8’ Mini Boat surface drive requirements

Im currently building a 8’ mini riva style speed boat and i’m planning on running a surface drive prop connected to a 4kw electric motor. However i do i have a couple of questions about the prop and positioning of it.

Firstly the motor is 4kw nominal power so around 5hp and i currently have a 3 blade 8’’ cleaver style prop aiming for 3k rpm which i believe for the moment is a good starter prop but what i’m really scratching my head at is the positioning of it. I want to keep it close to the transom and above the bottom of the hull to stop the prop from grounding purely for maintenance but i’m not sure that is great for efficiency?

I will be running a rudder behind the prop so no hydraulics for steering or trim so i believe it can be fitted close to the transom around 15/30cm is my currently plans. so really my main question is should it be completely below the hull for efficiency like many trolling motors on mini builds or not? or if not is there a minimum distance from the transom the prop must be at for greatest efficiency?

Cheers!

 

What am I missing.
Run it up close to the transom with the shaft on the planing water line.
No?

Huge rooster-tail!

 

Yes exactly that! i’ll plan on putting a plate over the top like i’ve seen some designs do to stop the rooster tail as much as i can but is there any reason why i can’t put the prop close to the transom? ie there no water flow?

 

The rooster-tail comes from the blades in the water, a plate's not going to stop it, and why would you want to?
A cover over the blades rotating through the air behind the transom would be a good idea though.
Perhaps you could provide some pictures , sketches or drawings of what you're proposing.
Yes, keep the prop close to the transom.

 

yes that’s what i meant a plate just above that water in the air and that’s good to know thanks. I’ll send a pic of a 1/10th model i’ve made and some cad models of the rear assembly tomorrow.

 

Don't forget that any plate letting water impinge on it will drive the bow down

 

ahh didn’t know that i’ll leave it, less parts to build then haha. Here is the design:

 

Thanks for posting some pictures.

The drive shaft is not at the " planing water line".
A surface drive relies on that.
The configuration you show is not going to get enough water to the lower prop blades when on plane.

 

Ok thanks for the info, so you’re saying the shaft should be as low as possible? i’m not sure of the waterline yet thats all

 

You're welcome.

No, I'm saying it has to be at the "planing water line".

The "planing water line" will be at the bottom of the transom.

I think you had best read-up on how "surface drives" work.

 

A question
I have only seen a couple of Arneson drives working in real life. Obviously, they have the ability for the prop to be moved up and down to optimize trim/speed/fuel consumption AND
permit loading of the propeller correctly. Ie a certain amount of adjustible vertical immersion of the prop cross section into the water stream

So, the question is this
What is the amount of the total diameter of the prop must be immersed for the prop to provide the best thrust?

I would think that close to 1/2 must be immersed as compared to say a 1/4 of the blade length.
The OP will not be able to get even close to having 1/2 the amount of the prop immersed if the shaft egresses the transom with the centerline of the shaft as shown.
Ie the centerline of the shaft above the bottom of the hull would have to be 1/2 the diameter of the prop shaft plus an allowance for the packing/shaft seal/ bearing the centerline of the shaft would have to higher.

So say he uses a 1 inch diameter shaft and the packing/bearing/shaft seal needs another 1 inch then his prop will be the 4 inch radius less 1 1/2 inches for the above, minimum leaving only the
outside 2 1/2 inches of the prop immersed on plane.

And hence my question above
I am hoping that the shaft diameter is not to scale as in Figure 1 as his first prop is only an 8 inch prop and the shaft diameter appears to be 3 to 4 inches in diameter and he is presenting 4kw power)

I am making an assumption that his 5hp available power will plane his 8 foot boat to allow the transom to ventilate

Which introduces another question
With the vertical axis of the prop fixed, the boat at rest, the prop totally immersed will 5 hp be enough to get the boat on plane with the prop fully immersed?

 

I completely missed these 6 words above the bottom of the hull when I put together post #12. I assumed that you were expecting the boat to plane and the only water into the inlet side of the prop would be water coming off the bottom of the hull with a ventilated transom.

If you do not ventilate the transom, and the prop is "close to" the transom and above the bottom of the hull, the water supply to the inlet side of the propeller will be limited.
If you want the prop to be surface piercing, the prop needs to be quite far back and the floating waterline at rest will become a major factor. Ie with lower speeds, the boat will lift, the bow will come up, but the transom will be flooded and the prop immersed. This does not make a lot of sense to utilize a surface piercing prop for a non planing hull.
I think that you are on an incorrect path.

If you want to make it surface piercing and not planing, (definition in this case, planing means that the transom is ventilating) the prop must be far back several feet and the prop will need
to have some ability to change its height to adjust to the variables as shown above.

 

Maxb99,

I realized there is a way to make your current configuration work.
Put a huge, 3-bladed prop on it.
But then you're going to be grounding it in shallow water.

Barry offers good advice.

Best of luck.

 

Another way to keep a surface prop at the proper level is to use a long tail for propulsion, which is used on some racing boats in Asia.

Longtail Evolution https://www.boatdesign.net/threads/longtail-evolution.68471/

There are pictures and videos of how this works in the above link.