Cruising RPMs for our Perkins 4-108

[Ernest Ashley]

We are about to head from Salem, MA to Nantucket through the Cape Cod Canal and Woods Hole. Both have strong current potential and it is a long(ish) trip so I expect we will have to motor for a good bit. We regularly motor along at 1500 to 1600 RPM (three blade prop) and move along at about 6 knots. I would like to know what RPMs other owners of the same engine regularly run and what they feel comfortable revving up to in case we have to deal with some nasty current. Thanks in advance.

 

[DGA]

My 382 and 4-108 with the original prop seems to like 2600 rpm, giving about 6 knots. 3000 rpm and I can hear the cavitation clearly, and it might do 6.5 knots, not worth it.

 

[MadameX]

My 384 has a 4-108 with 1.875: 1 transmission ratio. Prop is a 3-blade Max Prop set at 22 degrees blade pitch. I motor at about 2300 RPM and she does ~6.5 kn. Cavitation is slight at those settings, but gets to be too much at Revs above that.

 

[terry_thatcher]

I used to have a 4-108. Prop was 3 blade, 16x11, with a ZF transmission, 1.88/1 ratio. I ran at 2000 rpm for 6 knots . You will note that prop and reduction gear ratio are critical to performance.