After lurking here for a while and realizing just how little I know...I have another question for you guys. I will be reconfiguring my boat this summer. I have a 16' x 7" all aluminum hull (3/16 bottom and 1/8" sides). I will be adding 1/4" polymer. My engine is a Chev 350 with approximately 340 hp and about the same torque if my memory is correct. I've been leaning toward a 2:3 belt drive with a Sensinech 3-blade 78" K series. Do you think this is a viable setup with only 340 hp? I already have a 2:1 reduction unit that I put on about 10 years ago so I was hoping to upgrade the ratio without having to rebuild my engine stand. With this engine, would a gearbox be a better way to go? Also, I can only turn my 78 x44 paddle prop up to 2450 prop rpms.
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need advice on ratio change
- Thread starter airfan
- Start date
I am not sure if you will have enough torque to swing a 3-blade setup--you should probably call the guys @ Sens. I run a 2.3 on one of my boats(572ci) and don't have any complaints. It seems like most of the guys here have really experienced good results switching to steeper ratios. Good luck with whatever you decide.
sh
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Cchardt is swinging a 3 blade with a 2.68,and he has an almost stock 350,he says going to a higher ratio was the best thing ever. I'm looking for a gear box and to my understanding from questioning is the bigger ratio the more prop you can swing. with that "k" series you may be under propped. But that 2.38 would prolly do you justice.
c chardt
Well-known member
The motor that I'm running the truck it came out of the spec's was 210 hp and 300 foot pounds of torque and all I have done was bored the motor .030 over and put a very small cam and a aluminum intake with a holley 700 on it when I had the 2.12 with a 6 blade I could turn it 4900 wot and the boat got stuck all of the time.
So I got tired of pulling it out then I did the change and went to the 2.68 and the 3 blade super q 80" pitched at the #2 mark and now the boat will run every where I got stuck before and gets alot better fuel mileage and I only spin it at 4400 wot now and have run 4 adults and 3 dogs dry
I took my prop off and put it on a 15 or 16 ft by 8 ft with 29" side this boat was built to go to Alaska so that means it was heavy He was running a 454 with a aluminum intake and a holley 850 annular carb with a 2.37 and with my prop pitched at the # 2 mark he could only spin it 4000 rpm wot and it's a big block not sure what he had done to the motor but the boat would run on dry with 3 people no problem
THAT'S THE BEST THING I HAVE EVER DONE I JUST WISH I WOULD HAVE LISTENED SOONER
So I got tired of pulling it out then I did the change and went to the 2.68 and the 3 blade super q 80" pitched at the #2 mark and now the boat will run every where I got stuck before and gets alot better fuel mileage and I only spin it at 4400 wot now and have run 4 adults and 3 dogs dry
I took my prop off and put it on a 15 or 16 ft by 8 ft with 29" side this boat was built to go to Alaska so that means it was heavy He was running a 454 with a aluminum intake and a holley 850 annular carb with a 2.37 and with my prop pitched at the # 2 mark he could only spin it 4000 rpm wot and it's a big block not sure what he had done to the motor but the boat would run on dry with 3 people no problem
THAT'S THE BEST THING I HAVE EVER DONE I JUST WISH I WOULD HAVE LISTENED SOONER
Somehow I've Forest Gumped my way through the airboat world for 13 years. Now I need to learn the new technology. I'm going to rebuild my motor this summer so let's say that I will have apprx 380 hp and over 400 lbs torque. Why wouldn't I want to spin a prop as fast as I can? An engine at 4400 rpm with a 2:68 is only turning the prop around 1600 rpms and I am having a hard time grasping the concept of a slower spin but more power. I thought these propellers are rated up to 2800 rpms. I don't want to spend my money foolishly without understanding what improvement I will get.
fatboy0054
Well-known member
YOU HAVE MORE THAN ENOUGH PUSH WITH THE SUPERWIDES AT 1600 
FOUR PEOPLE 2800 POUNDS BOAT BONE DRY ! 15 MPH ! THEY ARE RATED AT 2200 BUT YOU DONT HAVE TO DO THAT 1600 TO 1800 IS THE SWEET SPOT !
FOUR PEOPLE 2800 POUNDS BOAT BONE DRY ! 15 MPH ! THEY ARE RATED AT 2200 BUT YOU DONT HAVE TO DO THAT 1600 TO 1800 IS THE SWEET SPOT ! Just as Thunder's signature says:
"Use your gearbox as a torque multiplier, not an RPM enabler"
Torque is what makes thrust, not rpms - As long as you have a propeller that will convert that torque at those rpms
That is what the Sensenich Superwide was designed to do. Make a lot of thrust at low rpms and high torque. That is why it works.
Just look at C Chardt's post above. By multiplying his torque and putting the right propeller on, he has gotten much more thrust at much lower propeller rpms. It's just a mindset that needs to change. Especially with the noise issues. Run your engine in the right torque range and multiply the torque with a high ratio reduction unit.
"Use your gearbox as a torque multiplier, not an RPM enabler"
Torque is what makes thrust, not rpms - As long as you have a propeller that will convert that torque at those rpms
That is what the Sensenich Superwide was designed to do. Make a lot of thrust at low rpms and high torque. That is why it works.
Just look at C Chardt's post above. By multiplying his torque and putting the right propeller on, he has gotten much more thrust at much lower propeller rpms. It's just a mindset that needs to change. Especially with the noise issues. Run your engine in the right torque range and multiply the torque with a high ratio reduction unit.