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Author Topic:   Long Shaft Short Transom Performance 13'
FrostyCold1 posted 06-17-2004 01:36 PM ET (US)   Profile for FrostyCold1   Send Email to FrostyCold1  
I see that many of you have built up the transom height on your classic 13 footers to accomodate a longer outboard shaft. To what extent did you improve performance? Would you do it again?

I am leaning towards keeping my Sport as vanilla as possible unless there is an overwhelming consensus to make the change.

BTW, I was going to do a Google search on Long Shaft Performance but I didn't think it would result in anything I'd allow my kids to read.


lerker posted 06-17-2004 03:24 PM ET (US)     Profile for lerker  Send Email to lerker     
the first hit on google when searching for "long shaft performance" is this page:

completely clean. i have no useful information for you though, i responded mostly due to the humor of a "long shaft performance" google search.

Bigshot posted 06-18-2004 01:45 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
It is not only hurting performance, it is hurting your transom. The stress that the extra 5" of lower unit being dragged around is tremendous, try putting your arm a foot under water at 25mph. If you don't want to build up the transom, get a manual jackplate or a correct engine.
orca25 posted 06-18-2004 03:34 PM ET (US)     Profile for orca25  Send Email to orca25     
If I recall, there are photos on this site of a nice whaler with a aluminum jack bracket with an infill panel that looked pretty slick. I agree that using a long shaft would destroy performance. Unless you want to spend the time and effort with the additional bracket or you're getting a longshaft motor for a steal, , I would just stick with a short shaft model. The jacking bracket would give you lots of adjustability though. My 13' with a new 30 johnson likes the second to highest position on the trim. I've spent lots of time in both styles of 13's and although I would prefer the straight transom, my buddy's 13 ran right with mine, with similar power. Btw, a short shaft is cheaper too!
FrostyCold1 posted 06-19-2004 09:26 PM ET (US)     Profile for FrostyCold1  Send Email to FrostyCold1     
Bigshot, your example of putting you hand in the water is only valid if you put a working prop on it. Seriously!

I agree that the longer the shaft, the greater the force is on the transom due to the principles of leverage. However, all shafts being equal, the drag created by extending the lower unit further into the water puts an opposing vector force against the force created by the prop which nets out to less force transferred to the transom.

In other words, the boat doesn't perform as well because less force is actually directed to the transom. My question asks, how much less power?

Bigshot posted 06-21-2004 10:17 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
As long as you believe that.
Samars posted 06-22-2004 09:55 AM ET (US)     Profile for Samars  Send Email to Samars     

Your boat is designed to accommodate a 15 inch shaft motor. Not sure what there is to understand. Anything different will place stress and pressures on the transom. It is recommended for 15 inch shaft motors or a modification is necessary.

FrostyCold1 posted 06-22-2004 10:31 PM ET (US)     Profile for FrostyCold1  Send Email to FrostyCold1     

I have a beautiful 1958 Johnson Sea Horse 35hp longshaft that came with the boat when I bought it two years ago. The boat is of similar vintage, although the Hull # and Serial # are long gone. It is in very rough shape but with a lot of work it can be brought back.

Needless to say, I am not going to match these two good old pieces of equipment with an aluminum bracket for the sake of performance. The hull was stripped down when I got it and I've never run it. It would reassuring to know that when the restoration is complete that the boat would be more than something that just looks good.

My first option is to find a short shafted lower unit for the motor. No luck so far.

Second, the transom could be built up with mahogany. To the untrained eye it would look original, but.......

Third, I could put the two together and hope for the best.

Tom W Clark posted 06-22-2004 11:38 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     

You need a 15" shaft motor if you want to use it on your boat as is. The issue is NOT transom strength but the hideous spray and lousy performance of the 20" shaft plowing through the water. Buy a 15" shaft motor or convert what you have if you are set on using that old 35 Sea Horse.


Bernie is correct. The lower unit is NOT being dragged, IT is pushing the boat! The stress on the transom is exactly the same as any Whaler with a 20' transom. While I will grant you that the torque the motor is exerting on the transom is one third greater than with a 15" shaft, this is true of any 13' Whaler with a 20 transom.

Given that the Whaler's transom is probably strong enough for ten times the stress a little 35 hp motor will exert, the extra 5" will not do anything to the transom. It will, however, slow the boat down quite a bit relative to a 15" shaft motor.

Bigshot posted 06-23-2004 01:15 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Especially when letting off the throttle. I bet if you did not lock it down, she would lift out of the water with no throttle being appplied.

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