Flickit

This seems to be a common issue with Yamaha outboards, I've just come across it myself, it appears most the smaller Yamahas are quite a bit longer than the standard 20" which most boats seem to be designed for.. I've been using a Stacer 380 SeaSpite with a Yamaha 15hp 2-stroke long shaft, both purchased new in 2005, the dealer recommended a 25mm/1" transom spacer, the buyer had a spacer welded in... The boat seems to handle ok unloaded and loaded up heavy, in very fast flowing turbulent rivers it feels like it's getting yanked around by the motor a bit but I figured that may be quite normal, (I'm not that experienced).. Top speed is around 37 km/h, 20 knots... The only real issue I've noticed is the fuel consumption seems quite high, I've read a review of a similar setup and they claimed around 4 NM/L (7.4km/L), I'm only getting about 2.6 NM/L (4.8km/l), that's quite a difference... The review with the fuel usage figures> https://www.northsidemarine.com.au/images/Yamaha/Yamaha-15F-Review-Trade-a-Boat-Magazine-Nov-2015.pdf ... The motor is basically brand-new, it only had 11 hours on it before I started using it, and I've put 32 hours on it in recent weeks, 470km ... After reading this thread I checked the motor height, it seemed to be about 40mm low: Investigating the issue I've found the Yamaha specs list the transom height for the motor as 567 mm (22.3 in), and using Archive.org to check the Stacer website from 2005 I found they paired this boat with a Mariner/Mercury 15hp LS which has a transom height of 508mm (20")... So even with the 25mm (1") spacer welded to the transom the Yamaha is still around 34mm (1.3") lower than Stacer intended... Still not convinced it was that much of an issue I searched through my video footage and checked how it looks in the water... I'm certainly no expert but the amount of water being churned up appears excessive, I figure this may explain the fuel consumption issue: The screenshot isn't too good so here's a vid: I can add in a 20mm spacer very easily so I'm going to start there, if that goes ok and doesn't introduce any issues I'll look at raising it more... My main interest is to get the fuel consumption down a bit... I notice the current Yamaha 4-strokes are longer than the standard 20" also. 567mm, 22.2" ... and Yamaha seem to happily throw them on boats and flog them off hanging VERY low >> https://www.mandurahboatshop.com.au/listing/stacer-379-seasprite#lg=1&slide=1 Thanks to the OP and the resident experts, this thread has been very informative