Extra long shaft to long shaft

[Chinewalker]

It's an inexact science. OMC's tend to fudge a little on the long side. This is because boat manufacturers tend to be all over the place in their transom heights. Very rarely is a transom built to exactly 15 or 20 inches. Also, keels, bottom shapes, hull designs, etc. can affect water flow around the lower unit. The end result is that outboards tend to run a little longer than the "standard". When someone says it's a 20-inch shaft, what they really mean is that it's for a 20-inch transom, even if the actual shaft length is a in inch or three longer. In truth, it is better to have a motor run an inch or two deep than it is to be an inch or two shallow. It is much easier to jack a motor upwards a bit to achieve better performance than it is to lower a transom to get the motor deep enough to pump water and keep the prop from blowing out.

If your motor is running as it should - both cylinder firing, compression good, prop hub isn't slipping, etc. then that motor should pop your boat right up on plane. If it's not, then something is wrong. Likely with your set-up. If you can lower the jack plate a notch, that may prevent ventilation blow out, if that's your issue. I have a similar motor on my tinny and it runs with the plate an inch or so below the bottom of the boat. When I've tried raising it, it will blow out in turns, and runs louder as the exhaust ventilates to the surface.

 

[fishdeer24]

The transom measures 20". The jack plate was added because the cavitation plate sat 5" below the hull. Does this not make the motor an extra long shaft motor?

 

[sutor623]

The transom measures 20". The jack plate was added because the cavitation plate sat 5" below the hull. Does this not make the motor an extra long shaft motor?

Dude, how many people does it take??? I'm outta here. Peace.

 

[racerone]

Fishdeer, you are the student !!!----Accept the statements made by people who have repaired motors like yours since 1968.----No more teaching attemps from me !!!!

 

[ondarvr]

Only the first pic in this last set is useful to tell the the actual AV plate height, which could be a little high, the other pics are at a slight angle and there is no way to determine the exact relationship of the AV plate and bottom of the transom. As was said before, the exact measurment of the motor may not be 20" or 25", it could be shorter or longer. I can't tell how much the jack plate raised the motor, but if the motor is 23" and the jack plate is set at 25", then the motor could be too high. I say could be, because on some boat and motor combinations that might be fine, on others it may not work well.

If you feel the motor is running correctly, then lower the motor (jack plate) and give it try

 

[Chinewalker]

The transom measures 20". The jack plate was added because the cavitation plate sat 5" below the hull. Does this not make the motor an extra long shaft motor?

It is not an extra long shaft. There was never an extra long shaft version of this motor offered. Period.

Arguing with us about semantics of terminology will not help your boat perform. The only thing that will make your boat perform as it should will be you getting a wrench out and testing some different set-up placements. Lower the jack plate. Trim the motor. Make sure the motor is running properly. Make sure the prop isn't slipping. Until you take some of the suggestions described above and try them out, I'm done...

 

[oldboat1]

transom is 15" to 17" or so. Standard transom. Engine is set up as a long shaft 20" to 23" or so, top of vent plate to mounting point on transom. Show a tape on the transom (mounting point in the factory cutout to the bottom of the boat). Motor vent plate would be about five inches below bottom of boat.

In any case, how is the motor running? Seem to be missing at all?

 

[fishdeer24]

I'm here looking for help, not criticism. To all that are trying to help me and not judge me, thank you!
To the last person that replied, I did measure the transom. The tape measurement is pictured in the first photos. 19".

 

[oldboat1]

well, failure to communicate anyway. no crime in that -- no satisfaction either. fishdeer -- hope you get it solved. :chargrined:

 

[jbcurt00]

Ok, thats enough fellas.

Fishdeer, lets start over. In my opinion, this is a 5inch extension that has been added to you motor:

The model # you posted is for a long shaft motor. If it has an extension added to it, it may measure long enough to possibly be an extra long shaft motor, but it is aong shaft motor. Its just had an extension added.

Bondz and another member both tried to make longer shaft motors work by adding jackplates or raising the transom, w little successs, similar to your results.

I'd like to know what boat you are working on. Perhaps a 25hp isnt the right motor for the boat or how you use it, or how you load it.

Pix show its an aluminum boat. Since its new to you, whats the frame of reference that leads you to believe it isnt reaching the right RPMs? A tach gauge?

I apologize that things got out of hand w some of the comments and posts. I definitely dont appreciate it.

You asked for advice, and got replies from some of the most knowledgeable outboard guys on the forum. They have lots of experience w these motors and many are professional marine mechanics or where. I suspect they mistook misunderstanding and repeated questioning of their advice as an arguement, and an afront to their knowledge and expertise.

