bertram31.com

Interested in finding a set of 4 blades for our 31 but need some help with the exact size..... Any suggestions??

Engine Info and specs
3208 Cat Turbo 280hp
Transmission gear Ratio 1.5/1
Top end RPM 2850

Why are you going to 4 blade?

General rule of thumb. Deduct one inch of pitch from your 3 blade prop and that will be your starting size for the 4 blade.
This works generally with same manufacturer same type props.

We went from Michigan DQX 3 blade 20x26 with some cup

To Michigan DQX 4 blade 20x 25 and added cup after the first season.

If your changing wheel types or designs within manufacturers all bets are off.

Hope it helps.

I think you'd be better off dropping an inch of diameter and keeping the pitch the same.

I heard you get a little bit smoother ride with 4 blades and possibly pick up a couple knots at cruise.... Any truth to this rumor??

CatEquipt wrote:I heard you get a little bit smoother ride with 4 blades and possibly pick up a couple knots at cruise.... Any truth to this rumor??

Smoother, better response at low speed, but loose a kt or so at top end... Always opt for the largest diameter you can safely swing, adjust pitch and cup to balance the RMP loading of the engine. You should be under propped by at least 50 - 75 RPMs

Cup, while primarily reducing slippage, also acts similar to increasing diameter. The goal is to get, as effectively as you can, a square relationship, (Xdiameter:Xpitch). A 20"dX24"p w/ a #5 cup will run effectively as a 21"dX24"p wheel with no cup.

The best two reconmendations are the DJX or the DQX as JP was talking but letters are a little different as posted for 3 blades.

You will get as close to a 4-blade smoothness feel with the 3-blade DJX and keep your speed up. but the DQX as JP mention will give you the 4-blade smoothness but will reduced your speed down a knot or so. I would suggest 21" on the 3 blade DJX and 20" on the DQX but as Capt. Pat suggested keep as close to square and adjust cup in stead of pitch.

http://www.michiganwheel.com/Propellers ... higan.aspx

I heard you get a little bit smoother ride with 4 blades and possibly pick up a couple knots at cruise.... Any truth to this rumor??

Chimera got faster throughout the rpm band up to about 2700, at that point the speed just flattened out with the 4 blades, the three blades would continue to add speed at higher rpms, this trade off seemed justified since the boat never spends any significant time over 2700.

At idle rpm the speed was up about a 1/2 knot. At cruise rpm almost 1 1/2 knots faster at the same rpm

Smoothness is a matter of engine alignment and propellers. There was a noticealbe difference between the 3 and 4 blade wheels on our boat.

I hope it helps good luck