TOPIC: outboard short/standard/long

quick question....whats the difference between short shaft, long shaft, and standard shaft outboards? i need a new motor and unsure of what i have and where to measure mine??????? looking at a 20 inch mariner 60hp, and i think my transom is around 16 inches

Sounds to me like you need a 15" shaft Delirious, are you taking the measurement in a vertical line (with the boat level) from the top of the transom to the bottom of the hull? Scott "Chinewalker" (I think?) did a great drawing on this, I'll have to see if I can find it). Generally short shaft is 15", Standard (sometimes called long shaft) is 20", and a true long shaft is 25" I believe.

Some jack plates will get a 20" motor high enough to run on a short shaft transom, but most won't. (My understanding, maybe one of the more knowledgeable folks can advise on this.)

Yep, Mark has it right. Transoms vary a bit, but mostly it goes 15", 20" and 25".

You need a 15" (short) one!

Frank

thnx for the speedy replies

Delirious:

I have a 1960 Glasspar G-3 Skiboat with a short 15 1/2" transom.
I like the original lines of the old G-3's but wanted a newer outboard motor and they all have either 20" or 25" lower units.

I bought a new old stock (NOS) "Mercury" 50 H.P. Two-Stroke. To mount the engine, I purchased a "Bob's Machine" Hyrdraulic Jackplate and then mounted the engine to the jackplate so I could have the new motor with the 20" lower unit. Here is a photo for you of the set-up:

Photo of the boat, motor and trailer:

Delirious:

I have had NO PROBLEMS at all with this set-up Sir! It has worked excellent even though I was told by some it would not work. You also don't have to go with a hydraulic jackplate as they make fixed jackplates too. They are cheaper than the one I bought. I liked the hydraulic jackplate because I can raise or lower it when I want to and it is real nice when you get into shallow water! This unit has been on our G-3 for three (3) years now.

very nice...thnx for the replies...all this is stressin me out...grrr...new motor??? new boat????? arrrrrgggggg!!!!!

The only thing I have against putting the 20" motor on the 15" transom with a jack, is I don't like the look of it sticking up higher like that. Obviously that would be an owner's preference kind of thing.

jepstr67 wrote:

The only thing I have against putting the 20" motor on the 15" transom with a jack, is I don't like the look of it sticking up higher like that. Obviously that would be an owner's preference kind of thing.

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Yes Sir, you are right on the "owners preference" issue. Personally, if you build a G-3 transom up to about 22" you have the motor "sticking up higher" than it normally would anyway.

Which was why I added that.

I think that is why I've always avoided Mercurys. I never liked the tall thin look. The longer, lower profile of the Evinrudes coupled with the Brooks Stevens styling always attracted me to OMC stuff.

Here is what Mark S was referring to.

That's the one, thanks Neil!

There should be a place in the library for this and how to set up an outboard on a boat for new boaters. Just a brain fart of an idea.

correct me if i am wrong,but i think herters offered a transom on the flying fish of 15 inches and for a dollar or so more,they would sell you one with a 16 1/2 inch transom just for mercury,if i remember right,john

My dad would always add a 1/2" or 3/4" spacer (wood) so the horizontal plate on the motor was even with the bottom of the hull. That was on an Evinrude. Sounds like Mercurys wanted a little taller transom.

Unfortunately, there is no general rule regarding brands. Some early Mercs had the water intake on the underside of the AV plate, so jacking it up so the AV plate was ABOVE the bottom of the boat could lead to a no cooling situation.

Motors were built with a bit of a fudge-factor due to variations between boat manufacturers and how they designed their bottoms and transoms. This is why a 15-inch shaft motor may actually come in around 17 or 18.

Bottom line is that no two set-ups will likely be the same. Best way to rig any boat is to start with the motor flat on the transom (assumption being that the motor and boat are both short/short or long/long) and then work your way up from there. Stopping points will be when you either run out of cooling water or start blowing out props (sucking surface air into the blades). Some folks alter the cooling water pickups to allow higher settings, and others play with different props (pitch, cup, rake, etc.) to allow the prop to hold its bite at higher settings, or a combination of both water and prop.

