Old Merc 650 Prop Question

[bryanj23]

I'm pretty new to boat ownership. My father in law gave a boat to my wife and I before he passed away a couple years ago. It's a 15' Crestliner Muskie 550 tri-hull fiberglass boat from the 70s. On the back is a Mercury 650 four cylinder (not the "Thunderbolt" version) from the late 60s. It's a 65 hp with a 13" 15 pitch prop that is 15 spline. I have the motor set all the way at the top for pulling the tube and usually down one notch if I'm just out running around.

We use the boat primarily as a pleasure craft trolling up the river, messing around on the lake and mostly pulling our three kids around on a tube. It used to work alright when I had a vented prop (the name eludes me, but I bought it on here) but not the greatest. Anyway, last spring I took it out once and everything was fine, the next time out the lower unit cracked (still not sure what happened there). I took it to a local shop and they replaced the lower unit with a used one and "lost" my prop. Now I'm back to my stock prop that has a really hard time getting the boat on plane with the kids on the tube (they are all quite small, our oldest just turned 13). Without the tube, according to GPS I can manage 31 mph with 16 gal of gas in the boat by myself and no extra gear (I'm about 200 lbs if it matters). I have no idea what my rpm's are as I have no tach for the motor.

I'd really like to get the boat on plane quicker and as long as I can manage around 30 mph I'm good. I've been thinking of switching to a 4 blade prop but am wondering if I should keep the same pitch or go down to a 13. I've been reading a lot and it seems there are times to keep the pitch and times to go down. I can't split the road and do 14 pitch as I can't find one!

Is it possible to determine this without knowing the RPMs? Or do I first have to invest in a tach and figure out how to get that on?

If you made it all the way through this, thank you! If you can help me, thank you even more!!

Bryan

 

[jimmbo]

Re: Old Merc 650 Prop Question

Here is a mercury prop chart from 1967
http://rides.webshots.com/photo/2139092110101354590rIzIGP

 

[bryanj23]

Re: Old Merc 650 Prop Question

Good info to have! Thanks! It looks like my current prop is even a bit off.

 

[steelespike]

Re: Old Merc 650 Prop Question

Hole shot with the motor all the way up(prop away from the boat)would usually be slower but might be faster once up.
Guessing the slip at 10% to do 31 your rpm would be about 4800. the motor is rated for 5300.
Guessing slip is a gamble so a tach would be really helpful.
An induction tach like a Tinytach will work with any motor just specify a multi cylinder.
Be sure to read all the info at the site before ordering.
You might check your setup.The anti vent plate(just above the prop) should be about even with the bottom of the transom
maybe as much as an inch above.If the motor needs to be a couple of inches higher you might gain a coup[le of mph and a whisker better hole shot.
On that style motor you can just put a stick of wood on top of the transom while you tighten the clamps.
Just be sure clamp have a good grip and transom is in good shape.Raise the motor about 1/4 to 1/2" at a time.
Carefully block the wheels on the trailer raise the tongue until you can rest the skeg on a block of wood. Loosen the clamps
just enough so you can move the motor ,continue to raise the tongue carefully as you watch the position of the motor.
The more level the boat is the smoother it should go.Be careful moving just a little at a time.
The motor may try to slip left or right so go easy.

 

[bryanj23]

Re: Old Merc 650 Prop Question

Wow! Learning a lot here! I greatly appreciate the input. I'll look at where the cavitation plate is when I get home and if it needs to be adjusted I'll try that first. I guess I figured at some point I'd have to get a tach on it and see what was what. I was kind of hoping to not stick a ton of money into this. I just had to replace the lower unit and spent more than I wanted to. I was thinking of getting a new motor next year. Something like an Evinrude 75 hp E-Tech. But I really want it to run well and have fun with the kids until that happens!

 

[bryanj23]

Re: Old Merc 650 Prop Question

So plugging in those last numbers on some websites, the suggestions seem to be to get more prop surface area with a lower pitch. Piranha Props suggest a three blade 14" prop with a 13 pitch. Or I could do a Solas of 4 blades 13" diameter and a 13 pitch. I'm sure there are advantages to each. But in the end would there be a huge difference?

 

[steelespike]

Re: Old Merc 650 Prop Question

I wouldn't be selecting any props based on my reverse calculation for your rpm.
You need to use some sort of tach to get an actual rpm.An optical tach is probably reasonable but difficult to use in a boat on the water.
Induction tachs are easier to use and I think priced at 30 to 50 dollars.
They can be hand held or installed A properly hooked up tach can really pin down your needs and with gps speed can tell if your setup is working as it should.
Looking at the props listed here at boats for 4 cylinder 650s a 15" pitch seems ok for a 15 footer perhaps a tad on the low side
when going to 13" pitch seems really low.
The list here lists props up to 26"
Another thought the boat being a 1970s may have absorbed water in the foam under the floor
adding a couple of hundred pounds right near the stern causing a slow hole shot.
If you raise the bow on the trailer you might get a constant trickle of water to the stern.
Otherwise maybe you can find some exposed foam to dig into to get a feel if it seems wet.

 

[bryanj23]

Re: Old Merc 650 Prop Question

And what steps need to be taken if that's the case? Would I have to dig out all the foam and replace it?

 

[steelespike]

Re: Old Merc 650 Prop Question

If its saturated you have to remove it and will have to pull up the floor to do it.
If the floor is soft anywhere suspect the foam.
You will need to replace as it holds the boat up if it gets swamped.