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Gunno

Underpropped (a little) help please.

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So I got a new Thunder Jet 186 Rush boat with a 90hp Tohatsu (Honda really). Anyhoo, i had the dealer set me up with a high altitude prop (i usually fish at 4000 ft, sometimes higher, sometimes at seal level). So i definitely want to get some increased top end speed. Right now I get on plane instantly so dont mind losing a little of that. Okay, so my question is... go from my current 3 blade 13.5x15p to a 3 blade 17 or, and this is what I'm curious about... stay with 15p and switch to 4 blade. I'm thinking it may be a real sweet spot. Any suggestions appreciated. 

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Need your rated max RPMs for the motor. Then need your RPM numbers at WOT with a average load on the boat along with the different altitudes your operating in for those numbers ,which may be different. Otherwise we/you are just guessing and may be lugging the engine or going over your redline , which in either case leads to the early demise of your power plant.

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Seeing as you used selling dealer for high altitude setup, I think I would go back to them with your question for additional top end.  Could very well be you need a low level prop too.  Dealer should have an idea of how the motor ecu adjusts to the altitude difference.  Out East here on Great Lakes we don’t deal with high altitude setups.

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53 minutes ago, Bozeman Bob said:

Need your rated max RPMs for the motor. 

Max rates are 5300-6300 for this motor. I could over rev this motor if I wanted to so I know I need to increase pitch but I am wondering if keeping the current pitch but adding a 4th blade instead. I've gotta have decent holeshot for when I take it to sea level (outrun a swell coming aft of me)

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13 minutes ago, mr 580 said:

Seeing as you used selling dealer for high altitude setup, I think I would go back to them with your question for additional top end.  Could very well be you need a low level prop too.  Dealer should have an idea of how the motor ecu adjusts to the altitude difference.  Out East here on Great Lakes we don’t deal with high altitude setups.

Yeah... the dealer in my case are pretty useless I hate to say. I only bought if from them because they had the boat i wanted. The ECU is going to help advance ignition timing but it can't get more air (high altitude symptom) into the motor. The standard practice for mountain lake fishing is get a prop with decreased pitch prop (which usually come in 2 in increments). What I'm wondering is that maybe going to a 4 blade instead of increasing the 2 inches. Like I mentioned above I could read line the motor as is, so it's slightly underpropped even for high elevation. 

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going from a 3 blade to a 4 blade is roughly the equivalent of dropping 1" of pitch. With such a wide range of max rpms and the fact you're over revving even with that wide of a range. I'd would guess you could likely go to a 4 blade and still go up an inch or two in pitch. I just put a 16p 4 blade on my boat so I know there are 16p 4 blades available (mine was a 15x16p). Goodluck

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Thanks. Didnt even think of a one inch bump to 16. Probably not gonna be a cheap prop.

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1 hour ago, Gunno said:

Thanks. Didnt even think of a one inch bump to 16. Probably not gonna be a cheap prop.


Depends on if you're fine with aluminum or want SS. The aluminum 4 blade 16p Michigan Wheel prop I just bought with hub kit was $145 shipped.

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