Jack plates and set back plates

phillnjack · 07 April 2010, 17:13

I can understand the need for a jack plate, as i am in need of making one for my longshaft engine to fit my shortshaft boat transom.

But what exactly is the benefit of the setback plates ?

Apart from it giving extra room inside at the back of the boat !

Is there a performance increase /decrease ?

phill

Nasher · 07 April 2010, 19:00

I'm no real expert, but I believe it allows the engine to run in cleaner water not immediately behind the hull. Of course the higher the engine is mounted the more reason to locate cleaner water.

Somebody will probably be along to call me a plonker in a minute and correct me.

Nasher.

Erin · 07 April 2010, 19:38

Plonker

No, really I think you're about right. When you increase the effective waterline it has some effect on the overall hull speed, and the cleaner water for the prop is of help too.

Cookee · 08 April 2010, 09:49

Quote:

Originally Posted by Erin

Plonker

No, really I think you're about right. When you increase the effective waterline it has some effect on the overall hull speed, and the cleaner water for the prop is of help too.

You're not increasing the waterline if you don't fill in the gap - by moving the outboard back you can raise the engine higher, this in turn reduces drag and as Nasher and Erin say the water is cleaner higher up further back.

Nasher · 08 April 2010, 12:43

Cookee

I have to own up to the fact that I only have an understanding of the principle from reading posts you've made in the past.

Nasher.

phillnjack · 08 April 2010, 19:38

So being as i am going to be making a jack plate for my 30hp suzuki, would it benefit me
to make it a setback plate at the same time ?

And what would be suitable amount of setback ?
Remember i need 5 inch of lift just to make it a normal ride height , so any setback will
need to be raised even more ! just how high do you go.

And also what is the cons to it, there must be some bad side effects or they
would be much more widely used !

Or is a 30hp not worth talking about as far as setback is concerned ?

phill..

C-NUMB · 08 April 2010, 20:29

No expert either, but with a setback You will have more torque on the transom, and also the weight distribution will slightly be changed. Are these significant issues, that I don't know.

I have a 56 cm(long rig) on a 50 cm transom, have been thinking about the same issue but decided only to extend the transom with a fixed plywood "Jackplate". Hope to post some pictures when completed.

phillnjack · 08 April 2010, 22:22

The more i think about the more im inclined to agree with C-NUMB and just
put on a simple bit of wood to raise it up.

If you use wood to raise the transom do you put it inside or outside ?

I need 5 inch lift (12.5cm), and thats a fair bit if lift for wood so might use steel plate.

If the weekend is nice ill also have pics to show , even if it dont work ha ha

phill

9D280 · 09 April 2010, 12:00

Just to chuck a thought in here, and not knowing what age your engine is, for the sake of buying a new leg / shaft / water toob and a couple of gaskets, might it be easier to convert the engine to short shaft? If it;s a 2-0 stroke I'll bet theres a few bits kicking about in the usual haunts.....

m chappelow · 09 April 2010, 12:36

i.m.h.o and i am only speaking from my own past experences if your only using your boat as a runabout and dont want optimum speed out of it i dont see what all the fuss is about ,i have run numerous boats with short transoms with longshaft engines and with only minimal drop in speed perhaps a couple of knots ,and with some types of engine a bit more spray getting kicked up , down side is that your draught is increased by 5 inches or so and changing or clearing a prop from weed or a plaggy bag you will have to stretch out a bit more and with some older engines with a small pressure relife hole it may have a bit more back pressure on the exhaust when at rest ,plus side is that in lumpy /choppy water conditions you dont lose power as the prop never airates as its always in the water ,and up to now i have never twisted a transom off a boat or sommersaulted one when setting off as in some of the myths .

phillnjack · 09 April 2010, 18:40

Well now come to think of it the above answer is pretty sensible realy.

Is it realy worth doing ?

The drag effect of 5 inches of outboard is not going to be that bad, as its not exactly a
flat or cupped surface.

Most of my use will be playing around on bouncy waves at the coast during the
very hot very long summer we are going to have this year( well i hope so anyway).

what i will do is make a jack plate and test it to see if there is much difference.
If not then ill just simply take it off and go back to long leg.

Buying bits and changing them could be very costly

would need the complete mid section including shaft etc and you dont see
1980's suzi short shaft 30's that often in bit form.

Dont want to sell it due to knowing the full history of the engine.

phill

Cookee · 12 April 2010, 08:33

If you just want a bracket and not a jackplate (a jackplate is a hydraulic adjustable bracket) we sell these - it will give you your 5" of lift and we have one in stock!

phillnjack · 31 January 2013, 22:47

Sorry its been such a long time since my last post on this subject.
I did make the transom taller by 5 inches and it transformed the boat from running with a longshaft on a short transom.
The steering was improved the splash comming over the back was crazy with it left down too deep.
speed increased from around 18 to 26mph and the boats attitude was also improved greatly.
Time to plane dereased by about 30% as well.
overall the boat was dangerous with it too long,i found steering very hard to keep it under control properly,but like said above its all better now.

What i done was to fit 2 3inch x 3inch lengths of angle to the inside of the transom and fit a board or same thickness as transom directly above the top to raise it up.
This was then covered at the back by a simply piece of ply all over the rear of transom for looks as well as a touch of strength.
overall it worked a treat.
pics below to show what i did.
i no longer have the avon and have now got a rigid orkney dory to give me a touch more room inside.
The avon was a good boat and done exactly what it was designed to do, a very good boat to own if your need a boat that can be packed into the boot of a car and can go anywhere you want it to go.
It wasnt what i would call a dry ride, but very good fun boat and could take a real good pounding that i dont a rigid boat could take without doing damge.
Would i have one again..........yes .

Thanks to those who wrote on this topic,it helped me decide to raise the transom and do a few experiments i would not otherwise have done.

great forum
keep it up (no pun intended)

phill

Locozodiac · 01 February 2013, 14:20

If you want to place a long shaft engine on a short shaft transom the quick way to go would be to get a 5" transom mount so to have engine sitting exactly on top transom and not extending/protruding out if using a external plate. Rising a transom works, but it's time demanding.

Using engine as it is will produce the following : unwanted water splashes at back transom, water could get inside deck at speed, boat will take longer to plane, excesive tail darg on water.

Happy Boating