help determining shaft length

[cbuie]

How do I go about determining the proper shaft length of an outboard motor for installation on my old i/o boat?I am in the midle of conversion but have little experience with outboards and their setup.Also,I have worked on cars and lots of motorcycles (2 & 4 strokes)but not many outboards.If I opt for a used or reman. unit,other than the obvious compression test,what are some tell tale signs of motors to avoid?I have a marine mechanic available to help but hes busy alot (like the rest of us)and I thought I could keep from wasting some of his time by doing an initial survey of my own.Any advise on reman. units?Best brands?(No big debate please!)Used?The new ones in the horses I will need(200 or so)are pretty much out of my price range so any links or numbers of good resources would be greatly appreciated.<br />Thanks in advance!You guys have been a great help so far.

 

[Franki]

Re: help determining shaft length

Not sure what you are asking, but that never stopped me before <br /><br />If you have 20 inches between the bottom of your keel and the center/top of your transom. then you have need of a longshaft..<br />more then that (say around 22 inchs) and you need a Xtra long shaft..<br />significantly more then about 24 inchs and you need to modify your transom or add a pod to the back.<br /><br />(15+ inches would be a shortshaft I think, but thats hardly relevant considering the size boat we are talking about.)<br />Those numbers may not be 100% accurate, but pretty close.<br /><br />As for the brand,, Johnson/Evinrude or Mercury/Mariner, I have met people who swear by one or the other.. don't think you can go too far wrong with either.<br /><br />and there are others what swear by Yamaha and Suzuki as well, and of the older 2 strokes, I don't think you can go wrong with any of these.. its only the newer models that have some issues, like VRO pumps (not an issue, but can be if not looked after) initial ficht models had some issues, as did merc optimax... <br /><br />The only advice I see given over and over again is "Don't get a Force".<br /><br />"Having said that, I have owned chryslers and think they are great (Chrysler became force)...<br /><br />Hope thats somewhat useful..<br /><br />regards<br /><br />Frank

 

[Forktail]

Re: help determining shaft length

You can figure exactly what shaft length you need by doing a little trig.<br /><br />Most transoms are 12 degree sloped (aft to fwrd). There are some exceptions, but you can always measue with a level and protractor. I'll use 12 degrees for this example.<br /><br />First measure the longest distance from the area where the motor bracket will rest to the boat bottom. This is usually the transom height to the boat keel. This will be your hypotenuse measurement. Lets say you measured 25.5".<br /><br />The adjacent side will be the measurement you will need for your outboard. Your outboard is measured from the transom bracket to the cavitation plate, straight down at a 90 degree angle to the ground. Remember, most outboards come in 15", 20", 25" and 30" shafts.<br /><br />Cosine of angle = adjacent side/hypotenuse side<br /><br />So...<br /><br />Cosine of 12 degrees = adjacent side/25.5" (or measuement you took)<br /><br />So...<br /><br />Cosine of 12 degrees X 25.5" = adjacent side = motor shaft length.<br /><br />So...<br /><br />.97815 X 25.5" = 24.94"<br /><br />Thus you would need a 25" shaft.<br /><br />When buying a reman, try to get the name of the shop and the mechanic that did the work. They can tell you more than the current salesman. The previous owner would be better yet. Hours on the motor are a main concern as are where it was operated (saltwater?) and how it was maintained. Good luck