What are the benefits of using a jet drive over an outboard motor?

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I've been looking into boat building. Kinda neat. All kinds of sweet plans out there. Yay, sailing!

Sailing isn't very /o/ though. And I'm not patient enough to be content with not having something under the hood.

An outboard motor just doesn't do it justice.

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Here is that catamaran, unfolded and sitting in the water.

I was thinking a jet drive, kinda like what you'd see in two seater jet skis, would be nice to put in the hulls.

I'm just wondering if it would be more efficient than outboard motors, otherwise it's not worth the trouble.

I see huge boats with tiny 25hp motors, and they only carry a couple dozen gallons.... it just doesn't feel adequate.

>>14087968
Nab a pair of Seadoos and somehow bolt their hulls into your cat,

Then slap on some hydrofoils to get that heavy front out of the water.

Now you are James Bond

>>14087968
It's essentially the difference between a fan and a fan with a cowling; the cowling allows the fan to make full use of its energy, and changes the fluid speeds as needed (basically tuning the fluid flow).

But a jet drive is obviously more expensive and complicated to install. Also harder to maintain, not because it's a complicated system (really isn't), but just because it's harder to tear apart.

>>14088193

I was thinking the only hard thing about it would be aligning everything and making sure it wouldn't leak. After that, it's all a piece of cake, right?

So, the jet drive would be infinitely superior? Reversing would suck, though. I'd have to put one of those scoops on it that redirects the flow.

Best to make the last foot of the pipe screw in and screw out-able, too. The part that would be beside the impeller.

Boat builder here.

Depends on the boat. I'd say the benefits is you can get into shallower water with them, but the downsides in that they don't have as much trim leverage which limits how you can setup and run the boat in terms of running attitude. Less speed and ride quality. They just run a lot different, they lift the stern too much a lot of times. You have to design for it.

Finally they are much noisier and burn more fuel compared to a similarly powered outboard.

>>14087968
>I was thinking a jet drive, kinda like what you'd see in two seater jet skis, would be nice to put in the hulls.
It would give you a lot of leverage for docking, having the motors far apart like that.

>I'm just wondering if it would be more efficient than outboard motors, otherwise it's not worth the trouble.
I'm pretty confident it wouldn't be. Just hang an outboard on the back and be done with it. Outboards have gotten really good.

>>14088419

But they still look like shit...

I'd probably want to rig them up so they stayed submerged and out of sight as much as possible.

>>14088419

Oh, I'm pretty confident two jet ski motors wouldn't lift it out of the water. If it would you could just change the angle of the outlet.

The risk of going into shallower water is it could suck up sand, rocks, etc. Much less risky going into shallows with an outboard IMO, you can run those pretty close to the surface and it'll be noisy and splashy but it'll work well enough, right

Jet drives are always less efficient, because the impellers are smaller than the props you would run on an out-drive and they require water to flow up and into the drive, normally getting some form of turbulence or interference from the hull, even with a good scoop intake that helps with cavitation in choppy water, but that scoop will rip off on the first submerged object you hit. I guess the best way to explain it is that the jet drive is mounted within the hull (hulls are normally built to have water flow around them), while a conventional prop would sit below the hull, so water moves better under the boat, as opposed to into it and through a pump. You do get the benefit of being able to run in very shallow water, but any vegetation or debris in the water will wear your jet really quick.

So unless you run in really shallow water, I'd say go with a conventional out-drive, and buy a jet ski with all the money you save over the years. Jets are for weird people and shallow water racing. Props are for normal people.

Listen to this guy:
>>14088419
>Just hang an outboard on the back and be done with it. Outboards have gotten really good.

>>14088525

B-but I wanna be special snowflake :(

I wanna be able to go faaaaast :((

I'm actually thinking about what >>14088186 said... would having small hydrofoils that were relatively unobtrusive be beneficial? I don't think the boat would go fast enough to be foilborn... but would lifting the boat a few extra inches out of the water reduce drag enough to actually be worth it?

>>14088504
I'm speaking generally. For pushing your hull absolutely not, but, the OP seemed to be talking about jet drives in general.


>The risk of going into shallower water is it could suck up sand, rocks, etc. Much less risky going into shallows with an outboard IMO
Shallow water is one of the main advantages of the jet drives, but yeah you absolutely can and will pick up shit and like >>14088525
mentioned it takes the life off fast. Yes, you can trim an outboard up and go through some pretty shallow water anyways. If your going to do that don't put high dollar props on it (It's a boat, shit happens).

>>14088588
You'll be faster with outboards. Put two Verados on there. You can get them up to 400HP.

Or get some Sevens (557 HP outboards), but I think those are kinda shit desu.

>>14088593

The thing I like about impellers, is they appear easy enough for you to cast in one piece. Before you install, make a silicone mold of the impeller, and if you need a replacement you can melt bronze with a propane torch and use lost wax casting to get the part you need.

I've been watching people on youtube cast their own parts for old automobiles, parts you can't buy, gotta make. It's really neat.

>>14087929
The jet drive is steerable, and it's powerful as fuck. No cavitation issues because it's basically a turbine. No CLEARANCE issues, because it's internal. Needs a shitton of power to throw a shitton of water out the back, but it's fine for high-speed applications.

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>>14088711

I know a transmission isn't necessary, but would it save gas while providing the same power during some circumstances?

I think folding catamarans are gonna me my new obsession for a little while...

I'm certain I'll never have the intelligence, money, and capability to build one.. but the idea that you can live in a boat and take it on the road without needing a heavy duty truck.... it's very appealing.