Short to Long Conversion.

[slader99]

Hey all,

I have an older 14' Aluminum, transom measures 19.75" Did a bit of reasearch before purchasing a 25hp 77' Johnson sea-horse short shaft (model 25R77H). I got the impression that as long as the cavitation plate was within 2" (above or below) the bottom of the hull everything was fine.

When the motor is on the boat, the cavitation plate is about 1.75" above the bottom of my transom. When I took the boat out this afternoon it seemed like the prop was cavitating past 1/2 throttle. I played with the tilt of the motor but it didn't seem to make a huge difference.

Based on the information I have provided do I need a long shaft? Do you think I would be better off financially to purchase a conversion kit (if available), or some sort of bracket (if it exists)? Any suggestions whatsoever would be most appreciated

Thanks in advance,

Colin

 

[wilde1j]

Re: Short to Long Conversion.

You need a 20" motor, but conversion is not very economical. I would consider a manual jack plate.

 

[tmcalavy]

Re: Short to Long Conversion.

You can convert to long shaft probably for less than $200, using used parts. You'll need the leg/housing extension, longer driveshaft and water tube(s), and the long shift shaft connector. A homemade jack plate would be easier, you won't have to lower it much. Search this forum/site for Tashasdaddy...he has posted pix in the past of how he made his own jackplate.

 

[slader99]

Re: Short to Long Conversion.

Thanks for all the replys folks. I have searched the forum for jack plates and found a couple threads with pictures, unfortuneatly all the threads are regarding rasing the motor. Can I still effectivly lower the engine 2" with a jack plate, and if so could you point me in the right direction (thread, pics or otherwise?)

Thanks,

Colin

 

[wilde1j]

Re: Short to Long Conversion.

It all depends on how high you mount the jack plate. When set up right, the cav plate will just be uncovered with water @ WOT.

 

[slader99]

Re: Short to Long Conversion.

So it sounds like a jack plate could potentially lower my engine the needed 2-3". Could someone please post a photo of such a setup? All the jackplates I've seen so far raise the engine. I would have thought that there would be interference between the powerhead/tiller and top of transom if you tried to use a jackplate to lower the unit.

Thanks,

Colin

 

[mikesea]

Re: Short to Long Conversion.

I may be wrong here.But I believe you OK ,Are you sure your prop doesnt have a spun hub.Basically you have a 20 in transom and the eng is 20.The older boats didnt have transoms for 25 inch.

 

[wilde1j]

Re: Short to Long Conversion.

Check w/ Bob's Marine for more info on jack plate install.

 

[slader99]

Re: Short to Long Conversion.

mikesea,

Its an older johnson and the measurement from the inside of the U mount to the top of the cavitation plate is around 17 3/4". Doesn't that make it a short shaft?

 

[mikesea]

Re: Short to Long Conversion.

But your talking about 2 inches.The basic sizes are 15 in which is your smallen engines up to 15 horse,then 20 and 25 in. 25 in.is certainly overkill.Has the transome been altered.Your numbers of 19.75 indicates a standard 20 transome,they arent always exact.Your engine,17.75 ?thats a 2.25 in differance from any standard,so perhaps the measurement comes up wrong due to the angle the tilt is adjusted at or?? but Its not a 15 in.doubt seriously that its a 25 ,leaves you a 20.the most common.Now,whats the problem.Your right ,the 2 inches shouldnt be that critical,especially going forward.My guess is prop slippage.Im also ASSuming ,your sitting at the stern ,giving more depth,if the boat got up to plane ,and cavitated Id say maybe.Performance boats are now using the water pickup even with the bottom of the hull for speed,and they arent experiencing the problem.Check your hub,make a mark on the hun and have it align with the prop.test run ,check to see if your mark moved.

 

[slader99]

Re: Short to Long Conversion.

I'm pretty new to boating so I could be out to lunch. I've posted some pics below, sorry none of the motor on the boat, but take my word for it, the cavitation plate is 2" above the bottom of the transom. Next time I'm at the dock I will take a shot of the engine on the boat.

I think this is a short shaft motor, perhaps it measures 17.5" because the waterpump is higher than normal on old johnsons? Anyway I am pretty sure the prop was cavitating at 1/2 throttle and waves seemed to make it significantly worse. Also when I started to open the throttle up it seemed like the waterpump wasn't moving as much water and the engine was heating up.

I phoned Lifetimer boats, (Its a 25 year old aluminum) and they told me they would have shipped it with a long shaft. 600 bones and they will cut out the transom and put a spillwell in.

Does anyone have pictures of some sort of jackplate I could use to lower the engine 2-3" without having to purchase an expensive leg, extension or transom modifications? So far all the jackplates I have seen raise the engine.

catch22... (It also seems that if the engine was lowered below the top of the transom the tiller would interfere with the top of the transom.)



Thanks,

Colin