Which brand and HP is powering that 3+ meter Sib ? I usually install these excellent induction tachs :
https://www.hardlineproducts.com/pro...ourtach-meter/
Comes with a long pick up wire, a matter to order one and check if reads well with an extra 5 Ft pick up cable of the same diameter prior routing and hiding it well. If doesn't read well can install it if there's available space on the lower motor's front pan.
Happy Boating