Low power tunnel, low speed drive

Hey there

Have a gander at http://www.atkinboatplans.com/, click on "Inboard Utilities & Runabouts", then "TWINKLE - 16' 2" tunnel-stern flat-bottom utility".

Anybody know anything about this sort of boat? I love the idea of using a little Briggs engine and fixed shafting without grounding the prop. Obviously we're talking about displacement speeds here, but I have always heard that tunnel sterns are meant for high speed, not low. What's the deal?

Thanks
--Rob

 

Twinkle is a tunnel STERN hull, not a full tunnel boat. The idea is to get very shallow draft. The idea will work for low or high speed but the tunnel design will probably be different for best performance at different speeds. Atkin was a pioneer in these designs. If shallow draft is wanted, this should make a nice little boat.

 

Yeah, it's the shallow draft combined with the ability to use any little engine I can find with fixed shafting. In the Midwest we have a whole lot of mudflats we call lakes and long, wet sandbars we call rivers! Sounds like Atkin's low power tunnel might be pretty handy.

I'm also trying to decide whether such a drive would work well on a flat bottomed, unballasted sailboat. The idea being that the prop is only fully in the water when RUNNING, so it should create less drag underway than a typical fixed prop. 'Course the tunnel itself might make some nasty turbulence.

 

IN theory "your NOT supposed to do it" but I have seen boats with small gas engines that powered props with in an external shaft that contained a car "U" joint.

The shaft was held straight under way , but after shut down the prop could be lifted clear of the water.

Since aeach HP will only give about 20 to 25 lbs of thrust , the loads of a brigs & stratton are easy enough to handle.

The engine makers have Roto Tiller engines , with a built in 6-1 gear box , that would make the setup more efficent, at little cost.

FAST FRED

 

Why not mount a WEED WACKER setup or something like one,- the the way they do in China and Japan-- Prop on end of a long enclosed drive shaft. Use a large oarlock and some means to anchor it at the stern? Should be easy to control for all depths of water. GO --007!! Rich

 

Fred: I was thinking the same thing on rototiller or snowblower gearsets. Thanks for letting me know the ratio!
Richard: I thought of the in-line drive approach, a la Go-Devil. Problem is that you can't have the prop in front of the sailing rudder to use the boat's regular steering. Otherwise not a bad idea.

 

Why can't the long shaft on the oar lock swivel, turn the boat very easely? It did in James bond's boat as they raced thru Hong Kong harbour. Leave the main rudder amidships when using the long shaft. You could swivel it more than 90 degrees, if stern design permitted. Lift the whole shaft drive off as one self contained unit. I think I am not understanding your requirements.

 

wheel steering

Ah, you're right. Wheel steering is the bit I left out. So the rudder is steered by the wheel, and it would be rather cumbersome to steer both with the wheel. True, I could lift a kick-up rudder and steer independently with the motor, but then I'd have to sit right next to the noisy damned thing, defeating the purpose of the wheel steering.

 

Why not steer it from anywhere foward with a simple, side of hull mounted steering wheel and a long flexable choke cable for throttle. The same 2 cables that steer the motor can at the same time steer the regular rudder. Why not use a small water cooled gas inboard motor normally- or a small outboard on the stern?

 

Responses

>Why not steer it from anywhere foward with a simple, side of hull mounted
>steering wheel and a long flexable choke cable for throttle. The same 2
>cables that steer the motor can at the same time steer the regular rudder.

Possible, but defeats the simplicity of simple rudder steering. More moving parts to maintain.

>Why not use a small water cooled gas inboard motor normally

Draft.

> or a small outboard on the stern?

That goes back to complicating the wheel steering. But the simplest is certainly to mount a non-steering outboard in a well in front of the rudder. It's what I have now. But you're stuck with 2-stroke unless you want to PAY. I'm not a fan of 2 stroke.