How fast will it go?

[Mark Emaus]

Building a pontoon boat. 18 foot long, the pontoons are (2) 18 foot by 15inches wide by 22inches tall. the pontoons are flat on the bottom and have a 16 degree pitch on the front end. I am putting a 65 hp engine with a direct drive shaft that is attached by a universal joint off the transmission. the shaft angle in the water is 7 degrees. the boat weights 1,200 lbs fully loaded. The pontoons will sit 6 inches submerged in the water at stationary. I will run the engine 1x1 ratio at 2500rpm, (this can be changed). What prop would you recommend for that rpm and stats for a 3/4 inch shaft and what speed do you think you could run at? thanks
Mark

[Jango]

Since max RPM is fairly slow, Prop will need to be 13 1/2 to 14 dia. with a pitch of 21 or 22, 3 Blade, min D.A.R. of 55% providing 11% slip.
Max speed should be 45 mph.
Prop size and predicted speed is based on max weight of 1200 lb with Driver and fuel etc.

[rasorinc]

http://www.go-fast.com/boat_speed_predictions.htm

http://www.glen-l.com/how-fast-chart.html

[Mark Emaus]

Since max RPM is fairly slow, Prop will need to be 13 1/2 to 14 dia. with a pitch of 21 or 22, 3 Blade, min D.A.R. of 55% providing 11% slip.
Max speed should be 45 mph.
Prop size and predicted speed is based on max weight of 1200 lb with Driver and fuel etc.

Thank you Jango,

What if we increase the rpm to 3000- to 3500. Can we put a smaller prop on her and maybe a 2 bladed and get somewhere around 20 to 25 mph? Is so what would you recommend?

[Jango]

Yes, As rpm increases, prop dia can decrease. Normally, props are sized for max efficiency. Is 65 hp max rated? at what rpm?
If you want to size for 20 - 25 mph, will need to know HP and rpm at that speed which can be estimated resonably close if max hp and rpm are known.

[Mark Emaus]

We can change the cam to slow the rpm down at still keep the hp at 65. I was under the impression that most props run at 2000 to 2500 rpm. Maybe I am wrong? We want to 20 mph with the smallest prop that we can turn. I was thinking of a 10inch or 11inch prop. What do you think?

Mark

[Jango]

If you're saying 65HP @ 2500 rpm? That's what I used to determine prop size of 13 1/2 x 22. Why would you want to limit max.rpm,s to 2500?
Allowing the engine to run up to 4500 rpm would calculate to a 10 x 12 Prop with the same 45 mph max. Higher max rated rpm would further reduce prop size - again, what is the rated rpm?
Slowing the engine down by decreasing the throttle to 2500 rpm (10 x 12 prop) would provide approx 24 mph with slip at abt 16% and utilizing approx 25HP.

[Mark Emaus]

Jango,

We can run the engine at 3500 rpm and we will use a 12 inch 10 1/2 pitch prop. So what will that come out with.

Thanks
Mark

[Jango]

With a 12x10 1/2 3 bladed prop @ 3500 rpm, speed should be 30 - 31 mph

If you're looking for small dia prop, @ 1200lb max, you don't need to go to 12" dia x 10 1/2. The 10 dia x 12 p. previously recommended will provide approx 34 mph at 3500 rpm and 24 mph at 2500 rpm.

[Mark Emaus]

J,

Thank you for your help! I appreciate your expertice. We will use this prop in mud and weeds. What would you recommend as a prop manufacture for something with that kind of use. I am thinking of a 3/4 inch or 7/8 inch shaft for the prop.

Mark

[Jango]

Unless your shaft is turning 4500 rpm+, stainless steel is not really necessary and is very pricey - 500 to $1000 or more. Alum. is not readily available for inboards, so I would use a std. Brass or Bronze propeller. I don't think you'll find a 3/4 shaft prop which I wouldn't recomend anyway. Stay with a 7/8 or 1" shaft. Any manf. should be OK . Search the web, look on Ebay.

[pistnbroke]

NOW EXPLAIN IT TO ME SLOWLY..

If you put a 65 hp outboard on this thing with its 2 :1 gearbox it will drive a 13/14 inch X 15 pitch and your speed will be 1mph x 15 x 3000 = 45 mph .Now no way will a 65 hp running at only 2500 rpm and with direct drive turn a 22 pitch prop more like a 10 pitch so you only get 10x2500= 25 mph.
AND I don understand changing the prop to slow down ..whats the throttle for ???

[Jango]

I agree, but the original assumption was 65HP AT 2500 RPM. hence the 13 x 22 prop. A highly unlikely situation unless the engine produced more like 150+ max.hp.

