What size prop for a 16' aluminum boat with a 70 HP Mercery

Hey guys,
I got a '74 Starcraft 16' SuperSport with a '78 Mercury 700 (3cyl.). It came with a 13 pitch prop on it which seemed a little small. I just installed a new tach and it shows 7000 rpm at full throttle. Either the prop is much too small or the new tach isn't working right.
Can anyone tell me what size prop I should be running?
Thanks

Re: What size prop for a 16' aluminum boat with a 70 HP Mercery

Check the settings on the back of your tach. I don't think 7000 rpm is right.

A 13/14 pitch prop sounds about right for that combination.

Rpm range is 4800-5300 rpm.

Re: What size prop for a 16' aluminum boat with a 70 HP Mercery

If you add gps speed to your test it will be easier to tell if the tach is right.Some cells have a gps app.
A agree with jestor68 as 13 or 14" pitch is probably about right.

Re: What size prop for a 16' aluminum boat with a 70 HP Mercery

The Tach is for 12 pole only. There isn't any settings on it.
The prop that is on it is listed for a 50 horse merc and not for the 70's. Mercury changed the lower unit gear ratio in the later models of the 50 from 1.64:1 (which is the same as the 70) to 1.83:1
I'm wondering if someone changed the lower unit to one of those.

Re: What size prop for a 16' aluminum boat with a 70 HP Mercery

I don't have GPS or a phone capable of it.

Re: What size prop for a 16' aluminum boat with a 70 HP Mercery

12 pole is correct for Mercs, at least current ones. The closer the gear ratio, the shallower the pitch requirement to run under the same conditions of rpms and speed....as long as something like boat weight, hull shape, or what have you don't skew the results.

Mark

Re: What size prop for a 16' aluminum boat with a 70 HP Mercery

Mercerman said:

I don't have GPS or a phone capable of it.

Click to expand...

No friends or boating buddies with a gps or compatible phone?
Looked up props for 1978 50 and 70 they are the same props.
If the motor is turning 7000 with a 1.64 ratio and 12% slip the speed would be about 46 mph.
This would feel mighty fast on the water.
Its highly unlikely you have the later gear case.

Re: What size prop for a 16' aluminum boat with a 70 HP Mercery

Mercerman said:

I don't have GPS or a phone capable of it.

Click to expand...

No friends or boating buddies with a gps or compatible phone?
Looked up props for 1978 50 and 70 they are the same props.
A 13 is slightly on the high side of the size list.The higher the pitch the harder to turn.
If the motor is turning 7000 with a 1.64 ratio and 12% slip the speed would be about 46 mph.
This would feel mighty fast on the water.
Its highly unlikely you have the later gear case.
I think a motor designed to run about 5500 max would seem mighty strained at 7000 if it stayed together.

Re: What size prop for a 16' aluminum boat with a 70 HP Mercery

Mercerman said:

I don't have GPS or a phone capable of it.

Click to expand...

No friends or boating buddies with a gps or compatible phone?
Looked up props for 1978 50 and 70 they are the same props.
If the tach is accurate at 7000 You would have to increase pitch 7" to get to 5600. Props only go to 17".
If the motor is turning 7000 with a 1.64 ratio and 12% slip the speed would be about 46 mph.
this would feel mighty fast on the water.
Its highly unlikely you have the later gear case.

Re: What size prop for a 16' aluminum boat with a 70 HP Mercery

Yes the 50 & 70 use the same prop from 77-84. What it turns out to be is an older prop #48-61818a1 (Dia. 11 1/2") from a 72-76 Merc 650 (or 402-500), where a 77-84 Merc 700 (or 402-500) used a 48-73136a40 (Dia. 10 3/8") or a 48-816704A40 (Dia. 10 1/2"). These all are 13 spline 13 pitch props and should be interchangable but have different diameters. Michigan Wheel even shows these later year props fitting motors from 25 hp - 70 hp. My motor has Merc 650 cowling on it but the serial number shows it is a '78 700. It also says 70 HP on the back down on the center housing. Someone must have had a parts motor at some time and swaped a few things. I guess the question is - does it have an older lower unit on it from a 72-76 650. I don't know how to tell the difference.

Back to the tach... I guess I'll pull it out and hook it up to my other boat which has a '69 Merc 1000 inline 6 and see if it matches rpms with the tach on it. Unfortuneatly the 100 horse tach is an old style airguide, so I can't try that tach on the 70hp motor.

My newest thought/question is if there is something wrong with the alternator, could it throw the tach reading off? And show a higher reading? I would think if anything it would show a lower reading.
This is the second tach I put on this boat and the first one worked while testing in the driveway but after taking it to the lake for testing it started rotating round and round. It was a new/used tach I bought off Ebay and just assumed it was junk and I got took for $20

Re: What size prop for a 16' aluminum boat with a 70 HP Mercery

You can check your ratio by removing the plugs and turn the flywheel by hand,count the revolutions for one revolution of the prop.
You'll need to mark flywheel and prop.
As far as the tachs without the a gps speed you can't evaluate the accuracy of either tach.When calculating prop performance
you need 4 of 5 parameters to figure the 5th one.We can usually get ratio off the net,prop size off the prop and speed by gps.
The rpm is easy to get but can be inaccurate many times. Only way to confirm it is with a known good tach like a shop tach
or to compare it with the known good numbers. With the possible leg change the only known good numbers we have is your prop
size.The gps is probably the most important development in accurate prop calculations.