16’ Mantra Tunnel Black Thunder

[Fonz69]

Well this next chapter begins with me testing out a handmade shoe for my Stealth after another successful racing season with UMPBA. I made a plan to drive the boat back to our dock and offered up rides to my family since we had some X85 fuel left in the drum.

Well I figured why not let both my sons take the wheel for some nice photos and give them a taste of the driving fun that I’ve known behind this alcohol guzzling boat.

Turns out the apple doesn’t fall far from the tree and both boys now want to go racing. When your kids take interest in your hobbies you really no choice but to fuel the fire before it goes out…right?

And so the search for the right boat begins, I found several OB powered ready to race boats that would be great for the JR racer or 65mph class but after showing them to my oldest it was a solid “no I want a jet boat that’s loud like yours“

We had a small budget to work with so our options were limited and everything we both would find meant driving out to the west coast, way over our budget or are too far gone to afford to save.

With some sponsorship help from his Grandparents we finally found a boat that looked promising. A 1986 16’ Mantra Tunnel down in Illinois.

Texts went back and forth and a plan was made to drive down and check out this prospect.

Some things were worse then I had thought however it was a diamond in the rough that just needs some freshening up to make it a reliable race boat. We negotiated a price and started the long drive home. He couldn’t wait to get behind the wheel so at our lunch stop this was his first shot.

And after we made it home his younger brother wanted to share in the excitement as he will someday take the wheel when he gets old enough.

Like I have done in my last 2 builds recorded on this forum I’ll do my best to update our progress. This will definitely be a work in progress so some things will just need to wait until we can afford to do them (sponsors are always welcome ) however it will always be our intention to make this build safe and reliable with what we have to work with. Hope everyone enjoys the build and he has already named it “Black Thunder”

 

[Fonz69]

Both boys were excited to start the tear down of the new race boat. My shop started to look like a bomb went off

 

[n2otoofast4u]

Seems to have some solid parts! Congrats on the new project!

 

[Fonz69]

The pump was supposed to be good to go however I discovered the impeller is shot when we looked at the pump in Illinois. The previous owner adjusted the price and threw in a better carburetor to help offset the known damage.

Did a quick compression test and it seemed to be decent.

Off with the headers.

Out with the seats.

Long day of hard work, they look tired

Time to pull the motor!

 

[Fonz69]

A little digging into the motor shows things aren’t always what they appear…. The previous owner was told it was a fresh build with forged internals. Hmmm not so much. Found a 2 bolt main cast gm crank with speed pro pistons and some Chinese red rockers. Well it did run so we’ll have to make this work.

Heads are gm large oval 241s

Unfortunately an old edelbrock rectangular port intake does not help air flow, otherway around would be fine but this will be replaced now by a rpm air gap manifold.

 

[Fonz69]

Was able to dig around online and found a post of a past owner in the 90’s when this old girl was in her prime.

The original trailer is still here and we still have the original back seat.

 

[Romans9]

Very cool!

You cannot go back and make memories, you have to make them now, good job dad!

 

[n2otoofast4u]

Post up what parts you need! I’m not sure about a sponsorship, but there’s a LOT of us with a LOT of shit laying around that would be happy to donate to put a new hot boater on the water!

 

[JUSTWANNARACE]

So Rad!!

Fallowing for sure!!

 

[Fonz69]

Post up what parts you need! I’m not sure about a sponsorship, but there’s a LOT of us with a LOT of shit laying around that would be happy to donate to put a new hot boater on the water!

I’m going to reach out to David and Duane at HiTech Performance also but big ticket pump item would be finding a nice used AA impeller. I have a new standard wear ring but the impeller the boat came with is toast!

Really need a flywheel cover and a safety loop or shield for the driveline too!

Anyone has anything decent please let us know. Thanks!

 

[Fonz69]

Put a few more hours in the shop today.
Boys went to town removing parts while I started the clean up and tear down of the motor.

Definitely had the wrong intake you can see the rectangle port gaskets stuck on the oval port heads

Goal to pull the pump was tougher than expected. Had to drill out the back two loader bolts and there was probably about 3 tubes of silicon around the race plates

Motor rails coming out. Plan on making new ones since the motor will move forward some with the Jetaway.

New intake came in yesterday so doing a dry run to see how things line up.

Pumps out, shoe will have to wait until it’s flipped over only 1 out 4 bolts came out and I bet the others will be real fun to remove.

