50hp, 15" shaft? do they exist?

[pcrussell50]

Probably shouldn't be a power hungry fool...but i can't help at least _considering_...

Anyway, my 1960 sporty little 13.5' span america does pretty well with its 35hp big twin, '59 super seahorse, rds21b. it's the one that fits 15" transoms. the rdsl is for 20" transoms. still, i'm a hopeless, opportunistic bargain shoper, and i comb craigslist all the time, looking for steals...especially on outboard motors. but again, my little boat calls for 15" shaft motors. so i was wondering if there are any outboards bigger than 35hp that still use a 15" shaft?

i looked at evinrude, mercury and honda's web sites just for reference, [i'm a johnson/rude kinda guy], and none of their modern 50hp or 40's have a 15" shaft...all 20" or some are 25". and as an aside, my but the "new iron" is heavy!!! i'm happy with my "old" light 35hp!
[yes, i know they're 4-strokes, but still]

sooo, anyway, if i were looking for a 50hp in a 15" shaft, is that a fruitless search?

-peter

 

[Chinewalker]

Re: 50hp, 15" shaft? do they exist?

Yep, they're still around. A friend of mine bought a new Merc 50 shorty a couple years ago for a vintage plywood hull he had. I recently had a 1980s version in my shop, too. OMC even made a 75hp short shaft 3-holer for a few years for high performance applications...
- Scott

 

[mthieme]

Re: 50hp, 15" shaft? do they exist?

They are rare.
This is the only thing I use on the hydroplanes.
I have a 20" 50 hp now and try to figure out how to design a transom to accomodate it.

 

[TN-25]

Re: 50hp, 15" shaft? do they exist?

The old OMCs as late as the late 1960s offered 15" shaft versions of their larger motors right up through the 90 horse V4. As was stated by others, they would be rare. Mid-range motors after that era that might yeild a 15" shaft are the 50 / 55 / 60 horse twins from 1971 into the early 1980s. The triples offered 15" shaft length on certain horsepower ratings, usually on the performance oriented versions. Even high performance boats like Sidewinder still usually required a long shaft. I'm thinking about a racing tunnel hull as being the original intended recipient of a motor of that type. One thing I have seen is factory type power trim was not offered for those short shaft versions.

 

[pcrussell50]

Re: 50hp, 15" shaft? do they exist?

Mid-range motors after that era that might yeild a 15" shaft are the 50 / 55 / 60 horse twins from 1971 into the early 1980s.

50 is about the absolute max that my little 13 footer could tolerate. Hell, It runs pretty well with the 35 it has now. The 35, according to some link about big twins supposedly weighs 136lbs. i wonder what a nice, light, pre-efi-era, 2-stroke 50 weighs? i'm not averse to slight overpowering, but i'm not reckless.

-peter

 

[TN-25]

Re: 50hp, 15" shaft? do they exist?

50 is about the absolute max that my little 13 footer could tolerate. Hell, It runs pretty well with the 35 it has now. The 35, according to some link about big twins supposedly weighs 136lbs. i wonder what a nice, light, pre-efi-era, 2-stroke 50 weighs? i'm not averse to slight overpowering, but i'm not reckless.

-peter

180 lbs for a 1975 Johnyrude 50 with a 15" shaft; 20" is 187 lbs.

 

[tashasdaddy]

Re: 50hp, 15" shaft? do they exist?

your solution is to add a jackplate so you can mount a 20 on a 15inch transom. this is a 50 20 inch on a 1966 14 ft, Sportcraft.

 

[pcrussell50]

Re: 50hp, 15" shaft? do they exist?

wow! very nice. i was afraid there would be no solution at all. can you tell me more about jack plates? are they custom made for the app? or are they available "off the shelf"?

also, barely on topic, can you post more pics of your boat and motor? or link me to where they are hosted? i'm interested in seeing your whole rig, not just the motor.

and also, i have to ask, is the jackplate solution "kludgey" or fraught with compromise? i was just doing a little online shopping and there's a commmon one, this "mini-jacker" that says it's good for up to 35 hp, so 50 is a stretch, unless it's really overengineered and using it on a 50 is ok.