Getting your motor higher not only makes you go faster, it's just plain more efficient. As hydrodynamic as a lower unit might be, having less of it in the water lessens drag significantly.

One thing I have always done was take a 4 foot level and hold on the center line of the boat bottom and see exactly where it is in the tilt of the outboard and where the water inlet is. You don't want to be sucking air into your inlet so make sure it is below the lowest part of the boat. My keel is internal so I don't have much of a keel externally. Every boat design is different.

Hi Guys,

I grew up with the cav plate at the bottom of the hull rule. I get it. Very conservative and solid rule.

But with my recent experience squeezing performance out of a boat, I've seen (what lots of others have seen before me) that rule isnt necessarily correct, with the goal of maximizing performance/speed from a boat. Merc says in my service manual, that for every 1" you can raise your motor, you see 1-2 mph increase in speed... for sure, I've seen it.

I have the same boat as 1960RedG-3, 1959 G3 with a 15-1/2" tall transom. If my motor was clamped to the transom, it's 15" shaft length would put the cav. plate 1/2" above the bottom of the transom.

Keeping in mind that a boat doesnt run... even on a plane, parallel with the surface of the water. The bow rides at an upward angle, meaning the water surface plane is at the same upward angle in respect to the transom. In the case of a jack plate, or my CMC tilt and trim unit, where the motor is mounted 6" behind the transom... the water surface plane (coming up at an angle) is much higher than the bottom edge of the transom.

On my boat with the 6" setback... I ran my short shaft Merc (15") cavitation plate at 2.5" above the bottom of the transom, and never cavitated. This year I have it now 4.5" above the bottom of the transom... where it breaks loose when the boat drops onto a plane for only a few seconds... and no loss of water pressure even with it trimmed way up at high speed. Soooooo... from my recent attention to setup, I have found that at 6" behind the boat (like a jack plate) 2.5-3.5" there is no cavitation, at 4.5" up there is about 4 seconds of cavitation, as the boat drops onto a plane.

At 45 seconds into this video, you can hear the motor cavitation and then grab traction at 4.5" vertical, with no trim (cav plate is parallel with bottom of hull).


...no big statement here, its just my experience this season and maybe something to chew on.

Dean

I like pics, but I LOVE video. Of course If I were looking for real speed I think I'd lose my 8 hp 4 stroke. Yes extending the outboard away does increase her bite.

As always,...GREAT POST Dean!

While my G-3 does not run as fast as yours Dean, my hydraulic jackplate also allows me to get that motor up in the area you describe. With just me in the boat, when I am "level" with the cav plate to the bottom I am running 44 MPH. But, as I raise the jackplate my speed increases to a top of 47 MPH. That is with a 15 pitch aluminum "Mercury" Prop. I just acquired a 17 pitch aluminum prop which I have yet to try. Not bad for a 50 H.P. motor!

Dean--So what kind of top speed are you getting with the 850? Also, are you running an aluminum prop, and what pitch?

All--Obviously the ideal would be PTT and a jack plate, but if you could only have one or the other, which do you think would be best for top speed and/or overall performance (and let's assume the jackplate is hydraulic)? Opinions?

Shannon in Crystal River

Hi Shannon,

Sorry about the long wait on the response, I've been out of town.

I recently hit 55 mph using an older Quicksilver stainless steel 3-blade prop with 19" pitch and cupped. I tried a lot of different types of props (diameters, pitch, aluminum, 2 blade / 3 blade) and this one is the fastest and with the three blades and cup, holds its bite pretty good. Not as quick out of the hole as my 17" aluminum Quicksilver 3-blade. The worst was a bronze 2-blade old Merc prop; couldnt trim much or make hard turns without it spinning loose.

Hi Shannon,

Sorry about the long wait on the response, I've been out of town.

I recently hit 55 mph using an older Quicksilver stainless steel 3-blade prop with 19" pitch and cupped. I tried a lot of different types of props (diameters, pitch, aluminum, 2 blade / 3 blade) and this one is the fastest and with the three blades and cup, holds its bite pretty good. Not as quick out of the hole as my 17" aluminum Quicksilver 3-blade. The worst was a bronze 2-blade old Merc prop; couldnt trim much or make hard turns without it spinning loose.