Johnson: 17" instead of typical 15 or 20" measurement?

[mystro]

I ended up buying a lightly used 98 8hp Johnson but what's weird is it measures 17" and not the usual 15 or 20"....the original owner mentioned this on the phone but I thought he had just mis-measured but it seems to be true was as this *another OMC better idea?

 

[tashasdaddy]

Re: Johnson: 17" instead of typical 15 or 20" measurement?

yeah, the work on either, like rental fishing boats. that can be either. most of the '50s motors were 18 "

 

[lifeisfun]

Re: Johnson: 17" instead of typical 15 or 20" measurement?

I got 2006 DF9.9 that is sold as 20"
When I measured the leg it is 22" ....

 

[Chinewalker]

Re: Johnson: 17" instead of typical 15 or 20" measurement?

Very few motors are right on the 15 or 20 measurement. They make them a shade long to take into account keels, strakes, variations in transom heights & angles, etc. Your motor is a short shaft (15-inch) and will NOT work on a long shaft (20-inch) hull - at least not well....
- Scott

 

[mystro]

Re: Johnson: 17" instead of typical 15 or 20" measurement?

Very few motors are right on the 15 or 20 measurement. They make them a shade long to take into account keels, strakes, variations in transom heights & angles, etc. Your motor is a short shaft (15-inch) and will NOT work on a long shaft (20-inch) hull - at least not well....
- Scott

Interesting which requires me to ask, what exactly is a 20-inch hull? I was under the impression it was based on where the kicker mount was located in the fully down position..I'll adjust my mounting to this "17 Johnson but I could of used either a short 15" or the long 20" since I'm reinstalling a new motor mount couldn't I? I get easily confused when I'm near this boat.

 

[WillyBWright]

Re: Johnson: 17" instead of typical 15 or 20" measurement?

The antiventilation plate on little motors rides below the surface. It's a shortshaft.

 

[mystro]

Re: Johnson: 17" instead of typical 15 or 20" measurement?

The antiventilation plate on little motors rides below the surface. It's a shortshaft.

What applies to the long shaft then or rather doesn't it also need to be below the surface and even with the hull bottom same as the SS?

 

[WillyBWright]

Re: Johnson: 17" instead of typical 15 or 20" measurement?

Only models with thru-hub exhaust have the antiventilation plate at the surface. It has to do with prop design. Your blades are swept back. Thru-hubs are just the opposite. It also has to do with the water intake. Thru hubs have intakes well below the antiventilation plate on the sides in front of the prop. Yours is behind the prop immediately below the antiventilation plate in the exhaust snout. Running yours with the antiventilation plate at the surface will starve the water pump and your prop will ventilate almost immediately.

 

[Texasmark]

Re: Johnson: 17" instead of typical 15 or 20" measurement?

A lot of smaller boats, alum especially have a keel that runs the full length of the hull which has to be considered in where the prop sits. Also in the old days, the water pickup on the engine was behind the prop on the front of the exhaust tube and also on the side, just above the antivent plate covered by a plate and 2 screws. Guess this is just a carryover.

Additionally, small motors usually have to push a large boat so there is a lot of prop slippage and all and if it were too close to the surface, with no cupping on the blade and all it would tend to ventilate....suck in air and rev up with little forward motion.

My 2c.

Mark

 

[mystro]

Re: Johnson: 17" instead of typical 15 or 20" measurement?

thx all..I'm starting to get a better understanding on how this all fits.

 

[Texasmark]

Re: Johnson: 17" instead of typical 15 or 20" measurement?

You asked about transom height measurement. There are pictures around here but basically with the boat horizontal, they draw a vertical line from the top of where the engine clamp bracket sits (on the transom....cutout) to the bottom of the hull usually independent of any keel....like you find on an alum boat.

Mark