Outboard shaft length

[nigelmercier]

Outboard shaft length: Flover 45TGS Electric Outboard

I have an Excel RT230 dinghy, like this:
http://www.excel-inflatables.co.uk/inflatable-boats/inflatable-boat-rt230.php

I have also bought a Flover 45TGS Electric Outboard, see here:
http://www.floveroutboard.co.uk/outboards.html

The TGS models have an "Xtra long 95cm shaft" and as such will not fit in my outboard locker! My old outboard was 68cm from the top of the bracket to the centre of the propeller, the new Flover is 86cm at the lowest setting. I need to reduce the length of the shaft by 10cm to fit the locker, but should I take off some more?

 

[winsbury]

My understanding is that for a normal outboard you want the length deep enough so that water covers the anti-cavitation plate but not so deep that the prop is so low in the water that 1) it puts the exhaust outlet so deep it causes excess back pressure or 2) it could dangerously lift the bow of the dinghy tipping out the passengers. Exhaust is not a problem for an electric motor and I very doubt you would have enough force generated by one of those electric motors to cause the latter effect so I would tend to think you should make sure the prop is just below the bottom of the transom so it is in clear water for maximum drive.

 

[Tranona]

The dinghy is designed for a standard 15" shaft length outboard - distance from top of transom to static waterline. The electric outboard you have bought was not designed for dinghy but as a trolling motor for something like a dory which needs a longer shaft to get the prop into the water and the controls high enough to operate. Unlike a petrol engine which can suffer if the prop is too low in the water, the electric one will work - just that it is too tall. Wrong product for the job.

 

[nigelmercier]

Wrong product for the job.

Which is why I am shortening it.

 

[Tranona]

So, are you just going to cut it, rejoin the cables inside and stick it back together? As I suggested, in the form as sold, not suitable for the application.

 

[nigelmercier]

So, are you just going to cut it, rejoin the cables inside and stick it back together?

Pretty much, except I'm not cutting the cables. I've now taken off 100mm and it still seems to long, so I'll have another go tomorrow. The dinghy is designed for a short shaft, so that means the top of the propeller would be a little over 15" from the top of the bracket, I think. The outboard manufacturer say the propeller should be a foot under water, so I'll do the sums.

 

[Tranona]

Pretty much, except I'm not cutting the cables. I've now taken off 100mm and it still seems to long, so I'll have another go tomorrow. The dinghy is designed for a short shaft, so that means the top of the propeller would be a little over 15" from the top of the bracket, I think. The outboard manufacturer say the propeller should be a foot under water, so I'll do the sums.

Set it to mimic the standard outboard with the anti cavitation plate 15" below the transom bracket. The prop will go a bit deeper when you are in the boat. If you have it too deep you will depress the stern of the boat if you apply full power and the bow will lift. Not sure what the manufacturer means by a foot below the water - is it a foot to the top of the prop, or the centre of the hub? The dimensions of a standard outboard setup are not there by chance. They are because that is what works and why it has become a "standard".

The trolling motor you have is that dimension because that is the standard for the boats it was designed for.

 

[VicS]

Set it to mimic the standard outboard with the anti cavitation plate 15" below the transom bracket.

It does not have an anti-cavitation plate ? ( or more correctly anti-ventilation plate )

 

[Tranona]

You are right. Should have looked more carefully. Would guess the point for setting at the waterline is about 2" above the prop. Probably not super critical, but the suggestion of a foot below the surface is probably too deep (depending on where it is measured to!). Not sure whether the motor kicks up if it hits the bottom, but clearly the lower it is the more likely that is to happen.

 

[nigelmercier]

...the suggestion of a foot below the surface is probably too deep

I thought so too, have contacted the manufacturer.

Whatever depth he comes back with, I should set the bracket top 15" above this, yes?

 

[nigelmercier]

I thought so too, have contacted the manufacturer.

Whatever depth he comes back with, I should set the bracket top 15" above this, yes?

Their reply:

... the top of the prop needs to be at least 15cm below the water surface from our tests...

 

[VicS]

Their reply:

That is to the top of the blades presumably. So 29 cm to the centre ... call it 30 cm

Add 38 cm ( 15") transom height to that and you need the inside of the top of the bracket about 68 cm above the centre of the prop.

 

[Tranona]

Their reply:

I expect that is because it does not have an anti cavitation plate, the purpose of which (in part) on an outboard is to stop air being sucked in from above and so allow the prop to run closer to the surface. In the application for which the motor was designed this does not make much difference as it is not expected to run at any speed, only slow trolling and holding a boat in position when fishing.

To give you an idea of how relatively deep they are suggesting, the distance from the waterline to the top of the prop on my Seagull 40+ is only 5cm and 35cm to the bottom of the skeg, compared with your total of around 45cm. So don't run in shallow water!

Will be interested to hear if it does actually work on your inflatable.

 

[nigelmercier]

That is to the top of the blades presumably...

Yes, and having cut the shaft down by 10cm, I now have about 8cm of shaft above the bracket. Not worth cutting again, but for new buyers of the Flover TGS models (with the "Xtra long 95cm shaft") it would be worth cutting off 15cm at the top of the shaft.

 

[rogerthebodger]

As seen in our marina