Poor man's 15inch Mercury race motor

[NZ Sidewinder]

Down here at the bottom of the world, the good Mercury stuff is difficult to find and very expensive.
So sometimes a man must improvise.
In this case I wanted to create a 15inch motor that would hang on a 20 inch transom, but also lower the center of gravity of the race boat.
The only way to achieve that is to lower the powerhead in relation to the transom clamp compared to a 20 inch motor.
This one is fitted to a 20 inch clamp.
I did away with the exhaust plate which dropped the powerhead by about 4 inches.
Then dropped the transom clamp hard down on the 20inch transom
( previously, the motor ran with the clamp 2 inches above the transom, which put the propshaft 2 1/2 below pad).
Then I set the mid section length so that when tucked under, the gearbox cleared the cross piece anode by 1/8 inch.
I could raise it more and place a trim pin in the clamp so that the down trim is limited.
I do that on my race motors anyway, so that when the driver gets the death grip on the wheel, they don't accidently trim in.
Been there, done that.
I'll post regular updates on progress
I think there is a Merc race motor that has a very thin exhaust plate with tuner attached.
Can't remember what it's called

[NZ Sidewinder]

Here is a progress photo.
This leg will carry a 260hp but will eventually get a 4 stroke around 400hp.
I've always wanted to build a dry sump 4 stroke for an outboard but the exhaust plates don't really facilitate it.
That's sorted now.
We have a few 300r boats racing here.
Hmm, this could be fun

[NZ Sidewinder]

The Clamp is tried on for fit.
It all seems to fit ok and I was able to do some measuring to see what I actually had.
One of the important measurements is the drive shaft length.
In theory it should be the same as a 15inch factory gearbox, but it's not.
It's half inch longer.
That's a nuisance.
It becomes obvious early in a project like this that the term 20 inch or 15inch doesn't actually mean anything outside the factory motor.
In this case the goal was to lower the powerhead realative to the top of the 20inch transom by as much as possible and maintain the 2 1/2 inches from pad to propshaft.
I've more or less achieved that but in doing so I can't use a stock 15 inch drive shaft.
So to deal with this I'll cut a few drive shafts down to 6 inch and make extentions so they can be used with this setup or a stock 20inch.
It's not a big job, but just another job.
The power head ended up 4 1/4 inch lower than a stock 20inch and with some persuasion could go down to 4 3/8.
Goat was 4 1/2 inch.
Overall happy so far.
It will get a snout soon and also big holes cut in it to reduce weight.
It will be fully welded and stress relived before getting Swiss cheesed though.
This F1 plate is the Merc version.
My one is just larger so that it can support the top cowl and wings

[NZ Sidewinder]

Bad boys, Bad boys
Whatcha gunna do
Whatcha gunna do when I come for you
Bad boys.

[NZ Sidewinder]

Almost done now.
This is really just an engine platform or foundation.
The nature of the build is such that a variety of power heads can be mounted on the leg.
In this case that will be a 260hp 2.5 Merc but it could easily be a 400hp Toyota 1600cc 3 cylinder turbo 4 stroke.
However in this Merc configuration, the goal is to radically lower the CoG of the motor on a 20inch transom for racing.
So today I got the leg assembled with all welding done, water tube in and anchored and clamp on for a final measure up to see what I got.
The power head sits 115mm lower than stock (4.5inch) and then the clamp sits down hard on the transom (thats 2 inch lower than racing position).
So Ive lowered the power head 6.5 inches and have raised the prop shaft 1/2 inch.
If that 1/2 inch is an issue Ill lower the transom 1/2 inch.(epoxy the holes etc).
Im running the poppet valve and stats but may add a dump valve piped to the exterior that can be adjusted.

[LakeFever]

Looks stout!

[NZ Sidewinder]

Yeah, won't break.
It needs to have cut outs yet to lighten it up.
Although it's not actually too bad for weight.
I can lift the clamp / mid/ gearbox

[NZ Sidewinder]

Mocked up

[David - WI]

[NZ Sidewinder]

What awoke the giant was him seeing his Son in a racing accident that could have killed him.
He was punching above his weight against a 300x and barrel rolled.
I decided then and there that the only safe place in a circuit race is out front.
We are going to see more of these big motors in our feilds, in particular the 300r.
You cannot race behind those boats, the holes are just too big.
So we need to be faster and more nimble.
https://youtu.be/W_sW6bKP8y0?si=cAsTGFfeP7Cku6dg

[NZ Sidewinder]

Nothing special, just a couple of random taper pipes.
Made from an old ski pole.
The first 4 inch of this tuner is normally the exhaust plate, so that makes this one very short.
This boat is designed to run almost continuously at WOT even through the turns, so maybe this tuner will work for that

[NZ Sidewinder]

406lb
Oiled up, including cowl

[NZ Sidewinder]

The spare powerhead for this leg this season will probably use old school 5250 rods and I can't get rod bolts for them, so was a fun day making and testing conrod bolts.
They are made from Taiwanese Socket Head Set Screws.
5/16 unc.
190,000psi (1300Mpa), with test certs.
Not from China.
The shank needs to be machined down to 1/4 inch which is a couple of thou smaller than the thread root diameter.
Only 50% of the set screws were concentric enough to achieve this.
Results of tests were impressive.
They will tighten to 35ft lb, with .003 stretch and no yeild.
At 40ftlb they stretch .011 and yeild .006.
So a torque of 35ftlb is really good and they can be used repeatedly at that because at that setting they aren't yielding.
Doing this primarily because the factory ones are unobtainable.

[LakeFever]

Are you related to Bill Munro? You have the same attitude and style of make whatever you need to make it work how you want. Ever see the world’s fastest Indian? Awesome story and great film

[NZ Sidewinder]

Lol
Yes I know of him and have visited a museum at the southern tip of New Zealand to see his motor bike and a huge display of his home made pistons.
Another great engineer from NZ was John Britten
He built the Britten V1000 race motorbike
https://en.m.wikipedia.org/wiki/Britten_V1000.
He lived not far from Bert Munroe.

Another Great New Zealander was Graeme Wingate.
I'm not sure if he is still alive but he built one of New Zealands greatest race boats over 40 years ago and is still racing to this day.
https://youtu.be/UfCBBNbZn3A?si=HzmYoZrLTlHwBsUL

https://powerboatmagazine.co.nz/clas...rboat-chindit/