Getting closer each day.

 

[Fonz69]

Motor is about 90% complete now.

Cleaned off the cheap paint that just melted off with carb cleaner
Fresh coat of etching primer and engine black.

New timing cover since the old one had one of the pan nuts missing so someone smashed a nyloc nut and silicone

Had to machine down the cam button to accommodate the new cover.
The oil pump drive shaft sleeve had sheered off and was rattling around at the bottom of the pump. When I pulled the oil pan and seen the stellar weld job on the pickup I knew looking closer would be important.

New HV Melling pump with matching pickup for the new 10q Champ Oil pan. Figured some of these upgraded parts just made sense to try and keep this questionable motor alive.

Champ pans are made in WI and I’ve used them before with good success and they are not terribly overpriced like some pans can be. They come with trap doors and a crank scraper built in.

Picture above is during cleaning with the scraper off.

Pan with Moroso 1 piece gasket and stainless ARP hardware.

Dusting off my original set of HTP valve covers. Have to make some custom tower breathers because of the blower setup they were on last time I used them.

Went with the Edelbrock RPM Air Gap manifold and went through the Holley 850 carb.
At least this intake will actually match the heads….picture is mocking up.

So the headers that came with the boat will need to be re chromed so I brought down off the wall my set I used on the 582 motor in my boat. Sanded up his scoop, set the intake, distributor and wires. I’ll have some brackets to make for the coil and 6AL box but that can wait for now.

 

[Fonz69]

Mission to complete the hull stripping is almost complete!

Fuel tanks out!

Pulling all the wiring out, past projects have taught me even if you think it’s ok you better check it. I found my harness in my stealth rubbed across a bolt and chewed off the coating on all the wires in the same spot! It’s just not worth it so this boat will get new wires installed in a poly conduit.

Max hard at work removing pounds of silicon.

New wheel came in so had to quick mock it up before we removed the steering.

All out!

Checked out the boat cover, it’s original and was nice but unfortunately has shrunk and only snaps the first half before it won’t stretch anymore.

 

[Fonz69]

Time to figure out this loose bow eye problem.

Marked out the light then drilled holes under the light area to expose the loose eye nuts. It doesn’t seem to be damaged on the inside but will definitely need attention on the outside.

Up she goes!

So this would make it 450lbs with the intake, carpet, tank straps, horn rub rail and rigging so probably around a 420 lbs shell.

Now we’re rolling.

Note to self, wait for help, this was not my best of ideas to attempt myself. When it goes back over again there will be more people involved as the cherry picker had a mind of its own and caused me some more gray hairs lol.

Well somebody loves to beach this ol girl

Will address the through bolt tank straps to make sure they don’t leak but thinking I’ll leave them thru hull still.

Have to address the water absorption issue, the bunks are rotted a
so that explains some of this.

We’ll time to fight with the shoe bolts and hope they will come out….I want to be able to adjust the hardware so now is the time to fight with it while she’s upside down.

 

[Fonz69]

Well after some drilling and broken tools the shoe is out. Looks like it was set when the intake was installed and never removed. Opening is 14 3/8” way larger then my stealth. At least now we can make adjustments as needed.

Old rusty tank straps need to go. Found some nice stainless ones online.

 

[n2otoofast4u]

Heck yeah! This thing is going to come out great.

 

[Fonz69]

Quick refresh of the trailer started today. It’s in pretty good shape but needs wiring, lights, axle seals, bunks, fender panels and tires.

Using some composite material left over for the fender skirts. It’s aluminum skin with a plastic core.

Stainless hardware and spray glue with some new rug.
Trailer selfetch primer and gloss black for now.

New treated bunks and rug with stainless hardware through bolted so hopefully they stay in and not fall out like the old ones.

Panels worked out great long as my boy doesn’t crash into them loading the boat. Part of the reason I’m not going with a high dollar powder coat finish lol.

Well no more room in the shop so you stay outside. At least I can use this canopy I bought for the races and somehow never end up using anymore.

Now it’s back to boat bottom and a goal to get her back on the trailer before the big snow hits!

 

[Kachina26]

One word, WOW!

 

[Fonz69]

Sanding fiberglass my favorite past time… not really but it’s got to be done. Not going to go crazy with it but address some of the sponson gel cracking which is in line with the main bulkhead so I’m guessing some wave pounding and past owners popping wheelies led to what I’m seeing.

