Cup or different prop?

[magster65]

2006 240 Sundancer. 5.7 mag mpi / bravo 3 (2.2 ratio)
Gps'd at 43.9 @ 5200 rpm with 2 people, full fuel, 3/4 water and a decent load of junk in it (and a full cooler). Merc says 4800 - 5000
It has 22" pitch props on it now... should I cup these or find some 24's?

 

[TilliamWe]

Re: Cup or different prop?

If you have a good prop shop around cup them. If not 24s would probably work fine too. I take it you are sure the tach is accurate? And that "as tested load" is a typical load for you, right?

 

[magster65]

Re: Cup or different prop?

If you have a good prop shop around cup them. If not 24s would probably work fine too. I take it you are sure the tach is accurate? And that "as tested load" is a typical load for you, right?

Lucky for me I have one of the best prop guys around. I'll see him later today and ask what he thinks. 300 rpm is a lot to try and cup but I'll ask anyways.
The tach... it has the 'smart craft' interface... I understand them to be very accurate. That load is typical, yes, although I do have an extra person or 2 on occasion... not for long though as I like my space! What's that saying... it drinks 6, eats 4 and sleeps 2.
Thanks

 

[TilliamWe]

Re: Cup or different prop?

Well, heck it's only hundreds of dollars for a second propset. 24s are very plentiful on E-Bay. Have fun.

 

[achris]

Re: Cup or different prop?

Personally I like to slightly underprop. You have greater 'driveability' and you're 'light loading' your engines. You'll save on fuel as well as the engines not working as hard. Unless you run a lot at WOT, I'd leave it all exactly as it is....

Chris...........

 

[magster65]

Re: Cup or different prop?

My prop guy said he could do it but I should try 24's first. The stainless props are 'harder' (haha) to work with.

 

[magster65]

Re: Cup or different prop?

Test results...
I found a set of 24's.
It's still quickly on plane and runs 46.4 mph @ 4980 rpm now with a similar load but I had the canvas up (about 16 - 18 square feet of wind resistance).
I think I should find some 26's for the 'poker runs'.
I'm really surprised at the engine's 'grunt'!

 

[TilliamWe]

Re: Cup or different prop?

Thanks for following up! You could have the 24s cupped instead of buying 26s. Unless you like having four spare props lying around. Then you'd be just like my dad!

 

[beanstyle]

Re: Cup or different prop?

just a thought; the cup on a propeller only really starts to perform at 3500+ rpm's. besides that it's only really loading the engines, producing a change in rpm with negligible results in speed.

 

[magster65]

Re: Cup or different prop?

just a thought; the cup on a propeller only really starts to perform at 3500+ rpm's. besides that it's only really loading the engines, producing a change in rpm with negligible results in speed.

With my experience... I typically see about 100 rpm less at wot rpm with a slight increase in speed. I'll pick up some 26's if I find some at the right price... if not the 24's are great for 'every day' boating.

 

[magster65]

Re: Cup or different prop?

Thanks for following up! You could have the 24s cupped instead of buying 26s. Unless you like having four spare props lying around. Then you'd be just like my dad!

hehe... I already have a set of 20's sitting there... so that'd be 6 props sitting around
If 4 = dad... does 6 = grampa?
haha

 

[beanstyle]

Re: Cup or different prop?

With my experience... I typically see about 100 rpm less at wot rpm with a slight increase in speed. I'll pick up some 26's if I find some at the right price... if not the 24's are great for 'every day' boating.

if i might pick your brain a little, do you know what size cup has said effects? i'm also assuming you're talking about the bravo 3

 

[TilliamWe]

Re: Cup or different prop?

hehe... I already have a set of 20's sitting there... so that'd be 6 props sitting around
If 4 = dad... does 6 = grampa?
haha

Well, I wasn't counting all my dad's props lying around. When I was there last month he had a set of 24s and 22s, plus at least 2 other SS props for an Alpha drive, on the floor, and there used to be 5 props in his cabinet. Now to be fair, he owns 1 boat with twin B3s, a second boat with a B3, and he has my sister's boat at his house that has a B1 and it has one spare prop.

Have you seen any of the Mercruiser cupped B3 propsets?

 

[magster65]

Re: Cup or different prop?

The 20's I have are factory cupped. The inner prop has quite a noticeable cup in it but the rear not so much.

 

[walleyehed]

Re: Cup or different prop?

just a thought; the cup on a propeller only really starts to perform at 3500+ rpm's. besides that it's only really loading the engines, producing a change in rpm with negligible results in speed.

Double bogus.......the added cup is how we increase efficiency and lift, which does and will increase speed.
Cupping is a feature that can be added in many station locations on a blade-not just the trailing edge...very mis-understood.
And for the original poster, if you're at the top of the range with a normal load, you are where you want to be..

 

[magster65]

Re: Cup or different prop?

And for the original poster, if you're at the top of the range with a normal load, you are where you want to be..

You betcha!
I meant if I come across a set of 26's cheap, I'll get them for the poker runs. I usually run 1/2 fuel, no more than 3 people and nothing in the boat other than a few refreshments in the cooler during those. Those runs are kinda' fast for my boat so... for the day I don't mind loading the engine a bit more for the extra few mph.
So far from cupped 20's to 24's I've lost about 300 rpm... say 100 rpm per 1" of pitch so if that stays true for more pitch, I should still be able to turn the 26's to around 4800 rpm with a light load. That'll be a nice cruise in the high 30's and a top speed close to 50.
I'm pretty happy with these numbers!

 

[TilliamWe]

Re: Cup or different prop?

The 20's I have are factory cupped. The inner prop has quite a noticeable cup in it but the rear not so much.

Cool, I'll know what to look for if i am ever in the market, thanks!