Correct Pitch for 69 Merc 650

Cariboo Milkman

Seaman Apprentice
Joined
Aug 17, 2011
Messages
33
I'm starting to think about getting my old boat and motor ready for the ice breakup in a couple months. Last outing last year, I finally got the old girl to run reasonably well after getting carbs rebuilt and finally playing around with the mixtures long enough to get it to both idle as well as accelerate to plane. However, it took between 15 and 20 seconds to get wound up to top speed, which according to the app on my i-phone was around 4400 rpm and 25 mph. It is on an old 14.5' fibreglass runabout, has a 15" pitch prop and the lake is at about 3700 ft. elevation. I'm thinking I should probably go to a lower pitch prop, but not sure if I should go to 13" or all the way to 11". Suggestions?
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: Correct Pitch for 69 Merc 650

Prop calculator says your slip is 34% figuring a 1.64 gear ratio.
At 3700 ft your down about 9% on power Still puts you at about 55+ hp.
Rule of thumb puts that at a 10" prop.Seems extreme even allowung for your elevation.
Maybe its just age but I do think your still down on power.
I think a 12" prop should put you at about 5000 and give a better hole shot.
Some things to check.Throttle opening all the way,Timing set right,Good compression?
Water in the foam under the floor.
Does the boat respond to up trim or does it seem to lay down like its glued to the water?
Is the bottom clean and smooth?
Is the prop in excellent condition?
Is the antivent plate(just above the prop) about even with the bottom of the boat?
 

Cariboo Milkman

Seaman Apprentice
Joined
Aug 17, 2011
Messages
33
Re: Correct Pitch for 69 Merc 650

Thanks for the info. Most of the questions you asked have a positive answer.
Compression is between 115-125 on all 4.
Played with trim and it did seem to make a difference.
Bottom is reasonably good for a hull of that age.
Prop is new.
Antivent plate is aligned correctly.
Pretty sure throttle is opening and timing ok but can't check until out of storage.
The one thing that has been brought up before by someone who knew a bit of the history of the boat is the idea of water in the foam under the floor. How would I check for that, and what is the fix?
 

jestor68

Commander
Joined
Jun 12, 2012
Messages
2,308
Re: Correct Pitch for 69 Merc 650

Dropping down 4 inches to the 11 pitch will gain 600-800 rpm(5000-5200 rpm).

Since your WOT rpm range is 4800-5300 rpm, the 11 pitch sounds like a good choice.
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: Correct Pitch for 69 Merc 650

Usually wet foam would be in the rear area. You need access to the foam. Hole saw carefully a couple of spots so you can check the foam. Sometimes in the stern area the floor ends and you can check the foam there.If the floor is spongy the foam may also be wet.
If you store the boat with the bow up and get a somewhat constant flow of water to the stern could be from the foam.
The fix is to remove the floor and replace the foam.
On a glass hull if the floor is spongy there could be other problems as well.
 
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