How fast will it go?

Well this idea of grinding the cam to reduce the rpm/output to the revs you are talkling about is a load of rubbish and certainly expensive rubbish .it will overload the crankshaft and rods ...and even if possible not going to give efficiency. What you usually do with a boat prop is fit as big a prop as is neccessary to hold the engine down to your WOT revs . then you go the speed you want by slowing the rpm, As you have been told car engines are not rated for continuous output ..so reducing the throttle opening reduces the load ...Naturally the prop cannot be so highly pitched that it will not turn at idle rpm. If you are going through shallow water you are not going to be going 45 mph . If you have a prop try that but as you will be having a left hand pitch prop then make sure to go for a shaft size that you can get a good selection of props for ...not an odd ball .
What VW engine is this you are using ....?? the thai longtails seem to keep the clutch and use the gear casing to support the prop tube ..not possible if its an old beetle engine as drive shafts at rt angles to crankshaft

 

Are we shure, that this thread is not just kidding?

 

Like I said P., We are not going to grind the cam, so we will trun the engine at 3600 to 4400. I can make the shaft turn right or left,so prop selection should not be a problem. Thanks for you help though. What is a thai md motor?

Mark

 

Thanks Jango. Speed would be great a 36 mph so I think we may go with the tiger prop that is a 12, 10 1/2. We have to be able to go through weeds and mud to get where we want to go.

 

WHAT IS A THAI LONGTAIL/MUDMOTOR I think you should do a google on mud motors and thai longtails ....basically its a car engine with a long tube to the prop which can be tilted and run in very shallow water ,,6 inches

If you look on DIY marinisation ..at the end of the weeedeater thread there are some photos from thailand and a small 6 hp one under " thail longtail with 6hp Lifan/Honda....." You did not say which VW motor you have that will run both ways....how are you doing that ???

 

Ok more stupid questions.

1. Is there a way to figure how much torque is need to sling a prop?
2. If so, what kind of torque is needed to sling a 12inch 12 pitch prop at 4000rpm, or the 12inch 10 1/2 prop?
I found the the vw engine develops 90lbs or so of torque. I don't know if that is accurate. But if it develops 65 hp at say 4000rpm Does that make the torque of 90lbs or so look right?

Mark

 

Power is what turns the prop ...now a 65hp outboard would usually run a 13 x15p prop ( maybe 17 on a light boat ) but with a 2 : 1 reduction ..say its going 2500 rpm . now if you want to run the prop at 4000 rpm you may will need over 100 hp as its not linear. So I think your 12 x12p sounds about right ...with car engines the torque is at a lower rpm than the max HP being at a higher rpm ...In the end you got to try it ...as the engine is not designed for continuous high output ( like and outboard) ..you only have shell bearings not roller bearing ....you may have to under prop it to reduce the load and watch it does not over rev...If its an old beetle engine remember a lot of the cooling is from the oil so keep the cowl and oil cooler ...
PS torque is measured in foot pounds not pounds
you did not saywhat engine it is and I still want to know how you are going to get it to run either direction as its clearly not a two stroke

 

I Thank you for your advise. I should have said foot lbs, but I thought you would figure that out. My bad. As far as turning it bot directions, I am designing a pulley system that will reverse the direction of the shaft. Something in the order of running a belt on the back side. If it becomes to hard to make, we may go to a bucket system for the reverse, (same as a jet engine).

 

The formula for Torque is:

T = HP X 5252 divided by RPM,s or 65 x 5252 divided by 4000 = 85.3 lb-ft @ 4000 RPM and 65 HP


From this you can see as HP increases and/or RPM decreases, torque will increase. Max Torque in an Engine is usually at a lower RPM than where max HP is developed. You would need to see the HP CURVE to determine at what RPM max Torque is achieved.
Based on the above calculation, it can be assumed that max torque is probably at least 90 lb-ft unless max HP is developed at much higher RPM,s

pistnbroke is correct about not enough power for a 12x12 with a 1:1 ratio. Even with only 1200 lb max you will most likely bog the engine down. The 12x10 1/2 you previously wanted to use will also limit max RPM but is probably useable

Hope this Helps

 

Are you building a VW version of this..if you are you have much to think about other than the prop.!!!

 

As far as prop size goes, you want a 10X7 or a 10X8. We run alot of subaru mudboats here and the prop for the 68 hp subaru mudboat is a 10x8 with a 1:1 drive. The subaru is supposed to turn 5000 rpm, which I thought was high, but thats what everybody runs. Boats are built 16X5 out of fiberglass. Max speed of these boats is about 30 mph. The engine does not produce enough HP at low rpm. I've seen the VW mudboats, but they have overheating problems due to being air cooled. Very simmilar engine to a subaru though.

 

mudman, I suspect your boat is heavier than 1200# fully loaded and also higher rpm? If so, that would account for the smaller Pitch.
See my posting on 2/26

 

No I think that I'm at 1200 lbs with one man, but maybe 1800 loaded.The props are heavy cupped though which creates more bite too.

It took me a while to find the right prop. I went with a 10X10 with heavy cup and could only get 3500 rpm. Switched to a 10X8 with no cup, and jumped to 5800. I put some cup on that which got me out of the hole faster, kept the same speed and brought the rpms to 5200.

Lots of things can be done with props, and unfortunately, the only way to figure out what is perfect for a particular setup is experimentation. Calcs can get you close, but I don't think that they are dead on perfect. Too many factors. 12 sounds a little big to me for that engine because the engine does not gain any signifigant hp until a high RPM. I need to find a hp curve. But the rigs I'm talking about are a little different.

I do have a rig with a 12 inch prop with the same size engine as stated above, but it is a surface drive. My boat loaded is about 1200 lbs.

 

My calculations are pretty much in line with your results. Used 4000 rpm with 1200lb max instad of 1800lb and got a 10 x 12 which should rev the same as a 10 x 10 with a heavy cup.
4000 rpm max will use more pitch than 5200 max as long as the HP is the same.

Based on your results, I believe as long as 65HP and 1200lb max are correct a 10x12 should be close.

 

Jango, What do you come up with if we raise the weight up to 2,000lb? I am ot that interested in speed as going through weeds. This will give me a ball park figure of where we need to be. and Mudman, have you ever heard of a prop by Piranha? They are composite and the blades are interchangable. So it would be easier to experiment with.

Mark