Raising - extending transom height.

phillnjack · 03 October 2009, 10:21

I have a Avon s400 and the transom height is short shaft.
The original boards are cut down a couple of inches to make these short shaft transoms.

Now being as the engine i have is longshaft, i am thinking about raising the
transom by 5 inches to accomodate the 20 inch longshaft engine.

The rating for this boat says 45hp on the plate, now that was for short shaft,
being as im going to be raising the transom by the needed extra 5 inches im thinking this
will possibly reduce the hp rating as there is going to surely be more stresses on the boat.

Im thinking that 5-10hp reduction would be ok for the longshaft ?
does this sound sensible for this type of thing or would there be little difference ?

Did they make longshaft transoms as well as short shafts for these boats ?

There is steel reinforcement bars attached to transom and floorboards to keep the rear end strong already fitted.

Would a third bar fitted central to the engine as far as possible to the top be over kill ?

or is the whole idea of raising the transom a complete no no, and dangerous ?

phill

m chappelow · 03 October 2009, 11:41

i personaly would try the engine on as it is and see how it runs first ,though with care ,even though it may be for a short shaft once up on the plane and doing tight turns or going over waves even flat calm water it could be that it starts to airate or cavitate at times ,alot depends on how large the inflatable keel profile is too as some boats can ride higher once up on the plane ,some large inflatables were prone to this even though they were perhaps set set up for a short shaft engine and transom ,,i knew a diving club that had to have a few inches taken off a short transom owing to it doing this,,,,,,even if you do decide to raise up the transom their are some metal transom/engine lifters on the market to convert short to long .,,going by an old avon brochuer the s400 is a standard/short transom and will take a 40 hp engine on remotes or 35 hp tiller steer .

palehorse · 06 October 2009, 04:46

on my avon 4.6 I chopped the transom to a 15 inch as it was a 20 now im hanging a 25 shaft outboard so putting the pieice I cut out glueing and three inch more and a adjustable transon plate and working with the right adjustment by need kim

phillnjack · 10 May 2010, 22:12

Well ive made the transom a 20 inch one now.
The 30hp longshafts sits perfectly on it and im now ready to give it a try out.

The rear of the transom now has a complete board across it to clean it all up.
The top has a fillet, but the actual strength comes from 2 X 7mm thick pieces of angle iron
cut to fit and bolted on. (DEFINITELY VERY STRONG)
The top bolts also hold the engine on as well as turenbuckles on the engine.

In the pic below you can see the fillet on the top of the transom and the angle iron used
for the strength of the whole raised transom.

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Here is the same transom parts painted and varnished and bolted on.

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pics below show new raised transom from behind the boat.
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Rear view before and after the cover board.

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And pic of engine on newly raised transom before final coat of varnish etc

phill

chewy · 10 May 2010, 22:35

Looks like you've done a nice job!

Nasher · 10 May 2010, 22:37

Nice work

Nasher

phillnjack · 11 May 2010, 17:12

The two black heavy duty brackets will get swapped for stainless steel ones when i
find some stainless ha ha
(ya never know what youl find in a skip on a trading estate !)
The transom support bars will also be replaced with stainless when i find some ha ha .

Now just got to give all the floorboars a good rubdown and revarnish them to make it
all
nice and weather proof for the new season.
And give the engine a nice new coat of paint and lacquer, might even get some new
decals for it .

so many ideas for this little sib, just need some good weather

phill

phillnjack · 23 May 2010, 20:32

Well i gave it a good try out on its maiden voyage with the raised transom.

I have had many boats before and some pretty big powerfull engines.

But this avon and 30hp scared the crap out of me from the first time i twisted the throttle.

I was not expecting the way it pulled away at all.
It just launched itself to flat out in a bout 2 seconds, up n woosh then stop.

The prop i have on the engine is far too small for the engine /boat set up, it revs
out too quick for one person on board.
I used a gps on the 3rd 4th 5 th 6th runs etc, 25 .9 mph was the highest speed attained.
But the acceleration is mental, only 3 seconds from staionary to flat out.
The engine is not getting any stress at all and i think its over revving, might need
to go up by about 4 inches in pitch.
The prop at the moment is 10.25 x 10 pitch, i think it might be better with 15 pitch to
give the motor something to work with.

acceleration once on the plane is mental, turning is a bit iffy as it wants to chuck you
straight out the boat.

is it normal for inflatables to be so bloody quick with just a 30hp on the back ?

and whats the handling like on these for pulling ringo's etc,

phill

Bigmuz7 · 23 May 2010, 21:44

Quote:

Originally Posted by phillnjack

the acceleration is mental, only 3 seconds from staionary to flat out.
The engine is not getting any stress at all and i think its over revving, might need to go up by about 4 inches in pitch.

Excellent

.. fit an altimeter too.. to check for altitude when leaving waves

.. now were talking

phillnjack · 24 May 2010, 00:16

I think ill be needing to fit some foot straps to keep me in the boat, it was very airy at
times for sure.
I did also try it with an extended tiller and found that to be a bit better as the steering
is very twitchy at full throttle.
(must remember to pack extra y-fronts for next trial !)

phill

phillnjack · 29 May 2010, 14:45

Here is a pic of the boat with me in doing about 20mph.

I find that about 20mph cruising speed is good for the handling of the boat, and gives me
a fair bit of reserve punch if needed.

The only problem im getting is that the transom rides a bit high .?
I maybe need a bit more weight at the back to keep the boat in the water !.
I find the outfit seems to ride a bit too flat on the water, and would of expected a more
raised bow attitude.
Might possibly need a bigger inflatable keel ?

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Flat out upstream on the river

gps readout on one of the first runs (upstream !!! )

The transom plate says max engine 45hp !! 45hp on this would be mental, and would
definitly need remote steering set up.

I found it far better with extended tiller arm, but still needs a bit of getting used to

phill

Cookee · 31 May 2010, 08:41

No speed limit on that river?

phillnjack · 31 May 2010, 18:29

Not when im using it

phill

woodcheck · 22 March 2012, 20:20

Phill,

What is your opinion on extending the transom when it is doesnt have the notch (saddle) in it as yours did. My transom is flat all the way across on top. So if I make a filet and use the same metal that you did should my filet be just as square as the original transom? or should it have curveature as yours? Also, I wont have anything to attach those bars to the floor, do you think I can accomlish this extension without those bars?
Thanks!