New to small boats.

[NovaNewfie]

Hi I've been reading posts here for the last couple of days. I recently bought a 1985 19' Sun Ray bowrider Seville 5.0. Its got a MCM 120 ser# 6854***. I've only had it out once. My question is about wot rpm. I've read that it should be 4300. With the 14x17 prop that's on it wot was around 47-4800 rpm and if the speedo is correct top speed was 28mph. Does this seem normal or correct. Boat had two guys in it around 200lbs each and a full tank of fuel 20gal. I'm a commercial fisherman and have run fishing boats all my life but know nothing about trim. These rpms were with the unit hard down. Does changing the tilt/trim affect rpm and speed. Boat came up on plane pretty quick (except for a shift interrupter switch problem) and stayed on plane down to around 3300 rpm. I've only had the boat for a week. It came with a new 15x15 prop. I don't ski and just bought the boat for cruising and taking my son fishing. I don't plan on running wot 36-3700 rpm gave me 20mph which I'll settle with. Would the larger dia. prop be better?

 

[cr2k]

Re: New to small boats.

Trim up until it porposes then trim back just until it quits. (basic trim guidelines) Yes it helps.
28 seems a little on the slow side but my 19 ft Regal with stringer and 3.8 V-6 does about that too.

 

[NovaNewfie]

Re: New to small boats.

Thanks. It seemed to run real flat at wot maybe bow down a little. My other boat is 50' long 21' wide weighs 85'000lbs and wouldn't plane if a Jet was towing it. Top speed is around 7 knots. My experience doesn't cover anything to do with performance.

 

[sethjon]

Re: New to small boats.

Hi I've been reading posts here for the last couple of days. I recently bought a 1985 19' Sun Ray bowrider Seville 5.0. Its got a MCM 120 ser# 6854***. I've only had it out once. My question is about wot rpm. I've read that it should be 4300. With the 14x17 prop that's on it wot was around 47-4800 rpm and if the speedo is correct top speed was 28mph. Does this seem normal or correct. Boat had two guys in it around 200lbs each and a full tank of fuel 20gal. I'm a commercial fisherman and have run fishing boats all my life but know nothing about trim. These rpms were with the unit hard down. Does changing the tilt/trim affect rpm and speed. Boat came up on plane pretty quick (except for a shift interrupter switch problem) and stayed on plane down to around 3300 rpm. I've only had the boat for a week. It came with a new 15x15 prop. I don't ski and just bought the boat for cruising and taking my son fishing. I don't plan on running wot 36-3700 rpm gave me 20mph which I'll settle with. Would the larger dia. prop be better?

47-4800 RPM sounds about right and that should be around 45 MPH. To know what the right prop is you need to know what the WOT actually is. If its around 4800 DO NOT go down in pitch. That will increase the RPM and thats the danger zone. I would think a 21 should be about right, maybe a 19 but I think 17 is too low a pitch. I also think that prop would be fine with 2 more people as well.

 

[NovaNewfie]

Re: New to small boats.

The other prop I have is only a 15 pitch but its also a 15 dia. If I go up in dia and drop pitch it should keep my rpm around the same? I've got to take my gps next time out and check my speedo.
This boat has never run in salt water. I bought it to use strickly in salt water. What anodes do I need? Are there anodes that go in the engine/manifolds as well?

 

[NovaNewfie]

Re: New to small boats.

My temp is staying at 125 after an hour of running so I'd say that's acceptable. What should my oil pressure be at. It was like 58/60 initially then dropped back to 40 lbs after it warmed up. I was going to add some Lucas oil treatment to it. Is that a good idea? I run it in most of my small engines.

 

[NovaNewfie]

Re: New to small boats.

Apparently I'm new to this forum as well. My last post came up twice. Is there a way to delete them?

 

[achris]

Re: New to small boats.

The other prop I have is only a 15 pitch but its also a 15 dia. If I go up in dia and drop pitch it should keep my rpm around the same? I've got to take my gps next time out and check my speedo.
This boat has never run in salt water. I bought it to use strickly in salt water. What anodes do I need? Are there anodes that go in the engine/manifolds as well?

With sterndrives the diameter (more or less) is set by the manufacturer of the prop... You buy the pitch you want, and the diameter is more or less fixed... and only changes less than an inch from a 13" pitch to a 23" pitch... Each inch of pitch change will change the engine WOT by about 200 rpm....

Some general information about your engine...

