Tunnel Hull Question

[Bill kubiak]

On a small Tunnel (12ft) using a single outboard, what is the ideal height of the motor? I am thinking that each hull is throwing a wake at the other near the very end of the tunnel. So that area might be full of water. I know at high speed, over 50 mph the tunnel is supposed to fly, but does the bottom of the tunnel actually clear the wakes of the two hull sides. Does anyone have any diagrams of where the propeller arc should be as a start point?
I do not see my little 15 hp motor pushing me any faster then 25 to 30 mph

 

[Silvertip]

Re: Tunnel Hull Question

Start at the lowest possible setting and continue to raise it until it begins to ventilate.

 

[Texasmark]

Re: Tunnel Hull Question

PM DeeJayCee from So. Africa. He ocean races inflatables which are setup on the tunnel configuration. If anyone knows, he does.

Mark

 

[longshanks]

Re: Tunnel Hull Question

I have a 15' aluminum boat with a 24" tunnel built in the back of it, designed to be run with outboard jets, to make a shallow-running boat. The majority of the time I use this setup with a prop (50HP). As Silvertip has noted, I moved the motor up hole by hole, and in it's present position, it can occasionally ventilate while running fast in heavy chop. AV plate is slightly above the top of the tunnel.

good luck

 

[Frank Acampora]

Re: Tunnel Hull Question

If you mount it too low on the transom, water will spray straight up off the lower unit and splash over the transom into the splash well. Too high, and depending on the position of the water pick up, you may lose cooling water and overheat. START with the cavitation plate even with the bottom of the sponsons (hull on either side of the tunnel) and gradually raise the engine. Depending on the engine horsepower and setting, and the speed, the tunnel may or may not be clear of the water surface. You may need to build up the transom too.

With my 10 foot Cougar Cub tunnel, it planes at about 15 MPH. A 15 runs it at about 18-20, and a 20 runs it at about 28 MPH. Even at that speed, because the hull is narrow (4 feet) and the engine and I weigh a lot, the 5 inch tunnel is not fully clear of the water. I have my 20 inch shaft engine set about 1 inch higher than the sponsons, and the transom was raised 2 inches to acheive this setting with the clamps fully seated on the transom. More horsepower does not necessarily translate into more speed. Because of the extra weight, a 55 on the same boat only yeilded 30 MPH. Sitting so low to the water, it feels like 100 though.

Now, if I could find a surface piercing prop, a nosecone, and a low water pick up for the 20, convert it to short shaft, then------

If you want to see the boat on a trailer, go to Chrysler Crew, join, and go to photo albums. Click on Frank's boats.

 

[Texasmark]

Re: Tunnel Hull Question

Hey Frank, when I was a kid, I put daddy's 10 hp Scott Atwater on an 8' hydro. At rest the stern area of the boat, including deck was submerged. Only thing that kept it from sinking was that the cockpit area had a box built around it.....to keep the water out. Ha!

I could have sworn I was doing 50, but like you said, it was only because I was sitting so close to the water. 20 was more like it.

Mark

 

[Bill kubiak]

Re: Tunnel Hull Question

The transon on my tunnel is 15 inches, the top of the cavitation plate is even with bottom of the top of the tunnel and the sponsons bottoms are about 10 inch lower then that. Just as the boat is about to plane the motor surges, revs up, and loses it grip on the water. I have a sting ray foil on it, that might be a problem by lifting it out of the water too much. I found one setting with the motor tilted in a bit that let us speed up but the sponsons were digging in and throwing a terrifc spray straight up from the sides which came back on us as it was accelerating like mad but the water spray got worse so at around 16 to 17 mph we had to slow down.

Just pulled the foil off and will try again in a day or so.

The 1988 15 Merc has three motor angle pre sets with 3 adjustments in each pre set, one has gotta be about right

 

[Frank Acampora]

Re: Tunnel Hull Question

If I understand correctly, with a 15 inch transom and the cavplate even with the botton (Top) of the tunnel, then you have a short shaft engine and it appears that when you start to plane the prop ventilates badly. If you do have a short shaft engine then you need to either build or buy a jack plate to lower it or convert it to long shaft.

Basically what is happening is that the sponsons are lifting the prop too high in the water as they start to plane. You need to get it deeper --probably by about 3-4 inches.

Actually, at this point it is not a prop question so maybe the moderators will move this post.

Once you get the engine and boat dialed THEN you need to experiment with props.

 

[walleyehed]

Re: Tunnel Hull Question

I will have to respectfully dissagree on it not being a prop issue...
I've just started playing in the Hydro racing classes and I have a completed 12ft Hydro with a 60hp 3cyl OMC. After only running 2 runs I can't say what the correct heigth is...obviously it will vary with speed as lift will be a concern and factor. I do believe the hydrofoil will cause problems in the long-run.
It will take a prop that will have a very minimum amount of ventilation and there's going to be a fine line of where the speed lifts the hull (as well as the prop) or a result of speed causing the air pocket. there is a very radical transition from "on-plane" to "wide open".
This type of set-up will not work with a standard type SS prop..it will require a custom cleaver for the best performance. My major concern is the style of tunnel you have and if it falls into the Hydro class or full tunnel. The available hp may be an issue as well.
The Hydro I have built is a Modified carbon fiber and wood Air Stream Special...with the 60hp tripple, this rig will achieve 110mph, although I have not found that mark yet...not even close, the performance sucks until about 40mph when it begins to come up and clean on the pocket at which point it accelerates like a rocket.
The answer may be in what you expect from this rig with this hp..???

