Tiny might propeller slip

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JhlFin
Posts: 21
Joined: Tue Sep 11, 2012 12:43 pm

Tiny might propeller slip

Post by JhlFin »

Hi, i have test drived my tiny might few times now, in im wondering why i got so much propeller slip.

I have 2100cc volvo engine with straight drive, put out about 120hp.
Engine weigth about 360lbs.
Shaft angle is 15 degrees.
Propeller is Mikado 11*15 bronze.

At 6000rpm i got around 35mph, it sound quite low?
I calculated slips(dont know if i did it right 😅:
4000rpm 52%
5000rpm 56%
6000rmp 59%

Sound really high? How much normal slip would be? Is it possible that my prop is just not made for speed or high rpm? I appriciate any help i can get!

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kens
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Joined: Wed Nov 12, 2003 5:25 pm
Location: Coastal Georgia

Re: Tiny might propeller slip

Post by kens »

Can you post some pictures of your build?
Can you show your 'direct drive' ?
I would like to help but since we are on opposite sides of our world, I have many question as you.
Pictures help a lot.
Ken
Oak is over rated, everything about it takes extra time; then it warps, splits or checks !!! :roll:

JhlFin
Posts: 21
Joined: Tue Sep 11, 2012 12:43 pm

Re: Tiny might propeller slip

Post by JhlFin »

Just dont look photos too closely, its ugly and not finished. Ill try to remember to take new and better photos.

That adjustable strut is going to bin, going to make new in winter.

I have clutch and bellhousing from car, i found it easiest way. Covers flywheel and nice surface to mount bearing. I have part of main axle from car gearbox, bearing and then rotex flex clutch.
Engine is too heavy and sits too high, because of oilpan.

Image


Image


Heres a short video of test drive, drives level and quite nicely. not painted yet and deck is only for testing purposes :D
https://www.youtube.com/watch?v=dVg_tA_xtNc

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kens
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Joined: Wed Nov 12, 2003 5:25 pm
Location: Coastal Georgia

Re: Tiny might propeller slip

Post by kens »

Hi,
That is interesting way to use the auto bellhousing, you can have a neutral with the clutch, and mount thrust bearing on the transmission flange, interesting.
I see in the picture some metal bracing, do you know how much does the boat weigh? The engine weight?
When you make a new streamlines strut, that probably will help some.
I have a 3.0L 4 cylinder engine, and it does not even make a full 120hp. Therefore I doubt your 2100cc is making 120hp either. Engine makers always make statement about their horsepower and that is very generous. For the purpose of sizing a prop I would use a number closer to 1/2hp per cubic inch as a starting number. Your 2100cc auto engine may perform in the boat as a honest 60hp.
In doing a lot of prop testing myself, I find that the more pitch you run on a prop, the slip goes up also. With the bit of information you give so far, yes, you are seeing about 50% slip.
Do you have a GPS you can get a factual speed number, and compare that to your engine RPM?
Oak is over rated, everything about it takes extra time; then it warps, splits or checks !!! :roll:

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Bill Edmundson
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Location: Birmingham, AL, USA
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Re: Tiny might propeller slip

Post by Bill Edmundson »

I estimate that the running weight is about 900 pounds. 120 hp should get you to 55 mph. 60 hp should get to 45 mph.

At 4300 rpm and 11" prop you may be getting blade tip cavitation. As you increase rpm the cavitation just spreads down the blades toward the hub. At 4000 rpm and 15% slip the 15 pitch prop should give 48 mph.

My slip calc comes 1-your number. So 1-52%=48% (still too high). don't know if there is much clutch slip.

I don't know exactly blade cup or blade rake affect all of this.

Bill
Mini -Tug, KH Tahoe 19 & Bartender 24 - There can be no miracle recoveries without first screwing up.
Tahoe 19 Build

JhlFin
Posts: 21
Joined: Tue Sep 11, 2012 12:43 pm

Re: Tiny might propeller slip

Post by JhlFin »

900 pounds running weight sounds about right. I'll try to get bit weight off.
Engine is marinized car engine, they say 90hp in car use. Same base engine in volvo penta application is aq125 117hp, they have slightly diffrent camshaft and manifolds ofcourse.
Horsepower wont be issue, im planning on getting more out, wery sturdy engines and lots of aftermarket parts available. Might go to 2300cc or 2500cc bottom end and most definedly better carb with choke(cold in Finland😂)

I think next im gonna make sure that clutch is not slippig, it should not be, but better to be sure.

Propeller was closest fit that i found, i planned to get it cut to 10", but didnt yet because it fits. Would cutting help? Might also try othet modifications.