MCM120 - 2.5 litre (153 cubic inch) 4 cylinder in-line engine, made by GM.
Recommended WOT - 4200 to 4600 rpm. (if you're getting a genuine 4700 with the 17" prop... leave that one on... It 'light loads' the engine, making it run better, and it's more 'driveable'... Mine tops out on the rev limiter, 4950.... Love the way that boat handles... You could use a little trim UP, you might find the revs going higher (and the speed, sure you can handle it.... ) In that case change to a 19" pitch prop...)

Drive.
Model designation - MR (Please, please, NEVER use the term 'pre-alpha'.... there is no such thing, and people like me go bananas when we see it)...
Ratio - 1.98:1

Given the engine speed and prop pitch you gave in the first post, my prop calculator indicates something is not right... Either your tacho is reading high, or the speedo is reading low... Did you use a GPS? Also check the calibration on your tacho. Most dash tachos just aren't accurate enough, you need a 'shop' tacho...

My temp is staying at 125 after an hour of running so I'd say that's acceptable. What should my oil pressure be at. It was like 58/60 initially then dropped back to 40 lbs after it warmed up. I was going to add some Lucas oil treatment to it. Is that a good idea? I run it in most of my small engines.

Temperature sounds too low... Check there is a thermostat installed... Some idiots take them out :facepalm: I have no idea what they're thinking (or if they are even thinking!).... If the thermostat is there, check it's working properly. Easier probably to just throw a new one in... Oil pressure sounds about right.

Spec for thermostat is 143?... Oil pressure spec is 4 psi at idle (minimum) and 30-60 at 2000rpm.

And just use the normal anodes supplied by your local friendly dealer (and they're getting harder to find too... )

Apparently I'm new to this forum as well. My last post came up twice. Is there a way to delete them?

Yes, you're new... (which is why everyone is so nice to you... ... Wait until you're an old fart like me... Then you get abused up hill and down dale... )

The moderators are the only ones who can delete posts... The magic 'zap' button... Used to get used on me lots, in the bad ol' days...

Chris........

 

[NovaNewfie]

Re: New to small boats.

Thanks for the reply.
So this isn't an Alpha drive?
I didn't use a gps but I noticed my speedo unit on the transom has black tape around it. I'll assume its not working correctly.
I'll check on the thermostat.
Both my props are Solas Rubex one on the boat is 14x17 other is 15x15.
Oil pressure 4psi at an idle seems very low. Mine is at 20ish cold and dropped to 15 after an hours running.
Are there "pencil" anodes in these engines? Can you add them.
I've got to get a manual for the engine and the drive. Any good ones around? Any to avoid?

 

[achris]

Re: New to small boats.

Thanks for the reply.
So this isn't an Alpha drive?

It's virtually the same, the Alpha One was introduced the next year, but 99% of yours is the same.

I didn't use a gps but I noticed my speedo unit on the transom has black tape around it. I'll assume its not working correctly.
I'll check on the thermostat.
Both my props are Solas Rubex one on the boat is 14x17 other is 15x15.
Oil pressure 4psi at an idle seems very low.

That's a minimum reading... Anything higher is better..

Mine is at 20ish cold and dropped to 15 after an hours running.
Are there "pencil" anodes in these engines? Can you add them.

No, and no need.

I've got to get a manual for the engine and the drive. Any good ones around? Any to avoid?

Avoid seloc and clymer, they're nothing more than a knee pad, or hand towels...

Try these (they're the Merc genuine )...

Engine -> https://docs.google.com/open?id=0B1TAvwZ3TjhfUVREOHFDUGZRR1NDelg5blYybmNRdw
Drive -> https://docs.google.com/open?id=0B1TAvwZ3TjhfOVRUNEk0anBUR3lQYTkwcUh5RElUQQ

Download and SAVE... and say hello to your new 'best friends'

Chris.........

 

[IanFS]

Re: New to small boats.

Pardon me for throwing in some thoughts to this......I have a little mechanical skill now thanks to these guys, mainly Chris, but having raced small boats in my younger days, props were important.

Firstly, are you sure you have a Sun Ray and not a Sea Ray, Seville's were made by Sea Ray, haven't heard that Sun Ray used this name.

Secondly, it sounds as if your boat is prop'd for skiing, a 19' sports boat of this type would have a 19" - 23" pitch prop. depending on the engine, so a 15 or 17" pitch seems very low. As Chris has said, 1" increase will give you a 200rpm drop.

