Raising transom for long shaft

[clarkbre]

I’m sorry my spelling is so terrible. I didn’t ever claim I was an English Major.

Here’s my sketch demonstrating the lift of the bow in comparison to the pivot point and the thrust of the prop. No matter what, when you rev the throttle in the forward position, the bow is naturally going to come up.

There’s no way that when you throttle up in forward that the bow pushes down.

 

[Ranchero50]

Perchins transom being weakly built allowed a pivot point causing the bow to plow... I can also see a weak hull flexing from the extra loads. A stiff hull is still pushing the boat by the prop.

I read some stuff posted on I boats #-o

Jamie

 

[perchin]

Then someone wouldnt' mind posting a video of them starting their motor, pulling up on the motor, putting their hand between the transom and the mounting plate and putting the gas to her.... :lol: Seeing how it won't do anything because it does not push there. :?

 

[clarkbre]

Then someone wouldnt' mind posting a video of them starting their motor, pulling up on the motor, putting their hand between the transom and the mounting plate and putting the gas to her.... :lol: Seeing how it won't do anything because it does not push there. :?

Funny you should say that...

Seems if some guy did that...
1. He wouldn't be real bright...
2. The pivot point of the motor would be at the top of the transom. In order to really clamp your hand between the transom and motor, you'd have to close that pinch point. To close the pinch point on your hand, the propeller would have to push the motor forward from the bottom with the propeller. That would sure hurt like hell but you'd start to understand thrust and lift. And, that's if the prop is set at the right height on the transom.

If that prop was sitting 6" lower on the transom, it would have more leverage to clamp onto your hand.

Oh, wait, you said the motor pushes from the top though...stupid me.

It's just like using two wrenches with different length handles. The short handle wrench will tighten to so many foot pounds. Use a wrench with a handle twice as long and it will deliver twice the torque on the bolt.

And again, the outboard and boat are working as a single unit at that point. It wouldn't matter if the pivot point on the motor/transom was 3 feet in the air. The motor pushes the boat from the prop, not the transom.

 

[perchin]

:LOL2: would you make up your mind .........a couple of posts ago, you posed a drawing disagreeing with whatever you just said..... for instance the pivot point.

And for like the fifth time your fixation with the prop.... this entire time I've been talking about the point of contact... I believe we are talking about the same Dang thing.....I'm only talking about where the motor meets the back of the transom (point of contact).....

Thanks for the obvious tip with the wrenches....as if I didn't already post it earlier about using a cheater bar..... again were saying the same thing, just talking about different areas.

 

[clarkbre]

... I believe we are talking about the same Dang thing.....

So tell me again how the bow would get pushed down into the water as opposed to being lifted out of the water if the prop was sitting too low?

 

[perchin]

again i never said that . i said if you build up the transom, that raises the point of contact.... and applies more leverage. Just like a longer wrench....

 

[clarkbre]

.... reason being is with the larger horse motors you'll have problems with it pushing the front end constantly deep....more so the faster you go.....

Yep, you never said that bow would get pushed down?

The point of contact does not matter. The reason a bow goes up when you gun the throttle is because the propeller pushes from the bottom. In the cases of an outboard, i/o, or an inboard, the prop pushes from the bottom creating the bow to rise when accelerating.

 

[perchin]

I meant I never said anything about the depth of the prop like you said I did.

I'm really not trying to make ya mad.... I enjoy a good debate, and also like learning new things, this constantly tends to give off the wrong impression. And also tends to tick a few off.... no harm was intentional...... I just wish someone could explain it so I can understand how my thought is incorrect...... as of now I guess I can see it both ways.... kinda :?

 

[Hard H2O]

Here are some pictures of my transom

 

[ditchen]

I have just been through this on my modified v jon. Had a 76 40hp johnny 15" mount to the 15" transom. But I had to have the tilt and trim and have always been a believer in the jack plates. So I obtained a cmc tilt/trim with a set back of 6". No changes of anything else. Big difference. With the motor set back and raise up while on plane back of hull ran bout 4" deep in the water.

Well I moved onto a 50hp, 20" shaft. Mounted it up with no changes in current setup. With the cavitation plate running 3" below the hull, the arse end was pushing so hard down while up on plane bow was high and back of hull ran about 12" (getting water spry from the anchor so I was pretty accurate on the running draft). It rode and handled like a brick.

