Propellor thrust - transmission to hull

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I read on another thread that 10hp equates to 100kg equivalent thrust. That got me thinking. I have a 50hp Yanmar but probably only use 35hp max. So that equates to 350kg of push from the propellor. That effort must be transmitted to the hull to facilitate forward motion. I presume therefore that the engine mounts provide this function. I had previously assumed that engine mounts only dealt with the downward force from the engine weight, but that can't be so can it? Unless somehow else the thrust is transmitted to the hull? But I can't see that it can be.

Schoolboy stuff, but interested nonetheless. Any knowledge out there to educate me?
 
Sorry, above posting is not 100% correct. Many AV mounts are designed to tske prop thrust. The gearbox on the engine is normally able to take the thrust as well.
 
A1GSS said:
Prop thrust bearing. Or saildrive equiv if you have that. The engine mounts / crankshaft should not be taking the loads...
Click to expand...

I don't know whether the bearing in front of the prop on a shaft drive transmits force down the shaft and up through the P-bracket but certainly on a saildrive the only fixed points between the engine and the prop are the engine mounts.

Richard
 
The 100kg per ten horsepower figure sounds suspect to me, but the principle is correct. A normal yacht shaft drive pushes the boat along by the engine, and with softish mounts you can usually see it shift forwards half an inch or so as power is applied. For the same reason you should have a small gap between the front of the prop and the back of the cutless bearing so that the two don't touch when it all moves forward.

I assume the thrust is taken in the gearbox; when the output flange fell out the back of our Yanmar I believe there was a conical bearing at that point.

Saildrives have their own designed-in mounting arrangements; I assume the load path is a combination of the big mount at the back and the two conventional engine mounts at the front. I haven't noticed our Volvo moving much, unlike the Yanmar shaft-drive.

The separate thrust bearings that A1GSS mentions do exist (for example incorporated in the Halyard flexible drive and similar) but they're not widely used on yachts.

Pete
 
Unless you have a cunning thrust bearing device, such as an Aquadrive, all the thrust is taken by the engine mounts.
 
Jcorstorphine said:
Sorry, above posting is not 100% correct. Many AV mounts are designed to tske prop thrust. The gearbox on the engine is normally able to take the thrust as well.
Click to expand...

+1

On my boat the thrust comes straight up the shaft, through the gearbox and engine and is transmitted to the hull via 4 engine mounts. When the mounts had failed, the engine would move forwards about 2cm when under load in gear - I could see the marks on the prop shaft - was time for a major engine mount overhaul....

P
 
Well I'm happy to stand corrected, but from an engineering perspective taking the prop thrust on the engine sounds daft to me... the effect of it will presumably lead to vibration in use, and more rapid deterioration of engine mounts and crankshaft bearings.
 
A1GSS said:
Well I'm happy to stand corrected, but from an engineering perspective taking the prop thrust on the engine sounds daft to me... the effect of it will presumably lead to vibration in use, and more rapid deterioration of engine mounts and crankshaft bearings.
Click to expand...

Thats why I fitted a flanged deep groove ball bearing on my propshaft between the PSS stern seal and flexible coupling
 
A1GSS said:
Well I'm happy to stand corrected, but from an engineering perspective taking the prop thrust on the engine sounds daft to me... the effect of it will presumably lead to vibration in use, and more rapid deterioration of engine mounts and crankshaft bearings.
Click to expand...

The thrust is transferred from the rotating parts to the fixed structure in the gearbox, so it makes no difference to the crankshaft.

If it leads to vibration and deterioration of engine mounts (and I'm not convinced that it does) then these factors are allowed for in the design because that's how 98% of them are installed, in accordance with the manufacturer's instructions.

I'm curious what basis you had for the blanket assertion in post #2.

Pete
 
Rogershaw said:
Thats why I fitted a flanged deep groove ball bearing on my propshaft between the PSS stern seal and flexible coupling
Click to expand...

Pretty well all of the smaller yacht engines transmit their thrust onto the gearbox and thus through to the mountings. This has never been a problem unless you have allowed diesel to leak over you mounts for years. (in which case you may get a horrible surprise in bad weather!)

I had a Victory 40 with a 75HP Perkins mounted on 4 level mounts. Behind that was a lorry propshaft with UV joints at each end. The propellor shaft itself ended (inboard) with a taper machined 6 inches from the end and a taper trust bearing attached to a substantial frame on the hull. The lorry propshaft was connected to a flange on the end of the propellor shaft. This meant that the engine - although sort of aligned, had a large tolerance to misalignment.

A little over engineered perhaps? Wonderful to have all that power on tap on a slow turning 3 blader. go any where vessel that was and fast once the wing had piped up.
 
