Bieker 24-Foot Single Outrigger Powerboat Build - Outboard and Prop Questions

[ecgossett]

I'm building a Bieker 24-foot single outrigger powerboat. I wanted electric, but the range couldn't match 1 gallon per hour or less with an outboard. If I thought a small 6hp equivalent electric motor with a 1:2 pulley, driving an inboard shaft with an RC-type prop, could push me at 20-25 knots efficiently, with lower battery costs and the same range as gas, I’d do it.

The plans call for a 25hp outboard. I've noticed some 30hp models have the same weight as their 25hp counterparts. I'm looking at Honda, Mercury, Yamaha, and Suzuki short shaft models.

This prompts me to consider whether opting for a 30hp engine could provide better efficiency without adding significant weight. Below is a comparison of some short shaft (15") outboard motors:

Brand Model HP Weight (lbs) Weight (kg)
Honda BF25 25 158 71.7
Honda BF30 30 158 71.7
Mercury 25ELH 25 132 59.9
Mercury 30ELH 30 172 78.0
Yamaha F25 25 126 57.2
Yamaha F30 30 214 97.1
Suzuki DF25A 25 137 62.1
Suzuki DF30A 30 157 71.2

Brand Model HP Standard Propeller Diameter x Pitch (inches)
Honda BF25 25 9¼ x 12
Honda BF30 30 9⅞ x 12
Mercury 25ELH 25 9.75 x 9.5
Mercury 30ELH 30 9.5 x 11
Yamaha F25 25 9⅞ x 10½
Yamaha F30 30 10⅜ x 13
Suzuki DF25A 25 10¼ x 9 to 10¼ x 15
Suzuki DF30A 30 10¼ x 9 to 10¼ x 15

Questions

1. If I upgrade from 25hp to 30hp with the same weight, should I change the prop to maintain or improve fuel economy?
   
   
2. Has anyone found recent fuel consumption data for motors under 40hp? A power/consumption curve would make comparing motors easier.
   

I ran a Mercury 3.5hp on my Core Sound 17. It hit hull speed at lower throttle and lower consumption than smaller motors, even with a small weight difference. I'm hoping to apply the same logic here.

Thanks,

Edward

 

[comfisherman]

I think the suzuki and honda shares the same block for the 25/30. On the suzuki they are the same basic block but the 25 is set around 5500 optimum and the 30 turns up to 6300. All things equal they both have the same cubic displacement and gear leg ratio. Guessing the added cost is some tune, top end and maybe some intake stuff (is the case for thr 9.9-20 hp outboards as well).

Think suzuki and honda are both setup in that regard. I'm not familiar with mercury as they aren't around much here. The yamaha 30 is based on the 40 and the yam 25 is on the smaller architecture. Ones a 432 cc displacement engine the other 747. That's almost a doubling of cylinder displacement for the extra 5. Guessing it would have noticeable more low end grunt compared to the other 30s, and of course having an appreciable weight pentaly.

Most of these are factory pitched to push along a dingy or small boat, your likely to need to loose some pitch if your boat weight is outside that realm to get the outboard to spin up to full rpm.