Prop Pocket

thanks for the feedback tunnels. when you refer to the other example, are you talking about the revised design i put up or someone else?

The problem i am finding is making a smooth transitition from a v shape hull into a concave surface. if it was flat it would be easy..

 

Ok, So i'm pretty sure i have nutted this out for a final design.

Please see images below:

1) Perspective View



2) Entry View



3) Side profile




4) Aft View



Would really appreciate peoples thoughts on these.

Cheers,

Nick

 

The rear end after the prop needs shape ! needs to be slightly reduced in size to form a jet kind of shape . also need to point down a little . !! at least 50% or close to of the prop needs to be up inside the tunnel

 

It would be nice if your drawing showed shaft, rudder, prop and all underwater hardware. Then when you measure everything out, you will notice different in draft not worth it in small boat. Tunnels work better on bigger boat. Had a friend with a similar boat, instead he developed a quick release rudder and shaft for trailering.

 

Ok - how about this one???!





Re: Improvements from Pocket - Shaft angle changes from 14 degrees to 8. And i have a draft improvement of 6 inches. Personally on a boat of this size, i believe any improvement of draft is worth it as inches count in my area!

 

This is a drawing i did long time ago for glass 36 ft fishing boat with a hold directly above the unit and the motor staying exactly where it was because of restriction of space !!, with a shaft drive so was able to simply relevel the motor to the shaft angle of the unit .
To get really good steering and almost be able to turn in its own length the rear end of the unit stuck out the back of the transom !!,had twin rudders and able to defflect more than 90% of the water flow from the propeller because the boat also did a lot of reversing so the twim rudders really were positive in either direction . The tunnel roof has its hightest point directly where the Propeller is and then aft it slowly begins to point down so to accomidate the rudders the rear end is almost rectangular shaped with a step inside from the round ! The front of the tunnel was part way under the Motor . the whole unit was to be made on the bench so to speak over a wooden mould and assembled then the hole cut in the hull and ground and compltely glassed in place .

 

your boat is not 26 ft, i had a 26 ft savage, it is a lot bigger than your boat. unless you have extended it you need to state the right length for working out angles, engine position etc: my prop pocket was only 1200mm long and rounded off at the forward end and a 10 degree shaft angle. why did you drop the sterndrive idea you had. if you stuck with the leg it would have more resale value.

 

Brendan, you are spot on. it has been extended by 1200mm. I dropped the sterndrive idea due to getting my hands on a jackshaft.

The shaftdrive allows for a far simpler propulsion method and an uncluttered transom. Resale value and boat rebuild should not go in the same sentence!!! Would love to see some photos of your savage if you have any lying around?

 

good points. my pocket drive worked well but it was time consuming, i made mine with lite ply over mdf molds and glassed it to 15 mm thick leaving the ply encapsulated, then i cut the hole in the boat and glassed the pocket in as tunnels suggested. i have never been as nervous as i was cutting the back out of that boat but it ended up okay. i will send you some pics of my savage. it was a 26 ft lancer with flybridge.

 

i have never been as nervous as i was cutting the back out
Glass is truely amasing stuff to work with and the things that can be done with it is amazing !!.

 

At this point the discussion has only been about curvature and length of the tunnel, but how about engine position and shaft angle? Longer is better, no doubt about that, but it also complicates the mechanical side.
Because this was originally a stern drive boat, the conversion requires a completely different interior layout. What will be the new engine position, which part will take the thrust?

I converted my 25' boat but made the tunnels shorter than they should have been because of space restrictions. The engines are positioned against the bulkhead and the tunnels extend 2' behind the transom, with platforms on both sides to prevent air entering the tunnels at speed (but that still happens). There is a long thread on the forum named "DIY tunnel drives".

 

Good points CDK.

savagescout, regarding the shape of your tunnel, you should imo round the line of attachment of the tunnel tube to the hull, to ensure a smoother transition of prop blades from the free-stream flow to the tunnel condition and to avoid the cavitation issues due to abrupt pressure field changes. Something like in these pics:



See the modification indicated with red lines (and sorry for just a quick rough sketch).

Also, check this thread which is very informative: http://www.boatdesign.net/forums/in...tunnels-gear-ratio-wheel-selection-40462.html and get in contact with Baeckmo. He will have many good advices to give you on that issue.

Cheers

 

Tunnel Design By Donald Blunt is interesting. I emailed his firm. They emailed me a few documents on tunnel design. I am trying to get the tunnel design pdf file uploaded to this site.