WOT RPM only 4000; my next move?

[tmh]

My "new" boat is a 1985 Mark Twain 19' Bowrider. about 2500 lbs unloaded with typical load around 700-800 lbs.

I have finally had a chance to get more info so I'm hoping for some guidance on prop selection. I have no problem trying a few different configurations but I want a plan of action before jumping in.

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

Currently has an Aluminum 19" 3-blade. Max WOT I get is about 4000 at 40mph. Obviously I need to lower the pitch to get into the "range".

I do mostly family boating, tubing, etc. I plan on getting Smart Tabs put on soon also. Hole shot inportant but not at a total sacrifice of top end as my tubers and skiers are mostly lighter kids.

I may get my hands on two Piranha props; a 16" and an 18" to try out for free (buddy of mine). Both are four blade. Should testing those two tell me a lot of what is needed long term? I'd be glad if one of them did the trick, but based on what i've learned here, the 18" 4-blade still won't give me high enough WOT RPMS and the 16" may be so little pitch.

Am I going in the right direction at least?

Thanks.

 

[Firestar]

Re: WOT RPM only 4000; my next move?

Sounds like you are to me. Nice you get to test those props.

 

[Texasmark]

Re: WOT RPM only 4000; my next move?

Is this the same thread where you (was it you) talking about this boat sitting in the water for years. If so, like someone else mentioned, clean up your hull before you spend a dime on props. Excessive hull drag, caused by marine growth, can suck the life out of your setup.

Mark

 

[John_S]

Re: WOT RPM only 4000; my next move?

Do the other checks others have suggested. While the testing of the props may prove interesting, you might be compensating for another issue. The smart tabs might add 100 rpms at WOT. Install them after the other checks, and fine tune prop with the tabs.

 

[tmh]

Re: WOT RPM only 4000; my next move?

Sounds like a good plan. Just what I'll do. When I pull the boat out in a week or 2 we'll clean up the hull, etc. then put the tabs on and make sure the engine is up to snuff. THEN we'll take it out for some prop/WOT testing.

I'll resurrect this thread at that time if i still need help, but I think I "get" all the basics now. If the tabs add 100 RPM and a hull cleaning chips in a tiny bit, with proper weight distribution we may find the current 19" Alum prop isn't too far from the recommended range.

Thanks to all for the help.

 

[Texasmark]

Re: WOT RPM only 4000; my next move?

Back when that boat and the Tri-Sonic 19' trihulls were new, I really wanted one. Seemed to be the perfect boat for a large family (4 kiddo's) and windy Texas lakes......but the old pocket book couldn't stretch that far.

Mark

 

[Chris1956]

Re: WOT RPM only 4000; my next move?

TMH, I run a '88 19' SeaRay Bowrider a bit heavier than yours, with a 205HP V-6 / Alpha-I. I normally use a 21" pitch SS prop and can get 4500RPM at about 42MPH. Your boat should run a bit faster than mine with that V-8. I am surprized you are not running a 21"pitch prop.

However, the gear ratios may be a bit different between our outdrives. You need to figure out the gear ratio of your outdrive, and use the Prop calculator to help you dial it in.

 

[walleyehed]

Re: WOT RPM only 4000; my next move?

tmh, your gear ratio should be 1.62:1, and the RPM is rated at 4400-4800.
Chris, your ratio should be 1.81:1, so you should be able to use the higher pitch.

 

[tmh]

Re: WOT RPM only 4000; my next move?

walleyehed, Ijust want to make sure of something - the engine says 4200-4600 and you say 4400-4800? I saw in a different thread someone (you?) said they have updated some of these figures from the original specs. Is mine one of those?

Not that the difference is that great, but just curious.

Also, do you work for a prop company or some sort of boat involved profession? If so, what company? I always like to give my business to folks who help me solve problems/questions. If you can't answer this sort of question in these forums, please PM me.

Thanks.

 

[walleyehed]

Re: WOT RPM only 4000; my next move?

Alot of the Mercruiser engines Max RPM has been relisted as higher on all that I've found. I'm guessing this is for the same reason we've found it best to run a bit higher with the outboards, except new....the fuel quality is just not as good as it was even 5 years ago.
Another angle is if they prop for slightly higher RPM, the bottom and mid-range performance will improve.
My family has been in the Marine business since 1971 and although I went and pursued my aircraft maintenance certification, became an FAA authorized inspector, then chose to specialize in turbine engines, I've always stayed (as much as I could) in touch with boating. I now do mostly powerhead O/H's and basic maintenance with a customer base. I also test propellers for certain manufacturers, and that has become my true love in boating...finding the right combination of variables to get the most from any rig.
I own my own business now, as the family has recently past it on to me and I am centering more around the Marine industry than what the business was in the past.
If your engine is a 5.7L of any style except MPI, the range is 4400-4800...the MPI is 4600-5000.

