Engines and Propellers

[Giovani Frisene]

Hello guys
I just bought two second-hand engines and I´ve got a question, the propellers spin to the same side?

[Ian R]

Interesting question, since some twin -screw  set- ups have counter -rotating props.

Are there any engineers on here to comment? Rare breed now chez nous in UK, possibly to get rarer in poor old Boris-laughing stock -Britain.

[Budget Boater]

You have not provided enough information for anyone to be of help.

However, if they are the same make and model number, chances are the propellers will spin the same direction.

[Giovani Frisene]

Thanks for the answer. I´d like to know if there would be some problem if the propellers spin in the same direction. I mean if probably the boat will drift to the spin direction.
I´ll share with you guys Hanneke´s answer:
Our two 9.9hp outboards spin both in the same direction. Seems OK.
Here is a piece from our Q & A file:

2) PERFORMANCE:

“By running one engine in forward gear and the other in reverse, the boat almost turns around her own centre.  By running her on only one engine she acts surprisingly well.  There is almost no extra drag on the helm.  By running her on both engines there is a little drag (not much; it would have been quite normal for a monohull) but whether this comes from the ‘turning momentum’ of the engine or anything else (i.e. the skeg may not be 100% in line) we are not sure. The propellers and engines run the same way.  It is preferable, if possible to get an identical gearbox that reverses the movements.  If you can get the propellers to turn opposite ways, most of the ‘turning momentum’ and the drag on the helm will more or less disappear. On Vahine we chose a two-bladed propeller.  The reason for this choice is that the propeller can be hidden behind the skeg when sailing. With a ‘mark’ on the shaft inside the hull and a brake mounted on the shaft you can easily block the propellers in the wanted position.  Why not let the propeller spin?  You may well ask.  Firstly, I have to say that tests have shown that a blocked propeller causes less drag than a running one.  But this is not the main reason for stopping the shaft and propeller. The reason is the gearbox and the bearings.  Even if the manufacturer says that it does not matter whether the gearbox is spinning or not when it is in the ‘free’ position some parts of the gearbox will break down a lot earlier than if the shaft had been blocked.”

 

Outboard Engine sizes for various designs:

 

We recommend a long shaft outboard for all our designs:

 

Hina                             5hp

Maui                            3.5hp

Hinemoa                      3.5hp

Tane                            7-10hp

Tanenui                       8hp

Tangaroa                     16hp 2-stroke engine or 10hp 4-stroke

 

Tehini                          1X 40hp or 2X 9.9hp 4-stroke (which have more thrust) It is the 9.9 4 stroke USL (ultra long shaft) This is the 25 inch shaft. The engine box is adjustable so if you are heavily laden you can set it at a different level.

 

Narai                            1X 20-30hp or 2X 9.9hp 4-stroke (which have more thrust) It is the 9.9 4 stroke USL (ultra long shaft) This is the 25 inch shaft. The engine box is adjustable so if you are heavily laden you can set it at a different level.

 

Ariki                             1x 25hp 2-stroke or 9.9hp 4-stroke high thrust (1 or 2) It is the 9.9 4 stroke USL (ultra long shaft) This is the 25 inch shaft. The engine box is adjustable so if you are heavily laden you can set it at a different level.

 

Oro                              1X 30hp or 2X 9.9hp 4-stroke (which have more thrust) It is the 9.9 4 stroke USL (ultra long shaft) This is the 25 inch shaft. The engine box is adjustable so if you are heavily laden you can set it at a different level.

 

H17                              1.5hp

T21                              3-4hp 4-stroke high thrust

T26                              4-10hp 4-stroke high thrust

T30                              6-8hp 4-stroke high thrust

 

T38                              9.9hp 4-stroke high thrust It is the 9.9 4 stroke USL (ultralong shaft) This is the 25 inch shaft. The engine box is adjustable so if you are heavily laden you can set it at a different level.

 

Pahi 26 (Tikiroa)          4-10hp 4-stroke high thrust

 

P42 (C Cook)    1x 25hp 2-stroke or 9.9hp 4-stroke high thrust (1 or 2) It is the 9.9 4 stroke USL (ultra long shaft) This is the 25 inch shaft. The engine box is adjustable so if you are heavily laden you can set it at a different level.

 

 

P63                              30hp (2X16hp) Minimum 2 X 9.9 hp 4-stroke high thrust. It is the 9.9 4 stroke USL (ultra long shaft) This is the 25 inch shaft. The engine box is adjustable so if you are heavily laden you can set it at a different level.

 

 

Do you have a building design for a longtail propeller shaft?

No we don't have our own design for a long tail propeller shaft. Sorry not to be able to help you.

[Bart]

With props as small as on outboard engines, rotatition direction does not matter, only with big propellers and lower revs..

[Graeme]

Giovani, that's a useful list. But why no recommended outboards for the Raka design? (Also for the Tama Moana, Child of the Sea, which is almost identical in hull dimensions, sail area, and weights as Raka.)