Warning: this post is a rather long post and has lots of assumptions ; you're basic house of cards
I'm looking for suggestions for prop selection for a thunderjet 185. I will likely only be able to borrow a few props for testing, but would like an idea of what might work. The dealer is recommending a Rebel 15.5 x 17 which seems to have too low a pitch.
A previous model of this boat weights 1800 lbs. My objective is to have a 50 mph boat that is reasonably fuel efficient at 30 mph cruise. I'm assuming that a 3 blade would be the best for my application.
I have a performance report on this boat by a Mercury 150 4 stroke. This motor has a 1.92 gear ratio and the propeller they used for the report was a 14.5 x 17 - but I have idea about the prop design.
RPM Speed (MPH)
The engine I have in mind is the 25" Evinrude G2 150 HO that has a 2.17 gear ratio. If I've done my math right, I believe that a 14.5 x 19.2 would give me roughly the same RPM to MPH when taking gearing into account. I'll assume that with a 14.5 x 19 WOT would be roughly 6040 RPM ( based on 200 rpm / 1" pitch), so I'd think a starting point could be a 14.5 x 19 or 14.5 x 20. In looking at the performance reports for similar boats, the most I saw was 15 x 21.
Not taking slip into consideration, and calculating volume for one revolution:
Prop Line Volume
14.5 x 19 RX3 3137
14.75 x 19 RX3 3246
14.5 x 20 RX3 3302
14.75 x 20 RX3 3417
15 x 19 Rebel 3357
15.25 x 19 Rebel 3417
15 x 20 Rebel 3534
15 x 21 Rebel 3711
Does this seem to be a reasonable short list? Should I consider a 4 blade prop?
03-01-2020 05:25 - edited 03-06-2020 06:12
For flat out testicles to the wall WOT top speed in a helm forward weight forward design, need a strong bow lifter prop to improve front half hydrodynamics drag in the water. Less boat in water =s faster. Don't be scared to trim it out.
Raker HO in 22P should give about 5500 rpms WOT full load. Any more than 5500 rpms WOT on these G2 V6s leaves some top end speed and fuel economy on the show room floor. May even be able to swing a 24P Raker HO to 5500.
Absolute minimum WOT rpms on these G2s for long engine life is 5300 rpms loaded. Overpropped Lugging kills outboards, overly hot alum pistons, rings, things start melting.
Adjust the vent holes for the hole shot you like.
RX3 will give you better mpg in most cases.
Merc 150 4 smoke is a huge motor, 3L vs 2.7L
Interesting. I would never had thought about a Raker HO. From my assumptions in my original post, and making a huge assumption that the prop design of the merc was similar, a 14.5 x 22 should give about 5600 rpm. Thats a big assumption but the raker is an option I'd not entertained. I would think that a 22" would make it move well faster than 50mph (maybe 55/60?) and I'm thinking that my hull is not really designed to be for that fast. I'll probably still try to optimize around a top end at or around 50mph
In any event, looks like I have alot of potential options out here...
I'l have to read up on the vent holes for the hole shot to understand how that works
The RX3 replaces the Rebel series and is a bit faster and more fuel efficient depending on the boat hull design.
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