I need a different prop for my 20 hp Merc on my 14' aluminum boat...

[pretty_amateur]

Hi,

I have a 1970's (not sure exactly) 20 hp merc (short shaft) on a 14-foot aluminum boat (probably just as old) that weighs about 150 lbs. The back actually seems like it may break off.

Anyways... My question:

The engine at full throttle seems to be reving too high. (I say "seems" because I can tell by the noise it makes - sounds similar to when a prop comes out of water.) So I want to fix this problem by putting a bigger prop on it... I just don't know what size to get. I also do not know what I have right now, but I would guess it's just a standard size.

[The engine is tiller-driven and it's basically just that on an old 14 foot, small and light, aluminum boat. I don't want the engine to rev too high and then overheat. I don't think the water-pump has been changed in a while too so that should probably happen soon, but I see a nice stream coming out of it still, so that is reassuring to me.]

***Prop size/pitch/type recommendations please and thank you!

 

[John_S]

Re: I need a different prop for my 20 hp Merc on my 14' aluminum boat...

Welcome to iBoats.

Get a tach like: http://trailtech.net/tto_tach-hour.html

Carefully examine the prop for merc part number. My 25hp is on the barrel. Last numbers are pitch.

Find gear ratio, and come back and post your wot rpm, and gps speed.

In the meantime, don't twist the throttle as far, to prevent the over-rev.

 

[pretty_amateur]

Re: I need a different prop for my 20 hp Merc on my 14' aluminum boat...

Hi,

Thanks for the reply and warm welcome.

I have tried for a while to get a tack and cannot. I live in a pretty remote area and it's too expensive - cost 1/3 of what I paid for the engine.

I'm not that particular about this, so if things are not perfect it's okay... I just need an educated guess. I could guess, but I'm uneducated.

I did find that the part number for the prop is: 48-33480-11

It is a 2-blade prop. I was thinking a 3-blade may be better with a 12 pitch and maybe 1/2 inch bigger diameter.

Also... The engine serial # is: 4055704 (1970's 20 hp merc/ 2-stroke/ short shaft).

***Again, I'm just hoping for an educated guess here.

***ALSO - I have two other unrelated questions that I may as well ask:
1.) The spark plugs are rusty... Should I buy new plugs? (I'm just not sure how much this affects performance. It could start a little better I guess, but it is 30-40 years old after-all.)
2.) The prop moves back and fourth along the pin (when I pull it out and push it back in) about 1/3 of an inch. It seems like way too much, yet the nut seems pretty tight. (I didn't try that hard to tighten it though.) Is this normal?

Thanks in advance for the prop type question and any help with the following two questions.

THANKS!!!