Prop help

[uspres2020]

I'm looking to get more hole shot to possibly tow a skier and get on plane faster for a more enjoyable ride.

My current prop has the serial #48 78122 A40 21P which I believe is a 14 1/4" 21 pitch 3 blade aluminum prop. It currently at WOT is about 4400 RPMs which I believe the engine can handle 4600 to 4800 rpm.

I was thinking of going to a 4 blade 19 pitch prop. Should I be concerned that that might push to many RPM.

it's a mercruiser 4.3L alpha one I/O starcraft 2012 1995.

 

[alldodge]

If your tach is accurate then you can go down in pitch to a 3 blade 19 pitch or a 4 blade 17 pitch.

Going from a 3 blade 21 to a 4 blade 19 is about the same, just get more lift

Going from a 3 to 4 blade prop, you need to reduce 2 pitch sizes.

 

[QBhoy]

Alldodge has it about right there friend.
If your numbers are right. A good 3 blade at a 19” will see you almost perfect all round I would think.

If you are mad keen on towing watersports. Get a hi 5. They are very good at this. But not much else really.

 

[Starcraft5834]

if you dont mind losing a mph or two, get a 4 blade.. best overall prop, especially for heavy under powered boats... ( I use to have one of those

 

[Texasmark]

The hole shot depends on the torque of the engine to produce the thrust necessary to spin the prop at the required rate for the desired effect. Since horsepower used to deliver thrust is governed by torque and rpms, your engine only has so much torque at a given rpm and the torque will be rising up to usually mid rated engine rpms. If you can get the rpms up to mid range faster, you will also get torque up faster and you have a 1-2 punch for a fast hole shot without affecting top end.

Therefore you might need to up your rpms to get your hp quicker for your hole shot. The 4th blade helps on the hole shot if the engine has the HP to spin it but that extra blade gets in the way at high rpms. Port holes in the prop will solve that problem for you. Size of the port determines how many rpms your engine will rev over a non ported prop in the hole shot. I liked to get an extra 1k. If you tune for too many, it amounts to blow out and you hinder the shot, not help it.

Once you get up on plane and get going, water pressure running by the prop's barrel will seal off the ports and the prop will function as if they weren't there. It's a 2 props in one thing.....simulated low pitch for hole shot and high pitch (prop rating) for WOT. I had a ported Laser (I) on my Ranger bass boat and the ports worked good for the hole shot when fishing and getting my arse up on slalom when playing. I had about 1k of rpm boost.....measured by the rpms when it locked up vs after and the boat had accelerated to the speed it would have had with out them...which didn't take but a heart beat...give or take. Grin.

 

[Texasmark]

Alldodge has it about right there friend.
If your numbers are right. A good 3 blade at a 19” will see you almost perfect all round I would think.

If you are mad keen on towing watersports. Get a hi 5. They are very good at this. But not much else really.

I have seen 1 Hi-5 in my life. Couple dozen 4s and 3 blades with ports on any boat that appeared to be expected to perform for the owner all the rest.....above about 75 hp.

 

[uspres2020]

Thanks for all the advice. I ordered a 4 blade 14.5" 18" pitch aluminum prop. I'll let you know how things work out. Probably only take it out once before I pack it away for the winter

 

[alldodge]

ordered a 4 blade 14.5" 18" pitch

Never mind, I'm mixed up, your 18 should be ok

but a 17 would be better