Outboard shaft length?

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dihwp734

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How do you determine the shaft length on an outboard? What would happen if a motor with a 25" shaft was rigged on a flats boat with a 20" transom?
 

ob

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Re: Outboard shaft length?

Without a jackplate it wouldn't be possible to get the cavitation plate of motor even with boat bottom .This would result in motor running too low in water and lead to operational problems.
 

Hooty

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Re: Outboard shaft length?

Measure from inside the clamp (the part that sits on the transom) down to the cavitation plate. Boats with short shaft transoms and long shaft engines normally don't perform very good. Too much engine in the water.<br /><br />c/6<br /><br />Hooty
 

Bear

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Re: Outboard shaft length?

shaftexmp.jpg
 

digimortal777

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Re: Outboard shaft length?

that picture contradicts what everyone onb the board says......<br /><br />the shaft there is longer then the transom height....<br /><br />everyone seems tro say 20 inch trransom = 20 inch shaft<br /><br />then you get people sayin the plate must be even with the bottom of the boat.....<br /><br />if thats the case then u need a 25 inch shaft on a 20 inch trransom
 

mellowyellow

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Re: Outboard shaft length?

sorry, but you are the confused one friend..<br />the shaft lenghth is the measurement from transom<br />to cav. plate... see the 2 red arrows in Bear's<br />pic.<br />hope this clears up any confusion.<br />regards,<br />M.Y.
 

radsrh

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Re: Outboard shaft length?

The shaft goes between the power head and the lower unit, so the measurment from the top of the clamp to the cavitation plate is roughly the bottom of power head to the top of the lower unit.
 

wilde1j

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Re: Outboard shaft length?

Too long a shaft motor will result in a significantly slower boat due to drag. The best is to match the boat's need for shaft length. If you measure a conventional hull from the middle of the transom where the top of the motor bracket sits to the bottom of the hull ... this is the dimension you want to match to the motor shaft length (15", 20", 25" and 30" are the standard lengths).
 

Forktail

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Re: Outboard shaft length?

Bear's description to answering the original post is very explanitary in basic terms.<br /><br />However, I will add that not all modern outboards come in exact 15, 20, 25, and 30" shaft lengths. Although they are sold as such, many are actually shorter or longer when actually measured.<br /><br />Likewise many boat transoms don't always match up to to the outboard's shaft length for optimum performance. This is because most transoms are at an angle of 12-15 degrees. For example on a 15 degree transom, an exact 25" shaft outboard would require a transom length (measured along the hull transom) of 26" to line up the bottom of the hull with the cavitation plate. I'm curious as to how red18 measured the 20" transom.<br /><br />Additionally not all outboards are designed to run even with the bottom of the hull. And not all boats are designed to have the outboard run even with the hull.<br /><br />So...it's a combination of the particular outboard's rigging specifications and the boat manufacturers rigging specifications, and proping. This will get you close. Then it's all about testing and making fine-tuned adjustments.<br /><br />That's why jack plates are so popular..... :)
 

dihwp734

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Re: Outboard shaft length?

I measured 20" from the center of the transom, however, there is a step where the bilge drain is which if measured all the way down to the keel would probably add about 5". Which measurement do you use. The boat is an 18' Hewes redfisher.<br /><br />On the motor do you measure the upper inside of the mount or the top of it?<br />Thanks for all the help
 

Hooty

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Re: Outboard shaft length?

Upper inside, the part that sets on the transom.<br /><br />c/6<br /><br />Hooty
 

Hooty

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Re: Outboard shaft length?

...and use the measurement from the top of the transom to the keel.<br /><br />c/6<br /><br />Hooty
 

ob

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Re: Outboard shaft length?

Red18, Yamaha Motor Company calls for this model engine for the 18ft.Hewes redfisher.F115TLRZ Boat specs top rig out with 150hp.
18_red.art
If this looks like your rig you will surely need a jackplate to accomodate a 25" shaft motor.
 

digimortal777

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Re: Outboard shaft length?

ok well my 15 hp johnson is a short shaft.....<br /><br />the measurement of the transom is 20 inches<br /><br />thwe measurement from the clamp braket to the very end of the shaft is also 20 inches<br /><br />no my boat is a very old design and the engine does not come up high out of the water i have a very narrow stern on my boat...is my setup wrong or right.<br /><br />It is the same setup everyone uses here
 

Bear

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Re: Outboard shaft length?

digimortal777,<br />There is nothing wrong with the picture or what it says. Maybe just that the cavitation plate is not exactly level with the bottom of the boat, but never mind. The standard setting is level or .5 inch below the keel. Then you can raise the engine a hole to raise it an inch for less drag, more speed at wot and better fuel economy. If you don't get any problems with water pressure, cooling, ventilation or cavitatation in sharp turns.<br /><br />In your case digimortal777, is the propeller above the bottom of the boat? Do you have a picture of it so we can see it? <br />What about the hull shape, like a sailboat, rowboat or a... banana. <br />Don't confuse the way shaft length is measured in this case, with the actual length of the shaft itself. (From crankshaft to propeller shaft.)<br /><br />I really want to understand what you have and what you mean. I picture would be nice. Is there perhaps a web site with one for sale?
 

Bear

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Re: Outboard shaft length?

Digi,<br />You said your transom is 20". Is that just the transom or from the top of the transom to the bottom of the keel. That is a big difference in your case. I belive you have a keel that ends about a foot in front of the transom. I have a row boat too. With a keel.<br />This was the best picture I could find. Not too good. But if you look close you might see what I mean about the keel and the transom.<br /><br />
boat385_details.gif
<br /><br />Edit: sorry about the distraction, but we are talking about the picture I posted. Digi said it was someting wrong there and I thought it was okay to straighten that out. I thought about it twice though. ;)
 

ob

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Re: Outboard shaft length?

red18,You still with us?Hope the rowboat topic didn't destract from your original post.
 

93bronco

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Re: Outboard shaft length?

i had a similar ? sometime ago on my bayliner cobra. it had a step hull design i think it was 18 or 19 inches at transom, then 10 inches back it dropped another 4 inches. it was a fish/ski and from what i got it was for shallow river/lake. also to use only a 20inch shaft length on it. hope that helps
 
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