1979 70HP Merc running 13p prop 16.6 foot Starcraft Deep V.

[MassillonBuckeye]

It's a pretty heavy boat with front and rear casting decks. NADA says 1300LBS and thats without gear? WOT at the moment seems to be around 4600 RPM. Top end is around 31 MPH. Does this seem right? I'm going to have to drop down to a 9p prop to get into the correct WOT RPM range? Previous owner has a hydrofoil installed. I porpoise badly when I take it off. Holeshot seems ok but I don't have much to compare to.

Thoughts?

 

[jestor68]

Re: 1979 70HP Merc running 13p prop 16.6 foot Starcraft Deep V.

Since your WOT range is 4800-5200, you're not that far off.

All you have to do is drop 2 inches of pitch (300-400 rpm) and you'll be set.

Check the trim angle of the motor to correct a porpoising issue. Sometimes the difference is just a tad of down trim.

 

[MassillonBuckeye]

Re: 1979 70HP Merc running 13p prop 16.6 foot Starcraft Deep V.

Since your WOT range is 4800-5200, you're not that far off.

All you have to do is drop 2 inches of pitch (300-400 rpm) and you'll be set.

Check the trim angle of the motor to correct a porpoising issue. Sometimes the difference is just a tad of down trim.

I'm all the way in. If I trim out at all while on a plane I porpoise. Without the hydrofoil, it was all porpoise, all the time. Thrust pin was in lowest position.

 

[steelespike]

Re: 1979 70HP Merc running 13p prop 16.6 foot Starcraft Deep V.

Is this a 3 cylinder or a 6 cylinder?My info indicates both with different gear ratios.
A 6 would probably run a 13" prop 4600 indicates about 6% slip possible but unlikely.
10% slip would indicate about 4750,more likely.
We need to get gps speed accurately stated.
Is the transom straight vertical or does it have about a 15 degree angle?

 

[jestor68]

Re: 1979 70HP Merc running 13p prop 16.6 foot Starcraft Deep V.

The 1979 Mercury 70 hp is a in line 3 cylinder motor. The only 1979 in line 6 cylinder motors were the 90, 115, and 150.

The last in line 6 cylinder 70 hp Mercury was made in 1961.

 

[jestor68]

Re: 1979 70HP Merc running 13p prop 16.6 foot Starcraft Deep V.

I'm all the way in. If I trim out at all while on a plane I porpoise. Without the hydrofoil, it was all porpoise, all the time. Thrust pin was in lowest position.

Well, that's one of the listed issues that a hydrofoil is supposed to help eliminate.

 

[steelespike]

Re: 1979 70HP Merc running 13p prop 16.6 foot Starcraft Deep V.

The 1979 Mercury 70 hp is a in line 3 cylinder motor. The only 1979 in line 6 cylinder motors were the 90, 115, and 150.

The last in line 6 cylinder 70 hp Mercury was made in 1961.

OK , According to the same source the 3 cylinder has a 2.30 ratio.13" Can't do 31 @ 4600
I suspect the tach is off perhaps set wrong.

 

[MassillonBuckeye]

Re: 1979 70HP Merc running 13p prop 16.6 foot Starcraft Deep V.

OK , According to the same source the 3 cylinder has a 2.30 ratio.13" Can't do 31 @ 4600
I suspect the tach is off perhaps set wrong.

It's a 3 cyl.
Broken tach could be an option. It does not sit on zero when motor off. Gah. It's an Airguide and I like the style, I was hoping not to have to replace it. GPS speed is confirmed at 31 MPH loaded, on a plane at WOT. Transom has an angle yes.

Not sure how I could test the tach beside buying a new one.

 

[ajgraz]

Re: 1979 70HP Merc running 13p prop 16.6 foot Starcraft Deep V.

Beg or borrow an optical tach to compare rpm at the flywheel vs the tach on the dash. Also see if the dash tach has an adjustable pole setting on the back, it could be on the wrong setting; IIRC you want the 6 pole setting for that motor. But I don't think that's your problem...

The gear ratio on your Merc 70 1979 is 1.64:1, and your WOT range should be 5000-5500. At 31mph at 4600 rpm with a 13p prop and a 1.64:1 gearcase ratio, you got about 10% slip, certainly believeable.

I see three options for you to get your WOT rpm into the right range:
1) Install a hydraulic T&T...yeah, I wouldn't either
2) Go down to an 11p prop...probably your place to start
3) Raise the motor an inch or two...would require fill old holes and re-drill/re-mount on transom; where is your anti-cavitation plate versus the hull bottom right now? I bet you got too much leg in the water, which is probably why the whaletail was added.

Couple more questions:
Does the motor feel like it's lugging?
Does the motor routinely wet-foul the plugs?
(Want to be you're in good mechanical shape first)

 

[Georgesalmon]

Re: 1979 70HP Merc running 13p prop 16.6 foot Starcraft Deep V.

FYI on the tach. It will likely NOT sit on 0 when key is off. It does need to go to 0 when you turn the key on though.

 

[MassillonBuckeye]

Re: 1979 70HP Merc running 13p prop 16.6 foot Starcraft Deep V.

