I’m Totally New to Jet Boats or the idea of building one.

[redfishjunkie]

Hey guys I really need direction! I have a 1984 Majek Skiff with a tunnel. It was originally built to run a 70hp Yamaha outboard. My dad was friends with Majek Sr. The builder and my understanding is it was the first of its kind! So as a test boat they offered to place a 90 hp Yamaha on it wanting to do testing and demos with it. The match was perfect and I had a 15.6 foot rocket that would run in a Wet Sponge at the age of 15! It would run 53 mph and without a jack plate and 2 people aboard you can run in 3” of water with hard sand bottom as long as you don’t slow down below 30 mph or lose a 1/2” of water for more than 50 yards. Well the old outboard is finally dead and I wanted to install a new one but the cost of such a motor is more then I can afford. I also would like to install a motor I know how to work on and won’t have as much out of pocket expense if I have issues. The boat by it self weighs in the neighborhood of 600 lbs. I’m actually thinking about re configuring center console and getting rid of it and want to build a top drive/casting tower expanded into a front casting platform. I’ll get pics next post but it’s a skiff with no sides so basically it’s a sheet of plywood with raised transom to give you an idea of shape and has a full tunnel starting from about 4-5 foot from the front of the boat. The tunnel is approximately 18-20” wide and starts flat with bottom and gets taller all the way to the back where it’s about 8” tall and over all boat thickness at the back is about 12” thick. The boat has wood stringers running front to back that basically splits boat into 3 equal voids from side to side that is foam filled. The raised transom is set into a pocket over the tunnel that is about 2 feet from back of boat. I’m assuming I’ll will be needing to get rid of tunnel to make it an inboard motor with a jet drive. From what I’ve read I may leave some of pocket to make sure the boat doesn’t porpoise or I’ll need trim tabs. The first thing I need I believe is direction on where to find info so i can learn what kind of motor I can use. I’m figuring I can go up to maybe 400 pounds on motor since the outboard weighed roughly 300 lbs and set on the back of the boat. Now I’m moving weight forward with the new inboard configuration. I really don’t want to lose any power or speed. If anything I would like more power, not that I want to go any faster, I don’t think the boat hull will really allow for more speed but the need for power in reserve with a shallow water boat is a Must. I like to chase Reds in backwater hard to get to places. I will not hesitate to jump small sand bars in areas I know has deeper water on the other side and substantial area to get back on plain to jump back out. As far as motor type, can I use a large motorcycle engine, rotary engine (Mazda RX-7 or 8, Honda 4 cylinder, or what other options are available? What horse power and torque should I be looking for? Also what brand and type of jet drive should I be looking at? To give y’all a little more info, I would love to do it cheaper and faster so I can try to get it in the water next February or March. So saying that if I can buy used parts, motors, and jet drive I don’t mind, but I want it done right and want the most range of adjustment I can build into it. So I don’t mind putting things off and waiting to spend more money on new equipment if that’s the best idea. But if used is fine and I can buy used jet drive that has the ability to be completely rebuilt and motor that can be bored out to increase power that’s fine too, if possible I want to creat the set up with a freshwater heat exchanger set up for the motor. I don’t like running saltwater through engine to cool it. I’ve had an inboard boat before and it doesn’t matter what you do as soon as you run saltwater through an engine it’s just a matter of time until you start having problems. I’m not sure if a freshwater heat exchanger will work with the jet drive or not. That’s another question I need answered. So any help and advice where to start or anything yall can tell me, if I’m totally nuts and nothing I’ve said will work and I need to be set straight as to a new line of thinking please set me straight now before I do something stupid! Please! ?
Thanks Guys! Tight lines and Cold Beer!
redfishjunkie standing by!
redfishjunkie@aol.com if you have anything you can forward to me that will help!

 

[AZMark]

You’re talking about a really complicated project! I’m not sure how much help you’ll get on this site, it’s definitely focused on later model Yamaha/Scarab/Vortex boats.

You’ll be substantially changing the dynamics of a boat that small by moving the weight from an outboard to the middle. You could put in a lot of work and end up with a boat that doesn’t perform like you hope.

Some of the new boats do have heat exchangers so that can work with a jet, but that would add even more complication to your build.