-peter

 

[tashasdaddy]

Re: 50hp, 15" shaft? do they exist?

mini jacker is not strong enough. photobucket is down for maintainance, will try to get you pics later today.

for the off the shelf Part # JP-4FA-DP is the one i would use. http://www.iboats.com/T_H_Marine_Hi...59951643--**********.883775848--view_id.40130

i had mine fabricated locally 3/16 aluminum total with bolts was $60.00, i had the plywood.

 

[F_R]

Re: 50hp, 15" shaft? do they exist?

Keep in mind that a jack plate raises the leverage on the transom. Unless it is overbuilt, which is doubtful, you might pry it right out of there. You are already overstressing the transom by contemplating a bigger/heavier/more powerful motor, and if you add to that by raising the motor you might be going for a swim. Or worse.

 

[pcrussell50]

Re: 50hp, 15" shaft? do they exist?

Keep in mind that a jack plate raises the leverage on the transom. Unless it is overbuilt, which is doubtful, you might pry it right out of there. You are already overstressing the transom by contemplating a bigger/heavier/more powerful motor, and if you add to that by raising the motor you might be going for a swim. Or worse.

in the back of my mind, i knew this would come up. i may be new to boats, but i have a mechanical engineering degree, and i understood what these jack plates would do. but thanks for not letting it slide as all newbs like me should know what they might be getting into.

little cocktail napkin calculating here...

putting an engine that's 50lbs heavier on, and moving/spacing it 4" off the back of the transom increases the moment on the transom by...well...not so much...let's see 50lbs x 0.33ft = 16 foot-pounds of increased torque trying to tear the transom off.

BUT, i imagine the REAL threat is not so much the increased weight, as the increased thrust multiplied by the increased moment arm. i haven't got the foggiest idea how much thrust a 35hp motor makes versus a 50hp. but the difference must be about 40percent'ish, which is the increase in power, [remember, cocktail napkin, not calculator]. and when you also increase the moment arm by a third, [the result of raising the motor by 5" over a 15"], i imagine the resultant difference in torque trying to tear the transom out would be significant. maybe nearly double. ouch. i'm going to have to put some serious thought into this.

on edit: remember, mine has a 35hp, but i'm not sure what the max recommended is. it's a 13.5" fiberglass, sport hull from 1960. sadly, i don't even know enough to know what the typical max power for a configuration like that would be.

-peter

 

[samo_ott]

Re: 50hp, 15" shaft? do they exist?

Hmmm, I was just reading my wife's mechanical engineering book today but I didn't see anything in it about Jack Plates... Lots of beam deflections, pinion gears, bevel gears, chains, clutches and other interesting stuff though...

 

[pcrussell50]

Re: 50hp, 15" shaft? do they exist?

mini jacker is not strong enough. photobucket is down for maintainance, will try to get you pics later today.
...snip...
i had mine fabricated locally 3/16 aluminum total with bolts was $60.00, i had the plywood.

any luck getting photobucket back up? i'm dying to see more of your classic looking rig. it reminds me of mine, from what i can see.

also, not to pit you against another senior member, but i'm curious as to your take on the idea of the increased stress a jacking plate causes. or rather, what thought process you went through to justify it. of course, if you weren't increasing the power much, if at all, it's probably fine. anyway, just wanted your thoughts.

-peter

 

[pcrussell50]

Re: 50hp, 15" shaft? do they exist?

Hmmm, I was just reading my wife's mechanical engineering book today but I didn't see anything in it about Jack Plates... Lots of beam deflections, pinion gears, bevel gears, chains, clutches and other interesting stuff though...

bet calculating moments is somewhere in the first chapter. fwiw, i'm not a practicing engineer, never was, really. just used the degree as a means to an end. i've forgotten a ton in the 16 years since i graduated, but moments will be with me forever!

-peter

 

[samo_ott]

Re: 50hp, 15" shaft? do they exist?

Hey, she said the same thing today when I found her book... Has never practiced it and that she has forgotten most of it in the last 20 years! Funny...