Gear ratio

captchuck

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26' seaway looking at putting in something around 220 - 250 hp diesel. How do I determine the best gear for the boat?
 

captainlarry84

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KristenFormer Charter Captain
Easy Math

First get the builders recommendations
You need to know:
The HP at what RPMs very important
The aperture to determine the largest wheel that will fit
The shaft Diameter to determine max wheel size for load
 

captchuck

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What I'm aiming for right now is finding a 4bt bta which is rated at 250 hp @2800 rpm. I am leaning towards this motor because its seems to be more readily available on the used market verse say a yanmar.
My current set up was a 350 with a 1.9:1 and a 1-1/4" shaft spinning an 18" wheel (don't have the pitch on hand)
Now I have not done nor know how to do all the math but I'm hoping and I may be wrong that I can stick with my current shaft or with a better quality 1-1/4" shaft like aquamet if I go to a 1.5:1
 

eyschulman

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Join boatdiesel.com and go to there gear ratio calculator it will walk you through process. You may also be surprised at all the other cool stuff on that site.
 

captchuck

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Join boatdiesel.com and go to there gear ratio calculator it will walk you through process. You may also be surprised at all the other cool stuff on that site.

Did like you said. Looks like if I go with the 4bta 250 hp with a 1.5:1 I can stay with a 1.25" shaft but would have to switch out my current one with an aqua met or something similar for more strength. Which would certainly be easier that going with a bigger diameter as I would have to pay someone to do so.
Thanks for the suggestion.
 

captainlarry84

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I would like to see 1 3/8 shafting when you go diesel. Ratio I am not sure on a 1.5 gear. Is 18 the max size wheel you can swing? We need more information.
 

captchuck

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I have 2 3/4" clearance between my hull and prop so could prob do 19"
 

captchuck

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Spin a 19 X 19 with the 2:0 gear & a v-6 FFI you will be very happy.

Is less that 2" of clearance except able? Or would using a 4 blade 18 with more path be the way due to clearance? Is there minimum clearance guidelines?
FFI?
 

captainlarry84

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If you already have the 18" us it and pitch it for load most likley with the 2:0 gear it will come in a about an 18 X 19 or 18 x 20. If you have a full 2" with the 19" wheel you should also be fine.
 
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