Im sure that new, more streamlined strut will help, also i will try other rudder designs. Rudder is only 2" from prop tip, im concerned if this would add slip? Ill try other mods first, and if nothing helps i go for transom mount rudder.

Lot of testing to do and not a lot of season left, soon ice and polarbears will take over the waters 😅🤣

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kens
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Joined: Wed Nov 12, 2003 5:25 pm
Location: Coastal Georgia

Re: Tiny might propeller slip

Post by kens »

My first concern would be clean smooth water going into the prop, as you said, you are to build a new strut, I sure agree with that.
Then make sure the clutch is not slipping.
If that car engine is 90hp in the car, then the engine maker is probably optimistic about his 90hp because of marketing pressure from the competitive other makers. By the time you get that car engine in the boat as is, you might see 70hp at the prop.
I would not cut nor modify the prop at this point.
There is too much trouble shooting otherwise before you cut the prop.

I really doubt the close distance from rudder is a cause of cavitation, it might give you a vibration in the rudder, but not, prop cavitation.
Oak is over rated, everything about it takes extra time; then it warps, splits or checks !!! :roll:

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Bill Edmundson
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Re: Tiny might propeller slip

Post by Bill Edmundson »

A 10" prop would get you to 4800 rpm before tip speed might play into it.

Bill
Mini -Tug, KH Tahoe 19 & Bartender 24 - There can be no miracle recoveries without first screwing up.
Tahoe 19 Build

JhlFin
Posts: 21
Joined: Tue Sep 11, 2012 12:43 pm

Re: Tiny might propeller slip

Post by JhlFin »

I have 10x10 prop with wrong taper. I guess ill try that after some machining. And Thanks for replies! It helps a lot, helps to think :D

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kens
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Location: Coastal Georgia

Re: Tiny might propeller slip

Post by kens »

I would devote time to a correct strut,
If the fat adjustable strut is disturbing water flow to the prop, then no prop will breathe correctly.
The faster you go, the more this will matter.
Oak is over rated, everything about it takes extra time; then it warps, splits or checks !!! :roll:

JhlFin
Posts: 21
Joined: Tue Sep 11, 2012 12:43 pm

Re: Tiny might propeller slip

Post by JhlFin »

Im sourcing materials for new strut, have not found yet. Well i could make it from steel, i have materials for that, i guess rust wont be an issue with good paint and boat always on trailer. Water around me is brackish i think. I live in an island, in smallest town in Finland :D
Just machined taper of 10x10 prop, its not good, i think someone have messed with it before and messed it up. I think after balancing it will be atleast good enough to test. Next week prop guy is going to look at both props, hoping to learn something usefull.
My engine is sitting quite high, can this affect on performance?
Also i looked both props and those dont seem to have that much of a diffrence in pitch, darker one is 10x10 and brighter is 11x15. Well i dont understand much about props, so i guess prop guy will tell me more :roll:


Image

Image

Image

JhlFin
Posts: 21
Joined: Tue Sep 11, 2012 12:43 pm

Re: Tiny might propeller slip

Post by JhlFin »

Tried other prop repitched, so now 10x12. Quite funny really, at 6000rpm prop slip was again 59% :lol:

So yeah, i guess that strut is just that bad. I ordered new rubber bearing, smallest i could find. Little bit machining and welding and ill guess try again :D

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kens
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Joined: Wed Nov 12, 2003 5:25 pm
Location: Coastal Georgia

Re: Tiny might propeller slip

Post by kens »

Hi,
you say that with either prop, you are at 6000rpm,
is that 6000rpm a WOT (wide open throttle) number, or is that different from a WOT?

and at 6000rpm what is the speed difference between the props?

in the pictures your setup looks like it should run ok
Oak is over rated, everything about it takes extra time; then it warps, splits or checks !!! :roll:

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Bill Edmundson
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Re: Tiny might propeller slip

Post by Bill Edmundson »

If there is little difference in speed, at some point the shaft torque hits a point where the clutch slips.

Bill
Mini -Tug, KH Tahoe 19 & Bartender 24 - There can be no miracle recoveries without first screwing up.
Tahoe 19 Build

JhlFin
Posts: 21
Joined: Tue Sep 11, 2012 12:43 pm

Re: Tiny might propeller slip

Post by JhlFin »

I cant drive wide open with either prop, engine will over rev(no limiter)

With 11x15 6000rpm speed was 35mph. 10x12 at 6000rpm speed was 28mph. With smaller prop it went worse at every rpm, felt it right away.

I know that clutch may slip, but i if thats the case, i think with smaller prop it should have performed better maybe(less slip)
Well if i have time to finish new strut and weather holds, next test drive i will measure shaft rmp, not yet sure how.

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