[Mark Emaus]

Ok, Just to make it clear. We use the boat specs at the start of this thread. Now, as for the engine. It is a vw engine and I was told that we could grind the cam and slow the rpm down to 2500 rpm, and still produce 65 hp. My bad, I thought that most props ran at 2500 rpm. That is why I wanted to slow the rpm down to max hp. Now forget that. Let us say that the vw engine will run all day at 3500- 4000 rpm. Back to the first question of the prop turning 3500-4000 rpm, what would be the best prop to run and what speed do you think we could get out of her? Please use the boat specs at the start of the thread. I would like to have a 12 inch prop and a 10 1/2 pitch?

thanks
Mark

[Jango]

The 65 HP VW probably developes max HP between 3600 - 4400 rpm.

The ideal Prop for a max of 4000 rpm would be a 10"dia x 13 1/2 pitch, 3 blade which will provide 45 MPH with 11% slip.

At the same 4000 rpm,a 12" Dia x 10 1/2 pitch prop will get you to 36 mph

Of course this is all based on a max weight of 1200# as previously stated

[pistnbroke]

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

[apex1]

Ok, Just to make it clear. We use the boat specs at the start of this thread. Now, as for the engine. It is a vw engine and I was told that we could grind the cam and slow the rpm down to 2500 rpm, and still produce 65 hp. My bad, I thought that most props ran at 2500 rpm. That is why I wanted to slow the rpm down to max hp. Now forget that.
Mark


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

[Mark Emaus]

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

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

[Mark Emaus]

The 65 HP VW probably developes max HP between 3600 - 4400 rpm.

The ideal Prop for a max of 4000 rpm would be a 10"dia x 13 1/2 pitch, 3 blade which will provide 45 MPH with 11% slip.

At the same 4000 rpm,a 12" Dia x 10 1/2 pitch prop will get you to 36 mph

Of course this is all based on a max weight of 1200# as previously stated

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.

[pistnbroke]

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 ???

[Mark Emaus]

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

[pistnbroke]

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

[Mark Emaus]

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).

[Jango]

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

[pistnbroke]

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

[mudman]

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.

[Jango]

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

[mudman]

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.

[Jango]

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.

[Mark Emaus]

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

[mudman]

First I've heard of them. The hub looks too big for my application (may catch alot of weeds), plus that composite wouldn't last too long for me when I get in oyster shells or sand. Good concept though, but it looks like it belongs on outboard raceboats.

The Tiger props that you are talking about are surface props and are designed to run half in half out of the water. We run em on the surface units. No good for a submerged deal, and forget about having reverse. Throws a rooster tail about 5 feet high. All commercial surface drive units do not come with reverse, except for the manufacturer of tiger props. They are talking about abandoning the reverse because it does not work. This was already known by the other 3 manufacturers. Check out mudmotor videos on youtube and see the difference of the 4 main manufacturers.

We've found that a good mud surface prop with good reverse capabalities is a regular SS mud prop whith extra heavy cup. Those prop guys work the prop to a sharp balde and cup it to death.

[Jango]

10 x 10 @4000 RPM, 65HP, 34 mph. Otherwise, exactly the same as mudman @ 5000 RPM.
I would follow mudman's advice, He's been there. Also it's almost exactly what my software shows.

[Mark Emaus]

Thanks Guys!
Jango was that last thread for 2000lbs?

[mudman]

I'd expect 10X10 to be about right for 4000 rpm. Personally, I run that thing at 5500 max. Probably never bring it there, but I'd prop it for that. Otherwise, the engine will stop at 4000 rpm and you'll still have 1/4 throttle left. Been there, done that.

Are you running a surface prop? I'm interested in how this will be set up.

[Jango]

YES, my last numbers are for 2000 lb

[mudman]

These are surface drives
http://www.youtube.com/watch?v=hPexWxFnRn8&feature=PlayList&p=CB879CE459048AA5&playnext=1&playnext_from=PL&index=16

These are longtails, like I suspect you are building.
http://www.youtube.com/watch?v=qimjHKRU7Ns&feature=PlayList&p=CB879CE459048AA5&index=13

Two totally different props. Both get the job done, but some prefer one over the other. 3/4" shaft is too small either way. The surface video is what we run through, is this what you are trying do also?

[Mark Emaus]

Mudman,

I appreciate your help. We have not finalized the exact way we want to go with. But to give you some background, we are looking at 2 different systems. One is a direct drive off a shaft and we will use a pulley system that is able to go into reverse. The other is a hyd system that will be eaiser to build, but is less effcient. Much advantages to this as I am not that interested in fuel effcientcy as power and mobility. Both systems will have a power trim system that we have designed. What prop manufactures do you recommend for props Mudman?

Mark