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Old 20-06-2008, 02:14   #31
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I assume you meant optimal not optional 2". Yes the difference is dramatic.
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Old 20-06-2008, 21:27   #32
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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
Sounds like a good thread to me. I have been looking for a less expensive solution to the Walker airsep. Anyone have any ideas??? I am still trying to limit the noise from the engine with out building up too much heat in the engine room. I hate that the engine room holds so much heat for so long right now. In a perfect world the engine would be cool and quiet.
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Old 21-06-2008, 00:36   #33
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Getting rid of heat means ventilation. There is no other easy and practicle way.
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Old 30-06-2008, 01:15   #34
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Here's an update on my prop investigation.
Finally got a clear, calm day on Saturday for some sea trials. Here's what I found:

1,000rpm 2.9Knots SOG (via GPS, averages of runs in 2 directions)
1,250rpm 4.1Knots SOG
1,500rpm 4.8Knots SOG
1,750rpm 6.1Knots SOG
2,000rpm 6.7Knots SOG
2,350rpm 7.1Knots Sog

For those of you who wanted to know if there is a difference between what I can push at the dock versus in open water ....there is ....and it was 1,900 vs 2,350 (i tried it at the dock as soon as I got back and tied up so the conditions were the same.

My maximum revs were 2,350 in calm water with virtually no wind . This is not what I am after. I'm looking for 3,400rpm or thereabouts, and I will now see what can be done about reducing the pitch.

Comments welcome.
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Old 30-06-2008, 02:02   #35
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Yes the reason you are seeing such a reduced amount between dock and open water is that the engine is heavily loaded and a small increase in load is resulting in a large loss. I would expect to see a 5% to 10% difference at the most. You certainly need to sort the pitch. It is a long way out.
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Old 30-06-2008, 03:10   #36
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Would you expect black smoke from a 4-108 Alan if the engine was overloaded by the prop and that overload causing the low revs? I'm not familiar enough with the engine to know.

Mike is getting roughly the same revs as before with the previous prop and reported no black smoke then (but appeared to be diesel in the exhaust). I am wondering that if black smoke would be expected from what must be a big overload from the prop if it is dropping the revs so much, and if no smoke is apparant same as before (Mike would need to confirm that) then is there a problem with the engine (fueling, timing, etc) or with its associated needs eg air supply to the engine compartment (but that would likely cause smoke so unlikely), fuel system, exhaust system?

I wonder about things like fuel pickup from the tank, etc?

John
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Old 30-06-2008, 03:34   #37
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Hi John and Alan

I don't get any black smoke at all, even when at max revs straining away with my big prop. I do get a little bit of steam which increases at high revs and virtually nothing at idle.

I thought I should get black smoke, too, if the engine was overloaded because the governer's delivering more fuel than the engine could use. Interesting? Sort of confusing as it makes you think lack of fuel but my IP and injectors were rebuilt (or so the invoice says) when I rebuilt the engine before Xmas. How would I tell?
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Old 30-06-2008, 09:51   #38
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If your engine manufacturer rates your motor for 3400rpm you have a long way to go. usually a pitch change is good for a few hundred rpm sounds like you may need a whole different prop set up - it may be time to consult with a good prop shop - I am assuming your motor can turn max idle rpm with no trouble - In that case you can exclude any motor fault for not getting the revs you need.
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Old 30-06-2008, 13:42   #39
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OK one BIG question. Sorry my memory is terrible and this may have been asked earlier. Out of gear, can you get the engine to rev to 3400??? If you can not, then it maybe that the engine has had the max rev screw-stop adjusted or the cable is not moving far enough etc etc. If you can reach 3400 out of gear, then you are overpropped. Not seeing black smoke is possible if you have a high cooling water volume entering the exhuast, but the water exiting the exhaust will carry the black carbon on it. This is where 95% of my black smoke ends up if I am pushing too hard. A big black patch on the water and it ain't from spinning the tires.
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Old 30-06-2008, 13:45   #40
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My thoughts exactly Alan, try the no load revs in case the governor stop has been set at 2350.
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Old 30-06-2008, 15:28   #41
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Out of gear I get 3,950rpm no trouble. So, I have concluded right from the start that I have been provided with a prop that is just way too big. I told my supplier before we started that all the prop calc programs I have used come out around 15" X 12" or thereabouts. They have supplied a 16.5" X 23 degrees on the basis that this is what they have used on 4-108's previously. Their initial comments, when I first told them that all I could get at the dock was aroud 2,000, was that there must be something wrong with my engine (just rebuilt), or a fuel or exhaust blockage. I don't want everyone pointing fingers and ducking for cover over this - I just want to have the right size prop to let me get to around 3,400 - 3,600 rpm to prolong the life of my engine 'investment'.

