Tiny might propeller slip

About inboard or outboard motors.

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kens
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Location: Coastal Georgia

Re: Tiny might propeller slip

Post by kens » Sun Oct 13, 2019 11:40 am

Whoaaa, NO, no.
You size the prop such that WOT rpm is held in check by prop size.
you have a too small a prop right now.
you need bigger prop not smaller;
if the engine is rated at 6000rpm, then you size the prop such that you do not exceed 6000 because of the size of the prop.

Bill, the clutch should be of enough ability to hold all the engine power.

A prior post said you had a 11x15 prop and 6000rpm. if at that point you were not WOT for fear of over-revving, then that is too small a prop right there.
Oak is over rated, everything about it takes extra time; then it warps, splits or checks !!! :roll:

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

Re: Tiny might propeller slip

Post by JhlFin » Mon Nov 18, 2019 9:48 am

Finally got new strut made, took long enough.

Had litte problems when welded strut, warpped strut housing. First bearing was destryed while installing. Little bit of work and i tought that now it would be good, but not... Second bearin installed, well guess what? too tight :D Well i sanded it a bit and now its ok for testing, just hoping that weather gets bit better.

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kens
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Re: Tiny might propeller slip

Post by kens » Mon Nov 18, 2019 7:52 pm

Very nice fabrication work.
Oak is over rated, everything about it takes extra time; then it warps, splits or checks !!! :roll:

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

Re: Tiny might propeller slip

Post by JhlFin » Fri Nov 22, 2019 8:11 am

Last testdrive for this season with new strut. New tachometer acted pretty much same as old, need to check wiring. Dont have accurate reading, but max speed was pretty much the same. So strut was not the issue. I feel like front of boat is too heavy or trim tabs were at wrong position. Lots of work to be done in winter, more testing in spring :)

https://youtu.be/cB0GuI2oyY0

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

Re: Tiny might propeller slip

Post by JhlFin » Wed Nov 27, 2019 9:12 am

Im thinking that my prop is not made for high speed/rpm, blades are thick and not even finished properly from what i can tell. i cant find any other suitable props in Finland. I would be very happy if someone could guide me to find and choose right steel prop. I think someone used 10x15 2 blade steel prop with honda 4cyl engine and it was fast, i have heavier engine, but horsepower i can match :) Would 2 blade propeller be better option, or atleast steel 3 blade?

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

Re: Tiny might propeller slip

Post by JhlFin » Wed Apr 01, 2020 10:31 am

Little update, i found propshop that was willing to modify my propeller. We talked a bit and we decided to cut prop to 10" as blade tip speed gets too high and causes cavitation, also prop will be cupped. They will also measure rake of propeller and change it to 5 degrees aft rake if it is less. Maybe too many changes at once, but should really feel in performance. Either good or bad, but something atleast :lol:

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kens
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Re: Tiny might propeller slip

Post by kens » Wed Apr 01, 2020 6:57 pm

Are you proposing to cut the diameter to 10", or, change pitch to 10" ? I not following that, sorry.
In your prior posts I believe you needed more pitch.
Perhaps by adding cup as you mention you can get the pitch you need that way.
I would not cut the diameter down though, because when you go down in diameter you generally go up in pitch, and you needed more pitch from the beginning.
Your new strut looks very nice. good work.
Oak is over rated, everything about it takes extra time; then it warps, splits or checks !!! :roll:

JhlFin
Posts: 19
Joined: Tue Sep 11, 2012 12:43 pm

Re: Tiny might propeller slip

Post by JhlFin » Thu Apr 02, 2020 9:29 am

Sorry, my english is not that good :oops:

So propeller diameter will be cut from 11" to 10". Pitch will remain 15" and cup is added. If it is needed, i will try more pitch. Prop guy said that with high rpm bigger propeller will start to cavitate from high tip speed. Smaller prop should have cleaner water and slip would decrease, he also said that cup is a must on this kind of application.
What i have searched from all around the internet, i think this sounds reasonable. Well i guess i will know soon :)

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kens
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Re: Tiny might propeller slip

Post by kens » Thu Apr 02, 2020 12:36 pm

Hi, what RPM is that engine rated at?
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 » Thu Apr 02, 2020 12:57 pm

Not just the engine rpm. But, the gear ratio/shaft rpm.

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

JhlFin
Posts: 19
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Re: Tiny might propeller slip

Post by JhlFin » Fri Apr 03, 2020 8:58 am

It has 1:1 Gear ratio, direct drive.
Marine engine based on this is rated at 5500rpm max, i have rev limiter at 6500(not stock engine). I think max cruise rpm will be 4000 and will run short periods of time at 6000rpm. Im not worried about running high rpm, these are sturdy little engines and it wont get many hours on it in summers time.

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Bill Edmundson
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Re: Tiny might propeller slip

Post by Bill Edmundson » Fri Apr 03, 2020 10:18 am

I think your prop guy is on the right track. A 12" prop will start getting blade tip cavitation at about 4000 rpm. The 10" will start around 4800 rpm. Pitch and cup get the power back.

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

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