latest on my duo prop problem

mikemcgregor

mikemcgregor

Well-Known Member
Joined
10 Feb 2004
Messages
403
Location
UK Spain
Visit site
Just heard from the engineer regarding the problem I had with vibrations from port leg due to bent props. He has now done a sea trial and the vibrations have now gone (phew!!!).
He said boat was flying pulling 4000 revs. engines are volvo penta ad41 turbos. my little booklet says max revs should be 3800 and should do 34 knots. Props are duo A7's. Anyone know what would happen if I re-propped with A8's? I think revs would drop and I may lose a bit from the top speed.
 
Mike
I used to have Targa 33 which had A5 props on when I got it, the engines reved to 4100 so I moved to A6 props which brought the revs down to 3900 and actually gave me about 1/2 knot extra speed.
It may be also worth checking the rev counters are correct.

Regards


ian
 
I've been told the step from A7 to A8 is greater than the steps with the lower numbers, from A6 to A7 for example. Therefore you could drop more revs than you bargained. Ours revs to over 4100 with A7s and had considered going to A8s. This is with AD31s (150HP) I think this is slightly over the max rpm range which I think is quoted at between 3900 and 4100. I was worried that A8 may drop me down too low, maybe to max out at only 3600.

There is a propellor repair place local to me where I had my current props refurbed for the begining of the season. He said he could mod the A7s to effecivly give me an A7.5. This may be a good compromise. Personally I'm not too worried about the top speed but would like to achieve a slightly more relaxed cruising speed.

It could be worth checking the rev counters. I bought a neat device of ebay that measures the speed at the crank using a light beam and a bit tape stuck to the front pulley. Measured up to 3600 rpm at 200 rpm stops. At all positions the rpm on the counters was within about 10rpm and the same across both engines. Quite surprised at the accuracy.

If you do decide to try different props I'd be really interested to know the results.
 
I am with you on not bothered about top speed. (It scares me s..tless when doing 34knts.) Like your concept about A7.5...I expect that I will remove props at the end of season and get them refurbished and may try altering to 7.5.
Thanks for the info
 
Going up to A8s might struggle to get up on plane as 41s only have turbos and no compressors. 41s are safe to rev to 3900 and how often do you do it probably not much. By the time the boat has a bit of weed on the bottom, full tanks loads of gear the revs might come down anyway.
 
Thanks for that. Now that I have had time to think about it and reasoning that Cranchi and Volvo would have selected a7's as the right prop for the boat.... I will probally settle for new/refurbished A7's. Judging by the amount of welds and filling that have been done over the life of the props (approx 12 years I guess) it seems reasonable to think the pitch has been screwed up anyway. Best price I have found so far is £415 delivered for a set of new a7's.
Thanks to all posters help and advice.
 
You must log in or register to reply here.

Share this page

Top