What Gear Ratio

WoundUpMarine

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Blitzen

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Question is, Do you have room for a larger diameter prop and maintain 15% tip clearance with the larger reduction. Keep in mind you want to try and keep the prop as close to square as possible.
 

WoundUpMarine

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traditions

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What do you have for a gear now?If you have a warner gear and want to upgrade to a twin disc or ZF I could see maybe changing reduction.If not ,the price of a new gear and wheel will probably deter you from switching.If you add a after cooler you will probably have to re wheel bigger any way,but a new or even used gear will be at least 2 or 3 grand.I like the bigger wheel turning slower,but alot of times it is what fits the boat and engine combo that works best.If you call a wheel shop like Nautilus they can run the numbers and tell you pretty close what would work best.
 

WoundUpMarine

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F/V First Team

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Top clearance looks good to go bigger, not sure as to the bottom clearance, angle is hard to view, 2:1 lets you swing more pitch or the same pitch + cup, every boat is different. Keep in mind how you plan on loading the vessel and how often it is loaded.
 

WoundUpMarine

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captainlarry84

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Cost???

A 1.5 to 2:0 gear is a very big jump you will reduce shaft RMPs by 25%. Therefore the new wheel will be 25% larger which could be any one of the following sizes:

20 X 24 if wheel clearance is not there not very efficient size

22 X 22 perfect

23 x 21 also good.

So these are you new numbers with a 2:0 gear, but here in lies the problemyour shafting needs to be bump up the 1.75.

New gear, shaft, wheel.$$$ I do not think it is worth the expense.

We can figure out your current vessels efficiency to better determine the benfit of a gear change. To do so I need one more item which is your RPMs @ cruise & Full throttle


Looking at your bottom with a 22 wheel you only lose 1 of clearance and you certainly have that no problem. 2:0 gears are the best if you can swing the wheel. Reduced shaft speeds increase running gear life, Larger wheels can grab more water because the field of the blade is not blocked by the keel, plus with a larger wheel the boats handle better & dock better.

However as stated you need to make a lot of changes. Shaft changing can get real expensive if your current tube will not fit the larger bearing, then you get into a tube change.

CCI12182009_00001.jpg
 

WoundUpMarine

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BillD

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right now i am cruising at about 2150 @15knts and 2600 for about 19.5 knts. no numbers on fuel burn as i havent really paid that much attention to it yet. do i really need to bump up to a 1.75 for a 22x22 if im only pushin 226 horsepower. i know ill be increasing torque on the shaft by making the wheel bigger but thought a 1.5 could handle that size wheel

Woundup,

I've been following your thread on changing props, changing gear ratios and all the variable running gear (shaft) changes required to do that.

If you change the gear, the shaft and the prop you still have the SAME hp in the 30 YB you have now.

Capt. Larry spelled it out pretty good. Not much to gain by changing anything.

If this was my 30 YB, (would love to have a DE boat) I'd stay put with any changes or $expense$ until a repower.

With a repower you have the extra hp to get some real speed/fuel benefits.

JMO,

Good luck, I enjoy following these posts.

Bill D



"why bother to do all this work and $expense$ to gain a 1/2 knot or so"?
 

WoundUpMarine

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captainlarry84

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These are my numbers...

Yes a 22 X 22 is a big wheel for 1 ½ shafting. HP does not break shafts reduction gears & wheel sizes do that. A 20” wheel has a min bore of 1 ¼ up to a max bore of 1 ¾

A 22” wheel has a min bore of 1 3/8 to a max of 2” so 1 ¾ puts you in the sweet spot with that size wheel. Plus with a single screw boat the last thing you need is a snapped shaft.

As for your boat, lets see how efficient she is:

Right now i am cruising at about 2150 @15knts and 2600 for about 19.5 knts.
With a 22 X 18 Wheel

At 2600 RPMs you are getting 77 % wheel efficiency 100% efficiency would put you at 25.69 knots, which you never get with a keeled boat. 85% is about the most, therefore you are doing pretty wheel at 2600 RPMs with your current wheel.

However at 2150 RPMs you really drop off to only 71% efficiency.

Based on the 71 % @ 2150 RPMs you would get better performance & speed with more RPMs at Cruising speed. I would try running the boat @ 2300 to 2400 RPMs. Your miles per gallon should also increase.

Based on all of this lets figure your boat out with a 2:0 gear a 22 X 22 wheel and 84% efficiency:

At 2600 RPMs with a 22 X 22 wheel at 84% you would top out at 19.78 Knots

At 2150 RPMs with the 22 X 22 wheel at 84% you would cruise at 16,36 Knots

Those are my numbers the reduction gear change you would pick up 1.36 knots @ cruising speed & .25 knots WOT.

I would say you are where you want to be unless you want to spend about $7,000 plus for a gear, wheel, shaft & bearings.
 
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WoundUpMarine

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captainlarry84

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These are my numbers with my math. It is a formula I use based on prior repowers and posted information.
If I was building a 30’ YB it would have a 2:0 gear and I would look for a motor with anywhere from 2600 RPMs to 2800 RPMs. If I went with a 315 Yanmar a 2.5 gear with be a very hot set up.
As stated earlier the big thing you get with a slow large turning wheel is handling in rough weather, better speeds with full load and maneuvering in tight spots like docking.
 

WoundUpMarine

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