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motion30 09-10-2012 05:06 PM

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 :eek:

timjet 09-10-2012 05:35 PM

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.

motion30 09-10-2012 06:58 PM

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

timjet 09-11-2012 04:47 AM

Quote:

Originally Posted by motion30 (Post 102723)
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

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.

FF 09-11-2012 05:21 AM

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

motion30 09-11-2012 07:29 AM

Quote:

Originally Posted by FF (Post 102760)
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

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

sunchaser 09-11-2012 07:54 AM

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.

Aquabelle 09-11-2012 08:03 AM

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?

sunchaser 09-11-2012 09:19 AM

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.

Larry M 09-11-2012 10:50 AM

Quote:

Originally Posted by motion30 (Post 102780)
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

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

Phil Fill 09-11-2012 11:25 AM

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.

yachtbrokerguy 09-11-2012 11:43 AM

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.

Fighterpilot 09-11-2012 10:14 PM

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.

honeybadger 09-11-2012 11:19 PM

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.

KJ 09-12-2012 01:00 AM

Quote:

Originally Posted by motion30 (Post 102706)
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

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

FF 09-12-2012 05:30 AM

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

Fighterpilot 09-12-2012 07:34 AM

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.

Nomad Willy 09-12-2012 10:37 AM

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?

FF 09-13-2012 05:59 AM

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.

motion30 09-14-2012 03:57 PM

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


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