It may have been frustrating, but please refrain from using foul language. It doesnt help you get the snswers you are looking for.

Guys, in the future, please remember that not all of us are experts. And that most come here for help, and many of those have little to no experience w outboards.

Quite frankly I am stupified that this devolved into a pickering match over long shaft vs extra long shaft and that nobody figured out that w the extension in place the motor could easily be misidentified as an extra long shaft. Especially to someone that isnt as familiar w this motor as you all.

I dont have time right now to clean up this mess, but I will.

Fishdeer, again please accept my apologies and try not to use foul language.

Now, what aluminum boat, how much gear and people are aboard and what RPMs are you running at WOT?

Since it appears to be a tyoical long shaft motor w an extension, I dont know what it'll take to swap back to regular long shaft because I dont know how the exension was added to the water tube, the driveshaft or the shift rod. May require opening it up and removing the lower unit.

 

[racerone]

??????----The long shaft model had the 5" extension right from the factory in the 1970's !!!----This motor only has one 5 " extension and not 2 of them.------Some newer 25 hp motors had a longer exhaust housing without an extention.-------The OP should remove the jackplate and install this motor as it is on that boat !!!!!!!!!

 

[jbcurt00]

Ok, that certainly makes sense. Fewer parts in the supply chain, and an optional extension would have been available to convert stock short shafts into long rather then completely different mid sections. Learned something, thanks.

Lets assume for a minute that Fish wants to leave the jackplate to be able to fine tune motor height, so he can run in skinny water. Isnt the cav/ven plate supposed to be a couple inches ABOVE the keel when running a jackplate? So removing the 5inch extension would let him continue to use the jackplate?

Although based on the pix he posted, the jackplate would need to be lowered some too?

But yes, much simpler to remove the jackplate, that does put the prop deep below the keel.

Is the plate adjusted up as high as it will go?

Because what he's really looking for, IMO, is the best performance he can get w the boat/motor/plate combo he has. Right Fish?

 

[Chinewalker]

If I may jump back in, in an attempt to clarify and condense some of the information given above.

The motor in its present state is a long shaft. Of that there is no question.

The transom of the boat, measuring 19-inches, is for a long shaft motor. Of that there is no question.

The addition of the jack plate may have raised the transom height too high. Of that there may be some question, yet to be determined.

The initial posting claimed the motor was not reaching top RPM. Of that there is some question, as the OP has not yet given us any data to support that claim. If the motor is in fact ventilating, .ie too high on the transom, then I would expect the RPM to be too high, NOT too low.

Which leads to our not yet determining if the motor is running properly. Of that, there is some question.

IF the motor is running as it should, removing the jack plate and placing the motor flat on the transom will work, but may not give best performance. It is always best to increase motor heigh incrementally to achieve best performance. Raise the jack plate one notch at a time, run the boat, note what the motor does or does not do. Note how the boat rides, with various loads. In general, you stop raising the motor when you either run out of cooling water or if the propeller loses its bite. This will require testing. Testing is a good thing - it means you're running the boat, out on the water, preferably on a picturesque day. Enjoy it.

 

[oldboat1]

Here are the photos asked for.

OP came to talk about what is, and the audience is telling him what it can't be. One or two of the pics seem to make his point. Some are inconclusive. (He apparently doesn't want to talk about condition of the motor.) One or two simple measurements, clearly shown, should clear things up.

Fishdeer, if still interested, I would like to see a couple more pics clearly showing measurements for the transom and the motor leg -- no extreme close ups, or side angles. No confusion regarding measurement points. Just a tape or measuring stick pictured flat on against the boat transom and the motor leg:

pic 1 (boat): show point to point measurement from top of transom at factory cutout to bottom of hull. need to see it all in a single pic, top to bottom, with tape or measuring stick in place.

pic 2 (motor): show point to point measurement from top of jackplate to top of anti ventilation plate on the mounted motor. need to see it all in a single pic, top to bottom, with tape or measuring stick in place.

From what I see, your motor mounted/raised on that jackplate is about where it should be (anti-vent plate about even with the bottom of the boat), as was noted by another poster.

My simple assumptions, give or take a couple of inches: Either your transom is 15", and the motor leg is 20", or the transom is 20" and the motor leg is 25". The latter, I think, is your belief. A couple of pics could clearly show both measurements, so there would be no doubt.

