Difference in SS 19pitch and al 21pitch

[Tfh383]

Fairly new to the prop world my question is with my al 21 pitch I can hit 55mph at 4500 rpm or more trimmed up with the SS 19 I can only hit 45 at 4,000 I thought smaller pitch is more rpm? Both are 14.25

 

[tpenfield]

What you have experimentally found is common in propeller science, because the ss prop is thinner than the al prop and is more effective at moving water. Had you used a 21 pitch ss prop you may have found the boat reaching its top speed at a lower engine rpm, probably around 4200.

The top speed, being the same for both props, is a good indication what the boat's engine is capable of.

 

[Tfh383]

I'm trying to get this hot out of the water better the 19 is a little better than the 21 not much I usually have full fuel 40 gal 4 adults and 2 small kids boats a 21 ft open now with 5.0mpi thinking of a al 17pitch?

 

[Sea Rider]

Which HP and brand engine is yours ?

Why do you need to run combo constantly trimmed up, just trim up a bit, full throtttle, when on plane trim down for combo and anticav plate to ride parallel to water level, for that to happen will need to distribute load evenly on deck. Run again both props as described and report back your max wot rpm and speed.

Happy Boating

 

[tpenfield]

You could go with something like a 19 P AL 4-blade prop. or a 18 P SS 4-blade to get the boat up on plane sooner. Your top end will suffer a little bit with a 4-blade. So, you have to decide if the 'hole shot' or the WOT speed is your priority.

 

[jimmbo]

Fairly new to the prop world my question is with my al 21 pitch I can hit 55mph at 4500 rpm or more trimmed up with the SS 19 I can only hit 45 at 4,000 I thought smaller pitch is more rpm? Both are 14.25

Something is not adding up here

 

[Tfh383]

Jimmbo that's my exact thoughts as well I'm currently changing plugs before I go back out this weekend I just got the boat this year plugs don't look to bad but giving it a shot.. The ss 19 does come up to plane a little faster but not like I expected and the rpm thing throws me off..

 

[Tfh383]

Just a update spent the day out yesterday after plug change and see foam running the ss 19 pitch prop toms returned and performance.. Wot 4200rpm and 48-50mph

 

[jimmbo]

Your rpms are quite a bit below your engine rpm range, most MPI, but not all, engines have a maximum in the 5000 - 5200 rpm range