Long or short shaft for podded motor?

[tunaticer]

I am currently designing a new 4.8m plate boat I will build in 17.
The boat will have the outboard mounted on an extended pod.
What is the advantage and disadvantages of long vs short shaft for this application?

This boat will be a very shallow draft vee punt for operating in shallow creeks and some bay work.
Primarily I will be using it in places like elimbah creek where there is heaps of travelling to do in about 30cm of water.

Shallow drive differences between long and short shaft would be a lot different I presume.

[lee8sec]

With the cav plate leval with the bottom of boat, shaft length makes no difference whats in the water, only height of mount from hull bottom.

[gofishin]

...there is heaps of travelling to do in about 30cm of water...

Something doesn't add up to me, unless you are planning on doing all this travelling at low speed, with the donk tilted up. Unless 60HP or smaller, a 4.8m boat will have a prop around 30cm in diameter won't it? Then add the skeg, clearance under the A/V plate (to the prop), and your hull draft... sounds like you need a jet to me.

...What is the advantage and disadvantages of long vs short shaft for this application?...

...Shallow drive differences between long and short shaft would be a lot different I presume...

As Lee has said, nothing except what's above the water (except for the few kg's weight penalty). If a shallow Vee you might get away with a S/S, however if you plan on fishing outside or in shitty Bay conditions occasionally, keeping the power head further above the water is a good thing in my books.
Cheers
Brendon


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[tunaticer]

Ok......presumably both long and short shaft engines both utilise the same shallow water drive components and measurements i.e. both tilt the motor at the same angle from the transom plane.
This should give a different lift in the water column between long and short shaft motors.

Is there a better optimum for this shallow water drive in the short shaft or the long shaft?

Yes, lots of my creek work does require walking the hull through the shallows, but i do not want to amplify that by selecting a motor that will increase that distance I have to walk the boat through.

Have thought about jet drive but the increase in weight makes it impossible to wade through when the hull drags on the sand......not to mention the impellor will likely grind away to nothing quickly.

[Noelm]

I will repeat for you, long or short shaft or even extra long do NOT increase draft, the "extra" length is above the bottom of the boat, not sticking deeper into the water, to cater for a long shaft, the top of the transom is 5" higher (20") the bottom of the boat does not change.

[scottar]

It will depend on the depth of your hull in combination with the extra mounting height required due to setback - this is dependent on your pod design. On a "standard" transom it isn't uncommon to have the AV plate above the bottom of the vee. The distance you can get away with is dependent on a multitude of factors - type of prop, deadrise, hull construction and as such it is difficult to provide a starting point apart from that recommended by the outboard manufacturers. From there the "typical" rule of thumb as far as I know is an inch up for every 12 inches back - if you have pod that is 24 inches long, the mounting height will be typically 2 inches higher than on a standard transom - provided the pod shape doesn't follow the transom - Rule of thumb only. Only you can work out how this factors into your design and the design of your pod. By going to a long shaft motor you may end up with a mount height that will simply not aesthetically suit the boat if the height required for the setback is added.

[tunaticer]

I will repeat for you, long or short shaft or even extra long do NOT increase draft, the "extra" length is above the bottom of the boat, not sticking deeper into the water, to cater for a long shaft, the top of the transom is 5" higher (20") the bottom of the boat does not change.

It is not the draft that I am trying to compare, it is how well they operate in shallow drive mode.
I understand the draft requirement for both motor and hull. The height of the transom or in this case pod is of no concern to me currently. It is trying to determine which leg length works better in shallow drive mode.

[scottar]

Given the brief for usage - personally I would stick with the shortest shaft length if you don't need the extra length to get the powerhead clear of water - less weight by a small margin and a lower centre of gravity for the same prop position, less height to intrude into fishing over the transom or reach to untangle if required

[Muzza]

Tunaticer,
I have done heaps of shallow work in smaller boats, trees across creeks , rock walls etc.
Lying down in the tinny at low tide to go under a tree and getting out to lift it over the tree on high tide.
I agree it won't matter, if the cav plate long or short is set right, when you hit it will hit, make sure your motor pops up and is not locked in
4.8 is a bigger than I've run but if flat should be ok, harder to get over obstacles cause its longer of course. We could rock our tinny over some logs.

Cheers
Muz

[Fed]

If you're designing & building from scratch why put a pod on it TT?

How about a jack plate?

[Owen]

I think what tunaticer is getting at is that given the two types have different heights of the pivot point, there will be some difference in the angle of attack of the prop relative to the water when both are trimmed to minimum possible depth.
I think in practice it would be only a couple of degrees as you are changing two sides of the triangle by the same relative amount.
I guess the long shaft would move the force further from the fulcrum (stern) which would push it deeper for the same thrust.

Just put wheels on the bastard and have a beer [emoji482]

[tunaticer]

Fed, podding the boat to maximise cockpit space.
Thinking seriously about a 4" hydraulic jacking plate in the design.
An extra 12.5kg isn't too much to accommodate.
If weight does become an issue i will downgrade to a 70hp yam 4st.

[spud477]

I didn't think you would get a short shaft in motors that size?

[scottar]

One thing that may be worth considering is a low power auxillary. On my last tinnie I had one courtesy of coming by it at the right price. With the offset and it not being mounted quite as deep as it would be normally, provided the boat would float, the auxillary could run. Was great for any extended shallow water stuff and it saved a tow once as well.

[lee8sec]

Fit a electric front mount, solves most of the shallow water problems.