WOT RPM only 4000; my next move?

[tmh]

Re: WOT RPM only 4000; my next move?

Gunner, did you get any more WOT RPMs from the smart tabs? What about mph? Thx

 

[tmh]

Re: WOT RPM only 4000; my next move?

bump - tailgunner, any more input on the Smart Tabs effect on WOT? I'll be putting mine on this winter and I can't wait to test them out next year.

Thanks.

 

[walleyehed]

Re: WOT RPM only 4000; my next move?

I gained somewhere around 75RPM with mine, but my light-weight 18ft alum. with a 200 on the rear was, shall we say.....somewhat un-controllable without the tabs.
I think as a general rule, 100RPM gain is not out of the question at all..it depends on the hull design and how it's used.
As for growth on the hull, I've seen as high as 2000 RPM on a 23ft searay we cleaned on Texoma...the boat would not even plane out to begin with, but it was very bad under water.
OMC done a test years ago on a boat in brackish water...they ran it every 2 days after putting it in the water (I think it was, may have been every day) but the end result was a slow trend to about 1000 less RPM in a few weeks.

 

[tmh]

Re: WOT RPM only 4000; my next move?

Thanks, Kenny. Looks like I have to wait until next season to get this figured out as my boat's been pulled and winterized here in Rochester, NY. I am very confident that I'll get much more performance from my new toy thanks to the knowledge I now have from this forum.

My guess is that my current props (19' Alum 3-blade and 18' Piranha 4-blade) may well be just fine. Seeing the boat out of the water, it looked like a reasonable amount of growth was on it and the outdrive looks pretty nasty. Clean that up and also I fully expect to remove some water-logged foam when I rip a section of floor out. Add my trim tabs and I may well pick up 6-700 WOT RPM all told. we'll see next season.

Thanks again, all.

 

[Crownie2]

Re: WOT RPM only 4000; my next move?

Sounds like a plan...

Bob

 

[tmh]

Re: WOT RPM only 4000; my next move?

OK, digging this thread up from last year.....even though it will be a few months before boating season up here, I wanted to confirm the WOT specs for my engine.

When the boat goes back in it will: 1. be clean of slime,scum, 2. have Smart Tabs, 3. Have at least 200 lbs. of soaked foam/wood removed. I'm VERY interested to see how it runs with these 'fixes". May solve the WOT issue without prop change, who knows.

From the engine plate: Mercruiser MCM 200; 305; S/N 6917823; 8 cyl. WOT 4200-4600.

Walleye said the WOT of this engine is : "your gear ratio should be 1.62:1, and the RPM is rated at 4400-4800."

So, Walleye or whomever, should I "shoot for" mid-rannge at about 4600 or high end/low end of the "new" range you gave me? Thanks a lot. Can't wait to test in May (maybe late April)!

 

[kp913]

Re: WOT RPM only 4000; my next move?

I am pretty sure the RPM is 4400-4800, you just want to be at the upper end of this range. You have taken all of the steps to ensure that your hull is clean and everything is in order, have you had the prop checked to make sure there is no damage to it and that the pitch it is supposed to be is actually what it is? Even minor damage to a propeller blade can affect performance. After reading through this thread, it does appear that with the mods 19 pitch should be correct.

http://www.propellerforum.com

 

[tmh]

Re: WOT RPM only 4000; my next move?

Well, another update now that the boats back in the water. Surprisingly (and a bit dissappointing), I did not gain much at WOT with the changes to the boat (wet foam removal, hull cleaning). I am only getting about 3800 rpm and 37 mph at WOT with my 4-blade 18P piranha prop. Even assuming the Piranha isn't the best performing prop, I am still a long ways off the recommended RPM. My 19" 3-blade aluminum prop had maybe a tiny bit more at WOT, but close to the same.

I had the marina mechanic check out my engine and he said it's running fine. He adjusted something on the carb but very minor, he said. However, I am quite surprised a 200 HP engine can't spin a 19P prop on this boat. Yes, I do think I have a pretty heavy hull, maybe heavier than I think as I've never had the boat weighed. Also, who knows with a 22 yr. old engine.

At any rate, I need to get the RPM up so I just bought a used SS Rapture 17P prop on eBay, in excellent condition. The lake I have a slip in has very few danger areas for ruining props/outdrive so I'm not too worried about the SS blade hitting something and ruining the outdrive. I'll be able to test it out with the new SS prop in a week or two I hope.

What do you folks think I should see from the new prop?

 

[tmh]

Re: WOT RPM only 4000; my next move?

Any opinions on the Rapture SS prop for my boat? It just arrived and I am anxious to test it out - soon hopefully. Will it provide some bow lift? More or less RPM than a similar pitched aluminum prop? I had a 19P aluminum and a 18P 4-blade Piranha which both got me about 3800 rpm or so. I'm figuring on maybe 300 more rpm with the 17" Rapture. I hope more because I have more to go to get into the recommended range.