Is my engine not doing it's job fully?

[tmh]

OK, I've been working on getting my new-to-me 1985 MT 19' bowrider set up properly. 200hp Mercruiser V-8, 2bbl; Alpha one (I think); chevy 305 small block. The boat is a deep V, old brochure sayd it weighs 2600# (I assume without engine). I took most the waterlogged foam and floor out over the winter, so it's likely a bit heavier than new, but not too much I'd say.

Anyways, I bought it with a 19P Aluminum Prop. Got about 3800 rpm at WOT and 37 mph. I just put a 17" pitch SS Rapture prop and got rpm up to 4100 and speed to 41 mph. Spped is NOT gps verified, but the marina mechanic DID check the tach and it was pretty much right on. So speed may be wrong - but it's rpm I care about. I've been told in prop forum I want 4500 rpm or so at WOT.

I'm asking this in the engine forum because I think I need to troubleshoot the engine more, as I certainly think a 200hp engine should at least be able to get higher rpm with a 17" prop. I don't most prop sites don't even have a 15" recommended prop for a 200 hp Mercruiser.

I was told by the person who rebuilt the outdrive that he thinks it's a 1.65:1 ratio. How do I check this? It obviously influences the prop calcs. The marina mechanic said my engine "seemed to run well" but I haven't had him dig into it much - he said he adjusted the carb a bit but not much else.

Anyhow, please confirm or disagree that my boat/engine shouldn't be underpropped with a 17" Pitch prop. Should I check more with the engine/drive before just going to a 15P prop to get rpms up?

Any help appreciated. I'm no mechanic so engine stuff will mostly be done by the mechanic, so it costs a lot.

 

[freddyray21]

Re: Is my engine not doing it's job fully?

4500 is about right. That much horse you should be able to get 4500 out of a 21 or 23 pitch prop in my opinion. 17 is way to low a pitch.

 

[kush3406]

Re: Is my engine not doing it's job fully?

17 is way too low im running a 23 on my 5.7 no complaints.

C.K.

 

[180shabah]

Re: Is my engine not doing it's job fully?

Before you spend any money on the engine, verify the drive ratio. Takes two people, pull the plugs out and turn the engine by hand. Count how many revolutions the engine makes for one prop rev.

 

[tmh]

Re: Is my engine not doing it's job fully?

OK, could someone explain a bit more on HOW I check the gear ratio? Exactly what do I turn by hand? Plugs out, OK, thanks.

Anything else specific I should check?

 

[Don S]

Re: Is my engine not doing it's job fully?

Put the shifter in forward, and crank the engine over (by Hand) 1 full turn to make sure the clutch in the drive is fully engaged. Put a mark on the prop hub and the drive housing and turn the crankshaft of the engine. Count how many many revolutions it takes to turn the prop one complety revolution.
It should be somewhere between 1.5 turns and 2 turns, so check carefully.

 

[tmh]

Re: Is my engine not doing it's job fully?

Don, sorry, I'm really "engine challanged" - HOW do I turn the engine over by hand? What do I turn?

So, it's one revolution of the prop, not the crankshaft, right. I'm checking revolutions of the shaft it takes for one rev of the prop.

 

[flashback]

Re: Is my engine not doing it's job fully?

Yep what he is sayin is that you will turn the engine over about 1 and half times for every complete rev of the prop. you turn it with a wrench on the bolt in the center of the crank shaft where the balancer is.. and be sure you turn it the right direction....

 

[tmh]

Re: Is my engine not doing it's job fully?

OK, I've had some work done and still the same rpm,mph numbers. Here's what's been done by a mechanic.

Compression check : 130,150x5, 140 x 2 Looks OK, 130 maybe a bit low.

Drive ratio : 1.5:1

Checked throttle - opening all the way. Carb rebuild/clean; Set timing; new Accel. pump; NGK Plugs - good spark.

I did have a hesitation issue also, these things fixed it (Acc. pump likely). The mechanic says the engine is doing all it can and it's just a bit "tired" but mostly just a heavy boat for a 19' probably. I did rip the wet foam out of the boat when I re-did the floor, so it's not full of heavy wet foam/water.

So, I'm leaning towards accepting this and going to a 15P prop from the current 17". I was sure it should AT LEAST be able to swing the 17P but facts are facts and it's not doing it.

Anyone think of something else that maybe the mechanic (great reputation, honest guy) might be overlooking?

Thanks as always for the help.

 

[Don S]

Re: Is my engine not doing it's job fully?

To check item #3 in the list above, thry this
http://forums.iboats.com/showthread.php?t=167561

 

[JasonB]

Re: Is my engine not doing it's job fully?

For comparison, I have a 19' deep v that weighs 2400 by the book with a 302 making 200-215hp through a 1.5:1 mercruiser. I get about 4100-4200rpm and 46mph. You have something going on.

 

[rad1026]

Re: Is my engine not doing it's job fully?

