Building a Thundercat (inflatabable catamaran)

[Tom JM]

59 with 50 Hp is very respectable. More is possible with major mods but it'd be less expensive to buy a bigger Hp motor if speed is ALL you're after. Remember, the more weight you hang off the transom the less predictable handling will be.

Can you use a jack plate?... Yes providing you install a low water pickup and extend your tiller handle. Again, it's weight.

Yes, some 90's Mariners were Yamaha's but short shaft Yamaha's of any year will be easy to find.

But... most power can be had from OMC SST60 spec motors. Build one with the right parts and you'll be making 110+ Hp.
Outlaw T-Cat boats have run them successfully.

Lots of info on http://www.hpbc.ca/index.php
Yes it's Canadian... a lot of T-Boat guys run small displacement OMC & Merc motors in our racing series. A few are testing Yamaha/Mariner. Dig around, you'll find valuable info.

We did try to get a T-Cat series going a number of years ago but not enough interest.

Top Gun Marine, Bradford Ontario is a popular motor supplier to our series. He'll likely have something that'll interest you and yes, he will ship. Do a search on this site, he posts regularly.

And please, don't have someone cut your mid down to 15". It's not a common mod and you don't want to be someone's guinea pig.

[race30]

59 with 50 Hp is very respectable. More is possible with major mods but it'd be less expensive to buy a bigger Hp motor if speed is ALL you're after. Remember, the more weight you hang off the transom the less predictable handling will be.

Can you use a jack plate?... Yes providing you install a low water pickup and extend your tiller handle. Again, it's weight.

Yes, some 90's Mariners were Yamaha's but short shaft Yamaha's of any year will be easy to find.

But... most power can be had from OMC SST60 spec motors. Build one with the right parts and you'll be making 110+ Hp.
Outlaw T-Cat boats have run them successfully.

Lots of info on http://www.hpbc.ca/index.php
Yes it's Canadian... a lot of T-Boat guys run small displacement OMC & Merc motors in our racing series. A few are testing Yamaha/Mariner. Dig around, you'll find valuable info.

We did try to get a T-Cat series going a number of years ago but not enough interest.

Top Gun Marine, Bradford Ontario is a popular motor supplier to our series. He'll likely have something that'll interest you and yes, he will ship. Do a search on this site, he posts regularly.

And please, don't have someone cut your mid down to 15". It's not a common mod and you don't want to be someone's guinea pig.

I've got a line on an SST60, and it's intriguing but i'm worried about a few things:
1. Supposedly you have to run 25:1 in it? doesn't pass my sanity test... it's a 2T motor... what makes it thirsty for oil?
2. it has the SST gearbox with the multitude of exhaust holes in it, and it's LOUD
3. it needs an electric fuel pump, which just adds the amount of crap in the boat, and reduces simplicity.
4. it isn't cheap, at first glance, its 2x the price of a yamaha 75HP and jackplate. (local on CL)

[Tom JM]

25:1 because it turns BIG RPM's
Yup, it'll be loud
Fuel pump cause it sucks gas

Quote from a 70's gear head movie... "speed costs money, how fast you wanna go?"

A 75 Yamaha short shaft will be more reliable

[race30]

do the SST's have reliability problems? or is it just a comment you made because they turn huge RPM's and nothing can really last at 8,000RPM
whereas a yamaha 75 is made to go fishing, so it doesn't really care if it parks at 5500rpm for 3 days straight...

the yamaha 75, and 90, and a merc "force" 90 that i found are all 260-280lbs whereas my tohatsu is about 185

I have to imagine that 100lbs extra is going to be a big problem for my 185lb boat!

[Tom JM]

You're right, nothing lasts long at 8000 rpm so reliability is open to interpretation... it's a race motor.

Think a Yamaha or Merc SS version will be slightly lighter but ya they're heavier thanks to all that power trim hardware.
The weight of a 90 might be too much to run one-up.

Our SS 3 cyl Merc 60 with trim runs fine one or two up with a 5 gallon tank and small Odyssey battery.

[olboatman]

I have a small SLT and I also run some OMCs at over 7 K rpm....so maybe I can shed some light here.
1. Most classic OMCs from the 70s and 80s with mods are happy at 7 K plus with GOOD oil at 25 to 32:1 ( I would run that in any make at higher rpm as oil is cheaper than rebuilds)
2. My Viper mota was built to run 10 one mile laps @ wot(over 7 K) and live season after season ( which they do)
3. All the SST motas (45, 60,100) were built as my V4 was for multi lap racing and to live at higher rpm
4. If the LU you get on the SST has the lower around prop exhaust just block the upper holes
5 I would bet that changing setup would get ya more speed....a water pressure guage is your best friend as the higher the better the faster you will be but watch the pressure!
6. My little SLT really woke up when I changed to a cleaver style prop....with more speed. I'm in low 40s as I'm only 11ft with 25hp.

Just wanted to share some info I have learned and hope it may help.

Gary

[Tom JM]

Good OMC info there. I'm in OMC's old back yard, lots of those small motors were built in Peterborough and hour down the road. Finding good rebuild candidates was easy till about 3 years ago when they started getting snapped up by racers. They're still out there just gotta know where to look.

Olboatman...Your Viper/V4 is a sweet looking ride, nicely done.
I'm building a Ventura/2.4 at the moment. Kind of a resto-ride... bit of this, bit of that. Hull structure is done, paint is next, no rush to finish as the build is just as much fun as the ride.

