12V CUmmins in an Airboat??

[JCrockett]

What are your opions on putting a Cummins into an airboat? A buddy and myself are toying with the idea, but we keep coming back to the weight issue and lag for this application.



Does anyone know what exactly a fully dressed 12V weighs? I was thinking around 1000-1100 pounds.



We were thinking twins around the 500-600HP mark for this application. Key to this is quick throttle response and power



Just thought I would throw it out there.



Thanks

 

[Forrest Nearing]

your'e about right on the weight estimate



port the head and maybe put a good cam in it to help w/ lag.



lag can be "overcome" by how you set up your prop



twins are a good idea

 

[JGrefenstette]

isnt one of the biggest factors in an air boat, RPMS?? ive been one ones with a crate chevy 502, fella told me it had over 800 hourses. from what i could tell it appears that the prop/props are mounted directly to the crank, no gearing. with the slow turning cummins would you just gear it way up, so get the prop singing???

the weight probly wouldnt be a huge issue if you properly ballanced the boat (especially up front) and you would probly want to have the centerline of the motor much lower than a regular one, just to keep your center of gravity lower to avoid rollover. that would probly create a good amount off room from the prop to the crank, to allow for some gearing... . just a few thoughts.



one more thing that comes to mind is... these motors tend to do best under a heavy load. if their just free wheeling will it make much boost.

 

[KSears]

I agree, it would seem rpm's and and a light engine would be the way to go , motor is only turning a air prop, don't think horsepower would be as important as throttle response and rpm,, may a built chevy small block

 

[Skydiver]

Huge ship diesel engines only turn around 600 rpms. Will need to have a prop that is pitched correctly and the fact that the prop can only be as long as the distance from the crank to the boat. While that distance is limited to how high you mount the engine. Get the engine too high and you what might happen.

If you could mount the engine low and have a jack shaft reduction unit mounted up higher to swing the prop you might have the best of both worlds. Are you contemplating using lake water for cooling the engine? If so remember you can run very shallow possible picking up moss and trash clogging the cooling system. Could use a heat exchanger also. I would install an audible temp alarm.

 

[JCrockett]

Thanks for the responses, Kind of what I figured though, not really the best option. Just thought I would see what others thought. I like Skydivers idea about mount the engine low and using a jack shaft, then you could run it throught a gear box to get your prop speed up there. Fun to kick aroun ideas like this.

 

[Gary - K7GLD]

I agree the engine weight would be a negative issue, but no reason the engine has to be mounted high for a Cummins - in fact, putting the engine down low, and using a multiple or cog belt drive and pulleys to overdrive the prop shaft would be the way to go, and allow lower engine RPM...

 

[Skydiver]

The engine speed would be fine with the correct prop. I have a friend that owns a 1953 T-6 trainer. The 9-cyl. 600 horse P&W Wasp turns slow. A fast prop will be too noisey. WHAT A RIDE!



I would go for a belt over a gear box. What effect will the lateral force have on the Cummins main bearings if you do mount it direct. Go for a 1:1 ratio with 12" pulleys.

In 1983 I had an Eipper double quick ultralight plane that ran a Cuyuna 430r with a belt reduction that was an option. If you speed up a prop you will have more vibration. No good! Spend more money on your prop and you will be happy. Your probably going to need at least a 60" prop with a kevlar leading edge. Could be much longer. Look at what others are using.



Southern Airboat



Airboat Trader

 

[JacobsD]

I too have thought about this. My father has an airboat with a Subaru 1. 8 on it and it has a belt reduction unit mounted to the flywheel. On that if you have the prop spinning over 3000 rpms your over spinning it. Not sure if its like that with all airboats or not. Maybe a 4bt Cummins would be the trick? and instead of running a belt reduction unit, run a 1:1 belt drive and mount the motor low, such as suggested before.



Maybe after one of my 10 million projects is finished I'll give it a shot :-laf

 

[JLEONARD]

Just remember that the duty cycle on a boat engine is much harder than on a truck.

You won't pump up a 6bta cummins to 500 hp and expect it to live very long in a marine application at that hp level. Marine duty is like hauling uphill all the time. You'll be needing to monitor EGT closely.

Cummins marine engines like to be underloaded a bit in order for them to have longevity and not score cylinder walls or worse.

Don't kid yourself thinking a prop won't eat up horsepower to drive it. It will and it will be under load all the time.

Right now I'm working on a boat repower using an engine out of an 89 pickup. Will "tune" it to about 200 hp.

If anyone has any leads on a marine flywheel or sae 3 bellhousing let me know.

Thanks.

 

[Steve St.Laurent]

Props on aircraft generally run at a maximum of about 3,000 rpm. The problem with RPM and propellers is that when the tips of the prop start travelling around the speed of sound the prop gets very loud and prop efficiency goes down. Not sure of the diameter of an airboat prop but the Piper Saratoga that I used to fly has a prop diameter of 80 inches and it's governed at 2,700 rpm. That has the prop tip traveling at 642 mph. Now if the prop diameter was larger than that (I think they are on air boats) then you'd want to be running at even lower rpm to remain below supersonic at the tips.

 

[Skydiver]

Good point on the speed of sound. That's why a bull whip cracks and is why when some people hear a deer rifle being shot there is a second pop. Many will say "I heard the bullet hit the deer" but it is actually the sonic boom.

I just sold an 89 I/O with a King Cobra 460. Was told by my friend that is a marine mechanic the big block had special rods because the engine was always running loaded. He then asked me if I had ever seen a boat running downhill.

 

[Gary - K7GLD]

Just remember that the duty cycle on a boat engine is much harder than on a truck.

You won't pump up a 6bta cummins to 500 hp and expect it to live very long in a marine application at that hp level. Marine duty is like hauling uphill all the time. You'll be needing to monitor EGT closely.

Cummins marine engines like to be underloaded a bit in order for them to have longevity and not score cylinder walls or worse.

Don't kid yourself thinking a prop won't eat up horsepower to drive it. It will and it will be under load all the time.

Right now I'm working on a boat repower using an engine out of an 89 pickup. Will "tune" it to about 200 hp.

If anyone has any leads on a marine flywheel or sae 3 bellhousing let me know.

Thanks.

That's true of conventional prop driven boats, due to several factors - but not sure that an aircraft prop drive (air vs water) is any harder on a airboat engine than an airplane is...

 

[MA2LA]

with the kinda torque these things put out you could run a good heavy pitch and since it seems like you don't want to run these props at to high an rpm it would work well, at least thats the way i see it.