 

[Texasmark]

Re: WOT RPM only 4000; my next move?

Hey Ken, did I notice another star on your board?

Mark

 

[walleyehed]

Re: WOT RPM only 4000; my next move?

Yes, I made another...d

 

[Crownie2]

Re: WOT RPM only 4000; my next move?

Yes, I made another...d

Always good to be involved in "Vice" :devil:

BTW - tmh has a 5.0...

Bob

 

[walleyehed]

Re: WOT RPM only 4000; my next move?

Yea, and as I said, the gear ratio is 1.62:1 and 4400-4800RPM range...according to the Merc book.

 

[tmh]

Re: WOT RPM only 4000; my next move?

Update. Please advise. btw, with the 19P 3-blade Alum it was last showing about 3800RPM (normal load) and a 19P 4-blade Piranha showed only about 3600.

I have yet to get the boat pulled out of the water. I may not get a chance to get it out, clean he scum off then re-test props this year. I REALLY am dieing to get this figured out, but looks like I may need to wait until next season. Darn!!!

Anyways, just curious if anyone cares to give a general estimate as to how much (WOT RPM) the following possible issues may be contributing to my over-propped condition. RPM at WOT change guesses please.

1. Scum on hull/drive. Hard to say how much, but I think the bottom paint is not new and it feels slimy all below the water line. The drive is coated in "gunk" gray, somewhat slimy also. How much effect ?(range) guess OK.

2. Wet floatation foam. Not sure of this, but I'll find out when I repair the floor this winter. ASSUMING about 2-250 lbs of water trapped (as others have reported on boats this age) how much might that effect WOT (like one large adult sitting near the back, I guess.

3. Adding Smart Tabs. The boat is fairly stern-heavy so how much might Smart Tabs help WOT RPM.

I know, more info needed....but I'm just looking for guesses from experienced boat folks. I'll be getting a few props off eBay to test next year and I'd like to have a basic starting point, which i doubt I have given these variables.

Thanks, prop guys!

 

[tmh]

Re: WOT RPM only 4000; my next move?

what? you guys can't even help me out with a guess on these issues? Don't worry, you won't get a grade on this assignment!

Others must have had some of these issues and seen how their boat performed when corrected. I KNOW their are many variables, but just throw out some ROM ranges or something.

 

[Crownie2]

Re: WOT RPM only 4000; my next move?

Well, guesses like this could have such a wide range and be so rough that they might not be helpful.

OK here are some WAG's - but don't hold me to them...

1) 100 - 300 rpm

2) 100 - 300 rpm

3) 100 - 200 rpm

Bob

 

[tmh]

Re: WOT RPM only 4000; my next move?

Thanks, Crownie! ....but if you're wrong I'm sueing you for the cost of a wasted SS Prop!!!!!!!!!!!! j/k

Anyone care to weigh in on the Smart Tabs effect they've seen on WOT RPM and/or MPH?

The way I see it, if I can get 5-600 more RPM with the boat in tip-top shape, then I'm OK with the props I have now (19P Alum 3-blade and 16"/18" Piranha 4-blade). I'd likely use the 16" Piranha blades for water sports, especially my beloved parasailing, and the other ones for general cruising.

Thanks again.

 

[Texasmark]

Re: WOT RPM only 4000; my next move?

Slime is slick so it may not be that much of a problem. Barnacles are for sure and in extreme cases could knock 40% (SWAG) off your top end.

Mark

 

[Tail_Gunner]

Re: WOT RPM only 4000; my next move?

Slime is slick so it may not be that much of a problem. Barnacles are for sure and in extreme cases could knock 40% (SWAG) off your top end.

Mark

A few months back i installed smart tab's on a 17' starcraft 3.0 I/O. I had limited use due a minor oversight, however they do "lift" the stern up out of the water rather well. Having my first two I/O's and comparing them to O/B's ive had they are a bit A@# heavy and the tab's really help off set that handicap..........a $100.00 well spent in my corner

I'm quite sure that they will also improve the ride quality on a choppy surface,even on plane they carry the boat higher (less wake was evident) allowing more of the v to cut a chop..wave etc