New SEI drive, need a new prop.

[baconbiscut]

I have an early 70s 17' Dixie Trihull with the Mercruiser 140. I recently installed a new SEI drive because the old drive had a very worn clutch dog in it. The new drive has a 1.94 ratio which I believe is a slightly taller gear than the older factory drive. Prior to the replacement I was seeing about 4500 rpm and 37 mph trimmed out in smooth water with just me and a full tank of fuel running a 3 blade 19 pitch prop. Now I'm topping out at 4200 and 37 mph. I believe the rpm range for my engine is 42-4600. I'm considering going to a 17 pitch four blade to get to the upper end of the rpm range with a light load. My old drive had a hydrofoil on it but I'm not keen on drilling holes in my new drive just yet. I've heard that a 4 blade provides more lift. Does this seem like a good choice? Also if it helps, I rarely exceed 3500rpm. When I'm running I usually keep it between 3-3500 but I spend a lot of time trolling and cruising a long at 1500.

 

[jlh3rd]

by book #'s : Dropping pitch by 2 inches raises WOT rpm's by 300-400 rpm's. Then adding a blade can reduce rpm's by 100-150..200 rpm's. So the net is about a 200 +/- rpm increase at WOT...that's by the book, actual results can only be determined by running results.
You have to decide what purpose you're propping for.
I prop my boat for engine requirements, max rpms at WOT by myself. And I went about 100-150 rpms higher so as to be at the higher rpm at WOT range when under load (towing teens on tubes). Speed, prop slippage, etc. is what it is.
I cruise at around 3500 rpm most of the time also but would never "prop" my boat for 3500 rpms.
Propping can be expensive.
There is a propping sticky on here.

 

[Scott Danforth]

I would file the points, set the dwell and timing and do a tune up to see if the RPM's come back.

 

[Pmt133]

Adding to that, the math isn't 100% but that shouldn't be that big of a difference... I'd first confirm the tach accuracy and move from there. If yor 4200 now is really 4400, I'd leave well enough alone. (I'd leave it if it were 4200 as long as it performs well and that was under normal operating conditions too)

 

[redneck joe]

what scott said. Attempt to find a root cause especially in a 50 or so year old motor.

One thing i never thought of was replacing all the plug wires, help tremendously. Also went to electronic ignition.

 

[baconbiscut]

Thanks for the advisement. I've done plugs cap and rotor last season but didn't mess with the timing or the carb as it runs so well already. Now that I've got the drive and shifting licked I'll move onto that. Just getting it back together piece by piece, last year I did bellows myself. I should be greatful that this season I'll be working topside.

 

[Stinnett21]

17" 3 blade, in the same prop brand/style, should put you right where you need to be. Instead of a hydrofoil consider Nauticus Smart tabs.