"Propeller pockets" or "Tunnels"

[67-LS1]

I'm assuming that they're the same thing? What is the purpose?
The reason for my question is that I see a manufacture that offers tunnel kits to fit older Bertram 31's. This is I would think a very deep V @ 24 degrees and a fairly heavy boat, not designed for allout speed as much as fishing.
Does this have anything to do with changing the angle of the shaft or is it just more clearance for a larger diameter prop? What would be the advantages / disadvantages?
Thanks,
Dennis

[Nomad]

Speed, better shaft angle, better effiency, bigger prop, more clearence, more level ride, etc

[gonzo]

The tunnels or pockets make the drought of the boat shallower. However, because they reduce the lifting surface, the speed decreases. In an installation where the power is marginal, the boat may not plane.

[Nomad]

Gonzo.... That depends on the design of the tunnel and the hull. In every case we has used them there has been a significat increase in speed. That and a better running angle

[tom28571]

Nomad,

Does that mean that you have modified an existing boat and found higher speed and better trim angle? Plus, which way did the trim angle move?

This is surprising. To what do you attribute the changes in performance?

[Nomad]

We have produced new boats from molds, both with and without tunnels. The boats with the tunnels get on a plane quicker, back down better, run faster, run flatter, get better economy, are more stable, etc. Also it allows for considerable tip clearence and bigger props.

The reasons you get better preformance: Better shaft angle.... Not pushing an an inefficent angle, less drag. This is where most of it comes into play.

If you need me to explain it better I will

[67-LS1]

I can see that the tunnels would allow for a better shaft "angle of attack". So I assume when you would add this to an existing hull, you would also change the shaft angle at the same time? This as opposed to just running a larger diameter prop?
Dennis

[Nomad]

Yes, you can do both. Also to consider, The size and shape of the tunnel is very important and the boat will sit alittle deeper in the water with a full tunnel.

[67-LS1]

Has anyone ever added tunnels to an existing hull?
Was it worth it?
Thanks,
Dennis

[Gypsy72]

my take on tunnels is that it decreases manuverability during docking situations, especially in reverse.
Being in South FLorida, draft is a concern, but 90% of my customers that run aground hit so hard and so often that tunnels dont make a big difference.
To add tunnels to an existing hull, in my opinion, would not make enough of a difference to make it "worth it".

[Nomad]

Originally Posted by Gypsy72

my take on tunnels is that it decreases manuverability during docking situations, especially in reverse.
Being in South FLorida, draft is a concern, but 90% of my customers that run aground hit so hard and so often that tunnels dont make a big difference.
To add tunnels to an existing hull, in my opinion, would not make enough of a difference to make it "worth it".

As for decreasing manuverability that all boils down to design. Most tunnels on boats are just the little worthless half circle design. That is not what I'm talking about......

I agree Unless you are in dire need of a tunnel and inlove with the boat it's not worth adding them to an existing hull

[Willallison]

Originally Posted by Nomad

As for decreasing manuverability that all boils down to design. Most tunnels on boats are just the little worthless half circle design. That is not what I'm talking about......

Not asking you to give away any trade secrets Nomad but can you give a little more detail?

I'm interested to see so many suggest a decrease in performance from tunnels. Sure, I can see why in shaft angle is likely to produce a corresdonding decrease in transom lift, but why wouldn't it also prdosuce a similar corresponding increase in forward force - ie increased speed...

[DaveB]

Originally Posted by Nomad

As for decreasing manuverability that all boils down to design. Most tunnels on boats are just the little worthless half circle design. That is not what I'm talking about......

Hi Nomad,

Could you tell us about the tunnel section shape you speak of? I'd assume that most are half circle designs.. just cut away the space that the prop needs... I don't know much about tunnels or their design but am interested!

Dave

[Tim B]

The entire prop-tunnel question revolves aroun what I believe you have come to, which is, of course, the tunnel design. The idea of putting a propeller in an enclosed tube (ie a waterjet) is to allow better efficiency at high RPM with less pitch (so each blade has a lower peak pressure, but thrust is greater due to 1/2*rho* V^2 *area*cl. The advantage of using a totally enclosed system lies in the control of vortices, and this is quite different in the enclosed and partially open situations. In an open case, the usual intention is to reduce draft, not to vastly increase efficiency (defined as Force forward/Power Supplied) so even if the thrust vector forwards increases, if the tunnel creates too much drag, the efficiency will decrease.

Cheers,

Tim B.

[Willallison]

I seem to recall that there was a good article about prop tunnells in Pro Boatbuilder a few editions back....