Ground out and added some glass back to the damage I found at the bow eye.

Max was back at silicon removal duty, his favorite

Owen was helping save my lungs while we drilled out and countersunk better fasteners for the new stainless tank straps.

80 grit fun

Some filler for the intake holes to smooth things out.

Much larger opening than my Stealth. Might make a shoe to close that up some since we don’t need any overcharge issues. And might try an open loader I had instead of the old rusted V blocker.

 

[17 10 Flat]

What a great project to do with your son's!!! No doubt boat will turn out amazing.

 

[Fonz69]

Worked a 70hr week this week so not allot of progress made though was able to finish the keel mods, speed coat the keel and finish the crack repairs. Hope to shoot the black gel coat next Saturday.

Modified the keel to less of a V hull design. Only testing will prove if it works

These cracks are right in front of the main bulkhead so I’m sure everyone’s popping wheelies in this didn’t help matters. Had to grind them down and build it back up.

Touched up the scratches and more bow eye detailing.

Gel coat seal the keel work.

Final filling on the crack areas in the front.

Scuff up for speed coating.

Hope the keel works.

 

[Fonz69]

Gel coat time.

Brought dad back out of retirement to spray a little gel while I mixed up the next batch.

Somehow I forgot how much fun wet sanding gel coat was when we sprayed some parts on my Stealth. Sand till your fingers bleed yep that kinda fun.

Speed coat similar to what I did on my Stealth and will help hide all the times I know he will be bouncing off the trailer fenders learning how to drive

 

[Fonz69]

Time to build a shoe and close that opening up a little. I’m going to start out with a flat shoe instead of the tapered one that was in it to help free up the hull some with the heavy BBC motor. Won’t really know where we are at until we make a few passes and see what the pressures end up at.
8x8x 3/4 and about 5 hours of machining and grinding to get the final result.

Starting to get some shape but still a long way to go.

Shape is good now.

Little detail cleanup.

Fits like a glove

New opening is 13 1/2” a good place to start compared to where it was at.

Here is a shot of the old shoe sitting on top of the new shoe.

Current depth is .050 above the keel so I have a little room to adjust if needed.

 

[Fonz69]

The time has come to roll her over and go back on the trailer.

Used an old Berkley suction that was junk anyway to rig up the back end.
And finally the Eagle has landed.

Max started ripping out the old nasty carpet.

I started the deck polishing which is taking more than it should considering this was advertised as “paint correction done” regardless it needed several passes on the starboard side and rear spoiler just to get it back to color again.

Buffing will continue and there are some Black Friday goodies on the way to keep rolling.

 

[17 10 Flat]

Gel coat time.
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Brought dad back out of retirement to spray a little gel while I mixed up the next batch.
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Somehow I forgot how much fun wet sanding gel coat was when we sprayed some parts on my Stealth. Sand till your fingers bleed yep that kinda fun.
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Speed coat similar to what I did on my Stealth and will help hide all the times I know he will be bouncing off the trailer fenders learning how to drive

That's some love you put into the new ride.

 

[Fonz69]

Well the top took some wet sanding and several passes with turbo cut and then some wax to bring it around but at least we are ready for reassembly.

 

[Fonz69]

While Max finished up scraping the old carpet glue which was a task I voted him to do since let’s face it he’s smaller and can fit into those tight spots . Only heard once “dad I’m stuck”

I started the process of filling in the holes where people thought they needed things mounted.

Drilled glued and covered in resin. Now it’s time to install the new fuel sender and both tanks.

New tank straps look way better than the old black rusted ones. Little polish on the fillers and check that off the list.

 

[Fonz69]

Time to build the ride plate. 8” structural channel. Had to mill the sides down to match the shoe.

Max learning on the mill making the final passes.

Matching the holes in the shoe and intake.

Going to order some longer bolts for the cradle but this setting is where we will test it out and go from there.

 

[Fonz69]

Ever have that moment when you take a second look at something and go yeah that’s not right. Well someone drilled the holes for the motor rails blindfolded and missed terribly on the rear ones so decided to plug them too and start over from scratch. I’ll add some nice plates on the outboard side of the stringers to dress it up a little and hide all the hole plugs.

Some more mock up since we installed the helm and I know he wanted to sit in it and make motor noises!

It’s going to be a long winter but should be rewarding.

 

[HST4ME]

excellent

 

[RichL]

Love it. Way to go Dad.

