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Tunnel Hull design on a mini???

PostPosted: Fri Jun 29, 2018 11:31 am

So now that I officially have the mini fever I've been doing a ton of research on intakes and I've noticed that know one is running a tunnel in front of their intake on these minis? Has it been tried and didn't yield any positive effects of is there some other reason people aren't doing it?

Re: Tunnel Hull design on a mini???

PostPosted: Sat Jun 30, 2018 9:14 am
by Ahusk
I highly doubt you will keep the pump loaded with a tunnel. The planning surface is not long enough and the back end is too lite.

Re: Tunnel Hull design on a mini???

PostPosted: Thu Jul 05, 2018 12:32 pm
by Ridextreme
One could just stick their head under a couple $150.000.00 boats and see what they do. In short you don't see it in the industry unless you are running an outboard. I'm sure it has been done and scrapped but I do see the interest in theory. Probably has to do with the water pressure under the boat while it is in motion . The outboards have a hard time scavenging which is why it is better to tunnel a steady flow of water . Inboards pull a larger volume of water it would be hard to calculate the amount of volume your tunnel would need.and if you did I think the tunnel would be too big and then inefficient . Loading an inboard can be achieved by a spoon shape delta in front of the intake . Never actually seen it but there are some photos online .

Re: Tunnel Hull design on a mini???

PostPosted: Fri Jul 06, 2018 1:05 am
by riverboater
I put a radius spoon and three inch deep tunnel in my last 14ft build. This was a 24 degree with a 773 and 350 chev. Not really a mini but no reason the same wouldn't work in 12ft 12 degree. When jumping gravel bars I rarely got rocks in the grate.
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Re: Tunnel Hull design on a mini???

PostPosted: Fri Jul 13, 2018 8:28 am
by Darth T8r
SJX uses a tunnel with their 21 ft boats and a sport jet, Custom Weld uses them in their 20 ft model - I don't see why it couldn't work in a mini.

One of the jet-john builds I read about with no tunnel had a problem with turning the pump into a suction cup crossing sand bars and acting like an unexpected parking brake.

You would lose a small amount of bouyancy - but the trade off would be that when you did touch bottom - if you still got water to the pump, the boat should be able to keep pushing over the obstacle.

I think one build just had a slightly recessed intake to help with. I have more reading to do - but it seems like a radiused keel ahead of the tunnel or recess helped push air bubbles to the sid & away from the intake flow while still giving a usable balance between lift and flow.

Other things to consider are cost and build complexity. I intend to try one, but I'm a designer/machinist/fabricator with a whole shop of equipment at my disposal, and I'll leave room to convert it to a flush intake if I have problems.

If I didn't have so much neat stuff and experience at my disposal, I'd build something simple/straightforward first and leave room to modify the intake & pump/motor position later if it was a nuisance.

Re: Tunnel Hull design on a mini???

PostPosted: Fri Jul 13, 2018 9:39 am
by Ahusk
Sjx and Customweld 20's benefit from a lot more planning surface and weight than a mini. On mine the difference between the half inch uhmw ending right at the intake verses stepping down with 3/16 steel six inches forward was noticeable.

Like you said you have the equipment and ability to do it yourself so why not try. It's going to aerate most likely and trim may be difficult to dial in. Keep the deadrise steep and the tunnel shallow.

Re: Tunnel Hull design on a mini???

PostPosted: Fri Jul 13, 2018 6:43 pm
by Darth T8r
Good point. My knee jerk reaction is to want to build something unnecessarily complicated. As small as a mini is - might just be too much work to justify fixing something that isn't a significant a problem.

You make good points about trim, and keeping the pump deep enough. The whole shape of the hull would have to be adjusted to increase the planning surface.

Untill I have a boat built to beat on - it's just speculation.

(Just put my application in to go back for an Aerospace degree - I partially blame you guys and projects like this where I know just enough to know what I don't know)