Going B5 to C4, but heard C4 is less coarse pitch?

[loco]

Hi all,
Quick background: My boat used to have a penta AQ205 with two barrel carb. I re-engined with a vortec 4-bbl, pushing around 220HP.

Drive leg has always been a DP290 with B5 props. With the new engine, it tops out at 37kts at WOT, and I think it hits 4800RPM, if I can believe my tachometer. Getting onto a plane is OK for a cabin cruiser, even when loaded up.

Props need replacing again, so I was thinking of going C4 which from the below document I thought would be a fraction coarser (quoting 39kts for the C4 vs 37 for the B5), but I've been informed that the PDF is old and not correct. The C4 props are in fact *less* coarse than the B5, and so I would drop top end speed, but have a better hole shot.

My hope was to have a similar holeshot (fractionally coarser pitch, but extra efficiencies of stainless), with an improved cruising RPM/slightly better top end.


Any advice welcome!

This is the document I've been basing my decisions on up to now: http://www.pentamarine.co.th/pdf/propellers.pdf

Thanks in advance

 

[Bondo]

and I think it hits 4800RPM, if I can believe my tachometer.

Ayuh,..... Cross-check the tach, with a known good shop tach,.....

If it's Right,..... You've got the proper pitch now,.....

Goin' to a taller pitch is gonna make it a dog outa the hole, 'n set up a Luggin' condition on the motor,......

 

[loco]

Thanks!
Yeah, I think the pitch must be about right already, but the issue is the numbers between stainless and alu props aren't a direct replacement, so I'm trying to figure which way to go. B5 to C5 is a change in pitch.

If it's a fraction coarser, will this be negated by the additional efficiencies of the stainless? It would be handy to know if others have done the swap, and what their results were.

 

[Scott Danforth]

Thanks!
Yeah, I think the pitch must be about right already, but the issue is the numbers between stainless and alu props aren't a direct replacement, so I'm trying to figure which way to go. B5 to C5 is a change in pitch.

If it's a fraction coarser, will this be negated by the additional efficiencies of the stainless? It would be handy to know if others have done the swap, and what their results were.

NO, the B series is aluminum and the C series is stainless version of the B series. B5 and C5 have exact same pitch. The stainless props simply do not flex as much at the upper end so you get a bit more bite, however not enough to worry about.

C4 would be less pitch (more RPMs)

looking at the chart, you need to make sure your looking at the correct line with the correct gear ratio.

there are also several mistakes on that PDF.

 

[QBhoy]

Funnily enough this is an exact consideration I had years ago.
I had a sealine 218 with the AQ205 and 290DP. It topped out perfectly at 4800 rpm and around 35mph (not sure gps) with the standard and original B5 props.
I often wondered about getting the stainless equivalent C5 and did a bit of reading up. From memory most accounts read that it wasn’t worth the huge expense needed to buy these things, unless you got a deal second hand. The other prevailing opinion was that those who had, noticed that similar set ups who did go like for like, dropped a fair bit of rpm and didn’t see any top end gain in speed with some cases even lugging and falling off the plane at their Normal cruising speed from before.
I decided not to change over at all given that the boat I had wasn’t really at the cutting end of performance and more for cruising and comfort.
I often wondered if the C4 would be a better choice and match the rpm of the B5 and maybe give a little more hole shot, but decided against it all, mainly for the price of them and that it might either be a dog with the C5s or raise the cruising rpm and fuel consumption with the C4s. Wasn’t often I was wot on that boat.

 

[QBhoy]

Yeah. The C5 is the stainless equivalent of the B5 as far as I was aware from memory.
Worth mentioning again. Your rpm sounds perfect. I’d reiterate perhaps what I said above. I don’t think you will see any real difference worth the money you pay for these things. You may even loose performance if you go to a C5, depending on the hull.
What boat is it ?

 

[loco]

Thanks for all the advice. I needed new props, and have found some second hand C5 props, which are only £150 more than a new set of aluminium props, so I've bitten the bullet and am given it a try. I'll report back how it goes in a couple of weeks...

The boat's a Falson 23SPC. As I mentioned, I've upgraded the engine a bit, and the props remain mateched tot he old engine, so hopefully won't affect her too much.


Cheers

 

[QBhoy]

Thanks for all the advice. I needed new props, and have found some second hand C5 props, which are only £150 more than a new set of aluminium props, so I've bitten the bullet and am given it a try. I'll report back how it goes in a couple of weeks...

The boat's a Falson 23SPC. As I mentioned, I've upgraded the engine a bit, and the props remain mateched tot he old engine, so hopefully won't affect her too much.


Cheers

Excellent stuff. Know these boats well. Great boats. They tick many boxes and do so much all round for their size.
Presuming you’re in the UK with that boat ? There’s one that’s just been rescued in my boat yard now. Been abandoned for years previously.

 

[loco]

Sorry about my terrible spelling in the previous post! Yeah I've had her for maybe 8 years now and she's been a great all-rounder. From threading in between rocks to go bass fishing, to overnight stays in France and the other channel islands. I'm based in Jersey, and she can handle the conditions here well - when you've got 11.5m/37ft tidal ranges, you have some crazy currents that you need to contend with!

Looking forward to seeing how these props perform. Worst case, I'm sure I'll be able to sell them again and revert to a known good.