Why no Catamaran Jet boat?

[Speedling]

So, in my ongoing quest for speed, I keep coming back to the hull.
Going for efficient hull designs, I keep coming back to a Catamaran hull(s)
Why not mate a couple of Hamilton or Berkeley drives to some V8's instead of the surface drive?
Or a smaller cat could be a couple modified Rotax engines mated to some bigger jets.

What would be the problem? When I do searches, everyone just keeps saying the same thing: jets take more power than props! I didn't think that would be the case. Perhaps more rpms which would equate to more horsepower, but not any extra fuel usage.

Pictures and links provided for clarification and enjoyment.


http://www.hamjet.co.nz/includes/files_cms/file/JetBrief/JB 413 - Tennessee Aquarium.pdf

 

[Julian]

The problem I've always figured with jetboats is that they require a scoop to pull in water for the pumps to function. As such, it is not as easy to configure a hull and intake for optimal water flow to drive the pump at every speed. When going super fast you need lots of water, which means a bigger "scoop" but make it slightly too big and it becomes drag, slightly too small and the pump starves. So jets are ok for boats that will have large wetted hulls, or that you can tune for an optimal range of speeds, but widen that range and it becomes a nightmare. Props on the other hand just sit in the water flow and have ZERO concern about intake volume....just spin them faster or slower while avoiding cavitation.

 

[tdonoughue]

I recall that there are two basic types of hulls: planing hulls and displacement hulls. We have planing hulls, designed to ride on top of the water. They are the least efficient, most rough, but fastest design. Displacement hulls are designed to sit in the water and cut through the water. They are the most efficient, gentlest, but slowest design. A cat is a type of displacement hull. It would indeed be efficient, but the drag of the water on the hull would mean that you would never really get it to go fast like a planing hull.

 

[eastcoaster]

A cat is able to exceed its displacement speed due to a narrow hull and fine entry. Think of a Hobie Cat sail boat . Go fast power cats are not displacement hulls but usually stepped hulls that run on a very limited wetted area at speed + use the area between hulls for added lift.They are in general faster than a monohull with the same HP. I agree with Julian's explanation of the limitations of a jet driven boat.
Hull speed can be calculated by the following formula:

where:

"

" is the length of the waterline in feet, and
"

" is the hull speed of the vessel in knots

 

[GiddYupJoe]

@Speedling this is old news but have you seen this set up. I think it may suffice in your quest for horsepower but probably not that practicle!

http://www.nyethermodynamics.com/squirt_2/

 

[Noko]

Cost. Simply that. You can make several boats on the same hull with a V hull, not so with a Cat.

 

[Noko]

Cost. Simply that. You can make several boats on the same hull with a V hull, not so with a Cat.

And size of the hull. Its easier to fit a V hull into a dock easier than a wider Cat.

 

[MikeyL]

A cat is able to exceed its displacement speed due to a narrow hull and fine entry. Think of a Hobie Cat sail boat . Go fast power cats are not displacement hulls but usually stepped hulls that run on a very limited wetted area at speed + use the area between hulls for added lift.They are in general faster than a monohull with the same HP. I agree with Julian's explanation of the limitations of a jet driven boat.
Hull speed can be calculated by the following formula:

where:

"

" is the length of the waterline in feet, and
"

" is the hull speed of the vessel in knots

LOL .............. This cracked me up .... Are we doing math lessons now ?
Forgive me, I have forgotten more than I ever learned in school. I am just foolin' around, so please don't take any of this serious !
I just needed a good laugh this morning and this is it. Best Wishes, Mikey Lulejian

 

[MikeyL]

The problem I've always figured with jetboats is that they require a scoop to pull in water for the pumps to function. As such, it is not as easy to configure a hull and intake for optimal water flow to drive the pump at every speed. When going super fast you need lots of water, which means a bigger "scoop" but make it slightly too big and it becomes drag, slightly too small and the pump starves. So jets are ok for boats that will have large wetted hulls, or that you can tune for an optimal range of speeds, but widen that range and it becomes a nightmare. Props on the other hand just sit in the water flow and have ZERO concern about intake volume....just spin them faster or slower while avoiding cavitation.

@Julian ...... May I ask what a WETTED HULL is, please ?

 

[GTBRMC]

Agree with Julian and add that surface drives likely have less problem with occasional loss of inbound water flow than jets (ie: minor airborne period crossing wakes, etc) that are inherent in most water race courses & conditions.

 

[Julian]

May I ask what a WETTED HULL is, please

In sailboating, the wetted area is the area of the hull which is immersed in water. (applies to all boats-but our boats have a wetted area that changes due to the fact that they reach planing speeds)

In aircraft, the wetted area is the area which is in contact with the external airflow. This has a direct relationship on the overall drag of the aircraft.

 

[007]

Please excuse me, I think I just wetted my hull!

 

[Jim Robeson]

That's what she said!

 

[maboat]

I often wonder why there is no jet powered pontoon. Not for speed but for safety. Most pontoons are family oriented slow cruisers with kids of all ages jumping off and swimming all around. Propless propulstion would sure be nice.

Since our boats have essentially jet-ski engines, how hard would it be to put one inside each pontoon? Each pontoon would have an intake grate on the bottom and a jet nozzle out the back. If I were more of a welder/fabricator (I'm not) I would sure be tempted to try it.

And with the triple-toons becoming more common it would be so easy to offer single, double, or TRIPLE engines. Man that would be so cool. But then again, I'm gettin' old and "cool" is much different to me now...

 

[Speedling]

Well, that was my idea on the high speed catamaran. The one i posted pictures of is a planing catamaran not a displacement hull. No one that has seen those things can argue it. Each "hull" will typically have a flat spot in the back for planing purposes where the pump intake could be put. The pumps would go where the surface drives are.

 

[Julian]

No reason you couldn't build a go fast cat with the type of V8 & berkley pump used in the "traditional" go fast river running jet boat. Stuff one in each pontoon and you'd go fast if you could fit them in! But it would be a ton of work! Maybe mother Yamaha is reading and this will be their next venture??? Probably more like what @maboat described though....a single engine jet tri-toon for the family party barge market! Or stuff one in each pontoon for one hell of a ride!!!!!! YEEEE HAAAAAWWWWWW

 

[OrangeTJ]

My guess is that they're just faster with the surface drives than they would be with jets.

 

[Julian]

My guess is that they're just faster with the surface drives than they would be with jets.

I agree, and also you have to think about handling too. We love our jet because they are so agile....cats are really not built for that same agility. Props in general seem to eek out more speed with less HP than jets...but lose in acceleration due to the lost power spent raising the bow (due to the offset prop). Each has its pros and cons.

 

[momark]

Listen I agree with the guy who just wetted his Hull. Also, the guy who needed a good laugh why don't you show me the formula that says you don't have to have a wider Cat if its powered, nor do you have to have thin hulls. Not only is it being done its being done really well! there are even Cats with foils in between the hulls that lift out of the water. You know what they run? Wait for it Jets I know you might have to go back to math to figure out some drunk red neck figured it out, took it out on the water had a ton of fun. Now you can to cause some manufactures saw this , now they can sell it to people who do really good math,

 

[momark]

(1) Statement! Marine 50' Catamaran - YouTube