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Re: Tiny might propeller slip

Posted: Sun Oct 13, 2019 11:40 am
by kens
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.

Re: Tiny might propeller slip

Posted: Mon Nov 18, 2019 9:48 am
by JhlFin
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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Re: Tiny might propeller slip

Posted: Mon Nov 18, 2019 7:52 pm
by kens
Very nice fabrication work.

Re: Tiny might propeller slip

Posted: Fri Nov 22, 2019 8:11 am
by JhlFin
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

Re: Tiny might propeller slip

Posted: Wed Nov 27, 2019 9:12 am
by JhlFin
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?

Re: Tiny might propeller slip

Posted: Wed Apr 01, 2020 10:31 am
by JhlFin
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:

Re: Tiny might propeller slip

Posted: Wed Apr 01, 2020 6:57 pm
by kens
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.

Re: Tiny might propeller slip

Posted: Thu Apr 02, 2020 9:29 am
by JhlFin
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 :)

Re: Tiny might propeller slip

Posted: Thu Apr 02, 2020 12:36 pm
by kens
Hi, what RPM is that engine rated at?

Re: Tiny might propeller slip

Posted: Thu Apr 02, 2020 12:57 pm
by Bill Edmundson
Not just the engine rpm. But, the gear ratio/shaft rpm.

Bill

Re: Tiny might propeller slip

Posted: Fri Apr 03, 2020 8:58 am
by JhlFin
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.

Re: Tiny might propeller slip

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

Re: Tiny might propeller slip

Posted: Fri Apr 03, 2020 5:52 pm
by kens
Bill Edmundson wrote:
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
What's the majic formula for that? Why a 12" prop @ 4000rpm?
There is sooo many boats, ski boats, mahogany's, that are running just fine with (Ford, Chevy, chrysler) small block V-8's, and the classical setup is the ubiquitous VelvetDrive 1:1 tranny along with the ubiquitous 13x13 prop. Doesn't matter be it a Ford, Chevy, Chrysler they all turn approx 4500rpm.
My point is that if a 12" dia @ 4000 is cause for cavitation, well then a 13" @ 4500 is an absolute no-go, correct?
Empirical data will agree with 13" @ 4500 runs just fine.

Re: Tiny might propeller slip

Posted: Fri Apr 03, 2020 6:21 pm
by Bill Edmundson
Just my observation Ken. That puts the tip at about 200 mph. I have just seen many start having problems about that speed.If the prop is in perfect condition maybe not.

Bill

Re: Tiny might propeller slip

Posted: Sun Apr 05, 2020 12:54 pm
by JhlFin
Kens, how much prop slip normally those boats have?

This is whole new world to me, it is very challengin but fun. Here in Finland usually inboard boats are displacement hull types and it is hard to find information because of it. This forum is totally life saver, nice people and lot of photos and information.
I have built one boat before this, it was 9' little speedboat, plans were from -69 and it had -69 20hp red stripe mercury. Then i got greedy and put 40hp yamaha in it, it was not a great idea 🤣. It was 15 years ago when i was 15, right the i decided that i want to build fast inboard boat. 2010 i ordered plans and it took seven and a half year to gain courage to start building.

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