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#1 Chrisc

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Posted 19 January 2017 - 02:19 PM

Going to drydock on Monday and I have a nice new prop to put on.
As I understand it, in calm water and with a clean bottom and underway, one should be able to just attain maximum engine revs at wide open throttle, this to ensure that no components of the engine and drive train are overloaded. In my case this would be 3600 rpm. So how theoretical is this? In other words am I doomed if the engine can only reach 3400? 3200? I figure that there must be a little bit of latitude with this.
Secondly,if I should find that the engine can't even get close to WOT, then I will have to take the prop off again for repitching, so don't want to ladle on $100 of Propspeed. Any recommendations for a reasonable, relatively short term prop antifouling?
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#2 Black Panther

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Posted 19 January 2017 - 02:22 PM

Plus your rev counter is probably lying. Mine is by heaps.


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#3 Chrisc

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Posted 19 January 2017 - 02:27 PM

Checked mine out last year with on of those handheld laser tachometers. The boats rev counter is actually not too bad..
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#4 waikiore

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Posted 19 January 2017 - 02:45 PM

Put MAXwax on available in a spraycan at most good chandlers - I got a year out of it, not quite propspeed but for your situation ideal.

And yes you should reach maximum 3600 with all things being right - clean bottom etc etc. We require that to sign off new engines as the boats always get heavier bottom fouling etc and you dont want to put a modern small diesel into lug. Depending on the engine an older one can have a little leeway.


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#5 Fish

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Posted 19 January 2017 - 02:46 PM

Soo,

Best short term option for prop antifoul is the Corrosion X stuff, we are using it on our shaft. there is a thread around here somewhere from when I was asking those questions. You can also use MaxWax, which is basically the same. The relevant question is finding a supplier.

 

We just re-propped with a much bigger propeller. Now we can't achieve max revs. We never did run anywhere near max revs due to noise and vibration anyway.

The main reason for achieving max revs is for warranty purposes of new engines. I did a fair bit of sussing and questioning of engine suppliers (and the kiwiprop supplier) on the need for this. It all comes down to your engine.

Ours is a 30 year old long stroke Izuzu that is so far out of warranty it is not funny. At the time of those engines being made, they were very conservatively rated. It's stated Hp was more than we need for hull speed, but more so, the stated torque was massive (and believed to be a conservative rating).

 

With the big prop, we achieve much higher cruising speed at much lower rev's, which reduces noise and vibration. With our very un-complicated engine, we can tell immediately when it is being overloaded, as it blows black smoke. Throttle back a fraction and she is fine. So we use that as a measure. I believe the risk of over propping an engine is excessive wear from heat, forces on bearings etc but those only occur when the engine is overloaded, which in our case gives black smoke. 

 

It is possible to up the prop to increase cruising speed at the expense of achieving max rpm. There is a good explanation on the Kiwiprop website (I can find it if your keen).

 

If you are fussy about your set up, I would recommend getting a digital tacho and checking your actual revs. $100 from the internet. Just noted you said you've already done though.

I've got a bunch of numbers somewhere of what revs and boat speeds we can get with our different props. Short story is I'm happy with our bigger prop but can't reach max revs now. Generally cruise at 2,000 rpm doing 6.5 knots (37ft, with a hull speed of about 7.9 knts) can get up to about 7.5 ish in flat water, smokes at 2,500 rpm, max revs should have been 2,800 rpm.


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#6 Fish

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Posted 19 January 2017 - 02:55 PM

Simultaneous posting. wow.

 

What sort of engine do you have Chris? as Waikiore says, the older it is the more leeway you have.


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#7 Chrisc

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Posted 19 January 2017 - 03:02 PM

Engine is a 3 cylinder 29hp Perkins Parama M30 driving through a mechanical PRM gearbox, 2:1 reduction.
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#8 Chrisc

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Posted 19 January 2017 - 03:26 PM

A little background.
I got the new prop a year ago,put it on the boat and zoomed all around the place at 6 knots at 2000rpm.
I didn't know then about all this max revs at WOT business.
Consequently, I destroyed the gearbox and had to shed a lot of money and tears on a new one. With that installed, I discovered that I could only reach 2500 rpm instead of 3600 rpm at WOT, and the gearbox just couldn't take the strain. The gearbox is correctly matched to the engine's HP.
So I put the old prop back on and had to be content with 3.5 knots. Now I've had the new prop repitched and want to put it back on. Can't afford to wreak another gearbox, though.
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#9 Fish

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Posted 19 January 2017 - 03:35 PM

The difference between 2500 rpm and 3600 is significant.

 

My Izuzu's max revs was 2,800, and we can do mid 2,000's fine (2,400 I think), smoking at 2,500.

I'm no expert, but I would have thought you'd want to get into the low 3,000's at least.

 

Why did you change the prop in the first place? did you want a higher cruising speed?


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#10 Chrisc

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Posted 19 January 2017 - 03:56 PM

The H28s came out originally with a 12hp Bukh which I think gave them around 5knots in flat water. With my 30hp engine I was getting around 4knots at 2000rpm with the fitted prop, whether the original or not I don't know.. I would have thought with that amount of HP driving the correct prop I should just about be able to go eater skiing behind her.
From what I have read, with a WOT of 3600rpm, normal cruising rpm should be around 2800. I can make 5knots at this rpm in flat water. With the old prop I can easily get 3600rpm with no smoke so figure she is under propped, hence the new one.
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