Help with Jack Plate please

[markov]

Hi there,
i have just picked up a 20hp Mercury 1999 to replace my old Evinrude 1982 9.9hp on my tinny.
Unfortunately the Mercury is a long shaft and the tinny has a short shaft transom. The Mercury was too good a deal to miss - so now I need to fashion a jack plate to raise my transom around 4 inches on my 11 foot aluminium Savage tinny. I have looked at a few images on the net and was hoping for a few opinions. I have fashioned a crude pair of brackets out of 5cm x 5xm x 3mm aluminium angle and have marked up my tinny to show where I propose placing my four stainless 3/8" bolts. I have attached some images. I am really after any opinions on the suitability of this plate and its ability to support and run a 20hp engine that weighs around 52kg. The plate will be fixed and not adjustable, and I will test out my plywood motorboard at a couple of heights before I finally fix it on for good.
I aim to have the cav plat around 1"-2" below the bottom of the transom. Is this appropriate?
My questions are:
The tinny is only thin skinned. The brackets on the hull will be 13" in length and I will drill my bolt holes about 1.5" and 10" from the top. Should I brace the inside of the tinny as well with angle? Is 3mm angle and solid marine plywood motorboard strong enough? Should I brace in any other way?


Should I be bolting my top 2 bolts on both sides into the reinforced cross member on the inside of the transom (which is behind the wooden board in the first image)? At the moment my bracket placement is just wide of this at 12" wide. The reinforced cross member is only 10" wide and I wasn't sure if I should drill into this?
All opinions welcome. Lastly, my tinny is for river fishing and not sea fishing so it is not going to be hammered - I just need a reliable motor to get me about.

 

[osborn159]

Re: Help with Jack Plate please

i have seen diagrams on these forums for a jacplate on a small tinny, just a week or so ago, i think i saw 2-3 different options, use the search feature youll probly get more than what you want to read, look for the diagrams, i cant remember the thread name, try the search
good luck

 

[markov]

Re: Help with Jack Plate please

Hi osborn,
Thanks for that.
I took a quick swing through but I can't locate anything that related to my thin skinned tinny and my particular questions. I am hoping to do this job over the weekend so any opinions are welcomed.

 

[lckstckn2smknbrls]

Re: Help with Jack Plate please

Try looking on www.tinboats.net

 

[markov]

Re: Help with Jack Plate please

Thanks lckstckn,
Had a quick squizz - but there isn't too much info on jack plates on there. Interesting all the same.

 

[bob johnson]

Re: Help with Jack Plate please

I would think you are going on the thin side for your material..

3mm is less than 1/8th" thick!!!

all the strees will be on the two bolts on each side!!

Id like to see you use square tubing so the inside of the riser will contact the actual transon for most of its length( minus 4 ")

and try to use at least 1/4" angle for the outer pieces...

this stuff will be REAL cheap...less than $20 for the lengths you need...

id like to see you some how get a cross members going from side to side..

what were you going to clamp against????

the widths of the uprights???

dont like that too much...I think you need some width and height for the motor to get squeezed up against!!!

is that what you were thinking about with plywood??

you should absolutely put some type of strap inside...I would use two pieces of 1/4 thick x 2: wide aluminum bar...one for each side...spread out the force and act as washers( id still use SS washer as well)

SS would be better for your straps!!

I think you are under built...and if you do err , it should be on the over built side!!!

you need to set the cav plate at or slight above the bottom of the HULL( i hope thats what you were calling the transom)..NOT below!

good luck

bob

 

[lckstckn2smknbrls]

Re: Help with Jack Plate please

I think the aluminum stock your using might be a little thin. Most of the home made are 3x3, 3x4 x.250 thick. with 3/8 SS bolts.

 

[tashasdaddy]

Re: Help with Jack Plate please

that will be fine for a 20 hp. you are 100% on the right track for what you are wanting to do. i would use 3 bolts instead of 2.

 

[markov]

Re: Help with Jack Plate please

Thanks very much for your replies guys, much appreciated. None of our local hardware stores run to more than 3mm - which is annoying. I live on a rural property so there are no fabricators locally. What I may do is double up on the 3mm for now and just test out the motor - taking it real easy - and wait until I can locate some decent angle.Then swap it over. I think over-engineered is a wise comment. 3 bolt holes it will be - thanks for your input tashadaddy.