Building a Thundercat (inflatabable catamaran)

[race30]

I've wanted to do this for years, but couldn't find more than 2-3 posts about the ins and outs of this sport/setup, so i'm posting this build thread in hopes that others will read it and save themselves some agony that i went through.

The hard part about finding info on these boats, is that the racers keep their cards close to their chest and don't divulge much, if anything, ESPECIALLY pictures of their setups.

Most of the good pictures i found came from overseas where this is a super popular sport.

First things first- you have to start with a boat.
I had my eyes set on a DUX brand boat in michigan, with plans to buy it and ship it to my house via uship.com
The night before i pulled the trigger on it, and the mercury 40hp mounted to it- i got cold feet and contacted Deon Victor at IBR.US
I had had a string of emails with him a few years before, and he seemed knowledgable.
I just had to bounce my plans off from SOMEONE...anyone, who knew more about this sport than my dog.
Turns out Deon loves to talk, is a giant advocate for the sport, and was more than happy to help me through my decision process with no promise of investment.
He hooked me up with numerous industry guys who know and love the sport, and are also happy to divulge their secrets.

I quickly learned that the Dux brand boat is a questionable investment.
The owner of the company has made many bad deals, and the internet is swarming his brand.
just google DUX problems, and begin reading... About 20 minutes later you'll need little convincing to stay clear of any new/used Dux boat that may or may not have issues.

Acquarius brand boats seem good, are light, but may use thinner materials than you'd hope.
I've never seen one up close, so take this for what its worth.

Gemini Zap Cat has good build quality, and good materials

Other brands exist, but i didn't do my homework on them, so i'll stop my list here...


Motors:
This is where i spent most of my time...
The tried and true race motors abroad are Tohatsu M50D2 or Yamaha 50
They are all short shaft (15") with a tiller handle
and as it turns out, extremely difficult to duplicate here in the northeast.

I had my heart set on working a craigslist deal on some unsuspecting guy who had some old clapped out fishing boat with a yamaha 50hp on the back of it... but it, convert it to tiller steer, and get away with murder (price wise).
Steal the trailer, modify it to fit the boat, and give the hull away.
This is where i began
After about a week, i think i found the end of the internet.
i had turned over every single web page
every ebay in the US, AUS, UK, etc.
I knew of every motor deal worth having, and none of them really fit my budget.

Then i went to version 2.0 where i was going to get a 20" long shaft and convert it to a 15" short shaft motor.
Ebay was tired of my searches by that point.
It's very difficult to find 15" mid sections and lower units, just like it is to find whole motors
Could you mod a 20"? cut the shaft? dice up the mid section?
maybe.
but i'm no machinist
so i wasn't going to risk it...
This option lasted about a day in my mind

[race30]

So i went to version 3.0 and i decided that i'd buy a long shaft (20") 50HP or 70HP yamaha (70's are far more plentiful, but would require some re-stickering on the cowl, since the hulls are only rated for 50HP...)
Under this plan, i was going to buy a manual jackplate - available from many different places, but they're rated to hold multiple hundreds of HP, and can raise the motor up to 6.5" (plenty enough to cover my 5" handicap (insert pun here))
I did my background checks on the jackplates.
The manufacturers of the jackplates had an assortment of options to choose from for $300 or so, and this option opened up my motor buying pool by a LOT.
The only thing the jackplates left to be desired was:
1. will the thundercat transom, glued to some PVC pontoons hold up with a 70HP motor, in the surf, at 60+MPH, with an extra 5" of leverage- more than designed?
2. Will it look goofy as fcuk?
3. Will the tiller handle be abnormally tall?
4. See number 2

While re-exploring the craigslists and ebays of the world, i got a call from one of the 50 people i had emailed about a motor.
He had a tohatsu 40HP, and it was a true short shaft
"garage kept"
"mint"
were words he used to describe it.
But how much?
$1,600...
Where?
15 minutes from my work

I rushed back to my computer to confirm what i thought: the M40D2 he had was the same as an M50D2 except for a few items:
1. Carbs
2. Reeds and reed blocks
3. Exhaust tuner
It is, however, the same block, head, lower unit, electrics, etc, etc, etc...
Sweet!
I scooped it up for $1,500 after checking the compression to be 150PSI top and bottom, 138PSI middle (normal: they lower the compression on cylinder 2 for "safety" from blowing up)
Even the flywheel looked new on this thing, even though its 18 years old.

