Pre Alpha Prop?

MikeySpeedR

Cadet
Joined
Jul 19, 2020
Messages
13
Hey all, so I’m in need of a new propeller (ran aground in a low tide, dinged it up pretty good). Running into a snag though...

Every prop/hub kit I’m finding it for post 1983 alpha’s/bravo’s. I have a ‘77 model (serial # 4792163).
Boat is a ‘77 Sea Ray SRV-240 with a Mercruiser 250.

Current prop is a 15 3/4 x 17, but it's not right for this boat. While it's got good top speed (33mph at 3200rpm, will not rpm anymore), it's slow to plane and will not reach WOT rpm (valve cover says 4500 iirc).

Anyone have idea’s?
 

alldodge

Supreme Mariner
Joined
Mar 8, 2009
Messages
34,300
Running that low rpm at wot I'm wondering if either the motor has low compression, or the drive ratio has been changed and its to high.
 

MikeySpeedR

Cadet
Joined
Jul 19, 2020
Messages
13
Running that low rpm at wot I'm wondering if either the motor has low compression, or the drive ratio has been changed and its to high.
Motor is fresh. Makes great oil pressure and on land I can rev easily to 4500.

According to the serial on the drive it’s correct for the year.
 

alldodge

Supreme Mariner
Joined
Mar 8, 2009
Messages
34,300
The drive is the right type but unless you bought it new, the ratio could have been changed.

Reving in neutral doesn't mean anything, there is no load

Can either remove drive and count the drive and prop shaft rotations
or
Bypass the neutral safety switch and disconnect power going to the coil.
Put in Forward
Connect water to drive
Mark the crankshaft pulley and the prop
Have someone crank the motor and count 10 revolutions
Have another count the prop revolutions
 

QBhoy

Vice Admiral
Joined
Mar 10, 2016
Messages
5,377
Hey all, so I’m in need of a new propeller (ran aground in a low tide, dinged it up pretty good). Running into a snag though...

Every prop/hub kit I’m finding it for post 1983 alpha’s/bravo’s. I have a ‘77 model (serial # 4792163).
Boat is a ‘77 Sea Ray SRV-240 with a Mercruiser 250.

Current prop is a 15 3/4 x 17, but it's not right for this boat. While it's got good top speed (33mph at 3200rpm, will not rpm anymore), it's slow to plane and will not reach WOT rpm (valve cover says 4500 iirc).

Anyone have idea’s?
Hi. I’d say your tachometer is way off. 33 mph would be about right for speed. But I’d say there is little to no chance it’s doing that at only 3200 rpm. Usually those rpms would be about minimum planing speed for a set up like that. Around 20ish mph. If for some crazy reason the tachometer and speed were actually as reported…it would mean it would have such a prop pitch and or drive ratio…that would surely mean the boat wouldn’t get close to planing at all. If you get me.
 

MikeySpeedR

Cadet
Joined
Jul 19, 2020
Messages
13
Hi. I’d say your tachometer is way off. 33 mph would be about right for speed. But I’d say there is little to no chance it’s doing that at only 3200 rpm. Usually those rpms would be about minimum planing speed for a set up like that. Around 20ish mph. If for some crazy reason the tachometer and speed were actually as reported…it would mean it would have such a prop pitch and or drive ratio…that would surely mean the boat wouldn’t get close to planing at all. If you get me.
I do. I’ll have to check the tach signal though. It doesn’t sound like 4500 rpm though. It could be off only a few hundred if that tach was wrong. Instead of 1000.

I am still searching for a correct prop though.
 

Scott06

Commander
Joined
Apr 20, 2014
Messages
2,102
I do. I’ll have to check the tach signal though. It doesn’t sound like 4500 rpm though. It could be off only a few hundred if that tach was wrong. Instead of 1000.

I am still searching for a correct prop though.
Unless someone swapped the wrong ratio drive on it, verify the tach first before playing with props. Unless the engine needs work/tune up, the hull is water logged would expect a 17" to pull way more rpm than 3200. What is the mantenace hsitory on the engine- I know you said compression is good, what about fuel system and ignition components? Reving under load doesnt take much power so that isnt an indicator of good running condition.
 

Scott Danforth

Grumpy old guy who plays with boats
Joined
Jul 23, 2011
Messages
36,648
what are the compression numbers? post them
what is the fuel pressure going to the carb?
what is timing?
what do your plugs look like?

Motor is fresh. Makes great oil pressure and on land I can rev easily to 4500.
motor with bad compression running on 1 cylinder will rev to 4500 RPM in neutral. that means nothing

define "Motor is fresh"
 
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