Building a stern tunnel into an existing flat bottom aluminum boat

Hi everyone. I have purchased this boat a few weeks ago and I am planning to modify it a bit to suit my needs. This is a river cruiser 9.5m long and 2.5 m wide build from aluminum, actually a converted military bridge barge build in 1944.

The boat as you can see in the pictures has two pontoons attached to the sides to increase stability.

At the moment the boat cruising speed is about 14km/h powered by a new Volvo Penta D2-55 (55 hp @ 3000rpm) with a reverse gear box 2.7 reduction ratio (8 degree down angle) and a four blade propeller 21” x 14”. The draft of the boat is 80cm.

What I want to achieve:
- 50 cm draft
- A better protection of the prop by reducing the keel depth.
- Same cruising speed

The questions:
1. Would a stern tunnel work in this case?

2. What would be the best shape for this tunnel?

3. Should I move the prop towards the aft a bit to avoid cavitation?

4. How do I preserve the reversing performance?

5. In order to lower the draft I will need to choose a smaller diameter propeller (and I will prefer 3 blades as they run better in weed infested waters) what would be the equivalent propeller so I can keep same gear box for a 40cm diameter – what pitch.

6. Do I need to modify the rudder for stern tunnel?

Any help would be appreciated.

Please find below the pictures with the boat and some plans I have made.

Daniel

 

Forgot to attach some stern tunnel pictures I got from the internet:

 

would a jack shaft and stern drive solve the problem?

 

Stern drives are complicated, expesive and unreliable so they are kind of out of the question. Thanks

 

Hi Daniel, I posted some sketches on the proportioning of tunnels in one of the threads where CDK was discussing his tunnel/engine modification project. The correct shape is essential for the operating performance of the prop. If you can't find the thread, I'll see if I can stir my sediments; the sketch should be there somewhere!

 

Hi baeckmo

I remember the thread of CDK about his tunnel. I have read it again...14 pages of struggle it is not encouraging.

I see you are "the" person to talk to about this subject. I have downloaded the pictures you have posted and found particularly interesting the tunnel build for the ex jet powered boat. My question is how would this system work with a 21" prop and a skeg for protection but without the closed ring as this will cause problems when going through weeds.

Could you tell me how was the manoeuvring for that particular boat? Reverse and steer?

I will make a sketch based on your data and post it later.

Thanks,
Daniel

 

Here is the tunnel with the same proportions given in CDK's sketches.

One question is - where should be the water line so the tunnel operates optimally also in reverse?

 

The tunnel outlet should be just covered when stationary. And check the pictures for the rudder arrangement! The rudder should be dimensioned as a spade rudder, i.e. no side forces taken by the heel. The heel profile forward of the prop should be rectangular (vertical) with proportions ~60 x 10, with corners VERY well rounded!

 

CORRECT

 

Is this more like it?

I can not go wit the rudder lower than the skeg as it will cause problems when hitting a log under water r in the shallows.
Would this give me trouble steering?

The boat will go slow most of the time and manoeuvring is very important to me as the canals are narrow and full of surprises.

Any comment on the above?

Thanks a lot,

Daniel

 

How about wraping a Cort Nozzle around the prop for protection?

 

Weedless props.

Use a RAZOR sharp 2 blade prop. Sharpen ALL the edges of the blade.

I would really consider a POWERED up & down, motor mounting plate assembly.

It will allow you to raise the prop high enough to get the stuff off of the prop with a LONG wood handled ...side walk ice scraper... Narrow the width of the blade from 6" to about 1 " wide. Also RAZOR sharp on ALL 3 sides of the blade.

WARNING !! These things WILL put a hole into ANYTHING. Even thin Aluminum bottom skins WEAR ankle high shoes or boots.

 

The "About Landing Crafts" thread has some info at the end of it about Higgins boats and their tunnel sterns and rudders. It may be helpful.

C.O.

 

Well cort nosels are complicated things and in the weeds they are useless just like jets. They also need special props.

Razer sharp props? That is far fetched... It should be made of steel and my boat is Aluminum. I want to keep it simple and efficient, I will use a line cutter to help with the weeds. I will make a separate post about line cutters.

Thanks for the tip about landing craft, I will take a look.

Daniel

 

I was pointing you towards the strong upper mounting of the rudder plus the method to make the rudder blade stiff in sideways bending, that you can see in the pics. You may very well use a traditional lower bearing to connect the blade to the heel, but don't expect it to carry side forces. Even better to omit the lower bearing altogether, and just let the aft part of the heel slip under the bottom of the rudder.