You're right about how cleavers run on a T-Cat, they do make the ride and drive much more fun and faster. They need more positive trim because of that extra transom lift at speed but that lets the hull level out and fly. Thoroughly enjoyable!

Racer30... have you looked into higher lower unit gear ratios for your MD50?

[jgadget]

Ive got an aquarius aquacat, what a blast, I had a 40tldi for a while and about 6 months ago acquired a m50D. Its been a long road but I am fully restoring the engine. Solid engine mounts, Fast cats steering damper, yamaha big tiller, new exhaust welded up on the lower, and soon the power head is going to alex p in maine for a full rebuild, welding crank, tsr head. Ive got 3 ron hill props, 2 are balanced, 14p semi cleaver and 18 and 21p cleaver.

yamaha 70ces (with the better gear box) is almost impossible to find, I met a guy that had one and was going to buy, but the mid had a modified exhaust which cracked and split all the way through one of the mid mounts.

there is also a these guys in australia called fast cats and they mod a tohatsu 70 power head to mate to a 50 mid and gearbox, apparently its stupid fast.

[Roflhat]

I've got a 70ces on my thundercat, Ceasar F10. Just set a new world record at Coniston this month at 70.10mph average over two runs, hit 74.9mph max. That took many years to get! Aiming for 80mph next year, got a few tricks to try
www.facebook.com/seumas.macleod.9/posts/2258939574147333?


A short shaft Yam 90hp would still be very heavy on the back of one of these boats, Aquarius make a 4.4m which might handle it better. The 70ces is the perfect engine for them really, just very hard to find, especially your side of the pond. Seen a few photos of BRF with SST60's on them, not sure how they go but the speed numbers seem decent at around 67mph.

The Mercedes steering dampers are a great solution and cheap too. You can make up a hydraulic system which works well, I've got details of mine and a whole pile of other stuff I did in this thread - http://www.rib.net/forum/f50/my-thun...-61912-36.html

[oldschoolltv]

Nice, was that record with two in the boat? I drove one of these years ago and it was a blast it reminded me of when I was a kid we had Fliver boats a small tunnel with tiller handle engine on them except the inflatable was much faster then any fliver I ever ran.

[Roflhat]

yeh that was 2up

[slowJEEP]

@FLATBTM I have thundercat hulls and short shaft merc 3 cylinders on hand if you wanna give a thundercat a go. I'm out of NW Florida.

Nice work Race30. Good luck on your quest for speed.

-Jim
Cracker Bay Thundercats

[race30]

Gentlemen - sorry for my absence.
I've put the boat away for the winter, and have been thinking about other things.
But alas- i'm back.

I've been scratching my head more and more about why a jack plate wouldn't work with a 20" shaft motor
It's about half the cost of buying an SST

OR perhaps i should spend my winter days sending off my lower unit for a low water pickup, and try an 18 or a 20 pitch prop on the 50.
My only concern with spending more money on the M50, is that it is already pretty boring to drive 2-up. I'm getting pretty acquainted with it 1-up, and with a recently added steering damper, it really takes the freight out of skipping over the boat chop.

Someone told me i'd need >2x the HP to go from 59mph to 70mph.
Perhaps Roflhat can refute that- i'd LOVE to be peaking out in the mid 70's but i'm afraid i don't have nearly the patience he has to keep buying/modding/building/testing things (I've been following your build thread...)

Most recently i have an opportunity to buy an SST60 built by "THE" fast Fred.
I'm skeptical about losing the benefit of power trim because it really wakes up the performance of my M50, but perhaps with another 60HP trim adjustments become a thing of the past?

[Roflhat]

The boat will physically work with a long shaft engine and a jacking plate as you say, I’ve not tried t myself but in a straight line the only difference would be the aerodynamic drag which will be negligible. But I’ve heard that it has a negative effect on the cornerning, moving that much weight up by 5” will certainly make a difference.

i wouldn’t bother with a lwp unless you know you need one, keep notes of speeds/rpm at each engine height and you will find an optimum. I would highly doubt that you need to go so high that you require a lwp.

its true you’d need a good bit more power, the relationship between speed and power required is exponential. By the time you’re approaching 70mph you’re fighting for tiny gains.
saying that a well built sst60 should do 70mph pretty much out of the box! I’ve never had power trim or really felt the need for it, moving weight on the boat has enough of an effect, and you say more so with the increase in power

[race30]

The boat will physically work with a long shaft engine and a jacking plate as you say, I’ve not tried t myself but in a straight line the only difference would be the aerodynamic drag which will be negligible. But I’ve heard that it has a negative effect on the cornerning, moving that much weight up by 5” will certainly make a difference.

i wouldn’t bother with a lwp unless you know you need one, keep notes of speeds/rpm at each engine height and you will find an optimum. I would highly doubt that you need to go so high that you require a lwp.

its true you’d need a good bit more power, the relationship between speed and power required is exponential. By the time you’re approaching 70mph you’re fighting for tiny gains.
saying that a well built sst60 should do 70mph pretty much out of the box! I’ve never had power trim or really felt the need for it, moving weight on the boat has enough of an effect, and you say more so with the increase in power

I think this answers my question- i just gotta $$PONY$$ up and buy an SST!!