After talking to CMC I purchase the vertical extension brackets designed for the jet drives and for short to long shaft conversions. No problems and quite happy with the results,,, well other than the one of the kind look.

 

[althedj]

My thoughts are aluminum boats (open style not the carpeted bass rigs) are generally 12 to 15 feet, max horse power on my Lund is a 10HP. I'm using a 5HP. There's not enough power to do much to begin with, driving the bow down, or making the boat unstable seems to be a bit of an over statement to me on a tiny little aluminum boat. Raising the transom 4" to bring the prop out of deep water shouldn't do anything. Just my thoughts. Raise that sucker up

 

[stevenmurray1987]

I know this is an old thread, but I wanted to add my experiences concerning raising the point of contact on the transom will contribute to the bow digging.

In my experience it DOES - but not drastically. It would be an issue if I did not have power trim.


Here is my new setup

 

[Bateman]

Man, this thread made my head hurt..

Hopefully heading to FL tonight to pick up a long shaft 50 for my short transom jon boat. I'll be using a jack plate. Been on several similar setups and never once witnessed a bow dig. Perhaps the jack plate's fulcrum being lower on the transom when compared to the taller jack plate makes the difference. But then, we'd be saying a factory tall transom boat would have the same negative side effects.. Hmm.

 

[stevenmurray1987]

Man, this thread made my head hurt..
But then, we'd be saying a factory tall transom boat would have the same negative side effects.. Hmm.

Yeah I agree with that logic and also about the headache lol.

I still don't really know the answer, honestly my setup has too many new variables to blame the digging on the transom raise.

-I also moved my console (my 200# body) 24" forward
-I removed weight
-Not sure of the angle my last outboard sat versus what I was trimmed at when experiencing the issue.

 

[vfourmax]

I understand the concept of how raising the engine mounting position to a higher point on the rear of the boat at the transom would have an effect of changing or making a difference in how those forces would relate to the hulls position as a result of the difference in height of those forces being applied.

From a simple explanation it is just simple physics but there are many ways to "offset" any negative effects of such a minor change.

As I think in this instance we are talking about a 15" to a 20" transom height difference and I think as a general rule that most outboards designed for a 15" transom are of 50 hp or less with the majority actually being of around 30 horsepower or less.

Granted if we were talking more of a using a 25" motor on a 20" transom then actual weight and the hp being used may be of a much larger effect as we could be dealing with multiple hundreds of lbs of weight and the effects of up to 300 horsepower.

Here I think we are looking at much less as far as the change of physics versus the size of the hull and height of the transom as long as the hulls maximum coast guard horsepower ratings are followed.

First off I would think those differences could be easily negated by utilizing a jackplate to not only lift the motor to provide the correct lower unit depth but provide a setback of 4" or so which should by moving the weight of the engine back further off the back transom should easily offset the change of the slight height increase difference of using a 15" to a 20" outboard with all other variables being the exact same.

The manual jackplate would also offer the benefit of having the ability to make smaller more precise height adjustments to truly dial in the engine to hull mating for the best results and adjustment changes can be made easily with a wrench rather than needing to remove the engine from the transom to change the engines height with every change.

Also the set back would conceivably put the prop back in cleaner water plus the setback would lengthen the hinge effect on the hull again helping offset any changes raising the engine may cause. Again here the largest effects would probably only really be noticed to any degree on the very largest of the engines in the class of mainly available 15" shaft motors.

Actually just doing a quick search of currently available new stock off the shelf jon boat models from LOWE, TRACKER and ALWELD the highest horsepower rated hull I found utilizing a 15" transom was only rated for 35 horsepower. Any higher rated hp models were utilizing a 20" transom.

I do know from personally researching newer 4 stroke motors in the 20-30 horsepower range that I have ran across several models that power trim was only available on the 20" model and not the 15" model.
I personally would not have a problem using a jackplate to fit one of the long shaft motors to a 15" transom hull rated for the horsepower if the motor I wanted to buy was only available in a 20"shaft.

Between the jackplate and the power trim on a motor of this weight and hp the adjustability would more than make up for the minimal 5" increase in mounting height and any performance or handling differences as a result of that slight height increase.

Without power trim using a set trim level every change or distribution of weight in the boat will compromise or enhance the boats performance. I would worry more about that than raising a motor of 50 hp or less up 5".