Tip Clearance

[nic]

Could I please have some views on what the minimum propeller tip clearance should be to the hull for a Yanmar 315 installation, ZF63A 2:1 3 bladed on our 63 fbc. I always believed 10% of dia was OK but I'm getting conflicting advice.

Also, I've searched the old site for 3 bladed propeller sizes but the closest is Acme which are their own style - what prop size do the faithful think so I can compare to the local advice.

Thanks in advance.

Nic

[scot]

I use 10% "plus a little". The boat I'm currently building will use an 18" wheel, I am setting the tip clearence at 2". I do this; 1) to be a little on the safe side, and 2) Just in case a 18 1/2" ends up working a little better.

I used this on my last build and I have had no cavitation and/or hull damage near the prop...bottom paint stays on, etc. And the prop is in a small pocket.

I don't know that 10% is the "official doctine" on tip clearence but it has worked fine for me.

Scot

[JP Dalik]

The way I've interpreted the Yanmar/Cummins repowers regardless of engine the output at the transmission is nearly identical. Cummins is usually 1.53-1 and Yanmar 2-1, the Yanmar operates at higher rpm but due to the gear reduction the shaft is turning at roughly the same speed as the Cummins. I'm pretty sure I remember Capt Pat writing this down somewhere.
With that in mind having Cummins 330's I would start with a 20" wheel pitched at 24". At 20" diameter you are not overworking 1.5" shafts so you have a nice safety factor. The standard 4-blade setup for 270hp Cummins is 20x22's. If your torque is closer to a 270 the equation is simple. Just add an inch of pitch to make it a 3 blade.

Hope it helps as with anything this is a guess the guys building the props would know the best.

[Bruce]

Stick with 10% of diameter and you'll be fine. Always worked for me for 30 years.