Confused with gear ratio & props

[Rallykid]

Hi everyone,
We just picked up our SA550 and hitting the lake tomorrow.

I’ve done a lot of reading and searching since last fall but still confused on the 1.5 gear ratio that we have in the SA550 and prop selection.

We ordered the boat with the 16x13.9 prop so it’s going to pull like crazy.

Why does the 550 have a 1.5 ratio vs the 1.76 in the 400/450?

Should we have gotten the 16x15 prop or should I get that prop as my back up/spare?

I understand the impacts of diameter and pitch but for the life of me can’t figure the impact from the gear ratio.

Thanks in advance for some education.

[MLA]

Why does the 550 have a 1.5 ratio vs the 1.76 in the 400/450?

I cant answer why Indmar has made this decision, but I can offer some info on why a lower gear ratio is an advantage.

A lower pitch prop offers better hole shot. The trade off is higher engine RPM. Going up in diameter reduces RPM, but the trade off is poor hole shot. When you put the 2 together, you get a happy medium of improved hole shot but not as much increase in RPM. With the larger diameter prop, you also keep the prop slip to a minimum, get better speed holding and less cavitation.

So what does this have to do with gear ratio you ask? It takes power to turn the larger wheels. So the lower gear v-drives makes it easier for the engine to run those larger props, like going up hill in first gear on a 10 spd bike v's the higher gears.

[larry_arizona]

The torque on the 550 is already overkill.

580ft lbs x 1.5 is 870 ft.lbs at the prop shaft

The 450 and 400 through the 1.76 are 711 and 795 respectively.

The 550 would be 1020 with a 1.76 and I would think the already larger prop shaft of the 550 would be at risk of snapping.

Are you sure the 16x13.9 fits the 1.25” 550 prop shaft? The 400 and 450’s use a 1.125” shaft.


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[Rallykid]

Thanks guys. I understand better now and yep it’s overkill but never gonna worry about too much weight or elevation.

Larry, I double checked and it is the 3591 16x13.9 L 1 1/8 prop. Not sure if/when the shaft might have changed.

1E585B9C-836F-4B2C-8634-71870DD703DB.jpg

[larry_arizona]

You will love that 550 and prop!!!!! stumpy McStumpersen.

I would seriously find a stump and try to pull it out.


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