Adding central log to aluminium pontoon for inboard diesel, straight shaft

Hi there,

I am about to buy a couple of used 24 feet long aluminium pontoon boats at a great price and I thought of modifying one of them. The idea is to fit it with a diesel inboard and a normal shaft but keep the low draft of the sterndrive or outboard engine.

I was thinking of designing a central log, big enough to accommodate the diesel engine (a 3-4 cylinder 50-60 hp). The problems I need to solve are:

1. Shape of the log - and here I was thinking of designing a log somehow looking like the Rescue Minor boat - I found a smaller model here -
http://www.boatdesign.net/forums/boat-design/how-design-outboard-atkin-seabright-skiff-16643-3.html see post 37.
that would fit the profile.

2. Position of the log - I thought of moving the log further back exceeding the transom but the question is how does a shorter log influence the general handling of the pontoon? I know this guys do it for floatability - http://www.pontoonwaterglide.com/PONTOON-BOAT-ACCESSORIES-specifications-Pontoon-Water-Glide.asp but my log will be a bit bigger.

3. What would be the max prop size for such design considering the shallow draft of the pontoon? Again this is a standard 24'x8.4' pontoon - see picture below.

Do you have any ideas and suggestions for the shape of this log?
Can you say if the presence of the other two logs will influence the prop performance by pushing air underneath the central log?

Any other viable shapes that would keep the draft shallow ?

Remember the requirements - diesel inboard with traditional fixed straight shaft. No other fantasies as jets, surface drives, outboards and so on...

This has to be as simple as possible and relatively inexpensive with low maintenance.

See you soon...

 

Simple as possible would be the motor in a well that doesn't touch the water and a long enough shaft held by a V strut and the aft end.

 

Thanks Gonzo,

I was thinking about that but I am not sure that is going to work as the diesel engine is twice as heavy as the gas one and there will be float-ability problems plus the prop is not protected. Also I am concern about the boat flexing too much and bending the long shaft. I would have to reinforce the whole thing I guess. I am not aware yet of how fragile these boats are... just thinking in advance.

I forgot to mention that the boat will travel most of the time at max capacity 12-15 persons so the central log will be needed to compensate the weight of the engine.

The boat will encounter debris and will be beached often plus will jump some logs if necessary...

 

You don't want the simple, low maintenance options and call them fantasies. So what's left is complicated and rather higher or more complicated to maintain. Use three logs, with the center one cut off for a tunnel.

 

I'd consider shaping the aft end of the ctr hull a bit like the 1st photo and a bit like an axe bow. Pinch the tube flat w the pinch vertical. Cut off the high and low ends of the pinch to make a narrow horizontal surface top and bottom. Choose a prop dia and run the shaft log through the pinch as low as possible (probably a bit below center) w acceptable prop clearance to a shoe attached to the bottom of the pinch filling the hole and reinforce this area. Reinforce the top to attach to the structure of the rest of the hull.

 

Thanks Rider,

I thought about this too just like a submarine propulsion but I am not sure if the prop being so close to the surface will ventilated and cavitation will be bad.

The tunnel Gonzo suggested is not a bad idea and actually the Rescue Minor its a tunnel prop design. The inverted V shape is also a tunnelled prop.

I wonder if anyone has experience with such things. I know CDK has his project done a bit like that but it tends towards surface drive.

 

With a tunnel, the shaft can be almost horizontal

 

Do you have any example of such tunnel on such a narrow hull? I was looking around and could not find anything inspiring.

I found this:

http://www.mcgowanmarinedesign.com/LeBlanc_24.html

They call it Chunnel Keels and seem a bit complicated... see pictures below

 

The tunnel is the same dimension regardless of the hull.