Chris will know the sterndrive ratio but assuming it is approx 1.8 and you are revving at 4800rpm thats 2594 revs at the prop, a 17" pitch will move (assuming in a solid, no slippage) 44,098" in one minute. Multiply by 60 and divide by 63,360" is 41.76mph, then calc slippage, I'm really guessing now at 20%, thats 33mph. A 19" pitch in theory should give you about 36mph.

Hope this helps a little.

 

[NovaNewfie]

Re: New to small boats.

Sorry yes the boat is a Sea Ray. My son has a Sun Ray.
The ratio is 1.98
Temp is running around 140
Had the boat out today. Noticed a few small problems.
Is the choke supposed to have a spring on the outside of the carb? Today it wouldn't start until I took the flame arrester of and closed the choke by hand.
With a through prop exhaust should there be water coming out the prop when running on muffs? Mine dosen't...
With my son and I in the boat about 340 lbs and full of fuel if I trimmed it quite a bit it would rev up to 5000rpm. With 5 adults in the boat it would only go to 3600 till it came up on plane then rev to about 4800 trimmed.
On the engine block below the exhaust manifold I have two lines from salt water between #1&2 and #3&4 cylinders. Manifold gasket? Boat was only run in fresh water until I bought it.

 

[achris]

Re: New to small boats.

Sorry yes the boat is a Sea Ray. My son has a Sun Ray.
The ratio is 1.98
Temp is running around 140

That's better.

Had the boat out today. Noticed a few small problems.
Is the choke supposed to have a spring on the outside of the carb? Today it wouldn't start until I took the flame arrester of and closed the choke by hand.

Did you give the throttle a pump first? The choke is sprung, but only inside the auto-choke module. When the engine cools down the choke doesn't close, because the idle stop screw is holding the 'fast-idle' cam (which isn't a fast idle cam, it's just where it would be if this was a car carbie.... Confused yet?) You need to open the throttle to release the idle stop screw from the cam so the choke can set itself...

With a through prop exhaust should there be water coming out the prop when running on muffs? Mine dosen't...

Most don't... Most of the water will run out of the idle relief holes, up under the drive, next to the transom, in the bottom of the gimbal housing...

With my son and I in the boat about 340 lbs and full of fuel if I trimmed it quite a bit it would rev up to 5000rpm. With 5 adults in the boat it would only go to 3600 till it came up on plane then rev to about 4800 trimmed.

Sounds like you need to use a 19" prop when you are just with one other person, and the 17" when you have a load on.. Small engines are a bit like that...

On the engine block below the exhaust manifold I have two lines from salt water between #1&2 and #3&4 cylinders. Manifold gasket? Boat was only run in fresh water until I bought it.

Arrr, that's not good news... No water runs between the head and manifold... If they are water lines, then it could mean a couple of things... Leaking head gasket (we hope), or cracked block (we hope not)... The only way you're going to find out is to remove the manifold... That's not a particularly difficult job, shouldn't take long, and it will give you an opportunity to check the condition of the exhaust elbow and the insides of the manifold.... (These are common trouble areas on older engines, and should be checked regularly)...

Chris........

 

[IanFS]

Re: New to small boats.

Here are some calculations I did earlier based on your earlier posts, I hope they help.

WOT speeds relative to Propeller Pitch
Current WOT Engine Revs PM 5000 4600 4200 3800

Prop Pitch 17 19 21 23

Drive Ratio 1.98 1.98 1.98 1.98

Prop Revs PM 2525 2323 2121 1919

Distance in Inches per minute with prop in solid 42929 44141 44545 44141

Calibrated to MPH 41 42 42 42

Slippage at 80% MPH 33 33 34 33

You can see that your earlier 28mph was not far out from the calculations.....only 5mph...not much eh?
In short it all depends on your prop design when it comes to high speeds. The reason the diameter is constructed by the manufacturer is because the greater the diameter the greater the tip speed. At low revs this is not important but at high revs it is as cavitation may be a factor.

In commercial fishing boats the displacement is the key to speed and economy, this is the water moved aside and underneath by the boat moving forward. With planing craft the bow wave moves from the bow to further aft underneath the boat near the stern, the boat sits on the bow wave moving faster. Consequently the propeller will move faster through the water and also rev faster.

I hope this all helps. The figures are only a guide......why? Because propellers depend a lot on their design and the hulll for which they are designed, therefore just because they are supposed to do a certain job doesn't mean they will. Consequently, going up 2" doesn't always mean a 200rpm reduction. I cannot give you the mechanics on paper so you have to try different props, but the fact that you have a Solas Prop with a Rubex Hub Kit says that your prop is fairly nice.