re stuart turner drop down g/box

[maggier]

thanks for your comments after my posting (22/01/06) now i know the the drop down g/box is infact a reduction gearbox am i right in thinking that if i install one i can go up on the prop size thus giving my boat more power i,m on the bristol channel and we have the second fastes tidal flow in the world and i,m a bit worried about fighting against the tide with just a 10hp engine, my boat is a 22 foot heavy old clinker cheers les /forums/images/graemlins/confused.gif

 

[Peterduck]

By having a slower propshaft one can use not just a larger diameter prop, but a coarser pitch one as well. The selection of prop specifications is very complex, and you really should talk to Brunton's to get it right. 'Skene's Elements of Yacht Design' has a brief treatment of propeller design, Dave Gerr's 'The Propeller Book' is more thorough but might lose you halfway through. If you are 'fighting the current' of the Bristol Channel you are not planning your boating trips properly. Plan your trips to avail yourself of the beneficial flow of the water, out and back. Over the years it has given rise to the expression 'Go with the flow'. It may mean that a certain destination may be unreachable on a particular weekend. That makes it the wrong destination; pick another. Similarly with the wind. If there is wind against the tide you're going to have a miserable passage. Smarter to wait for a better slant of wind, even if it means curtailing your boating and leaving your boat in a strange harbour. In the years before powerful engines, yachtsmen such as Maurice Griffiths used to sail to a port on one weekend and catch the train home. The following weekend they'd sail to another port or sail home again. Don't fight nature; you'll get tired of it quicker than she will.
Peter.

 

[oldharry]

You can go up on the prop size, thus making it more efficient, but you cannot increase power - the engine is rated 10hp, so around 8.5hp is what you will get at the prop.

A larger slower running prop will keep you going against wind and sea, better than the smaller faster turning 1:1 ratio prop, but will make little difference to boat speed as a rule.

Why only 8.5hp at the prop? The rest is taken up in drive train losses - gearbox friction, shaft friction, and driving any ancillaries such as generators or alternator. Engine Hp ratings are normally quoted as the maximum output available at the crankshaft , without driving any ancillary items. A big alternator working hard may take several Hp in itself.

 

[Katouf]

Just as amatter of interest, my dad had a 22ft clinker open pleasure trip boat from Shoebury beach.
Originally fitted with a 4 cylinder Morris Navigator engine, it was replaced with a Stuart P55MR driving a 17 x 17" prop. Loads of power even with a head sea on such a beamy hull. Maximum hull speed was about 6 knots as far as I remember.

Alan