Nothing to get excited about. even in big MOBOs with hundreds of HP the thrust is still transmitted through the engine mounts. They are designed to do just that, but as already suggested the thrust can be taken by a separate thrust bearing such as an Aquadrive or even a fixed bearing. The main reason these are not used is because of the lack of space in many yacht installations so the engine mounts do fulfil the function of taking the thrust and dealing with the movement of the engine. Using an aquadrive does mean you can use softer mountings as they no longer have to take the thrust.
 
I have a strange set up on my Mobo. The prop shaft passes through the stern gland then there is a massive ball bearing in a huge casting that transfers all the thrust to the hull. Then there is a car style prop shaft with universal joints on each end. This little shaft connects to the gearbox with absolutely no thrust...... otherwise the universal joints would fold up!!!
This allows the engine to fit horizontally on its mounts even though the shaft with the prop on it is at a steep angle. As a consequence my engine mounts last forever!!!
 
Alpha22 said:
I have a strange set up on my Mobo. The prop shaft passes through the stern gland then there is a massive ball bearing in a huge casting that transfers all the thrust to the hull. Then there is a car style prop shaft with universal joints on each end. This little shaft connects to the gearbox with absolutely no thrust...... otherwise the universal joints would fold up!!!
This allows the engine to fit horizontally on its mounts even though the shaft with the prop on it is at a steep angle. As a consequence my engine mounts last forever!!!
Click to expand...

That's not strange, that's good. Think a lot of it, and bless the person who designed / installed it.
 
A1GSS said:
Well I'm happy to stand corrected, but from an engineering perspective taking the prop thrust on the engine sounds daft to me... the effect of it will presumably lead to vibration in use, and more rapid deterioration of engine mounts and crankshaft bearings.
Click to expand...

The thrust is most likely to be transmitted from shaft to anti vibration coupling to gearbox flange to gearbox output shaft to gearbox casing to engine block to engine mounts. I cant see why any axial vibration will carry past the anti vibration coupling.
 
In my case I have a rigid propshaft so to have a flexible engine I have to have a flexible coupling and I did not like the propeller thrust being transmitted to the gearbox through the flexible coupling.

The thrust is of cause in both directions (forward and reverse) so any thrust bearing must take thrust in both directions so if you go for a thrust bearing that resists the load in both directions.
 
Robih said:
I read on another thread that 10hp equates to 100kg equivalent thrust. That got me thinking. I have a 50hp Yanmar but probably only use 35hp max. So that equates to 350kg of push from the propellor. That effort must be transmitted to the hull to facilitate forward motion. I presume therefore that the engine mounts provide this function. I had previously assumed that engine mounts only dealt with the downward force from the engine weight, but that can't be so can it? Unless somehow else the thrust is transmitted to the hull? But I can't see that it can be.

Schoolboy stuff, but interested nonetheless. Any knowledge out there to educate me?
Click to expand...

You are correct that on a typical shaft drive AWB, the thrust from the propeller is transmitted to the hull via the engine mounts. This means that the mounts have to be stiff enough to transmit the thrust, meaning that they are stiffer than required to dampen vibration, and more vibration than necessary is transmitted to the hull.

Posher boats have a thrust-bearing (Aqua-drive or similar) which transmits the thrust to the hull, and allows the fitting on softer engine mounts and makes for less hull vibration.
 
Chris_Robb said:
Pretty well all of the smaller yacht engines transmit their thrust onto the gearbox and thus through to the mountings. This has never been a problem unless you have allowed diesel to leak over you mounts for years. (in which case you may get a horrible surprise in bad weather!)

I had a Victory 40 with a 75HP Perkins mounted on 4 level mounts. Behind that was a lorry propshaft with UV joints at each end. The propellor shaft itself ended (inboard) with a taper machined 6 inches from the end and a taper trust bearing attached to a substantial frame on the hull. The lorry propshaft was connected to a flange on the end of the propellor shaft. This meant that the engine - although sort of aligned, had a large tolerance to misalignment.

A little over engineered perhaps? Wonderful to have all that power on tap on a slow turning 3 blader. go any where vessel that was and fast once the wing had piped up.
Click to expand...

We have the same set up on our Trintella 44 with an 86hp engine. The industrial Plummer block houses a 50mm roller ball bearing that supports the shaft. The shaft is grooved with a locking sleeve machined to match the groove. That locks the shaft from moving longitudinally. The inboard end of the propeller shaft then flanges onto a Lorry prop shaft. The lack of longitudinal movement allows me to use a deep sea seal that never leaks. I know these are not everybody's idea of the perfect seal but it works very well on this particular set up due to the total lack of longitudinal movement.
 
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