section to better absorb thrust and make the prop-
shaft turn at a constant velocity.
If I add a universal joint between the engine and the thrust bearing it
seems like a splined slip joint might also be needed ?
I think this could be done with automotive components:
https://www.dennysdriveshaft.com/p96..._1310_sli.html
I would connect the forward u-joint to the engine at near 0deg.(1deg.)
and the aft u-joint would split the angle to the prop-shaft.
The thrust carrying u-joint would operate at half the angle of the current
one.

Thrust Bearing:

Changing the mounting of the thrust bearing to split the angle between the engine shaft and the prop-shaft.
I don't know what to use here ?
Change the current pillow block to a flange mount ?
I see mounted roller bearings at McMaster, like this:
https://www.mcmaster.com/standard-mounted-bearings
But shaft sizes start at 1.25".
Or maybe this:
https://3bgsupply.com/bearings/mount...22416h-rexnord
1"shaft, locking collars on either side ? Or just mount the u-joint collars
snugly ?
I don't see stainless options.
Perhaps I can mount it inside the engine compartment.
I am also thinking of trying to isolate noise by mounting it with these:
https://www.mcmaster.com/9311k142
Any problem doing that ?

Stainless U-joint with rubber boot. from McMasterCarr
https://www.mcmaster.com/2455k26
If I end up with the Aft most U-joint outside the engine compartment I will use a SS u-joint.
Do you think this one would be OK ?
I would be inclined to drill through the u-joint and shaft and either insert a
spring pin or a bolt.

[Rumpi]

JM, since you have multiple problems I will try to explain this in steps so it's clear, and offer some solutions:
Problems:
1. Vibration: this has two sources wiplash of the long shaft and the U-joint not beeing CV.
2. Axial load: having the torque bearing after (or before if you count from the motor) articulation. U-joints can carry some axial load, but they are not happy doing it.
Solutions:
1. The elegant one is to replace the U-joint with a CV-joint designed to transmit all axial loads. Such things exist but are speciality items.
2. Move the torque bearing before the articulation and use a true CV-joint that does not carry axial loads. This will require articulation of the thrust block.
3. Remove whiplash by adequatly supporting the propshaft. This you already figured out but there is room for improvement.

I will deal with wiplash first. In order to support the shaft adequatly put a bearing every foot. If you want waterlubed bearings (cutless) you have to insure they get enough water, meaning a pressure system is needed injecting water at every bearing. A superior system is to use sealed roller bearings, in wich case you need to fit a waterseal at the prop end. Every single outboard and saildrive uses one so it's not as if it can't be done. Since most of the propshaft is not submerged you can also use a cutless at the prop end and sealed roller bearings for the rest. To assure a rigid shaft I propose: take two 1-2" fiberglass tubes and glue them togheter like the barrels of a shotgun. One will serve as water pickup, the other as exhaust. On this double tube you can now glue roller bearing carriers to support the driveshaft. Then the whole thing gets a fiberglass fairing glued onto it that aditionally stiffens the assembly and cuts the drag. Think of it as building a wingmast, rudder or daggerboard.

Now for the rest. The brutal option is to use the biggest U-joint you can find (something from a big truck maybe) to transmit the axial loads to the thrust block. It will still vibrate and be unpleasant but will last another circumnavigation. Not something I endorse but possible. You don't need another U-joint on the motor side, you already have a flexible coupling there.
You have to decide by yourself if you have the money for the elegant option.
Articulating the thrust block is not so complicated as it seems and can be acomplished cheaply. If the shaft is to only rotate up and down you simply mount the thrust block on a round bar that pivots on the frame in plain bearings. If you want it to also pivot sideways you need those plain bearings mounted on pivoting arms, basicly creating a gimball. The propshaft - motor connection is via a true CV joint, if necessary with additional length compensation. You can have the whole thing custom made in stainless steel or you can go cheap and use automotive components. Front wheel drive cars have all components you need on the front wheel. Power comes from the motor via a driveshaft with CV-joints at each end. The wheelhub is a bidirectional thrust block. The hubcarrier is made to rotate in two directions. A double whisbone suspension with kingpins would be the best for adapting to your boat since all you need to do is mount it on it's side.

[FPNC]

This whole thing scares me. With that said, I would say slow down the prop shaft with a lower gear reduction. Spin as large diameter a propeller as practical. The torque on the shaft will go up so be prepared to go larger in prop shaft diameter. Slower shaft speed and larger diameter shaft will reduce potential for harmonic vibration.