I have recently refitted my Coronet 21 with a brand new Volvo Penta KAD 32P/DP (170 hp). The boat is 18 years old and was orginally fitted with Volvo Penta AQAD 31A (turbo diesel - 130 hp - duoprop Z-drive) .
We have tested the boat with 2 sets of propellers. When tested with B7 props, the boat gave a top speed of 34 knots at 3700rpm. The boat cruised at 30 knots revving 3100rpm.
When tested with B5 props, the boat planed more quickly, however top speed was reduced by 2 knots (to 32 knots), whilst cruising speed reduced by 6 knots to 24 knots. We also noticed that with these propellers, the engine revved a maximum of 3900 rpm instead of 3700 rpm.
Whilst testing the mechanic (who fitted the engine) told me that the B7 props were too big for the engine since they disallowed the engine from acheiving the full revs i.e. 3900 rpm implying that they were creating a 'load'. He thus advised us that the best props were B5s. He also told us that the 'load' could damage the engine in the long run.
I am concerned about the correctness of his advice. On one hand, I would like the boat to acheive the best performance. On the other hand, I would not like to damage the engine. I have gone through the owners manual which states in the propellers section that " For best performance and ecomomy, the engine speed should be within the wide open throttle (WOT) range". The WOT range for the KAD 32P is 3700rpm to 3900 rpm. In view of this, I presume that no harm should be done to the engine if it revs a maximum of 3700rpm.
Another mechanic also told me that I should also try B6 and that 'A' props are normally used on diesel engines. On one hand I would like that the boat acheives its maximum performance. On the other hand, I would not like that the props could somehow damage the engine. Can someone advise me what type of props I should use.
Whilst testing the boat we also noticed that the boat pulled to the right hand side. The boat pulled mostly (and quite unsupportably) with the B7 props. The pulling decreased significantly when we tried the B5 props. The pulling was virtually reduced to none when the Z-drive was trimmed to -4 (with the B5 props)
The initial reaction of the mechanic was that the previous Z-drive was not installed in the perfect place. Having installed the new Z-drive in the same place (without modifying the hole on which the transom is mounted), the problem cannot be solved. Another mechanic confirmed this, however he added that the pulling might have been caused due to incorrect settings to the Z-drive or due to the fact that a power steering is not installed. I am aware that the boat did not pull to neither side with the previous engine except when the Z-drive was trimmed below -4. This might conclude that the previous Z-drive was fitted in the right place.
Having spent a hefty £12 grand on this new engine, I believe that I should acheive better results i.e better performance and no pulling. Can someone advise me what I should do or maybe try?
Thanks in advance