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Old 17-06-2008, 18:46   #16
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I'm guessing you are around 3.0:1?
2 to 1.

Low slow and lazy, that be me and my boat.
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Old 17-06-2008, 19:24   #17
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Just a point of clarification( a % of max engine rpm may be a rule of thumb) the more correct way to prop is based on the manufacturer's rated rpm at hp- so if you have a 52hp rated motor at lets say 3200rpm or(what ever specs are) You must prop your boat to a minimum of that rpm(off dock) full load at wot. to be safe and to account for some bottom growth etc 100-150 rpm above that. This rpm # has little to do with the max rpm of a motor that can vary and be governed and is not an indicator or helpful in determining loading. The max rpm is only important as a first step to determine if your motor has some functional problem restricting performance before running wot under load.
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Old 17-06-2008, 20:03   #18
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Okay, lets go again:

Static RPM tied to dock: 90% of MAX.

Cruise RPM: 70% 0f max continous. (NOT MAX)

Max RPM in Neutral: 100% of MAX rated. As per the governor.

Not sure that any formula has to account for growth on the prop..?
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Old 17-06-2008, 20:57   #19
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a little growth on your prop or bottom will overload your motor just as well as a over pitched prop so you takes that into account by under pitching the prop by 100-150rpm(the initial pitching procedure is done with clean bottom and prop) Its how hard your motor has to work at given rpms that leads to internal overheating and shortened life and you wont see that on your water temp gauge you need exhaust heat gauges and other more sophisticated gauges to monitor that
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Old 18-06-2008, 00:47   #20
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Ok, here is what I've learned, and my thoughts and a plan of action...

First, I know the KiwiProp website has examples of other 4-108 installations and that my new prop seems to be consistent with these. However, none of these give enough details. I mean how do I know if they got to max revs? Every boat is different too (displacement, waterline, reduction ratio, etc) - it ain't just the engine.

From what you have all said the plain facts are that if I can't get to at least 90% of max revs in flat conditions, with a clean bottom (which I've just done btw), I'm overpropped and I need to get this right or I will overload my engine and not achieve its full potential. Propping it to achieve 100% seems a good extra to aims for to allow for weed growth, but I'm going to aim for 90% first off.

When I get back to NZ I'll do my sea trials and report back.

Thanks everyone!

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Old 18-06-2008, 01:52   #21
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I have only heard good things about Kiwi prop. But I have never had personal experience with them. The pitch difference maybe because the blades are straight and a very different shape to anything else on the market I know of. However, pitch is pitch and I would not have expected that much difference. For instance, a three blade prop with cupped blades has about an extra 1" of pitch applied to any equations you work with, than a standard blade prop. I can't see how the Kiwi prop would be approaching double what a standard prop pitch should be called for.
For sailing vessels, tied to the dock or traveling through the water will make very little difference between the max achievable RPM. Moving forward simply does not add enough water flow to that which is being swept over the blades by it spinning.
There is some info in our study hall on props. That may help you somewhat as well.
The important thing is that your engine should be able to reach within 10% of the max RPM range. You most certainly do not want to have your engine peak out at 2000RPM.
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Old 19-06-2008, 01:32   #22
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Thanks Wheels
I read your study hall post before I posted this thread.
Glad to see I'm not losing my marbles!!

Mike
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Old 19-06-2008, 03:02   #23
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I have just replaced cutlass bearing, shaft and coupling and fitted a PSS Shaft Seal and a brand new 16.5" X 23" KiwiProp.
I've broken the code - KiwiProps give the pitch in degrees so prop is actually 16.5" x 23 degrees not 16.5" x 23".
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Old 19-06-2008, 04:17   #24
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That would make sense. I suppose they use degrees of angle because of the blade shape. So are they factory set? or can they be user adjusted?
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Old 19-06-2008, 05:29   #25
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That would make sense. I suppose they use degrees of angle because of the blade shape. So are they factory set? or can they be user adjusted?
They can be adjusted for forward but not reverse, this is set to maximum angle. Have read that this can cause smaller motors to stall going into reverse. As I was intending to use 2 of them on 20 hp motors this worries me a bit.

Mike
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Old 19-06-2008, 07:40   #26
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Thanks for the great responses.


Easterly - I presume your 3-blade is a fixed prop rather than the feathering props I'm talking about? However, you are much closer to what I calculate I need. Glad to hear you can get to 3,500 at 8knots. That's pretty much what I'm after (8.5 knots in my case).

.
It is a fixed 3 blade. I could go up a little in size to get a little more speed at lower rpms but I don't see a need for it. I can cruise at 4~5 knots at 2000~2500 rpms and the engine noise is bearable, you can have a conversation and still hear the VHF.
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Old 19-06-2008, 14:20   #27
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Back to the concern of the RPM though, the Engine must rev to its rated speed. In all instances I have been involved in, that can be achieved tied to the dock. However, seeing a I have not had experience with Kiwiprop, I shall consider that I am wrong in this instance and I suggest you therefore confirm your speed underway. If the same result occurs, then you are over propped and that angle needs to be reduced.
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Old 20-06-2008, 00:13   #28
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Wheels

Thanks for confirming what I have observed re rpm at the dock.
I'm going to check it out at sea when I get home next week. Will post the results.

Been basking in some lovely English summer weather this week. I thought there was no such thing!
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Old 20-06-2008, 00:17   #29
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Hey MidLandOne

Now that makes more sense - 20 odd degrees is a heck of a lot more believable than 20 odd inches. Good dectective work.
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Old 20-06-2008, 00:22   #30
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Easterly

My next project - once I get the propping right - is to work on reducing the noise my trusty 4-108 makes. I have picked up a Walker Airsep on our local auction site. I've already started installing soundproofing but I used the 1" thick material not the optional 2" (foolishly listened to someone at the local marine store who said it would be fine). Maybe this is a good subject for a new thread as there are heck of a lot of us 4-108 owners in the forum?

Mike
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