hydraulic pod drive 30' displacement hull

Gday all.
Thinking of fitting a pod to my 30' steel displacement hull cruiser and using a hydraulic pump and motor to drive it. This would allow me to mount my motor lower (by doing away with the shaft etc) and do away with the rudder and associated fittings. There is also the added benefit of making the prop horizontal, decreasing required h.p. (?).
Any help or advice would be appreciated as this is still very much in the planning stage but needs to be built in now to progress with the project.
Cheers.

 

You will have to increase the HP by about 25% to account for the power losses.

 

Thanks Gonzo,
I was hoping that the efficiency increase due to having horizontal prop allignment might make up for the decrease in power(?) due to the hydraulic losses.
I wonder if you or any other subscribers might know of any other displacement speed, low power vessels using pod drives( even better using hydraulics to power them). There doesnt seem to be a lot written about this type of arrangement on these types of vessels(25-35',6-8kts). We dont all race around at warp speed making smoke, noise and wake (must be fun though!).
Maybe there are some articles or other forums out there that might be able to help?
Cheers.

 

If efficiency is not an issue and the extra weight and complication is something you can live with, a hydraulic pod can be built and installed. Usually, the only application for hydraulics are as thrusters for occasional use.

 

Thanks Gonzo,
I forgot to mention that the boat is steel, weighing in at approx 5.5 metric tonnes (give or take) and only requires 36hp to propel it at hull speed. Proposed engine size is 50hp to run at best fuel and torque ratings around the 36hp range, never using full throttle.
The reason for thinking hyd./pod is to allow me to get the engine as low as possible into the keel section, do away with a troublesome prop shaft, do away with a troublesom rudder post and increase manouverability.
I dont know if this can be achieved but I live in hope.
The pod would be mounted 1.5 mtrs forward of the transom with the box (?) keel offering protection.
Maybe another option would be a transom mount stern leg or even an ugly diesel outboard (yuk).
Any thoughts on electric drive motors?
Cheers.

 

The hyd pump looses 5% to 15% , the plumbing costs and the hyd motor looses a similar amount .

So losses are built in that a lower shaft angle will never compensate for .

If the engine is in the middle perhaps a V drive , swinging a nice big 24 ? inch prop would work for you.

A 30 ft boat will seldom be operated at >hull speed< as it will take triple the fuel of just the sq rt of the LWL (in ft) for speed in K. Knots.

At a bit over 5K your boat should require 11 to 15 hp , 1/2 to 3/4 GPH , a nice sweet quiet operating range.

 

Thanks Fast Fred,
The waterline length of the boat is 25'.
Draught is approx 3'.
Beam is 11'.
Weight may go as high as 6000kg (the boat will go over the scales on its way to the water).
I have had a look at v drives but i dont have the space under the floor in the stern to mount the engine and I definately dont want an engine cowling in my way back there, hence my search for other options.
The lowest hp requirement I could come up with was 26 hp giving hull speed of 6.7kts with approx 22" wheel. Is this in the ballpark?
I will try and get some pics of the hull over the next few days and get my kids to help me load them on the site.
Cheers, Matt

 

What is so unusual about the design that a std engine layout is not acceptable?

One concept if the run is fairly flat, is to place the engine in the forepeak.

Std (cheap truck drive shafts ) hook up to the tranny (so you can use its thrust bearing) .

Nice as the rotten motion at the bow does not have to be endured by a human.

 

With a rather heavy boat, the weight of the engine being a bit higher shouldn't be a problem. Is the shaft angle your main concern?

 

Thanks Fast Fred and gonzo,
serious rust issues around the prop and rudder tubes requires some replating and not having to reinstate these fixtures would be beneficial. Current shaft angle places the engine too high requiring a cowling in the wheelhouse, not something i can live with. Changing the prop angle would reduce blade clearance from the hull.
Still looking at options, any suggestions and advice are being taken onboard.

 

You could use a U-joint to lower the engine. The aft part of the shaft will need a thrust bearing. With the torque of the engine any automotive u-joint would work fine. They are quite cheap.

 

Perhaps a flat motor , a Subaru flat 4 would lower the engine box height enough?

 

This is not to your scale, but I have a little 6hp diesel sitting in the basement, and I'm picking away at a little steel boat so I have been doing a lot of thinking about how I can easily (that is without a machine shop) build a transmission for it to run as an inboard. So I've been doing a lot of thinking on hydraulic for a while now.

Basically my thought is to build a hydrostatic transmission. So run a pump and motor with different displacements per shaft revolution to give the "gear" ratio. Then the diesel spins the pump, pump sucks from a reservoir, after the pump is a by-pass valve back to the tank (ie Neutral).

When not bypassed, oil goes to the motor via a reversing valve, but when this is connected 100% fluid from pump goes straight to hyd motor with the least restrictive valves and fittings (must include pressure relief back to tank here in case of prop jam or something).

At which point you vary the rpm on the motor to change speed like a normal gear box. To me this seems to be the easiest way you can build a half decent "transmission" with off the shelf readily available hydraulic parts. And losses shouldn't be too bad.

 

Have you considered a yanmar or volvo saildrive .

 

if you go to all that trouble why not use a variable flow pump or motor. Swashplate are infinitely variable and super reliable. But a 6 hp will be about the same as 3 hp with shaft drive.