 

[Fonz69]

Time to work on the suction housing. Added vent connection for jetaway that I’m donating off my Stealth.

Data ports for High and Low readings.

Keel port.

Hard to see but there is an 1/8” hole in front of the loader pad.

Drill and tap for the jetaway. Fasten with the existing bolts and mark the extra 4 required.

Drill and tap.

Little rattle can paint job.

 

[Fonz69]

Suction installed with new base gasket and hardware. Staked the new Ultimate Wear Ring. Suction also received a double lip bearing seal and packing swap to seal kit from HTP.

Jetaway gasket measuring. Install the jetaway measure the gap then add gaskets that thickness plus .005-.010 for crush and no leaks.

HTP loader installed.
Thru bolted and nylocks on top.

That MPD impeller won 2 Championships so the kid better take care of it.
Checking depth to bowl clearance to setup for the stuffer install.

 

[Fonz69]

After measuring the clearance from the clay test we determined the amount that needed to be removed from the stuffer plate. I’d rather take it from the stuffer than the bowl to maintain the value of the bowl without doing something that could never be undone later.

Max drilling out the stuffer plate holes getting ready to tap the holes.

Stuffer in place.

After some careful measuring and setup the bowl is on and filled up with lube. Stole the prime a jet from my boat as the bowl had a breather that would get the air out but would also keep spraying water after which defeats the purpose of an efficient bowl.

This might not be the final setup but more of a brainstorm of placement and mounting ideas. The ball valve would be more of a water line emergency shutoff than any flow control. Will have -8an line into motor and -10an out along with the psi overcharge valve.

Disassembly of the Place Diverter for some maintenance and fresh paint.

Also upgrades to the hardware.

Pump is coming together nicely now.

 

[Fonz69]

Started the rail kit install. Most all the hardware is just for mock up.
The stringers are remarkably close to parallel and good on height however the starboard side leans outboard and I’ll have to make some metal shims now to keep everything true.

Used the driveline in place to establish my rail thru bolt locations so I don’t run into where the motor pillow blocks bolts get installed.

Since the boat could be sitting unlevel on the trailer I established my zero from the pump intake at the shoe pad.

Rails are in and temp bolted in place.

The old motor plates had a bunch of shims under them and had been redrilled multiple times. I had hoped I could have used my old rear plate but nope. The amount I had to shim everything to get into alignment of the pump I had to older up some material to make new plates

Now it’s up to height and at least I can establish the pillow block locations and drill out the rails to accept the new motor plates when the material arrives. Maybe Santa will bring it early? Na probably not doubt I made the nice list this year.
So I based my zero off the intake under the boat and checked it off the jetaway which matched up nicely.

Verified my motor was at the same 4 degree offset to keep the driveline angles in parallel.

Mocked up the flex plate cover and driveline shield as they were both used and need attention. Have to drill and tap new holes and true up the driveline shield and shorten a little to make sure the jetaway collar will slide correctly.

I used the jetaway radius as a guide to help align the rear of the motor. Tough to hold the tools and take pictures of that setup but it seemed to work well.

 

[poncho]

I could write a book on how impressed I am with your work and the lessons you are teaching your Kids, fantastic.

Real glad to see that Jet-Away going in, those boats don't like to shut down nicely sometimes.

I believe your friends with Kilgore, pretty sure he knows his way around that hull pretty well.

 

[Fonz69]

I could write a book on how impressed I am with your work and the lessons you are teaching your Kids, fantastic.

Real glad to see that Jet-Away going in, those boats don't like to shut down nicely sometimes.

I believe your friends with Kilgore, pretty sure he knows his way around that hull pretty well.

Richard Raderchak has one called Mighty Mouse that’s a twin turbo deal. We chatted for awhile and my conclusion to decrease the intake opening was right on as his opening is the same I changed mine to. He had a little accident recently though but said it wasn’t the boats fault but a driver error not hitting the pop off valve and didn’t realize his motor shut off. Boat was unhurt though he was still recovering from the exit into the water.

I only plan to run this in the 65mph class but agreed the Jetaway is a great safety device.

 

[poncho]

Richard Raderchak has one called Mighty Mouse that’s a twin turbo deal. We chatted for awhile and my conclusion to decrease the intake opening was right on as his opening is the same I changed mine to. He had a little accident recently though but said it wasn’t the boats fault but a driver error not hitting the pop off valve and didn’t realize his motor shut off. Boat was unhurt though he was still recovering from the exit into the water.

