I was wondering if anybody else has this problem. We cruse our Beta 13.5 hp at about 2400RPM. This will put us to just about hull speed assuming we're not bucking into a chop. If we push the engine up to 2800rpm then we start getting a prop wash noise. The max engine RPM that we can push is 3250 and the boat will be making 7.2 knts. The boat is a bristol 29 full keel and were running a three bladed michigan 12x8. So, I'm wondering are we pitched right? Should we look into a smaller prop to increase the RPM? It does seem like the engine can hanle the load because if we do have to push hard there is no smoke comming out of the exhaust or any atomized fuel.
Propwash noise is indicative of insufficient propeller blade tip clearance to the hull of your boat. Also, as you can only reach 3250 rpms this indicated to us you may be over propped.
We ran a prop calc based on your boat with the BZ482 with 2:1 transmission and we believe that you get away with reducing the propeller diameter to 11" and this will probably eliminate the prop wash noise and allow you engine to perform closer to specification.
This is going on the third season on this engine. What would your recomendation be if that was the prop the dealer I bought the engine through recomended? Is the prop wash noise causing any damage to the boat if the prop is too close to the opening?
Prop wash in itself is not damaging, merely very annoying. It is the result of a lot of water being pushed past the hull at high speed. This is often found on a retro-fit on an older boat which has a very limited propeller aperture and one is trying to get the power of the new engine to the water as efficiently as possible. I would guess that your old engine was an Atomic 4 and that a compromise was struck with the sizing of the new propeller.
Propeller sizing is not an exact science and occasionally the sizing of the propeller using experience and recognized formulae do not always produce the best results for a particular boat/engine/transmission combination. Sometimes a further adjustment of a propeller is warranted to get the improved efficiency or performance and I believe that you would benefit from the 'tweak' to you propeller suggested in my first reply to your post. Any good propeller shop should be able to accomplish this for you at a reasonable cost. But first, tell them about your problem and let them come up with the calculations as on many occasions the results do not always meet expectations, as noted above, and they may then, if possible, re-adjust your propeller without further charge.
