Building an Aluma-Jet

I have a tendency to push the envelope when it comes to running through narrow channels and shallow water. Consequently, I need a boat that can do that. A couple of years ago, I bought a Sea-Doo Jet Boat....not realizing that when it is idling, it draws as much draft as an average 18 foot fiberglass center console with a 75 HP outboard.

I recently bought an old 16 foot Dura Craft, with the center keel nearly destroyed from electrolysis (previous owner had painted it with copper paint ) I re-built the damaged area, and I have been planning on turning the boat into an aluma-jet.


The first thing I want to do is go all the way around the gunwales with 8 or 10 inch Sch. 10 (thin wall) aluminum pipe, similar to a Zodiac design, but with the added protection from punctures from oyster shells, etc by using an aluminum sponson instead of an inflatable type. The sponsons will provide added bouyancy in the event of the boat being swamped, and they also will act as pitch limiters during a turn with a lot of weight on one side. Instead of the gunwale dipping to the waterline, and taking on water, the sponsons should keep the gunwales well above the waterline during a turn, or while 2 or 3 people are boarding from the side.


Then, I want to put a stand up/sit down center console in it, far enough forward that the boat will sit perfectly level when idling.

Lastly (and this part will be a PITA) I want to take the engine and drive system out of a 1,000 c.c. Tigershark Jet Ski, and mount it in this aluminum boat.

My Sea Doo has twin 85 HP engines, weighs 1200 lbs, and will do about 60 at full throttle. This aluminum boat will weigh about 6-700 lbs after modifications, and will have a little more than half the horsepower of the jet boat. I'm figuring half the weight, half the power, the speed should be about the same. No outdrive to hit partially submerged logs or oyster shoals. I'm guessing about 4-6 inches of draft at full throttle, and maybe 10-12 inches of draft at idle.

I have no problem cutting, welding, and modifying any part of the boat, as I have been welding aluminum for many years. I have unlimited access to aluminum, a plasma cutter, and a TIG welder, as well as a spoolgun, and soon, I'm getting a milling machine.

My problem will be when it comes time to mount the engine and drive system.

I'm trying to figure out what is the best approach to mounting the engine and jet drive. Would it be better to put a heavy (1/4) aluminum plate in the bottom of the boat, affix motor mounts to it, and work from that....or, would it be better to cut the bottom out of the jet ski, cut a slot out of the bottom of the boat, and somehow, mount the whole system, already connected, into the boat?

I'm thinking mounting the whole thing as one unit would be much easier....but how do I mount something that is made from fiberglass, to something made from aluminum? If they were both aluminum hulls, it would be too easy, and I wouldn't be asking these questions. But, this is not quite that easy.

I'm wanting to make REAL sure I've got it right before I drill the first hole, or cut anything. Any advice will be appreciated.

 

You have found that jet boats are not the answer to many shallow water conditions.that is some thing many people do not understand.
You may be interested in some of the photos in my photo gallery of boats in shallow water.

 

True enough, at least, in the case of the Sea-Doo jetboat I have. Like I said, it draws about 2 - 2 1/2 feet of water at dead idle, because of its V-hull construction. I know that I can make a very shallow draft vessel out of the 16 foot johnboat, and I know it will work. I just have to get the engine and drive system mounted correctly to make it work.

Even though the drive system I want to use has the jet nozzle mounted on an adjustable trim system....I'll probably still put a set of trim tabs on the transom, to assist in planing the boat off in a short distance, as coming up on plane will draw the most draft (which should still be no more than 1 foot with the flat bottom hull)

I have a Kawasaki 750 Jet Ski.....and I know how shallow it can run....so, a johnboat should be roughly the same, especially considering that the weight will be distributed over a longer, wider hull than what a jet ski has.

If I can build a press brake, a slip roll machine, telescoping booms, and all the other gadgetry I've designed....I know I can build a jet boat that is faster, has a more shallow draft, and a longer cruising range than my Sea Doo.

But, I'm still stumped on installing the engine and drive system. I know the alignment and everything else is very critical to function, so, I want to make damn sure I've got it right.

So, what details am I missing? Anyone who has installed a jet drive system in a boat, chime in. Like I said, I want to make sure I've got everything covered before I start welding, cutting, and modifying.

 

I consulted with a few people, and so far, this is what I've found out.

Basically, I'll need to cut a rectangular hole in the bottom of the boat, from the transom forward (roughly 2 feet long) I'll need to box that area in, so that the jet pump can sit flush with the bottom of the boat. Then, a ride plate will bolt over that, just like on a jet ski.

Not sure if I'll need to use a longer drive shaft, or if the factory shaft will work. If I use a longer one, I'll have to mount a carriage bearing about halfway down the shaft, to minimize vibration.

The nozzle assembly will have to be mounted off the transom, just like on a jet boat.

And, there was a mention of the fact that I may need to go to a different pitch impeller, for better top end.

Also, I'll have to use an aftermarket performance intake scoop, to force more volume of water into the pump, due to the flat bottom.

So far, that's what I've got. If anybody else has any other helpful info, I'd appreciate it. Thanks.

