Two years ago we replaced the Bukh DV20 engine on our H28 with a Yanmar 3MY20 engine. The old Bukh was rated 20hp @ 3000 rpm and the new Yanmar was rated 22hp @ 3600 rpm.
During the commissioning sea-trials a very noisy rattle was evident at engine speeds above 2100 rpm and after various checks the "experts" considered this to be propeller cavitation. With the engine change the original Briski 3-blade15x11 prop had been re-pitched to 15x10 to match
the new higher speed engine.
In aiming to correct the prop noise issue Briski added some "cupping" to the blade tips (as common on large outboard motors) which they said made the prop equivalent to a 15x11 size again. The result was a part improvement with the noisy rattle evident at higher engine speeds above 2800 rpm. At 2800 rpm boat speed was 6 knots so this seemed reasonable for the short term although I did want to eventually eliminate it at all speeds.
Over the past year I researched information on prop sizing, cavitation and prop noise causes etc. I found a useful website tool on prop sizing and this
convinced me that the prop size I had was ok for our H28 arrangement. It also convinced me not to reduce the prop diameter.
From other sources I found that while cavitation is one common cause of prop noise, other causes are propeller tip noise, prop shaft/bearing mis-alignment noise and prop-shaft overhang. I was sure the cause was not due to the new shaft/bearing arrangement and the overhang was within the recommended limit so I focused on cavitation and prop tip noise.
Cavitation is the imploding of bubbles of water-vapor against the prop blades and comes from inadequate blade area. If severe it will erode the blade surface. It is a complex topic and references indicated very "technical" measurements as needed to address its cause.
Prop tip noise comes mainly from inadequate clearance between the prop tips and the hull or rudder with the minimum gap recommended as 0.15 x prop diameter. This was an easier item for me to address so, after checking Sandbourne's prop tip clearances and getting photos/measurements from numerous other H28's, we planned a modification at haulout.
On our H28 the clearance between hull and the top of the prop swing was just 0.5" so well short of the recommended 2.25" (based on 0.15 x prop diameter). Other H28's checked had from 1" to 2.3" gap so this was a strong indicator for me. The modification carried out involved cutting a section out of the stern/hull area to increase the prop tip clearance and this opened up a reasonable hole in the hull which needed filling and reglassing.
The finished job increased the gap from 0.5" to 1.75". And the result was totally successful with no noticeable prop noise up to maximum engine revs. So now we can "put the hammer down" when we have to.
The Sandbourne Scribes