Learning the ropes

[nickstick]

Hello sailors,

As I am new here you will hopefully make allowance for ignorance,BUT i am struggling to understand my Hurth 100 gearbox.It is like this.

The technical data says that the box is in forward drive when the input and output rotation run counter to each other.I can follow that

but having measured the gear multiplication manually I find that one input turn of 360 gives 120 degrees of out put in forward drive and

240 degrees in reverse.Is this as it should be?I had assumed that the forward gearing would be higher than reverse.I am wondering if

this is a modified box, or is it like that to overcome the inefficiency of reverse prop thrust so the o/p revs are higher in reverse to counteract this.

The reason for my worry is that I am putting a diesel engine/transmission in my 25 foot Broads cruiser and gathering

knowledge and parts as I go.So far I have a 25 hp Ruggerini diesel engine,a drive plate and a gearbox.The engine has no flywheel as yet

and everything beyond the gearbox is still to be purchased.I don't mind admitting I feel a bit at sea.Perhaps I am worrying unduly.

Any comment and guidance would be very much appreciated.

 

[rafiki_]

Welcome to the Mobo forum. I'm sure someone will give you a sensible answer. However I'm pretty sure on my boats that the fwd and reverse ratios have been the same. The gear selector changes rotation, nothing more. Happy to be proven wrong though.

 

[[2068]]

Having a reverse ratio that is more coarse than forwards is in the "weird, but true" category.

http://www.nathape.com/res/Logoundanderes/hurth_hb_manualowner.pdf

http://www.ericsonyachts.org/infoexchange/archive/index.php/t-9137.html

 

[nickstick]

Thanks for that, all gen will find a mark in my empty head.Does it follow that one could run the gear box in reverse position for forward drive of the boat?I wondered if the gear box was expressly Not designed to
run backwards for extended periods.If this were true it would solve one of my problems in that I do already have a left handed prop and running the box in reverse position(as indicated by the Manufct. data)
would give me the result I need.

 

[[2068]]

Thanks for that, all gen will find a mark in my empty head.Does it follow that one could run the gear box in reverse position for forward drive of the boat?

This isn't a good idea: in "reverse" the power is transmitted via an extra gear, and the power limit is lower.

 

[macnorton]

I thought the Hurth gearbox's always used the layshaft to transfer drive and could be used in either rotation.the ratio is different on these.
They can also (with a simple mod) be used on there side to get an offset drive, although Hurth gearbox's say this is not possible I have done a couple.

 

[[2068]]

 

[nickstick]

Thanks for those comments , very instructive and food for thought.It does look as though it could be run in 'B' position to give forward drive to a LH prop.and at the power rating of my
engine (23 HP) any extra transmission losses if applicable will hardly matter.Just so long as the gearbox life will not be reduced.On the Broads we have few opportunities to exceed
4 kts or come up against serious tidal flows.

 

[[2068]]

At 4 kts, I don't see it's going to be a problem running in either gear position.
You could check the gearbox temperature after a longish run: if you can fry an egg on the casing, it's become too hot

 

[Nautibusiness]

At 4 kts, I don't see it's going to be a problem running in either gear position.
You could check the gearbox temperature after a longish run: if you can fry an egg on the casing, it's become too hot

Surely the egg will slip off?

 

[nickstick]

Turning the right way

At 4 kts, I don't see it's going to be a problem running in either gear position.
You could check the gearbox temperature after a longish run: if you can fry an egg on the casing, it's become too hot

Thanks for the comments boatyfolks.I have now done the sensible thing and bought a right hand propeller.I also managed to a find a geezer on Ebay(loc.Sunderland)selling new pressure
plates for the Hurth gearbox at extra-ordinarily reasonable price so feel I am making progress.Now investigating the mysteries of stuffing boxes,glands and flexible couplings.The whole
business is turning out to be way more complex than I thought(and more expensive)However I will save a fortune on 2 stroke fuel.

I will have to find some other way of cooking my eggs I suppose.There is always a downside.

Fair winds be among you..........................Nickstick