rotary engine

[NIGEL]

Has anybody seen any rotary engines in boats. i have a worn out mazda but she will turn 7 grand all day long. with a almost flat torque curve seems like it would be cool in a jet boat. watcha think.

[BLOWNDRAGBOAT]

Hey Nigel.....I am assuming you were being serious so I will answer your question as such. Although your rotary engine probably revs to 7000 grand all day long, it won't work in a jet. Heres an example. I have a blown big block in my boat and it will hit my rev limiter of 7500 rpm and higher if I let it but this is on the trailer. Once you put it in the water and put a load on the pump everything changes. Now, considering my engine puts out A TON more torque than your rotary engine, I am turning it around 6200 rpm. Where did the other 1300 rpm go? The pump just won't let the motor spin that hard. We can go into this for days on theory of operation but I will make it short. JETBOATS LOVE BIG TORQUE ENGINES. They NEED torque to turn a loaded impeller. Think of your pump as a water brake on a dyno. Thats exactly what its doing. The pump is loaded all the time with water and torque is needed to turn it against the resistance water places on it. Your rotary engine just doesn't produce the TORQUE or horsepower. PERIOD. Hope it helps.

[canuck1]

A guy in Invermere BC had and was selling Outlaw sprint boats with the rotary engine in them. The boats worked quite well, my guess is the weight was about 1000-1200 pounds.

[BLOWNDRAGBOAT]

bet they weren't big jet boats were they?

[NIGEL]

I WAS SERIOUS AND I APPRECIATE YOUR ANSWER. THANX NIGEL

[froggystyle]

BDB is exactly right. More than a flat curve, it is an exponential curve. It takes a lot less torque to turn 3000 than 4000. Actually, to the Nth power. 5000 is exponentialy more than 4000 etc... What this means is you can afford a peakier torque curve, as long as it stays in line with the torque required to turn the pump. The energy required to turn the pump at "X" rpm is damn near exact, and as long as at that RPM the engine has greater than that for power, you will continue accellerating. At a certain point though, these lines will intersect, and that is your maximum RPM for that motor.
Cutting the impeller only prolongs this curve. If you think that you can keep the lines apart for longer, you cut your impeller to reach a range where the engine becomes more efficient (peaking) and will pull the lines back apart for a while, but inevitably they will come back together.
This was probably very cryptic and vague, but if you stayed with me, you get the picture. Bottom line is, 500 lb ft of torque will turn an "A" impeller around 5000 rpms or so. No less than 500 lb ft will do.

[froggystyle]

After thinking about it though, it would probably make a jet ski pump in a small boat run like hell!

[BLOWNDRAGBOAT]

WazupFROGGYYYYYYYYYYYYY. You guys need ME to come out thier and work for ya. I could sell a ton of those boats. Theres NO reason I can see why nobody out here don't know what an Ultra is. If Nigel really wants to put his rotary engine in a boat, I think a rotary would be PERFECT for one of those swamp boats. You know, the ones with the airplane propeller on it? I think those things are KOOL!!!!!

[canuck1]

Nigel, I made a call to a buddy out at the lake the little sprint will turn about 55 MPH with 4 people in it.no turbo no mods just a stock motor. Yes these boats are smaller but they look good and burn half the fuel mine does for only 15 mph less

[canuck1]

Also I think it had a berk with a C or D impeller

[jeff-in-ky]

Just happened to be thumbing thru a back issue of HB Mag (Jan. 96) and guess what, an article on a rotary powered jet boat. The boat was built by Ebbtide in TN it was a 19 ft. with 8 ft. beam and weighed 2300# it was model RXR-19. It came with a Mazda RX-7 engine marinized by Rotary Power Marine in Spokane, Wash. it was avail. with a supercharger also. The pump was a Dom. 12 TD top speed 45 mph, or 55 mph (blown). It produced 270 ft/lb of torque at the pump.