Rewind the clock 1 week from this moment in time.
I found a pair of Honda tiller handles on my local craigslist for $225 for the pair.
They're fully loaded with shift lever and trim button
I snagged them up having no idea if they would work on something else, but figured i'd give it a whirl.
Worst case- sell one of them on ebay for $400++

[race30]

With the motor in hand, i was pleasantly surprised to find out that the Honda tiller bolted right up... zero modifications to the mounting bracket (I think the 2 mounting holes are relatively universal)
the big challenge was wiring it in, and connecting the cables for the shift, and throttle... which were both push/pull cables.
I ended up fabbing a small alum. bracket with some basic tools (jig saw, drill, hand file, vice, hammer, MAP gas)
And i had to buy some M5x0.8 threaded rod, jam juts, and threaded sleeves to extend the end of the cables from the honda tiller to the Tohatsu engine.
It worked out real well. I'm pleased.

Then i had to wire it in.
The Tohatsu had remote controls, and the honda tiller had a 15+ prong plug.
I couldn't find comprehensive wiring diagrams anywhere, so i had to do it the old fashioned way.
using a multimeter i was able to determine which wire did which movement.
Added some real nice OEM bullet connectors to complete the wiring harness and it all works great!

Now the motor was ready to mount onto the boat.

I snagged a Zapcat demo boat from IBR.US and mounted the motor.
Bought a 12 gallon gas tank on craigslist, and fabbed up a fuel line.
I figured i'd rather have the capacity and not need it, than need it and not have it...
6 gallons is probably more like it.
but i couldn't find any for sale.

First run out, i mounted the motor 1" higher than the Highjackers (center of prop to center of highjackers)
it cavitated pretty consistently and struggled to stay on plane.
with 2 people it was better, but still needed some tweaking
I determined the motor wasn't revving out all the way... probably stuck in the 3000's RPM and had a stock 12.5 Pitch prop, so it only went 20-25MPH
Pretty disappointing but nice to get out on the water.

Test number two lowered the motor 1 hole, and adjusted the carb butterflys to be fully open at WFO.
It took off like a banshee and cavitated a LOT less
but the RPMS' kept dropping over the course of the 5 min test run
I brought the boat home and flushed the engine- it was running lean at idle.

Enough fiddling with those carbs anyways.
New 50HP carbs arrived in the mail!
Time to install those, the reed block (intake manifold), the new Reeds, and delete the oil pump.
More on this (and pictures) later.

[race30]

Time for some pics!

The honda tillers, complete with trim, Start, and shift


The motor


Motor on stand, tiller mounted, lots of wiring to clean up, and cables to adapt


Cable bracket, and extended cables


The boat






50HP carbs, intake manifold and reeds... oh! and a TLDI head gasket for 10-15psi extra


Mounted with 16P Ron Hill semi cleaver prop

[race30]

So, as it turns out i was having trouble getting the 40HP to run well
The carbs weren't getting full throttle (butterflies weren't all the way open)
and i have no idea where the timing was set.

So when i installed the 50hp carbs, i read the manual and set the timing, linked and sync'd the carbs, and went for a test drive
It ran great on the hose.
But when i got to the water, it acted loaded up.
i pulled off the hood and found the chokes were stuck 90% closed on the bottom 2 cylinders, because i was still using the 40hp air box, and the 50hp carbs are bigger. The butterflies were stuck!
So i pried them open, restarted the boat, and HOLY CRAP!
It was a WILD ride.
(I LOVE finding smoking guns!)