I have a 1981 2 stroke 90 horse Johnson and it’s really not very hard to work (with a service manual) on or even get parts for these old OMC motors. All the major components are fairly bulletproof. I’d go that way if you really want to get out on the water in 6 months.

 

[Julian]

Not sure a tunnel hull will be compatible with a jet drive at all. I would find a used outboard and stick with what you know works REALLY well on that hull!

I'd hate to see you put in a jet, only to discover that at running speeds the intake is out of the water due to the tunnel hull!

 

[redfishjunkie]

Ok, that’s really not what I wanted to hear. If I wasn’t worried about 6 months and you were going to give me any hope, what would you change to go in thee we direction I want with the inboard? I’ll say this about that boat, I’ve had tons of weight on it at once and the boat basically preforms the same way. I’ve had 5 guys on the boat in very windy conditions wade fishing a local tournament in Corpus. We were fishing an area right on the edge of the bay with 40 mph and the waves were rolling 3-4 foot and it stayed stable. With th Ed power it has getting on plain was still possible in 12” of water. The boat doesn’t dig or bow up to get on plane. I saw an aluminum John boat build where the guy dismantled a wave runner and cut it apart then cookie cut the John Boat to fit that part of the wave runner into the boat. With the pocket on my skiff if I remove pocket and push the transom back that 24” the weight of the new motor should sit where the outboard sat originally. If that all worked out in a perfect world, what kind of wave runner or is there a wave runner or jet boat with a motor that would put out the kind of thrust I want? Even if I need to get rid of the fresh water heat exchanger. What size motor should I look for?

 

[redfishjunkie]

Not sure a tunnel hull will be compatible with a jet drive at all. I would find a used outboard and stick with what you know works REALLY well on that hull!

I'd hate to see you put in a jet, only to discover that at running speeds the intake is out of the water due to the tunnel hull!

I was figuring I would have to get rid of tunnel. I just wanted to describe what I had Incase there was a reason not to get rid of it. I figure with jet I want need tunnel and the bottom of boat is just fiberglass along with tunnel so none of that will be hard to get rid of. The wood is in the stringer, transom, and deck. The stringers run down the sides of the pocket so attaching a wooden engine mount structure wood fit right between stringers and help fill in the tunnel. I guess sitting and talking to you and last guy, y’all are right on target, maybe I’ll just find old John boat to play with the jet drive idea on and wait till I can find an old 90 hp Yamaha or save enough to buy a new one on the Majek. It ran so perfectly with the Outboard that it is stupid to take a chance on ruining that! So changing my mind, Thanks! What’s the best wave runner or jet drive motor should I look for to push a 16’ wide and deep aluminum John boat around 45-50 mph? ? I’m a big guy and need it to be over powered a bit. And again Thanks for talking sense into me.

 

[Julian]

I was figuring I would have to get rid of tunnel. I just wanted to describe what I had Incase there was a reason not to get rid of it. I figure with jet I want need tunnel and the bottom of boat is just fiberglass along with tunnel so none of that will be hard to get rid of. The wood is in the stringer, transom, and deck. The stringers run down the sides of the pocket so attaching a wooden engine mount structure wood fit right between stringers and help fill in the tunnel. I guess sitting and talking to you and last guy, y’all are right on target, maybe I’ll just find old John boat to play with the jet drive idea on and wait till I can find an old 90 hp Yamaha or save enough to buy a new one on the Majek. It ran so perfectly with the Outboard that it is stupid to take a chance on ruining that! So changing my mind, Thanks! What’s the best wave runner or jet drive motor should I look for to push a 16’ wide and deep aluminum John boat around 45-50 mph? ? I’m a big guy and need it to be over powered a bit. And again Thanks for talking sense into me.

That tunnel is providing you stability and "cornering". If you remove it you risk changing the way the boat drives big time. As you say "It ran so perfectly with the Outboard that it is stupid to take a chance on ruining that" - THIS is why I'd stick with an OB. You have all the parts- steering etc to make it work - perhaps even plug and play.

Yes, a jet john boat is cool, but that is a HUGE amount of work. We have a thread on here where someone did this....I'll see if I can find it....but it was a TON of work.

 

[Julian]

This isn't the thread I was thinking of....it is a thread about a mini jet rebuild.....but it gives an idea of what a rebuild of the same propulsion is like .... let alone switching....

https://jetboaters.net/threads/mini-jet-build.23028/