fixed surface drive shaft length

[seamy]

Hi guys ,does anyone know how important shaft length is on a fixed surfave drive, will a longer shaft have any effect on the hull running at speed,ie,push the bow down or can it create any lift,the normal,or average length would seem to be 1000mm to prop nut from transom,any thoughts,cheers.

[Olav]

Without any practical experience with surface drives (but a strong interest in them anyway) I have the heart to claim that - provided everything else on the boat is the same - the boat would run flatter with a longer shaft due to an increased bow down moment, as the lift force (in vertical direction) produced by the propeller now acts at a longer lever. Same is true for transverse forces as long as you don't have a twin drive installation with counterrotating propellers.

Just my $0.02...

[pistnbroke]

clearly steamy it depends on the length of the boat ..1 metre would be excessive on a 4 m boat ....

[Frosty]

It would appear that unless you are really into high performance it does'nt matter.

Strange thing to say? well not really,. The prop position in the water is the most important.

Sonny levis last design (the name of which escapes me) was very close to the transom meaning ease of fitting and supporting the shaft.

I have designed and built such a system and I am very satisfied with it.

Take a look at the thread "shafts thru the transom"

There is however something called wake fraction which takes advantage of the slowing wake behind the boat for increased performance but I dont think we are there yet.

What are you building?

[seamy]

Thanks frosty,currently working with a fixed shaft setup,whilst i do agree that the prop height is always important,my set up is a single shaft through the transom,I dont believe that a surface prop can carry the bow when it is only partially submerged at speed regardless of cupping or rake,sure it will help to a degree but when speeds exceed fifty knots i encounter a new set of problems ,Torque steer being the first,bow down being second which scrubs of top speed big time,I have tried different cleaver props with poor results, and the only conclusion that i can come to is that cleaver type props will not get results on a drive of this type regardless of weight behind cg,any thoughts.

[Frosty]

Well then you need minimum shaft angle, and rake will keep the bow up plus mounting engine as far aft as pos

[seamy]

How can minimal shaft angle be achived while keeping engine back,The further aft the engine is located ths steeper shaft angle will become,hence asking my original queestion.

[Frosty]

keep it to the minimum,--- not zero. If your concerned about bow down keep weight back, you can move it forward later or fine tune with batteries etc.

Arneson do a shaft extension so you can jack up the back of the motor and minimize shaft angle. Then I imagine it must be well over a meter.

Its my own thoughts that the prop would then be in the stern wave.

Ive done a bit of this designing and building surface stuff and its hard work , you are basically on your own, its unlikely no one else has done it to your boat. and if they have they wont tell you.

As I say I have a reasonable success and performance better that the manufacturers with my drives with 100HP less. Its expensive to keep hauling out and I beach to tinker and tweak when I can.

At the moment I am tinkering with the rudders which you hav'nt mentioned yet. I just recently modified the tunnels to a one legged tunnel and put 3 knots on top- 17 to 20,-- not your speed of 50.

Beware you could throw a lot of money at it and have a dog.

I like fixed shaft because they are agricultural and strong no moving parts and can be antifouled and left in the sea. If you a trailer sailer then an Arneson is the way to go.

By the way a single installation will have a lot of side walk, I know you are aware of it but it will prob be a lot.

[Steve H]

Lets see some pictures!!!

[Frosty]

Shafts thru the transom thread.