B3 Prop pitch recommendation needed please

[Hai Nhi]

Good morning all.

Please help with prop selection.

My boat is 1999 Cobalt 292. Manual shows twin 350 Mag MPI with B3 @ 2.0 having 28P.

I'm repower it twin 6.2L 350 HP, B3 @ 2.0 ratio. What prop pitch should I get?
Dry weight is about 7500lbs.

Uses:
- mostly touring;
- pull wakeboard & tubes once in a while.
- I'd run fast to get to places only, not for racing or anything like that.
- We boat mostly in lakes around Nor Ca and around the San Francisco Bay.
- We also boat in Lake Tahoe, which I understand needing a separate sets of props.

Thanks much.

 

[alldodge]

The 6.2 350hp will actually be 320hp at the prop. The 350 MPI is 300hp at the prop, so your getting 20 extra hp and good thing is a more torque.

You should have a bit better hole shot, but the WOT will have 200 extra RPMs at 5200

Will probably need 2 inch less pitch for Tahoe
For now stick with what you have and then decide if it needs to change

 

[Hai Nhi]

The 6.2 350hp will actually be 320hp at the prop. The 350 MPI is 300hp at the prop, so your getting 20 extra hp and good thing is a more torque.

You should have a bit better hole shot, but the WOT will have 200 extra RPMs at 5200

Will probably need 2 inch less pitch for Tahoe
For now stick with what you have and then decide if it needs to change

Thanks for the quick reply.

I currently don't have any props for this setup yet. It'll be all new.

The existing setup are 350 MPI with B1, 19P. I have a pair of B3 2.0. So I'm thinking about swapping the B1's out and install the B3's.

I have a set of 20P and a set of 28P for the B3 (not a pair for each). Not sure how to determine which one is better for my general boating. I called Signature Propellers and they said start with 22P. Is there some rule of thumb or an online calculator for figuring out all these?

Thanks.

 

[alldodge]

Was the B1 the same ratio as the B3?

 

[Hai Nhi]

Was the B1 the same ratio as the B3?

The B1 is 1.5. B3 2.0

 

[alldodge]

The B1 is 1.5. B3 2.0

Oh boy, thats going to hurt, but if you stick with Hill marine not that much

Look at it like this.
B1 is 1.5 ratio, with lets say 19 pitch prop, turning 5000 RPM and getting 50 MPH. This equates to a prop slip of 16.6%

B3 is 2.0 ratio, turning same 5000 RPM, and 50 MPH, and slip is the same at 16.6% this means you need a 25 pitch prop to achieve the same speed

The B3 has less slip then the B1 so if the B1 and B3 had the same prop set, and the motor can achieve the same RPM, then the B3 will be faster. That said there is more drag with the extra prop blades in the water. My guess is you need to stick with B1 or get same drive ratio

I went from 425HP to 530HP, B3 drive and changed drive ratio from 2.0 to 1.81 thinking all the extra HP I need to change the ratio. Was I wrong about my 11K pound cruiser. I installed the drive and with a set of 22 pitch props and it barely got on plane. Remove the brand new drive and reinstalled a 2.0 and it came out of the water great. Installed Hill 24 pitch 4x4 and doing good. Later stripped the paint off the bottom and epoxy coated it, and had to shelve the 24's and now have 26 pitch 3-blades

The Good
Hill Marine will swap props with you until you get it right. Your cost will be the return shipment. They did great by me

 

[QBhoy]

Hi. I’ll throw this in. I think some 350 mpi are rated at 4600-5000 rpm and others at 4600/4800-5200 rpm (or something similar). The 6.2 (if it’s the gm version and not the newer in house built one) are something similar. So what I’m saying is that if your 350 is a 4600-5000 and the 6.2 is 4800-5200 rpm…you may not need or want to go for more pitch perhaps. Just a thought.

 

[Hai Nhi]

so when I have them installed, what’s the basic testing procedure to see if a pair of props are perfect pitch for the boat?

 

[alldodge]

so when I have them installed, what’s the basic testing procedure to see if a pair of props are perfect pitch for the boat?

Load the boat as you would normally, then see if it will reach close to or at 5200 RPM at WOT