 

[Bill kubiak]

Re: Tunnel Hull Question

Yesterday I was hanging off the back of the boat as my wife drove, I wanted to see what was going on back there. The speed was about 7-8 mph which was a speed just before it Ventilated. It appeared as if the hydrfoil was barely running just below the surface. The stingray foil is slightly curved down at the rear of the wings which sort of acts like flaps on an airplane.

Water was coming off all the bottom surfaces pretty evenly. I can only guess that as we added power the foil rose to ride on the surface sorta like a water ski causing all kinds of a ruckus back there.

If I need to lower the motor a few inches how do I do that without cutting up the transom? How do I make a home made jack plate.

I took the foil off and am trying to get the wife to agree to a test run again later today. She is complaining something about it is christmas and visiting the kids yadda yadda yadda.

 

[Bill kubiak]

Re: Tunnel Hull Question

I think I have found my problem. using a straight edge from the bottom of the tunnel out to the cavitation plate, the plate is about 1/2 inch higher then the bottom of the tunnel. I gotta lower that motor by at least two inches.
Any suggestions?

 

[Frank Acampora]

Re: Tunnel Hull Question

What engine are you running?If it is older, there were some that had conversion kits to change a 15 inch leg to a 20 inch leg.

Lacking that, I don't have any photos but just make an engine plate out of 1/8 inch thick aluminum (should be sufficient for a 15) and bend it into a "u" or rectangle shape. make it a little wider than the engine clamps and a little taller than them. use two layers of 3/4 inch plywood attached to it as an engine clamp surface. If you search other threads in other parts of the forums, there are some photos of home-made jackplates that you can look at.

Picture a cardboard box without the top and bottom. one side bolts to the transom, slightly lower than the top is now and the other side carries the engine. Then you can use multiple holes to raise and lower it on the transom. It only needs to be wide enough to clear the clamp screws on the inside, but you could make several and experiment with various set-backs.

This is temporary and once you find the combination of set-back and height you want then use something more permanent than plywood or treat it for water resistance.

You would also need to search and find if there are any type of performance props for your engine. I think the stock prop will always give you some sort of problems on the tunnel hull. But hey! I could be wrong. Ask my wife!

NOW---I did the same thing on my 10 foot tunnel with a short shaft 8 HP engine. IT RAN LIKE CRAP! even after I lowered it on the transom. --Rather much like you are describing. It was not until I put on a 10 HP long shaft engine that I started to get any performance. That's why I say that at this point it is not a prop problem. At 8 mph your performance is so poor, no amount of fooling with the prop is going to help. FIRST you need to get it buried in the water deeply enough at planing speeds. Then you may find you can achieve 20-25 mph.

But just for the hell of it----Did you check the prop to be sure it was ventilation and not the hub slipping? 7-8 mph is not a lot of speed.

 

[Bill kubiak]

Re: Tunnel Hull Question

I had the prop re-hubbed so I am pretty sure it is not the hub slipping.
I saw a mount in a WM catalog for a Garelick Fixed Mount Bracket for up to 15 hp. Got it on order and it will be here in a few days.
The Motor is an 88 Merc 15 and at around 8 to 9 mph it seems like the prop just loses it's bite on the water and revs up. For the hell of it I let it rev and as the boat slowed it just grabbed the water again and then did it all over again.
We angled the motor so it would push the bow down and that boat zoomed right up to 15MPH but we had to cut the power becauase the bow spray went straight up off the two sponsons and dropped down on the cockpit area drenching us.
I know it will get up and boogie, the trick is to get to go without digging the bows in. I am hoping to see something between 20 and 25 MPH when I get it right

 

[walleyehed]

Re: Tunnel Hull Question

Go to Hydroracer dot net and you'll see what I mean about the prop you will need...Check into the classes running your HP...although you may not be wanting to "race", the technology of the prop must meet the design you have.

 

[Bill kubiak]

Re: Tunnel Hull Question

I went to that site but did not find any prop info. lots of great pictures tho.
Do you have a specific link?

 

[Frank Acampora]

Re: Tunnel Hull Question

Hey Wally: Thanks for the link. I joined the forums and while I don't intend to race, I'm hoping a few well chosen questions will help me maximise performance on my 10 foot tunnel hull.

 

[walleyehed]

Re: Tunnel Hull Question

A wealth of knowledge exists there for the specific boat we are playing with now....I've been dealing direct with one of the members in central NE due to the short distance he lives from me. There is a spot on that fora for questons from new folks....there is a gent that will usually provide the needed info on the prop you will need to try and where to get it.