Standard shaft (20") on a short-shaft (15") transom?

wolfee

Seaman
Joined
Aug 20, 2007
Messages
71
I've been looking forever for a good deal on a 50hp short shaft tiller outboard and have never bought one yet. I have a 25hp Mercury short shaft that just can't get the boat on plane and the max HP for the boat is 50hp. I saw a listing from Yamaha about a 50hp short shaft (new) for 3,800 and it seems too good to be true. I'm thinking of buying a standard shaft (much easier to find) and using a jack/plate/whatever it's called to adjust the prop height. Does anyone have any advice on a good jack plate that I could put on my transom? It's a 16 1/2 ft Sea Nymph, aluminum "Fishing Machine". Any help appreciated.
 

Chris1956

Supreme Mariner
Joined
Mar 25, 2004
Messages
27,036
A 20" shaft length is called "Longshaft".

There were some motors in the 50HP power range that were shortshaft, but not sure how many there were. Many 40HP motors would be shortshaft, and should plane your boat fine.

A jackplate could make a longshaft work for you. It would need to have the correct height adjustment and be mounted in the proper place on your transom.
 

jimmbo

Supreme Mariner
Joined
May 24, 2004
Messages
12,944
I haven't seen a 15" 50 hp since the early 70s, except for some racing engines by OMC. I'm surprised a 16 1/2 ft boat would come with a 15" Transom.

The Yammie sounds like the Answer to your Prayers. Is a Tiller, or can be converted to one? Yamaha makes a good engine
 

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
Some issues :

So it's a long shaft motor mounted on a short shaft boat ? what's the transom height, is it possible to shoot a pic of said motor showing the lower leg configuration ?

OTOH if the boat is used light / medium light loaded don't need a 50 HP motor, If the motor runs strong a prop maximization to a less prop pitch will do the trick. to plane faster. For that will need to install an induction tach to play safely with less prop pitches if currently available...

Happy Boating
 
Last edited:

JimS123

Fleet Admiral
Joined
Jul 27, 2007
Messages
7,952
Back in my youth I had a boat with a long shaft transom and a long shaft motor. A young fella down the street had the same boat, but with a short shaft motor and a short shaft transom. Both were same HP 'Rudes.

We were talking one day comparing performance and mine was better. He assumed that it because my motor was longer and down lower in the water, so he took it off and cut the transom down 5".

Now, obviously he wasn't a brain surgeon and he did this without consulting me about transom height, etc.

Anywhoot, before his boat was a little slower, and now it ran like crap.....LOL.

Sooooo, to the OP, use the jack plate as recommended, put the engine at the proper AV plate height and THEN maximize the prop. No prop change will fix your problem.
 

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
There's the erroneous idea that a short shaft motor willl 100% match a short shaft transom and a long shaft motor will match same a long shaft transom. Will be extremely lucky if that has been achieved on your current combo. The usual perfect spot on motor/transom height match won't be dialed as there are several hidden tech aspecs the boat and motor manufacturer has not mentioned whatsoever to avoid tech conflicts betwen both parties, owner include, that's if wanting to get the max water performance out of your prefered motor brand powering whichever boat brand model you choose to boat with. To achieve that will need some to fine tune the motor height with respect to the transom.

In the previous example of 2 similar boats model and HP motors, if both lower legs were set at same equal height with respect to the aft boat's bottom no matter if being a short shaft on a short transom or a long shaft motor on a long shaft transom should have run equal assuming that both motors were running 100% strong, both with same trim position, with deck load well distributed and boaters having the exact same weight for a true 1-1 tech comparison. Did you know that besides an S and L height transoms there are (M) middle heights ones....

Running a long shaft motor on a short shaft transom will produce excessive water showering at back transom at speed along huge lower leg drag which will inevitably slow the combo down. Will be only good for fast displacement speeds..

Question : Define which is the proper AV plate height ?

Happy Boating
 
Last edited:

JimS123

Fleet Admiral
Joined
Jul 27, 2007
Messages
7,952
Question : Define which is the proper AV plate height ?