With a co-pilot, i saw 44mph on the open ocean
I have been over 60 on standup jetskis, over 50 on other boats, and nothing was quite this thrilling
seeing that huge rooster tail, and feeling right on the hairy edge of control was really a rush!

Some minor idle adjustments are needed, a steering damper, a tiny tach.

Once i document the baseline - i'll swap out the TLDI head gasket (which is thinner)

then i'll pull the powerhead and put on the M50D (not D2...) tuner - at which point i will officially have a 50+hp M50D2

[71V153]

Yes, I see ...

[Tom JM]

Yes, it's been difficult finding complete T-Cat info since the demise of Thundercat AU website.

I've been running an Aquarius/60Hp Merc SS for 5 years as a lake playboat so I understand you're frustrations.

Tohatsu is a great choice but I can't help you much with that. BoatRacingFacts forum is a good place for more advanced info.
Could also try HPBC in Canada, good wealth of knowledge on small cube T Boat racing there.

You didn't mention where you ended up on motor height but it looks low to me.
I run propshaft CL 2 1/2 - 3" above HiJacker running surface. That cleaver will want to be as high as possible.
As to the cleaver... it'll be a brute on your arm making for short days. Steering damper will help with control but you'll want a bungee setup to counter the devastating torque steer.
You'll find a low bow lift round ear prop easier on the arm and generally more fun to drive.

You'll discover the boat is very weight distribution sensitive. Get a smaller gas tank then spend the time dialing in your setup for preferred running be it one or two up.
I didn't bother with a jackplate, just run motor as high as I can without sucking air and... use body english to control general boat attitude.
It's a participation activity and the most fun you can have in any boat, period!
Good luck.

[Tom JM]

Inexpensive and effective steering damper here... https://www.ebay.com/itm/MEYLE-Power...-/311416246352
You'll have to fabricate one bracket to connect to transom, other end to steering link.

[race30]

yes sir TOM JM!
I have one of those on the bench as we speak!
Thanks for the advice on props and setup. I started out 1" higher (center to center) prop > hijackers
had to come down 1 hole to get it on plane (I think that's 1/2"?)
but that was with a stock prop
more testing saturday...

[Tom JM]

Props are The Key to getting T Cats performing as they should. Most if not all stock props I tested were just plain scary as they don't like to run on the surface. Ill handling is an understatement.
Have fun Saturday.

[home made tunnel]

I love the backyard mechanic threads! Nice job!!! How much to the inflatables run you? They don't say on the site.

[race30]

I love the backyard mechanic threads! Nice job!!! How much to the inflatables run you? They don't say on the site.

He had options from 3k (used) to 6k (approx) for a new one, made to order

Email him!!

[FLATBTM]

This is awesome. I was very close to pulling the trigger on a Ceasar, but just didn't think it would be well rounded for all my needs.

I can't imagine how absolutely ridiculous this thing would be at the beach though! Good work man. Looks like you're getting it figured out. If I didn't own a long shaft Merc I would probably be on this train already. It just stressed me out thinking about selling my engine and then looking for the short shafts. Those things are about as rare as hen's teeth.

[race30]

This is awesome. I was very close to pulling the trigger on a Ceasar, but just didn't think it would be well rounded for all my needs.

I can't imagine how absolutely ridiculous this thing would be at the beach though! Good work man. Looks like you're getting it figured out. If I didn't own a long shaft Merc I would probably be on this train already. It just stressed me out thinking about selling my engine and then looking for the short shafts. Those things are about as rare as hen's teeth.

You have 2 options
1 buy a jack plate for $300 and run what cha brung
2 buy a tohatsu from ibr.us ... They have the ones you can't otherwise get in the usa...tohatsu M50d2 short shaft , pull start, manual tilt, short shaft... He had a few in stock last I talked to him

[FLATBTM]

Yep. Not sure what to do yet. I need multiple boats to be honest haha. Trying to find that one that does it all is hard.