 

You have found that your sea-doo needs considerable draught to float,AND for the jet unit to pump continuously under all conditions.If you fit similar units to a very shallow draught boat you may find the jet pump may need re-designing.

 

Well, I'm hoping it doesn't come to that. Hopefully, the performance intake grate will force more water to the pump than a standard grate, which should keep the pump running properly. Also, the pump I'll be using isn't from a Sea Doo jet boat....it's from a 1,000 c.c. Tigershark ( 2 seater) The Tigershark doesn't draw too much draft, probably 6 inches at the most, maybe 8-10 inches with 2 people, at dead idle. While it doesn't have a completely flat bottom, it's not a vee hull, either. So, I'm hoping no re-design will be needed.

 

EEEeeeeeeeeeeeeeeeeee! Forcing water into the pump?? no no no no no. the pump inlet has to have a given amount of immersion, but it is at least not correct to say "forcing" water into the pump. You definitely don't want to do that.

 

Has some one suggested forcing water into a water-jet?Many have several stages in their design.Just back from a weeks holiday at the sea side and there were several boats using water jets as an enclosed propellers,fully emmersed all of the time.There is even a paddle boat using twin jets like that.If it does your job why not.Some were disembled for manitenance,and were full of sea shells in their intake grills,and much wear grinding up sand and shingle,some had plastic bag parts wrapped inside and needed dis-assembly to get it out.What fun boating is.

 

Hello All,

I am new to the forum. I am also about to undertake the conversion of a 12' 1960 v-hull john boat to include a 1987 Waverunner motor. I am new to all this. Any advise would be greatly appreciated.

 

Pictures Of The Aluma Jet

Here are some pictures of the project so far:


http://community.webtv.net/VRaven/Miscellaneous


Now I am in the process of cutting out the transom and bottom for the jet drive assembly.

The only issue I have run across is how to join the 2 half shafts of the drive system, being that they are 2 different diameters (.635" and .700") I thought about putting a sprocket on each half shaft, and joining the 2 with a section of double strand stainless steel roller chain. But I don't know if that chain will hold up to RPM as high as 7 grand, sustained. Anyone with mechanical knowledge, I could sure use it.

 

In New Jersey, U S A, long ago we called jet drives by the true and accurate name of "Vacuum Cleaners". They tried to follow flat bottomed, shaft and propeller drive boats across 6" sand bars, covered with weeds and junk. 2 things did happen. The prop boats USUALLY made it across. The Vacuums RARELY did. They choked and slowed, as they reached a critical speed the suction would yank the boat to the bottom and a almost instant stop. A few drivers went to the hospital. There is no way to stop your body from being tossed foward. Moral. "Vacum" drivers date I C U nurses.

 

This is true (what cyclops said) Because the jet is a suction device, if you run through too shallow of water, it can actually 'vacuum' to the bottom, and come to a dead stop.
I have been on my jet ski before, crossing a sandbar through an inlet, and she stopped, but I didn't I've also run across oyster reefs on falling tide, and have been shallow enough, I could see the shells passing under me...the whole time thinking, "if this thing stops, or if I fall, I'm hamburger"

An outdrive hanging off the back can slow you down pretty quick, too, however. The one advantage is that having that outdrive hanging off, acts like a limiter, and will only allow you to run into a certain depth of water, before you have to stop. Once you trim the motor up, you float off. With a jet drive, the system is flush mount. This means if you run aground, you are screwed.

It all boils down to having an experienced navigator, who is familiar with local hazards to navigation, running the vessel....not some fly-by-nite, weekend idiot sailor (around here, there is definitely not a shortage of those)

 

True comments from experience obviously,and there is much more you can add I am sure.

 

Right,but please explain why.

 

I don't think I'll need to modify anything, such as a high performance scoop or impeller. I was looking at where the cut-out falls in the floor of the boat, and realized that even with the max horsepower rating on an outboard, trimmed all the way out at full throttle, that part of the boat will still be in contact with the water. So, it should be the same for the jet pump. Especially considering that a jet pump, at speed, has a pressure of about - 65 PSI at the intake, which keeps it 'sucked' to the surface of the water. Output pressure is about +325 PSI at the nozzle.

I know one thing...after all the cutting, grinding, cussing, and headaches, it was an absoloutely beautiful sight last night when me and my friend test-fitted the pump and scoop assembly, and finally saw the system sitting in the boat. It's like the light at the end of the tunnel.

Once the scoop and pump are installed, it's simply a matter of hooking up the control cables and wiring harnesses, and the boat will be done. I'll have mixed emotions about that. On one hand, I'll be glad to have it out of my hair, and get my shop space back (boat is taking up all my available room) But on the other hand, I'll be kind of depressed, with nothing to burn up free time (besides running in the creek in the Aluma-Jet)

So, I'll have to think of another project to start on. Maybe building that .223 caliber PSG-1 that I've been talking about. I already have one in 7.62mm (.308), need one in 5.56 (.223) to complete my collection.

Anyhow, I'll be updating the pictures again soon, now that the drive system nears completion. After that, the next pictures you'll see will be the Aluma-Jet running down the North Santee River on its test run.