Which Prop -- Just Starting Out and Expecting to Have to Change

Buddy just called who has been doing the old boat resurrection thing all summer. His and mine have been somewhat twin/dueling projects. He is now very close to putting his in the water the first time. Neither of us has a clue as to the voodoo that props do.

He's got two props in hand. Rather than just flip a coin as to which to start with I told him I'd ask here to see if there's a recommendation. He knows to check WOT and speed details when it gets on the water for later changes if necessary. Just trying to figure out which of the two he is best to start with.

Both our old boats are Mercruiser 165. 1.65:1 gear ratio behind inline six cylinder Chevy engines. Bowrider with dry weight of about 1100 pounds. Wet and loaded weight about 1900 pounds with all the kids aboard.

He's got an aluminum prop that is 14.5" x 19 pitch and a SS prop that is 13.5" x 21 pitch. He's got to start with one or the other. Which is the better option to start with? Also, as my own curiosity kicks in, how will the characteristics of the two differ?

Thanks again.

Re: Which Prop -- Just Starting Out and Expecting to Have to Change

Unless your boats are really small or unusually light I think your I/Os are probably closer to 1500 lbs. or more.
Speaking very generally a ss prop is usually more efficient producing better performance than a similar aluminum prop.
" Disregarding the material" A prop of lesser pitch ,the 19,will produce better hole shot and will rev easier,may stay on plane better at reduced rpm but likely will produce less top end at the same rpm. A 21" will produce slower hole shot but higher speed at the same rpm if the motor can twist it up there.
After all that its very difficult to compare props of different materials.
I say pick one and do a careful test,then do the other.
Usually a ss prop will produce better numbers than an al. But there are al props that a very competitive.

Re: Which Prop -- Just Starting Out and Expecting to Have to Change

Going strictly by the gear ratio you posted, the 19p should be the best of the 2. I highly doubt that the 165 engine will even get on plane with a 21p and a load of gear and persons.
That being said, I don't think that gear ratio is correct, or at least not correct for that motor (but I could be wrong). I have a 170 hp engine in a 21' boat and have a 1.84 drive ratio using a 19p alum prop.
You may find that you need to go to a 17p prop to get the best overall performance.

Difference in alum and SS props:
Alum will flex and deflect under a heavy load where a SS wont, or at least not as much. This flex changes the way the prop presents the blade surface to the water. This can cause adverse affects. Now that doesn't really mean anything unless your trying to get the very last mph out of the boat.
Alum is much more forgiving to hitting the bottom or a stump than a SS prop. It can be the difference between just replacing the prop and replacing the lower unit.

Re: Which Prop -- Just Starting Out and Expecting to Have to Change

Every reference I've read for the Mercruiser 165 drive and inline six combination has said the gear ratio is 1.65. Spinning the shafts by hand suggests that it's that or close to it. The weight spec mentioned was taken from the owner's manual for the boat. It hasn't been over a scale but that manual was in hand for a source.

If in the end the boat's performance suggests going to a 17 pitch prop when that change is made (and the money is spent) what about the diameter? Do you shoot for the largest that can fit or is there some other factor in the equation more important?

Re: Which Prop -- Just Starting Out and Expecting to Have to Change

What year,brand and model/size of boat? When selecting a prop if it is intended for your motor and outdrive the diameter will be correct.
Sometimes boats with particular problems a diameter change may be necessary.

Re: Which Prop -- Just Starting Out and Expecting to Have to Change

steelespike said:

What year,brand and model/size of boat? When selecting a prop if it is intended for your motor and outdrive the diameter will be correct.
Sometimes boats with particular problems a diameter change may be necessary.

Click to expand...

17' Glastron SSV177 from 1980 with Mercruiser 165.

Re: Which Prop -- Just Starting Out and Expecting to Have to Change

ihaveabrownboat said:

17' Glastron SSV177 from 1980 with Mercruiser 165.

Click to expand...

Wow, that is a smaller boat than I expected. If it were me, id start with the 21p. For no other reason to see what it does. With that boat motor combo it will probably perform well. That has a lot of motor in a small boat. My old 17' Starcraft did 45 with an 85 Johnny on it and a 21p (maybe it was a 19p, been a long time).

As for the diameter, don't get hung up on that. Just go with a "p" that works best for all around. Let the manufacture worry about that.

Re: Which Prop -- Just Starting Out and Expecting to Have to Change

bifflefan said:

Wow, that is a smaller boat than I expected. If it were me, id start with the 21p. For no other reason to see what it does. With that boat motor combo it will probably perform well. That has a lot of motor in a small boat. My old 17' Starcraft did 45 with an 85 Johnny on it and a 21p (maybe it was a 19p, been a long time).

As for the diameter, don't get hung up on that. Just go with a "p" that works best for all around. Let the manufacture worry about that.

Click to expand...

Will pass that along. My boat from mine and my buddy's dueling resurrections this summer is nearly identical. Only differences are the color and I added a trim gauge and GPS/Sonar unit to mine and his upholstery is different. So they really are mostly twins. If I end up needing to consider prop changes this will help me too.

Re: Which Prop -- Just Starting Out and Expecting to Have to Change

An 80 SSV177 is 1100 lbs with a 120 hp.4. I find a ratio of 1.7 for the 165,could be mistaken for 1.65
Anyway at 4600 rpm a 165 hp.1.65 ratio could do about 48 with a 21"

Re: Which Prop -- Just Starting Out and Expecting to Have to Change

I had a 140 hp Mercruiser 4 cylinder on my first inboard outboard--a Aristocraft v hull. It used a 21 pitch prop.

You'd probably be okay with the same. The 165 hp 6 cylinder was a nice running engine--smooth and relatively fast. My uncle had one in an Aristocraft.