Anyone know how I can rule out (or confirm) lack of fuel or blocked exhaust so we can move on?
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Old 30-06-2008, 16:22   #42
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I am not familiar with the engine as I said, but I would imagine that its fuel use with it out of gear will be considerably less than when under load. So I would not be inclined to think that because it revs to near max revs unloaded that there is not some fuel (or other) problem unless it is known that there is little fuel use difference between loaded up fully and out of gear. But I may be wrong with that for that particular engine.

I also wonder why the prop it replaced was "overpropped" and now the same again with its replacement. If it is an overpropping issue then the same mistake has been made twice in prop selection it would seem. It is also unusual, in my experience, for a well regarded prop builder to get it so far wrong, usually pretty much spot on or only a little off.

I am not saying it is not the prop but rather saying that if it were me I would keep an open mind at this stage.

Regarding checking fuel system and exhaust outside of the engine what I would do is blow back through all the fuel lines from the engine back to the bottom of the pick up pipe in the tank to check they are clear. If there is a primary filter then this would have to be done in sections rather than blowing back through the filter. If it is hose and original I would be inclined to just replace it to be sure. And if there was a restriction in the tank pick up it is likely that the problem will repeat itself if the cause not removed from the tank. I would also check that the tank vent is clear.

For the exhaust system the first thing I would check is any hose sections for internal delamination and without knowing anything else about that system that is likely to be the most likely cause of a problem should there be one. In fact I know a boat that just recently had this problem - new engine installed and gave the impression of being under propped from the folding prop fitted so they replaced it with a fixed 3 blader. Immediately ran into overload problems (including, in their case, overheating on longer runs as they were away cruising in the tropics) - after much heartache the problem was found to be a small delaminated section inside a section of exhaust hose restricting flow. Replacing the hose fixed the problem.

I have never dealt with KiwiProp themselves in Auckland but I have heard good reports as to how responsive they are to sorting issues. Becomes a lot easier talking about such problems if when talking to them you have already covered and discounted the non prop related possibilities they may suggest be looked at.

Again, I am not saying it is not the prop, but from experience sorting problems on new builds can say it is valuable to keep a very open mind when trying to identify the cause - can be from the most unexpected casue.
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Old 01-07-2008, 00:10   #43
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Hi John

Thanks for the detailed responses.

First a quick backgrounder as to why my old prop was selected wrong. The PO had replaced a 2-blade folder with a (beautiful) 3-blade Flex-O-Fold. He had ordered it through a Waikawa shop. On the prop order form they got him to fill out he had virtually everything wrong (including displacement, engine HP, size of old prop, etc). When I brought the boat it wouldn't rev past 1,700. I contacted the prop supplier (Henleys) and got them to do the calcs for me. Guess what they said? Yep, 15" X 12". They could have supplied new blades but they were just too expensive so I decided to go down the KiwiProp route as I had a Kiwi on my last boat and I liked it.

Hopefully that explains why the previous prop was wrong. Interestingly the Henleys guys rate the Perkins 4-108 no better than 40HP @ 3,000 for calcs (despite the claims Perkins make of 52HP at 4,000rpm).

I have already replaced all the fuel lines and installed a new Racor 500 so I doubt its that. The only re-used line from primary FF to IP was cleaned out.

I'll check the exhaust (I might as well replace the remaining bit of old exhaust from waterlift to transom just to make sure).

Once I've tried that I'll check the fuel supply from the tank.
Then I'll talk to the prop guys. Makes sense ....just very frustrating.

Thanks again for your help.
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Old 01-07-2008, 00:38   #44
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My boat is a fat and heavy little cruiser with a 27' waterline and a displacement of about 23,000 lbs "as is".
How did you calculate that displacement? I'm wondering what these 33's weigh typically full. And isn't the waterline 25ft?
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Old 01-07-2008, 03:23   #45
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Displacement isn't a major issue. That is more an issue in regards to working out the required Hp. The essential points are for diameter... the distance between the prop tip and the hull bottom and for pitch...the speed of the hull through the water and the shaft RPM. This is affected by gear ratio remember.
They reason why Henleys de-rate the Perkins engines Hp is because there are many ratings used. It is a case of knowing the correct one to read. All the ratings are usefull and are all correct. But it is important to know which to use for a real world example. You can bet that a dealer will use the highest rating of course, which is why these engines end up being sold as a power rating above what they can produce in our real boating world.
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