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Old 09-15-2012, 02:20 PM   #21
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You need to recheck your maximum wide open throttle rpm. Every 3208 I have seen, installed or inspected during a survey since 1980 or so has been rated to turn 3000 rpms no load and 2800 loaded. If you prop it to reach 3000 rpms loaded you will be underwheeled.

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I have a 44' chung wha with twin 3208 cat 320 hp I just gonged my props in bimini, Anyway all the prop charts I see call for something around a 23''x 16p The current 4 blade props have no markings but measure 27'' in diameter I can not reach rated rpm of 3000 only around 2400rpm Anybody have a simular set up that can help my gear ratio is 1.81 Oh buy the way the bottom in bimini harbor is hard coral with scattered BIG rocks
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Old 09-15-2012, 02:26 PM   #22
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Quote:
Originally Posted by bglad View Post
You need to recheck your maximum wide open throttle rpm. Every 3208 I have seen, installed or inspected during a survey since 1980 or so has been rated to turn 3000 rpms no load and 2800 loaded. If you prop it to reach 3000 rpms loaded you will be underwheeled.
you are correct thank you 2800 it is
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Old 09-15-2012, 11:02 PM   #23
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There is another view that says 100 to 150 rpm over at WOT, fully loaded is the safe number, in that bottoms get dirty, and sometimes the weight grows as well and the combinations will slow you down to the 2800 rpm number at WOT but you will not be overloading the engine. Your call, however.
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Old 09-15-2012, 11:06 PM   #24
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like many the way I use this boat is as a trawler running at 1300-1400 rpm so my confusion is what diameter should I choose for slow speed ecomomy The two props I have talked with say the perfect prop is the one they have in stock the two online prop calculators call for a 24x18 4blade
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Old 09-16-2012, 12:26 AM   #25
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M30 - For prop sizing, you may want to pose your questions on boatdiesel. You will get feedback from some very experienced diesel and prop people. Also read the props move boats article posted there. I can tell you that propping correctly to reach rated RPM plus 100 extra or so will provide the best economy and better engine life for your 3208.
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Old 09-16-2012, 02:48 AM   #26
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When we thought we needed new props we called the best (we were told by many sources) prop shop in Seattle and asked them what information they needed to determine the best props for our boat. They said they wanted to know the make and model of the boat, the type of engines, the horsepower of the engines at max rated rpm, the rpm of each engine at WOT, and they wanted to see the current props.

From all this they could determine the best props for our boat. How this compares with the chart or formula on boatdiesel I have no idea. But since props are expensive and the potential penalty of installing the wrong ones could be even more expensive it would seem to me that the smart course of action would be to turn the matter over to a qualified and experienced prop shop where their work is backed up with a warranty.

In our case the shop found on examining our current props that they could be extensively reworked to achieve the proper performance. Which cost us several hundred dollars instead of nearly $4,000 for new props. An Internet forum is not going to present you with this sort of solution.
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Old 09-16-2012, 09:46 PM   #27
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Not all prop shops will either, but boatdiesel would be a good source to compare the prop shop solution. Boatdiesel requires water line as well as the above mentioned items and would be a second opinion.
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