Prop Selection-gctid377849

[Guest]

I need new props (twin VP 220 hp), and am curious if there is an advantage to going to a 4 blade vs. 3 blade props for a boat this size ('85 2850 Contessa Sunbridge).

I understand the pros and cons, but mostly discussed on bass boats, or high speed demons, not mid sized cruisers like this. Any real world, seat of the pants advantage, one over the other?

[2850Bounty]

I ended up going with 6 blades, and wow....what a difference!

Seriously.... all you can do is give it a try. If you can meet your WOT RPM, and if your speed improves, it's a win win.

[Guest]

Ha!

big expense to take 2 props out for a test drive. Big expense to change over 2 sterndrives to DP as well. Has anyone tried 4 blades on these boats, or is it a waste of calories?

[Guest]

I had a Wellcraft 2800 with dual Alpha drives some years ago. It's a 28-footer with 10' beam. I went from aluminum three-blade to aluminum four-blade, and that was a very nice improvement. Then I went to stainless four-blade and that was a HUGE improvement. Do it. You will gain traction and handling around the docks, plane faster and at a lower RPM, and gain fuel economy. It was a win all around.

[itsabowtime2]

I did it on my 3055 and LOVE the difference. Our boats are similar in size. This is what I got:

Time to plane is now about 7 seconds. Not bad for a cruiser.

I can hold plane at 20mph.

Mid range acceleration is fantastic.

I did not loose any top speed.

[Guest]

great feedback, I may try it.

Question, how does a prop keep the boat on plane at a lower hull speed? Do the 4 blade props increase lift at the stern or???

[itsabowtime2]

dpoelstra wrote:
great feedback, I may try it.

Question, how does a prop keep the boat on plane at a lower hull speed? Do the 4 blade props increase lift at the stern or???

Exactly.

[2850Bounty]

dpoelstra wrote:
big expense to take 2 props out for a test drive.

You may find a shop that offers prop testing.

Ya leave them with your CC information, and test the props with an arrangement that you'll purchase the new props from them.

Test props returned w/out damage to them........ and there ya go!

.

[Guest]

Know any good prop shops in the L.A. or Orange County areas? I am right near Huntington Beach and work in Downtown L.A.

[Guest]

itsabowtime2 wrote:
I did it on my 3055 and LOVE the difference. Our boats are similar in size. This is what I got:

Time to plane is now about 7 seconds. Not bad for a cruiser.

I can hold plane at 20mph.

Mid range acceleration is fantastic.

I did not loose any top speed.

What Diameter and Pitch did you go with?

I know I need to take the props in and have them checked for pitch, but it is 15" diameter, and if I hold a measuring tape to and rotate the prop, i get a measurement of 5.75 inches from the back tip to the front tip on my 3 blad.

now I am looking at a 14.5 diameter x 17 pitch 4 blade.

No exchange policy with these so I am interested in feedback.

[2850Bounty]

Dan, with your 1985 Bayliner 2850 Contessa Sunbridge w/ twin Volvo 225/280 drives, you really need to be comparing props to those of the same boat/drives.

The final drive ratios between Merc and Volvo are substantially different.

FYI: with the 280's, you can go up to the Volvo Penta 16" diameter prop.

Not sure if you were aware of that.

.

[Guest]

good point.

Thanks.

What performance differences in dropping down, or going up in diameter. I understand pitch is like a gear ratio, but what about diameter?

[Guest]

I was reading on the Piranah website, and they mention that going from 3 blad to 4 lade requires a drive that has power trim, to trim up once on plane or performance really suffers. I do not have power trim, so has your experience been that you need power trim to trim up once at speed?

[2850Bounty]

dpoelstra wrote:


What performance differences in dropping down, or going up in diameter. I understand pitch is like a gear ratio, but what about diameter?

Man.... that's a tough one to comment on, and it's above most our pay grades....... well, mine anyway!

I'd have to say that changing pitch, at first glance, would appear to be similar to a gear ratio change, but it is not due to the unique characteristics of hydro-propulsion.

The engine determines the over-all gear ratio via it's horse power and torque output.

The prop pitch is derived by the engine being capable of reaching the engineer's WOT RPM.

IOW, there is no substitute for a correct gear ratio, so we can't prop ourselves out of an incorrect ratio.

Your over-all ratio will be 1.61:1.

There are exceptions to gear ratios for high performance and light weight hulls that deviate from our cruiser requirements.

dpoelstra wrote:
I was reading on the Piranah website, and they mention that going from 3 blad to 4 lade requires a drive that has power trim, to trim up once on plane or performance really suffers. I do not have power trim, so has your experience been that you need power trim to trim up once at speed?

I've not tried anything but the three blade props on the Volvo Penta s/p drives.

Make sure that your "set pins" are in the center positions.

.

[Guest]

my set pins are good, I really dont need much trim even with the tabs unless i have 4 of my big 250+lb buddies on board, and they are all sitting on the port seats, or a wind is pusing me to the side. I like all the benefits of going to a four blade, until I read the need for power trim. I would love to hear if anyone knows about this. It seems that the additional trim is only needed once up on plane, so changing the set pins won't solve the problem, if it is in fact, a problem.

The more reading I did, it seems that the diameter has little effect on performance, and its all in the pitch and blade design (cup, etc.).

Anyone go from 3 blades to four, and found that they needed more power trim?