Searay 330 EC Re-Power Yanmar

[John-Eric Buckheit]

I have a 1997 Sea Ray 330EC repowered with twin  rebuilt 2002 Yanmar 6LPA-STP 315HP. 2;1 gear and 18″x21 pitch 4 blades with cup. Listed below are the current specs: No load RPM 4200, loaded RPM’s 3600, top speed 24knts, boost is at 18 psi and I’m told on these engines 20 is tops. My issue is that the performance of my boat is far less than the performance of the sister boat set up with all the same drive gear. One variable I’m not sure of yet is the props on the sister boat. The sister boats @ 3100 is 24knts, @ 3800 is 28knts.  My numbers tell me I’m over propped but my history says that if I lower the load on the engines to get the rpm’s up I will lose more speed. I’m I thinking wrong that lessening the load to get the recommended rpm’s back up that I will lose speed? Is there a prop that I should be looking at? DO I have prop slip? seems that if I did my rpm’s would be higher. Looking for more advice. Thanks, John-Eric

Topic

[Rob Schepis]

5 year difference in boat model year can change things, weight being a big one.  Have both boats been weighed on the same lift?

The sistership has more prop diameter and more pitch and is turning more rpm’s than you?

Have the tach’s on both boats been verified ?

Your boat is overpropped and/or has a low power issue…

 

 

[John-Eric Buckheit]

Info Clarrification

Thanks for your reply Robert. 

  The things I do know are they are the same exact boat, full of fuel and water, same hp Yanmars although 5 years different, same gears, fresh bottom jobs and fresh props. The helm furniture is still off my boat and that adds up to a little over 700 lbs. making my boat even lighter. What I’m not sure of is the props on the other boat. He states 19″x23 4 blades DQX but that doesn’t jive with my 18″x21’s 4 blades so more info coming. I also figured if my engines are loaded up they would smoke but I get none at any point above an idle. I also checked my hull for moisture and have none so no extra weight. 

[pwrobert]

Even sisterships can have pretty big differences in setup. To compare two they both need to be in same condition as far as bottom and running gear cleanliness. Similar fuel/water load etc. 

But yes everything else being equal when you decrease pitch and or diameter the RPM will go up and speed will drop slightly. But the engine will not be lugging along and live longer. <br /><br />

There is no changing the prop slip. All props slip and that is built into the hull and running gear setup largely. Using math you can determine your prop/running gear efficiency. Using pitch and RPMs, transmission ratio, etc you can see how far you would go in one hour if there was no slip. Then compare that to what you actually do and see the difference. Mine looses about 25% from what would be theoretical possible. <br /><br />

With the performance differences you tell about I suspect there are different engine models. Prop changes alone will seldom if ever give you a 4kt speed increase. To get a 4-5 kt speed increase I had to repower from twin 200HP engines to twin 355 HP engines. Now I do cruise at higher speed and burn less fuel per mile.  But keeping same well running engines and playing with prop settings may give 1-2 kt difference. <br /><br />

Tony can give you his thoughts and realistic expectations. 

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