Saltram40 propellor

[Trevoronlara]

I have a Saltram 40 with a Perkins Prima 50hp engine running at a max 3000 rpm and 2.15 ratio gearbox. The propellor is a 3 blade 18" x 13" but above 2000 engine rpm it makes a "rumbling" noise. Does anyone else have this problem? What size of fixed blade propellor do you use for this boat?
Many thanks
Trevor Scott

 

[Kwik Decision]

I regularly sail with a friend who sails his Saltram Saga 40 out of Chichester. There is always a bit of a rumble while motoring, has been since we sailed her back across the N Sea in 2016, just after he bought her. He's replaced the prop shaft flexi coupling, and the prop with a 4 blade feathering, and it still rumbles, so he now lives with it...

 

[Tranona]

If you can achieve 3000 rpm+ then the propeller size is correct for the boat. An 18" 3 blade is a common size for that engine/reduction combination. Assuming there is no excess wear in the cutless, the rumbling is almost certainly due to inadequate propeller tip clearance in the aperture. That is a common issue on that design of keel/rudder with poor water flow across the keel and blunt end to both the deadwood and rudder.. Tip clearance should be minimum 10% diameter ab-marine.com/knowledge-base-propeller-basics/what-is-tip-clearance-and-why-is-it-important/

 

[elenya]

We have owned our Saltram 40 for about 25 years. It has had a three blade 17x13 inch prop most of that time. We replaced engine mounts, prop shaft, cutlass bearing and prop. All aligned very well. We went 17 to give a bit more top clearance. We get a rumble when we get above 2400 rpm. There is a very wide stern post in front of the prop also so water flow is affected above 2000 rpm which is about 6 knots for us.

 

[Tranona]

If I remember rightly the original engine was a Perkins 4/108 and a 2:1 reduction, so higher shaft speed than the OP with a Prima. A 17" would be more sensible for the 4/108, but as you say the smaller propeller just moves the rumble to higher rpm but similar speed.