No one has asked you where you are located??? What is the altitude you are running the boat at. I'm at about 3500 feet and my 21' chaparral with a 5.7L and a 1.62:1. I run a 17 or a 19 depending on what lake I am at. If he is running at any kind of altitude his outdrive ratio may need to be taken in to consideration.

 

[tmh]

Re: Is my engine not doing it's job fully?

Jason, what prop do you have on it? pitch, dia., material? Thx.

Well, I guess there are still a few things on Don's list I can check. For one, I have bottom paint (albletive ??? - the stuff the slowly comes off) and it was last done maybe 2-3 years ago so it's pretty worn down. However, I started the season with no growth on it so i assumed the bottom paint being old wasn't slowing me down - but maybe it does a bit. Anyone know if this could be a big factor?

I'll ask the mechanic about the other thing on the list to see if he chacked them also.

Thanks for all the help.

 

[JasonB]

Re: Is my engine not doing it's job fully?

My primary prop is a 14 1/4 x 19 that has extra cup. I also have a 14 1/4 x 21 Michigan with regular cup that I can turn to the same RPM, but get 47-49mph.

 

[tmh]

Re: Is my engine not doing it's job fully?

OK, thanks. I'm way short of your power for some reason, but maybe that's all I'm going to get.

Altitude - I know this is a factor, I'm in Rochester, NY which I assume isn't any high enough altitude to make much difference. Good idea though.

I may put a 15P prop on this weekend to see what I get. It will help hole shot, which isn't that bad now, I may not lose anything on top end if the RPM make up for the drop in pitch. Even if I do lose a mph or 2, most important is letting the engine run in the proper range so it stays happy!

 

[tmh]

Re: Is my engine not doing it's job fully?

Well, I talked to the guy who rebuilt the drive unit and he says he put a 1.65:1 ratio gears set on. Also, I used gps to confirm speed and I am maxed at 39 mph, trimmed out. If my tach shows about 4000 rpm that doesn't really add up on the prop calc I use.

IF it is actually a 1.65:1, and I assume 9% slip w/ 17" pitch at 39 mph then my rpm must be in the 4400 range. And that 9% slip is pretty generous for this heavy boat.

I THOUGHT the mechanic who did the tune-up had verified the gear ratio manually, but turns out he didn't. I'll make sure to do it next time I have it out of the water.

So, I may not be as short on power as i thought. However, I would assume I should still be able to get a 19" prop into the 4400 range with a 200 hp engine. That would put me around 43-44 mph which sounds about right.

I'll be checking some fuel line "stuff" tomorrow - pick-up hose, water seperator/filter.

Thanks to all who have helped.

 

[Bry21317]

Re: Is my engine not doing it's job fully?

I have a heavy boat also (NADA States 2900lbs) that is a 94 Crownline 19 foot 6 inches and has a beam of 8'6". I also have the Mercruiser 305 with the Rochester 2 bbl Carb on it. Currently I have a 21 Pitch Aluminum Michigan Wheel prop on it and with the boat loaded and me and the wife on it, I got it up to 5200 rpms, to high I know, and 51mph. So I have ordered a new prop, same pitch just has cup on it, to see if it will bring me down closer to the 4800 rpms.

You should get more out of that motor for sure. I also have the 1.65 Gears on the Alpha 1 lower unit. So we are pretty close to the same weight, engine and gearing.

The one thing I did see was that one Cylinder that had the 130 for the Compression, that was low compared to the others. You might want to try spraying some Deep Creap, made by Sea Foam into the carb, to decarbonize the engine. I would do this for sure, and then see if that will bring your RPMS and compression up.

I am surprised that none mentioned this on here yet. You can get Deep Creep from Autozone, its an aerosol can and works awesome, just watch the smoke cloud after you shut the engine down and allow it to sit. Then you can fire it up and run it. Sometimes you might need to pull the plugs out and clean them up or replace them once you have done this. It should not hurt, but can always help with the engines.

Thanks,
Bryan

 

[Bry21317]

Re: Is my engine not doing it's job fully?

I would also say that you said your Bottom paint was chipping off or worn down, and this could be causing alot of extra drag. You might also want to take care of this before trying for a new prop.

My bottom is all gel coat, but perfectly smooth and waxed also. It will scoot on the water.

Just trying to help you get some ideas of what your should be doing.

Bryan

 

[paulrfrancisco]

Re: Is my engine not doing it's job fully?

A couple more thoughts..
If it is trailerable... Take it to a place where you can drive it on a scale, un hook it from the vehicle... Then later with just the trailer do the same... figure the actual weight of the boat...

Also... what the motor ever rebuilt... maybe they put an automotive cam in that doesn't produce the torque or HP that the moter is really supposed to be?

 

[qystan]

Re: Is my engine not doing it's job fully?

If its 1.65:1, you should be able to reach 4400-4800 or overspeed with no problems. This lighter ratio presents a lighter load to the engine for the same prop size.

Your problems are elsewhere. If compression is good, its likely fuel related. Another possibility is a loose/worn timing chain that will mess up the inlet/exhaust sequence - not sure how much power would be lost for such a situation.