I only plan to run this in the 65mph class but agreed the Jetaway is a great safety device.

I have several friends that are running some hard hitting jets, I've always worried about them if the pop off didn't work, my flat had a C02 shifter and it leaked out on me once, no big deal in a flat.

Your deal is perfect for that Jet-Away.

Not to mention the ease of running on the trailer.

 

[Fonz69]

Building the pillow blocks and cutting them for the 4 degrees.

Before and after. Shimmed up hacked up garbage to solid and symmetrical.

With the new blocks installed I will have to order material to make plates that will be long enough to work.

Max got his helmet for Christmas so he’s one step closer to racing.

Time for the driveline cover work.

 

[Fonz69]

Got the parts used so not really sure if the driveline shields were in a fire or something but they should clean up ok.

Little weight reduction just like I had done on my set in the Stealth.

Fits nice.

 

[Fonz69]

Installed the switches and set the gauges in place to check spacing.

Horn and starter button.

 

[Fonz69]

Today is race plate fabrication day.

Aligned the first plate. This side will have the steering tube but I’m eliminating the water inlet through the plate and it will be going higher through a hose to a bulkhead fitting.
Also made the plates taller to give a little more rigidity for the steering tube since the transom is thin.

All sealed up and good to go.

Steering install and some ARP fastener upgrades along with a HTP extension tube instead of the thin wall pipe that was used prior.

Wheel and helm locked in place. I would like to see the dash a little more rigid so might make a brace to help stabilize it more.

 

[Fonz69]

Since I had a New Years Day off figured might as well keep rolling on the project.
Installed the forward and reverse cable and realized the one that came with the boat was bent on the pump side and did not function smoothly. Ironically enough I had purchased the exact cable for my boat and never had the time to install it yet so guess where that’s going

If you noticed the bulkhead fittings those are from Menards and are 1/2” electrical cord tight fittings. Only $4 each and worked out nicely for both control cables.

I had originally thought about installing the ignition switch between the horn and starter button however it’s not easy to see when you’re behind the wheel so this seems to make more sense.

Some mock up again to locate the mercury kill switch. So far right side of the stringer in front of the drivers seat seems to make the most sense and should be out of the way so the lanyard won’t get caught up or in the way while under way.

The rear light base electrical insert was shot however finding a matching base with the same screw layout was proving difficult. Also the original metal base was in great shape. I was able to find a similar looking unit at Walmart of all places for $9 and popped the insert out and into the old metal base. Good as new and still looks original.

Max trying out the steering and pedal distances.

Boat came with this heavy carpeted plywood insert in the center which served its purpose but all that weight doesn’t make for a quick boat. I’m fabricating a section of composite material that’s 1/8” thick instead. I’ll have to reinforce some areas with foam or channel but should still be strong enough to stand on after the carpet covers it.

 

[TimeBandit]

I feel like I just read a master class in jet boat rigging, beautiful work!

 

[Fonz69]

Finished up the motor plates today. $150 worth of 6061 aluminum plate transformed into BBC mounts.

Rear plate cut and ready for final holes.

Marking with 3/8” center pin punch.

Bolts slip right in as they should.

Had some old stringer washers from a 21 Daytona I used to have. Want to use them for the front mounts and had to drill them out for the 7/16” bolts.

Test fitting the RH mount.

Sanded and debured.

Checking alignment and adjusting the LH mount.

Finally ready to pull the motor and detail the rail setup.

Two steps forward one back. Some people never realize what it takes to rig a boat. The cost is one part but the hours it takes to do it right I know that I could never afford to pay a shop to do all this.

 

[Fonz69]

Had a couple hours Sunday afternoon so started to detail out the rail parts.
Tried a couple ball mills I had but just didn’t like the results so I ended up with a straight 4 blade end mill instead.

Nothing too fancy but makes the parts lighter and adds a little character I suppose.

 

[Fonz69]

Max practiced on the 2nd run tonight.

 

[Fonz69]

Well parts are all made so it’s time to prime and paint. Max wants everything painted black so guess I’ll be painting instead of polishing.

 

[Fonz69]

Started the painting process. Going for another coat of black but ran out last night.

 

[Kachina26]

I gotta ask, (because I'm jealous of your energy) are you retired? Or are you doing this after a long day of work? Either way, I'm loving the build and the updates.