1978 Cobalt 19CD with a car motor??

[jotisbrown]

Looking for a boat to buy, came across a 78 cobalt. Clean boat, but when test driving it seemed a bit underpowered and revved a little too high at top speed. 5000 rpm and only 41 mph. When checking the motor it's a gm good wrench 305... Says the 350 that was in it froze up, and he replaced it with the 305. The prop is a 19 pitch, but the boat didn't have a lot of bottom end... Or top end.. And 41 seems slow!

Ideas or thoughts? It's a clean boat but I don't like the idea of a slow v8... Especially one that's not marinized

 

[jotisbrown]

No Title

Pics of the motor

 

[HT32BSX115]

Looking for a boat to buy, came across a 78 cobalt. Clean boat, but when test driving it seemed a bit underpowered and revved a little too high at top speed. 5000 rpm and only 41 mph. When checking the motor it's a gm good wrench 305... Says the 350 that was in it froze up, and he replaced it with the 305. The prop is a 19 pitch, but the boat didn't have a lot of bottom end... Or top end.. And 41 seems slow!

Ideas or thoughts? It's a clean boat but I don't like the idea of a slow v8... Especially one that's not marinized

Howdy,

Well, there's other reasons why the boat may not making top speed.

But if the TACH is correct, you're probably turning approx the right RPM at wide open throttle (WOT)

Is it an inboard or a sterndrive?

If it's a sterndrive, it's possible that someone replaced the drive with one of the wrong ratio. Also, a 305 (5.0L engine) would probably be "ok" for that boat although it might be a little underpowered.

If it's a "Goodwrench" engine and it has brass core plugs (AKA freeze plugs) and it has marine type starter, ALT, and distributor, it's marinized.

If it was originally built for a truck(truck/RV camshaft) ,the cam is likely close enough.

Also, if it's a sterndrive, and it has the correct drive ratio (around 1.47:1) , 5000 RPM, 41mph, indicates that something is wrong (the slip is 32% and should be around 15%)

On the other hand, if you have a 1.94:1 drive ratio, (totally wrong for a 350 V-8 in a run-a-bout type boat, ) you would only get about 40mph at 5000 with a 19p prop.

There's a couple of things you should do to deterimne what's wrong........

1. verify the speedometer (use a smart phone GPS)

2. verify the tach. (have the tach tested or use another known tach)

3. Verify the drive ratio. If it's not printed on the side, you can remove the sparkplugs and turn the engine and count 15 revolutions of the crankshaft. If you have someone count revolutions of the prop, it should 10. (That would be a 1.5:1 ratio.........you could also turn the engine exactly 1.5 turns [1turn + 180 degrees] and get exactly 1 prop turn etc...... )

If the drive is 1.94:1, (approx 2:1)you could turn the engine exactly 2 turns, and get very close to 1 prop turn.

The wrong ratio can result in less than optimum performance. You could try a 21p prop, but it might make your holeshot performance suffer.

You need to know the RPM, speed and drive ratio though to determine what if anything is wrong.

Regards,

Rick

 

[jotisbrown]

Thanks for the response! Speed was verified by my iPhone gps, I'll check that ratio though. It had a 350 in it before he swapped motors and I asked, he said the outdrive is original to the boat from 78, so I'd bet it has the drive ratio for the 350. Can't believe 5000 rpm is okay, seems so high! I asked and yes the original 350 pushed a 21 pitch prop, but he said the hole shot was a little weak so he got a 19. I'll probably go ahead and buy the boat though if that 5000 rpm is near normal. Runs like a top except the shift interrupter kills the motor when shifting into reverse.

 

[jotisbrown]

Says the drive ratio is 1.65??

 

[Bondo]

I'll probably go ahead and buy the boat though if that 5000 rpm is near normal.

Ayuh,.... Ya want the motor to be Able to run 4800 rpms at a light load, light chop, flat out Gettin' it,....
That's propped perfect,....

Runs like a top except the shift interrupter kills the motor when shifting into reverse.

That says the lower shift cable is draggin', 'n needs to be replaced,.....
Check the bellows, ya might as well do 'em all at the same time, goin' that deep,....

 

[jotisbrown]

Ayuh,.... Ya want the motor to be Able to run 4800 rpms at a light load, light chop, flat out Gettin' it,....
That's propped perfect,....


That says the lower shift cable is draggin', 'n needs to be replaced,.....
Check the bellows, ya might as well do 'em all at the same time, goin' that deep,....

Well he's got a 21 pitch so I guess I'll have to give that a try. Not wanting to break any speed records or pull 3 skiers at a time, just something to cruise on a nice day.

 

[jotisbrown]

No Title

Just before the test drive...

 

[HT32BSX115]

Just before the test drive...

Nice looking boat!

ANY Cobalt is a great looking boat!

I would try the 21. It might be ok for what you want to do.

Once you install the 21, with 2 people, full fuel, get it up on a plane and take it up to WOT. ......... trim the drive for maximum speed in light chop.

Note the (GPS) speed and WOT RPM.

Then plug those numbers into a prop-slip calculator.....any will do......I like http://www.go-fast.com/Prop_Slip_Calculator.htm

Use approx 15% slip to estimate what your numbers should be. (15% is approx correct for most run-a-bout planing type boats)

With your previous numbers, (41mph, 5000 RPM, 1.65:1 ratio and 19p) I got about 25% slip................ That indicates something is wrong (tach, ratio, prop pitch, or speedo)

If you're using GPS you have the speedo covered. I would STILL verify the ratio though. It's very possible that someone replaced//re-geared that drive in the last 38 years.
(Here's a chart for ratios. http://www.sterndrivereplacement.co...r/alpha-one-gen-1/complete-sei-106/gear-chart )

THEN use a good known tach. Yours might be way off at the upper end.

Even with a 1.65:1 drive & 19p , you should be going approx 47 mph (trimmed for max speed) at 5000RPM. If the other "stuff" s correct, high slip in a planing type boat can indicate overloading (too many people, or other load [water-logged]) OR, the drive is trimmed too high or too low.

Don't trim based on the gage, trim (in small increments) for max speed. Too high[drive-OUT] a trim setting will result in porpoising (and loss of speed) and too low [drive-IN] will cause the bow to "plow" and result in loss of speed.

 

[jotisbrown]

I definitely had problems trimming during the test run. The lake we were at was a private lake and it was tiny, more like a glorified pond. I could have probably gotten a few mph out of it had I not been limited by space and had time to play with the trim

 

[HT32BSX115]

I definitely had problems trimming during the test run. The lake we were at was a private lake and it was tiny, more like a glorified pond. I could have probably gotten a few mph out of it had I not been limited by space and had time to play with the trim

You really cannot do any meaningful performance testing until you can measure speed & RPM accurately and you know the drive ratio and prop-pitch. It also might be useful to drive that boat over a scale and see if it weighs near the expected weight or is way over weight.

 

[jobrown]

Just an update: I bought it! I don't know where the old man got the idea it was at 5000 rpm. It sits beautifully at 4500 rpm and trimmed out runs 43 mph! Cruises comfortably at 30 mph and 3500 rpm

 

[jobrown]

That's 13% slip! And with a dinged up aluminum prop at that.

 

[HT32BSX115]

Sounds like it's running ok then.........

 

[444]

Sounds like you're right in the ball park. That era of 350 is not a very powerful engine, and the 305 is a notch down from there, so all in all I think you're doing quite well.