Another Prop Issue

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

Sorry to hear about your dinged props. I was very careful while in the Abacos in May and watched the tides closely.
boatdiesel.com has a prop calculator that should give you a good idea of what the proper size prop and pitch should be.
If you're not reaching proper WOT rpm there are other issues at play too. boatdiesel can help with that too.

Tim
I messed up, it was after dark and I let myself get out of a narrow carved channal. boatdiesel call for A 22'' diameter 4 blade, the current props are 27'' diameter just perplexed how the fomer owner could be so far off

motion30 said:

Tim
boatdiesel call for A 22'' diameter 4 blade, the current props are 27'' diameter just perplexed how the fomer owner could be so far off

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I would do a little more investigating if the current prop is that far off from what boatdiesel recommended. However, that may be why you are not getting WOT rpm.
From comments I've read on boatdiesel, it's not uncommon to find a boat propped improperly for a variety of reasons.

I had an inch of pitch taken out of mine per the prop calculator on boatdiesel. I've been told that many times a manufacturer will prop a boat to attain advertised speed which may be overloading the engines, the number one reason for diesel engine destruction according to Tony Athens, a moderator on boatdiesel.

current props are 27'' diameter just perplexed how the fomer owner could be so far off

The prop recommendations are not written in stone.

A larger prop with less pitch than "ideal" may serve a purpose.And be more efficient!

Sounds like the PO was knowledgible enough to spec cruising props.

Not good for ultimate top speed , but most cruising is not on the pin , but at lower RPM.

The cruising prop concept is to give up the 2 x 300+ hp cruise and have a very efficient lower cruise RPM. Less noise , less fuel burn, works for many that understand it.

If you prefer 50GPH cruising and top speed, the factory prop will be your choice.

FF

FF said:

current props are 27'' diameter just perplexed how the fomer owner could be so far off

The prop recommendations are not written in stone.

A larger prop with less pitch than "ideal" may serve a purpose.And be more efficient!

Sounds like the PO was knowledgible enough to spec cruising props.

Not good for ultimate top speed , but most cruising is not on the pin , but at lower RPM.

The cruising prop concept is to give up the 2 x 300+ hp cruise and have a very efficient lower cruise RPM. Less noise , less fuel burn, works for many that understand it.

If you prefer 50GPH cruising and top speed, the factory prop will be your choice.

FF

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In fact I use the boat as a trawler running around between 8 and 10 mph so how do I pick a new crusing prop ? the current props have no markings

Props are not "picked." Since you are buying new ones, I suggest you use the calculator on boat diesel and post your question there as well. FF aside, you should be able to pull full rated RPM on your new props without overheating.

If you normally cruise at around 1600-1800rpm to achieve hull speed and rarely exceed 2000rpm except briefly, over-propping can be acceptable for a full displacement hull, as FF indicates in post #5. Use the boatdiesel calculator with your displacement hull speed and cruise rpm's to guide you on prop size. Are your props really a write-off or can you send them off to a PropScan-equipped propellor shop to have their diameter and pitch accurately measured and potentially, repaired?

When performing a sea trial during a boat shopping endeavor, if full rated RPM cannot be achieved without overheating or black smoke, it is a red flag. Cruise (wrong) props have and will result in a failed sale simply becase the prospective new owner does not where the overheating cause(s) lie.

There is no upside to a cruise prop on a vessel with twin 3208s. Maybe on a true blue water single engine passagemaker the logic is better suited. Too many buy into the cruise prop approach because selecting the right props for full rated RPM is time consuming or too expensive.

motion30 said:

In fact I use the boat as a trawler running around between 8 and 10 mph so how do I pick a new crusing prop ? the current props have no markings

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In addition to looking at Boatdiesel, contact a propeller manufacturers/repair centers. These guys match props to boats all the time.

Equally important is the engine operating temp is with in the normal cruising speed range, not to low and not to high. Especially if the boat is a go fast semi displacement boat you plan on operating at lower hull speeds.

The Eagle’s, full displacement long range cruiser, prop are over sized for max cruising speed, 7 to 10 knots, and range, 2,500+ nm, at a low rpm of about 1500, so I really do not care what the WOT is, as the engine’s temp is within the recommend operating range of 180 to 190. The engines that last long are ran at recommended operating temp range with a required load on them.

Your old prop can be depitch/sized for less than 50% of new props. A driver might be able to take off/on which is also cheaper than pulling the boat.

Since it happened in Bimini you can not be far from Florida. I suggest asking at some of the prop shops what their suggestions might be. Try Frank and Jimmies in Fort Lauderdale, Lauderdale Propeller, or a shop local to where you keep the boat. Propeller sizing is an art not a science. If you pick a prop size from a chart and it is not right the prop shop will not want to help you.

I believe the larger dia. prop is better. Make sure you have the proper tip clearance distance recommended and reduce pitch to make your rated RPM when loaded and at WOT so you don't over load the engine. boatdiesel.com beside the calculator also has a good article on props with the title something like "Props move boats" written by a very respected author, Tony Athens.

On my last twin wheel boat i ask why i could not put larger wheels on it as i had room. It was explained to me that being so close to the bottom would cause a disruption and cavitation due to the blade passing so close to the bottom. Reworked the original wheels. Then i could reach wot max rated RPM and at 75% was just past hull speed. Changed my fuel burn to the better and she was more responsive docking and was a noticed drop in engine vibiration at cruise speed. I had to change the thermostats as then i was not reaching full operating temp at cruise also. Read and talk to a real prop shop who has the ability to size the prop with boat deminisions and weight with true shaft H/P.

motion30 said:

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. my gear ratio is 1.81

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I have a Chung Hwa 46 with twin DD 8.2's. The props are 26x22 four blade.
My gear ratio is way different from yours. KJ

My gear ratio is way different from yours.

A simple Rule of thumb is a prop at 1000 shaft PM will travel a theoretical K for each inch of pitch.

This has to be reduces for slip, on a trawler yacht 50% slip is a start.

FF

Honeybadger is correct. Dave Gerr's Propeller handbook has a chapter on installation considerations. It lists minimum tip clerance at different RPM ranges with high-speed planing craft being the upper number at 20% of prop diamter. With trawlers one is looking at some number less than 20% of the prop diamter but as he stated never less than 2 inches. He also said "Tugs and Trawlers frequently accept the additional vibration of propellers with only 8 to 10 percent tip clearance to gain attitional thrust at low speed from the increased propeller diamter." Maybe 15% would be a good starting place.

Re honeybadger and fighterpilot,
That may/probably explains why my larger 5 blade is not as smooth as my smaller 3 blade. Every time a blade passes by the hull it "slams" water thrown off the blade tips at the hull bottom. The drumming sound can be quite noticeable to very annoying. I'll stick w my small 3 blades.

Interesting "rule of thumb" Fred. How much do you think the slippage varies from trawler to trawler?

How much do you think the slippage varies from trawler to trawler?

Not too much as the speed of advance doesn't change that much. 6-8K ?

Rotten high speed lines at displacement speeds , poor prop location , poor installation will increase the fuel burn .

A foul bottom will increase the slippage as more water is being pushed.

after using 2 different prop calculators on line I came up with with a 23''x18'' 4 blade Now two different props come up with something different, one a24''x21'' 4 blade the other a 23''x 20'' 4 blade so I am wondering if this is what the have in stock or ?? heres are my specs 44' 44000lbs two 320hp cat turning 1.97 gears

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.

motion30 said:

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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bglad said:

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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you are correct thank you 2800 it is

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.

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

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.

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.

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.