FYI on the tach. It will likely NOT sit on 0 when key is off. It does need to go to 0 when you turn the key on though.

It used to. Then after or during replacing a rectifier and coil, it decided it wanted to start out at about 175 RPM key on or off. So I subtract 175 from my values I've given.
Tach at rest, no key:

Tach at idle:

Heres a pic of the cav plate vs hull bottom.

I'll double check when I get home, but I'm about 100% positive that was trimmed all the way down. Not the best pic. I'll take a better one.

No lugging. Plugs do get a bit oily, but I troll with that motor. So a lot of idling.

 

[phillnjack2]

Re: 1979 70HP Merc running 13p prop 16.6 foot Starcraft Deep V.

if that is all the way down then you need to get some transom spacers for the boat.
the spacers are say about 1 inch at the top and about1/4 inch at the bottom, from this pic your trimmed way out too far.
those 3 cylinder mercs from the 70's were nice engines, easy to work on with the clamshell lids.


phill...

 

[MassillonBuckeye]

Re: 1979 70HP Merc running 13p prop 16.6 foot Starcraft Deep V.

if that is all the way down then you need to get some transom spacers for the boat.
the spacers are say about 1 inch at the top and about1/4 inch at the bottom, from this pic your trimmed way out too far.
those 3 cylinder mercs from the 70's were nice engines, easy to work on with the clamshell lids.


phill...

Had to work overtime tonight, so I'll make sure and post better pics in about 2 hrs or so.

Thanks for everyones input thus far!

 

[jestor68]

Re: 1979 70HP Merc running 13p prop 16.6 foot Starcraft Deep V.

Not certain what brand of hydrofoil that is; but it appears that you have the bolts in the wrong way.

They should go in from the bottom. That way, most of the nut and bolt end will be in the recess in the top hole, and not cause so much drag, as well as turbulence in front of the prop.

 

[MassillonBuckeye]

Re: 1979 70HP Merc running 13p prop 16.6 foot Starcraft Deep V.

Not certain what brand of hydrofoil that is; but it appears that you have the bolts in the wrong way.

They should go in from the bottom. That way, most of the nut and bolt end will be in the recess in the top hole, and not cause so much drag, as well as turbulence in front of the prop.

That makes sense. THey also had the tips dog eared down... Cant only imagine what kinda drag that creates. I cant tell if they formed em that way on purpose or they backed into something. I'd think if they backed into something, they would have broke and not form nice little uniform bends.. Or is that how these things are designed??

That above pic was actually with the thrust pin in the Second hole from bottom, so yeah, pretty good level of trim. These are with pin at in the first hole. Closest to stern.

Dog eared tips: My first inclination was to break em off..

When I first got it this spring I wanted to remove the hydrofoil asap to see how it performed without it. Looking back, I realized the thrust pin was in the first hole during that run so I could have had that much trim back. It was a pretty bad porpoise. Weight could have been distributed a bit better as well. I don't think hook is a big issue here because he hull is in really good shape for it's age. You can tell it hasn't been beat us too bad. I'm guessing I'm going to drop a couple inches of pitch, pick up some RPMs and speed? And be able to take the HF off... Thats my hope anyhow. I'm kind of curious though, will the added RPM equate into top speed or just more hull out of the water? I guess it'd almost have to make you a bit faster through the water eh? But thats all assuming my tach is correct.. Haha. So whats gonna be cheaper, test equipment or a new tach?? Anyone want to rent me someting? lol

 

[steelespike]

Re: 1979 70HP Merc running 13p prop 16.6 foot Starcraft Deep V.

A hook in the hull would pull the bow down what you need to look for is rocker .Use a straight edge 3 or 4 feet.
It is possible that being slightly under powered your at a sensitive speed.
You might check out the Turning Point Hustler It is said to perform like a SS.
Supposed to have good grip. I guess in theory as the prop turns faster it might be more effective.

 

[MassillonBuckeye]

Re: 1979 70HP Merc running 13p prop 16.6 foot Starcraft Deep V.

I took the HF back off and I'm gonna run it tomorrow with motor in lowest position and see what happens.

You guys think I need to raise the motor? More pics.

 

[steelespike]

Re: 1979 70HP Merc running 13p prop 16.6 foot Starcraft Deep V.

Here's the way I look at it;If there is no venting problem raise the motor.
Looks like there is something sticking out of the bottom on the starboard side of the keel.
Might cause some disturbance in front of the prop.
Prop looks like it could stand replacing. Sometimes a 4 blade will help dampen porpoising.

 

[MassillonBuckeye]

Re: 1979 70HP Merc running 13p prop 16.6 foot Starcraft Deep V.

Update: I ran the boat yesterday with myself, half a tank of gas and some water in the livewell and it didn't porpoise. With motor in full down, I hit 33.4 @5000 RPM or so and was able to trim out a bit and gained almost 2 MPH. My dad was with me when I first ran without the hydrofoil and I was pretty new to the boat. Hes a bigger guy probably around 250 but I thought I tried moving him around the boat but maybe not. May have just been sitting towards the rear. So it would seem I'm just about where I should be unless I add more weight. She seemed to want to porpoise a couple times after hitting a wave but she came out of it. I was pretty happy! Thanks for everyones input.