Simplistic response....

What is your objective? Are you an Engineer that is designing a system to be so awesome that you will generate enough data to support your patent application? Or, are you an average Joe that simply wants to take your boat out with the family and go fishing and have a nice day on the water? In the latter case, performance if of little concern and AV placement is not even a consideration.

To reply with a concise answer, put the AV plate wherever it lines up with the transom height. (Probably down low because that's the way the manufacturers designed them). That's the average Joe and he doesn't care. Family happy.

If you are an efficiency addict, raise it up one notch at a time, collect data, and put it where you find is the best position. "Best" is whatever you personally consider "performance" to be.

If you have a rubber boat, save your pennies and buy a real boat.

(PS - I'm still waiting for the authoritative all encompassing treatise on engine position)
 

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
Yep, that's the cast on stone answer was 101% sure to receive as all swears by it.

You're so experienced with many motors through your boating years that totally forgot that Clamp screw motors can't be raised or lowered one-two holes compared to Bolt On motors to start with, for that will need to modify the transom height raising it or shaving the transom down to dial the cast on stone height setting to happen. No wonder why everyone here throws clamp and bolt on motor on same sack and expects to water perform exact same which is a complete fallacy..

What's the point ridiculing the powered boats I like to boat with. Aristotle once said : The perfect boat it's the powered cork which most pleases you. For traditional boaters understanding, Pro Ribs rely on their deep V hulls as any other traditional boat, not on their inflatables tubes compared to small tender dinghies which are water toys. Why be a Mediocre Joe Boater of the bunch when you can be a Super Joe Boater....

Tomorrow will post my "authoritative treatise"...

Happy Boating
 

JimS123

Fleet Admiral
Joined
Jul 27, 2007
Messages
7,952
To the OP...

You were talking about buying a 50. Since most (not all) are bolt-ons I didn't discuss how to adapt a clamp-on model.

Regardless, since your intention is to use a jack plate, bolts and clamps are moot points since you have the ultimate adjustment setup.

The key to a proper setup is a combination of height optimization and max engine rpm maximization.
 

Faztbullet

Supreme Mariner
Joined
Mar 2, 2008
Messages
15,589

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
Where did you get that was buying a 50 HP motor, was just referring to your post about comparing the water performance between a S and L shafts, did not posted which HP motors were.

Nope, never was on my mind nor intended to use a transom extension just wanted to corroborate the usual AV plate/ even with the boat's bottom cast on stone answer. Count with an expert fiberglass techie that can rise or shave by the millimeter any transom for less than $ 80.00 peanuts cost, includes hand labor, material and 2 cold ones. Playing by the millimeter is a zillion times better than installing a fixed height transom extender which have different fixed heights if plan to fine tune the water flow passing under the boat's bottom through the lower leg for best water overall performance.

Happy Boating
 

JimS123

Fleet Admiral
Joined
Jul 27, 2007
Messages
7,952
Where did you get that was buying a 50 HP motor, was just referring to your post about comparing the water performance between a S and L shafts, did not posted which HP motors were.

My last response was directed at the Original Poster and no one else. In post #1 he indicated that he was looking for a 50 horsepower Outboard and he wanted to put it on a Jack Plate. My comments were directed at his desires, and i suggested how he could optimize his boat's setup.

Actually, a transom extension is not a bad idea. Hanging the guy without making any specific adjustments would probably make 99% of the boaters happy. We all just want to have a good time with the family. Optimizing is for the other 1% that are just boatnuts....:)

For anyone that may still be cornfused, simply read the whole thread from the beginning and it should all make sense.
 

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
Yep, sorry for the mistake, realized late that were refering to the Original Poster and too late to errase it. The OP turned out bit confusing, probaly threw the towel already. LOL !! Tomarrow morning will post what wanted to know about the lower leg plates interaction, was gathering info to post and understand the differences without too much writting.

Happy Boating
 
Top