I looked up your vent. It appears that it is located at the front of the tunnel (towards the bow) - is this correct?
I don't know much about the tunnel design, but does this vetn only air or do you get some water back in the boat as well?
XF20 what pitch 4 blade
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Chris Sopotnick
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Ken, yes venting the tunnel.
With the vent installed in the tunnel I saw a dramatic performance increase. On my initial test run I had a moderate load and with the vent open we could reach over 24 mph, almost 25. When I put my hand over the vent to block it off, the speed fell to 21.5 mph w/o changing anything else. Also the RPM's increased as the prop started to slip in the turbulent water when the vent was blocked. As I removed my hand the RPM's would slowly drop and the speed picked up as the prop started biting again. There was a lot of suction at the top of the vent when my hand was next to it. With my leg next to it, the suction was pulling my leg hair into the tube. Hard to judge vacuum, but it was substantial!
The vent was simply a 1 1/2 inch conduit threaded collar and a long sweep ell. I cut a 2 inch hole in the front of the tunnel and sealed the collar in with 5200. I have designed something more permanent and less obtrusive in my new tunnel using a SS through-hull fitting.
Here are two pics of my test vent in the old tunnel, which is just a more narrow version of the XF tunnel.
Chris,
The vent is at the front of the tunnel just as it starts to make the slope upwards. By putting it here you break the suction that is being formed by allowing air to be sucked in. The vent will puke back water on you when you drop off plane and the hull settles down in the water. To prevent this, you can build a baffle and/or run a piece of flex tubing over the side to give the water a place to go. I have built a baffle on my new one which will also help with water flow.
The new tunnel changes width from the front to the back to reduce drag. The purpose of the tunnel is simply to redirect water and I am trying to limit the amount of turbulence induced into the water. The old tunnel seemed to compress the water with its design, as Jacques mentions it will do in the XF-20 study plans. This creates the vacuum that I relieved by putting in the vent (venturi effect). I am attempting to redirect the water w/o compressing it by allowing a greater volume of water in the throat with the wider mouth. I calculated the volume at the throat and at the exit. While I was not willing to give up enough planing area at the front to make these volumes equal, I am closer than the original design.
A local aluminum fabricator with much experience and success with tunnel design on this size & style of boat recommended this design. He has built one similar with great results.
If there are any other questions I'd be happy to answer them.
Tim
With the vent installed in the tunnel I saw a dramatic performance increase. On my initial test run I had a moderate load and with the vent open we could reach over 24 mph, almost 25. When I put my hand over the vent to block it off, the speed fell to 21.5 mph w/o changing anything else. Also the RPM's increased as the prop started to slip in the turbulent water when the vent was blocked. As I removed my hand the RPM's would slowly drop and the speed picked up as the prop started biting again. There was a lot of suction at the top of the vent when my hand was next to it. With my leg next to it, the suction was pulling my leg hair into the tube. Hard to judge vacuum, but it was substantial!
The vent was simply a 1 1/2 inch conduit threaded collar and a long sweep ell. I cut a 2 inch hole in the front of the tunnel and sealed the collar in with 5200. I have designed something more permanent and less obtrusive in my new tunnel using a SS through-hull fitting.
Here are two pics of my test vent in the old tunnel, which is just a more narrow version of the XF tunnel.
Chris,
The vent is at the front of the tunnel just as it starts to make the slope upwards. By putting it here you break the suction that is being formed by allowing air to be sucked in. The vent will puke back water on you when you drop off plane and the hull settles down in the water. To prevent this, you can build a baffle and/or run a piece of flex tubing over the side to give the water a place to go. I have built a baffle on my new one which will also help with water flow.
The new tunnel changes width from the front to the back to reduce drag. The purpose of the tunnel is simply to redirect water and I am trying to limit the amount of turbulence induced into the water. The old tunnel seemed to compress the water with its design, as Jacques mentions it will do in the XF-20 study plans. This creates the vacuum that I relieved by putting in the vent (venturi effect). I am attempting to redirect the water w/o compressing it by allowing a greater volume of water in the throat with the wider mouth. I calculated the volume at the throat and at the exit. While I was not willing to give up enough planing area at the front to make these volumes equal, I am closer than the original design.
A local aluminum fabricator with much experience and success with tunnel design on this size & style of boat recommended this design. He has built one similar with great results.
If there are any other questions I'd be happy to answer them.
Tim
Ran my boat with the new powertech prop today. No cavitation. 10" pitch four blade. It has a ridge on the prop edge. must really make it bite.
I ran through some really shallow water today. Just wanted to see what it would handle. I did run hard aground I mean the boat was stuck. Had to wait about 30 minutes for the tide to come in before I could push it over to a little deeper trench. When I got out to push the water was just over my ankles.
Caught about 30 redfish and a dozen nice trout. Would have never gotten where these fish were without this boat. The only other boat I saw was an airboat.
What do I need to do to vent my tunnel? Drill a hole and put a piece of 3/4" pvc in it and seal with 5200? Do you need to put some sort of valve on it to control the amount of venting you get?
I'm running about 20 mph. Sure would like to get another 5.
I ran through some really shallow water today. Just wanted to see what it would handle. I did run hard aground I mean the boat was stuck. Had to wait about 30 minutes for the tide to come in before I could push it over to a little deeper trench. When I got out to push the water was just over my ankles.
Caught about 30 redfish and a dozen nice trout. Would have never gotten where these fish were without this boat. The only other boat I saw was an airboat.
What do I need to do to vent my tunnel? Drill a hole and put a piece of 3/4" pvc in it and seal with 5200? Do you need to put some sort of valve on it to control the amount of venting you get?
I'm running about 20 mph. Sure would like to get another 5.
I don't know if 3/4" would be large enough, but it would be a good starting point. You could always go slightly bigger if needed.
As far as a valve goes, some boats have them and some don't. If you decide to put one on you should have it closed when getting on plane so that the prop will slip a little more and get the RPM's up. Once you're up on plane and picking up speed, open the valve to relieve the vacuum. If you think about it, just prior to coming off plane you can close the valve to prevent water from puking back up the vent. This would eliminate the need to run a hose over the transom or side.
T
As far as a valve goes, some boats have them and some don't. If you decide to put one on you should have it closed when getting on plane so that the prop will slip a little more and get the RPM's up. Once you're up on plane and picking up speed, open the valve to relieve the vacuum. If you think about it, just prior to coming off plane you can close the valve to prevent water from puking back up the vent. This would eliminate the need to run a hose over the transom or side.
T
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stickystuff
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Chris Sopotnick
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Sandy Claws
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Vented tunnel on the XF-20
I would sure like to see what Jacques had to say about that vent!
Rog...
Rog...
Rog,
He breifly discussed it with us in this thread last week, but I didn't get a reply if he read the test results on my GF-16. If I can't claim the free plans, maybe he'll give them to Pita since he vented his XF-20.
http://gallery.bateau2.com/forum/viewto ... 5d82face07
Tim
He breifly discussed it with us in this thread last week, but I didn't get a reply if he read the test results on my GF-16. If I can't claim the free plans, maybe he'll give them to Pita since he vented his XF-20.
http://gallery.bateau2.com/forum/viewto ... 5d82face07
Tim
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