[MHml] OutDrive Legs (was PRM Transmissions)

[Allied]

----- Original Message ----- 
From: "Dan Frenette" <Dan.Frenette at Sun.COM>


> Be careful. I have talked a number of owners that tried hydrolics and
> There are clearly good adaptations of this sort of thing on land but
> something about boats is a bit different.
>
> Dan Frenette
>
I have twin hydraulic drives that are attached to retractable (up & down, on 
vertical SS pipes, attached to the inside, aft, of each hull) outboard legs, 
as I did not want anything un-necessary in the water when sailing, or 
parked. I sized my twin pump and motors and gearing to give me what I 
considered at the time the correct RPM at the prop. Though with a swash 
plate hydraulic pump, I can choose what RPM the motor goes and what I want 
at the prop, this is completely variable and instantaneously reversible, on 
either drive. This make for a very manoeuvrable boat, great for picking up 
pots, nets, or moorings. If everything is sized correctly and you use piston 
pumps, then losses in the hydraulic system are not too much different, from 
a gearbox. As far as reliability goes, if the system is in good condition 
and there is no dirt in it, filtration is good, then it should go 20 years 
un-attended. I had my local foundry cast me some Alloy props, which I then 
turned down to the diameter that would fit in under my cavitation plate, 
leaving a broad outer end to the blades, which seems to work well for me. I 
did a lot of hand shaping of the props and balanced them whilst doing so.

Well I have said the advantages, the disadvantages are; that it is 
expensive. Just the hoses and SS fittings, things I couldn't make, came to 
NZ $6,000. It can be noisy, though this can be improved.

Things to see to if you ever consider hydraulics; You will need cooling in 
the high pressure side of the system. I suggest that if you have the motor 
central, then you pipe this aft, under the boat, using SS pipe, with a 
second pipe over the top, with cooling passing through this second pipe, you 
will then maintain a cool system. You can then noise insulate all the 
hydraulic components, thereby keeping your noise level down. I had to fit 
high pressure coolers as an after thought and now intend to make some noise 
covers for the motors, which I hope will work.

Would I do it again? Well it does work well and apart from the noise, which 
I hope to overcome, is as good as any other system I can think of. You do 
need a full workshop and machinery though, I would hate to think what I 
would charge someone else, to replicate this system.

Colin