 

[Chinewalker]

Oldboat1 - he has already done so with the measurements. The transom measures approx. 19", making it a 20" transom. The motor measures approx. 23", which in OMC terms is for a 20" transom. As I have tried explaining several times, the motor IS NOT an extra long shaft. It simply isn't. Period. They never made one. Even if some aftermarket company made an extra long shaft kit for this model, this motor does not have one installed. Fourth photo in post #37 shows the extension is only 5-inches. Suggesting otherwise simply confuses the issues at hand.

 

[oldboat1]

no, believe that begs the question, Chinewalker. Know what was said and what the specs. are, but would like to see the actual measurements I described -- point to point.

 

[fishdeer24]

Wow! I've created a monster here. I am sorry for all of the misunderstandings.
1) I measured the transom. It is 19" which makes it a 20" transom which is for a long shaft motor.
2) The measurement from the mounting bracket on the motor to the cavitation plate is 23". Aren't short shafts 15" and long shafts motor 20"?
3) Do you see my confusion now? The motor mounted without the jack plate sat way too low in the water causing too much drag and not allowing the boat to plane. Even with dolfins mounted on the motor. It had no high speed RPMs. Like the motor was working too hard to push the boat. I've been around motors long enough to know if it's reaching WOT rpms.
4) I added the jack plate and it did improve some in planing the boat but not enough. It is just myself and my wife in the boat which is a 1985 14' Lund Pro Angler with raised floor. But most of the time it's me by myself in the boat.
5) Can it possibly be the wrong prop on the motor?

I hope I've answered all of your questions and I do appreciate the effort to help me.

 

[fishdeer24]

I will try and take more photos this weekend.
Thanks guys!

 

[Chinewalker]

2) The measurement from the mounting bracket on the motor to the cavitation plate is 23". Aren't short shafts 15" and long shafts motor 20"?

Yes.... and no. As stated above, due to a lack of industry conformity, boat transoms are seldom exactly 15 or 20 inches in height. With that in mind, outboards are built with some wiggle room to allow for different transom heights, transom angles, keels, etc. The engineers at OMC (Evinrude/Johnson) apparently provided 3-inches of wiggle room on the 18-25 hp models, of which yours is one. This allows their motor to be run on any design of transom. If this motor were a short shaft model, it would measure approx. 18 inches and work just fine on transoms that are described as short shaft. Your motor, being a long shaft model, would work just fine on transoms described as long shaft.

4) I added the jack plate and it did improve some in planing the boat but not enough. It is just myself and my wife in the boat which is a 1985 14' Lund Pro Angler with raised floor. But most of the time it's me by myself in the boat.

That said, I have run long shaft 18-25 (same family of motor as yours) motors on my short shaft boat (14-ft Starcraft tinny) and they work just fine. Plenty of power, plane off easily. BUT, they do throw a ton of water, and the do not perform quite as well as the correct short shaft versions due to drag. With your boat not performing well with the motor flat on the original transom, I would say something else is at play.

5) Can it possibly be the wrong prop on the motor?

Yes, it is entirely possible you have the incorrect propeller. You noted you have a raised floor in your boat. This adds a some weight, and depending upon factors as materials used, etc. could be a good deal of additional weight. If your motor is running the 11-inch pitch prop, you may see some improvement by dropping down to the 9-incher. Take a look at the hub for either a part number or even the diameter and pitch.

Also need to find out, for sure, if the motor is producing all 25 ponies. A properly propped 25 on a 14-foot tinny, even with a hefty floor, ought to get up 'n scoot.

 

[oldboat1]

here's the thing (or one of them). 3-4 inches of drag shouldn't cause the symptoms you describe on that boat. That COULD mean the transom was manicured to allow use of a shorter shaft motor -- say 15" or so at the mounting point -- that your longshaft motor actually has, say, five to eight inches of drag. You rule that out if the 19" measurement is at the center of the transom -- at the cutout for the motor in any case. Specs for the boat aside (and to a lesser extent for the motor.....), I see it as a simple empirical issue that takes no expertise to resolve.

Given a longshaft motor on a longshaft transom, your performance issues could be prop related -- would think first of a slipping hub, given the lightweight boat. A standard prop should move it with no problem (I have a 12' Starcraft with an 18.), so wouldn't think a change in pitch would necessarily be what you are after for normal performance. But if not the prop, think there may be just the standard mechanical issues with the motor. That's something that folks would continue to be interested in, myself included.

(edit. correction: 14' Starcraft, similar to the OP's. Had a 12' with a 10, and that was plenty fast too.)