effect of close clearance tunnels on gear ratio and wheel selection.

Baeckmo,
thanks for replying so extensively to both DanV's and my questions. Always so much to learn from you.
Cheers!

 

I remember there was an article written about tunnel design. I could not find it however.

The article describe the longtitudinal section as S shape, with the S extending past the prop, sort of a hook. The clearance at the top of the propeller is close but opens up at the hull intersection with a soft radius reccomended.

As far as I can recall, there were three types, shallow, medium (half of the propeller dia), and deep (more than half the propeller dia.)

There were other rules also. Hope somebody recalls the article. Maybe it is an old issue of Professional Boatbuilder.

 

Got it. PB number 44. On the same issue is about propeller selection.

 

Nice find RX, thanxs.

 

Thanks for all the info and thought put into my problem. Unfortunately the only photos I have are of the original tunnel prior to reconfiguration. It's actually now alot closer to the tunnel you describe, Baeckmo. I will try to post pix next haulout. Thanks daiquiri for the excellent article on tunnels.

 

Guess it's just a typo. You probably meant to say "thanks Rxcomposite" and I share your thanks. This is one very instructive thread, an excellent example of why this site is so great!
Cheers

 

Building a tunnel

Great reading baeckmo!

I'm building a 32ft trawler (pleasure use) Need you expertise for this tunnel shape, have read the thread with CDK and his major problems and looked over the drawings you did for him to solve his problem, so, I have this drawing and drawn it on the hull (still in building and fortunately up side down)

Now, I have as yet to chose the diesel engine, about 300 Hp I think to get around 30 Knts seldom and 15 to 20 knts often, in the Indian Ocean out side waters of Perth Western Aust.
I want a V drive to get the emgine 'out' of the cockpit and probably half the engine will be under the aft deck.
The V drive will be 8 deg, not sure what I need for the reduction box as the engine is an unknown.

Can I still build the tunnel with information I have put in this propcalc.xls attached, gives when I punch in figures that I think will be about right for the hull to get to required speeds. (economy is top of the list with top speed needed to get the "heck out of here" when the weather looks to be going bad.)
View attachment Propcalc.xls

I have attached Spec's of the trawler design, alas, I have added an extra 4 ft to this designs 28 ft version!
View attachment Sheet 000f - General Specifications.pdf

This will be the V drive:
http://www.cassellmarine.com.au/p/324026/argo-v-drive.html

 

Shipyard workboat tunnel

 

Oh heck....!
Another shape tunnel..... more towards the better thinking with the tunnel being full width for it's length, but then there's that keel running into the forward end of the tunnel, to quote beackmo, water don't like changing shape - or words to that effect.
My hull is also 13 Deg deadrise, single tunnel being planed.
May work better if those transitions were shaped more fluid? (no abrupt changes in angles)

Interesting the rudder being in the tunnel, I would have thought water thrust straight back would be dam'ed by the rudder swinging, also redirecting water within the tunnel? how would that work?

Another observation, the tunnel shape around the prop, the tunnel isn't a perfect radius centered on the prop shaft, I read that it is better to have the lower sides shaped like this to avoid cavitation?

May look at building a water canal to test hull/tunnel forms... glass sides and bottom, dye streams, just to see what works.
A lot of work, but fun!

Now to google test tanks.... oh heck!

Cheers Michael.
Do you know how this hull/tunnel works out on this work boat? Or is she coming in for a major re-shape?

 

Dont know how she performs. Just out of the water for an antifoul. Occasionally I see her zipping around...perhaps 20 knots.... mostly it tows small boats around the port.

 

20 knots is good! Still, don't know if this setup has the engine running economically @ 20 knots.
Thank you for running the tape over the prop area too Michael.

 

This seems very strange. After depitching, you lost 3 kts! That is why you have only gained 50 RPM from a fairly significant change. With that large a loss, the inflow was slower and the prop loaded up more. At this point you are likely have pretty bad cavitation and a propeller that is no longer very effective. This appears to be pretty heavily loaded with 450 HP and the tip speed is high, so cavitation and vibration could become a problem. What is the current gear?
The tunnels as described are not ideal, but if you got to 32 kts with the power installed and the vibration was not too bad, then it works. The trick is to get the prop set right so that the engine is loaded properly.