Inboard engines

[MrRob]

As I want to go faster in my Marshan I have been thinking of ways to do it!
So I was thinking of using 1000cc Motorbike engines.
You can pick up fireblade engines cheaply enough these days that have a bhp of 150hp with a max rpm of about 11000rpm-13500rpm.
I think you would be looking at about 90-95mph on a 21p prop.
As these engines are small and light weight it should be a snap to install one into the back of a boat.


WHAT YOU ALL THINK???? AM I DREAMING??? OR JUST PLAIN OLD MAD????

 

[MrRob]

at 10000rpm, 1.63 gear ratio, 21p prop, and 15% slip that would + 104mph!

 

[Dangerous]

Keep takin the pills!

 

[kiwiracer]

how do you propose to get the power into the water?

 

[BluFin]

Been there tried it, with 2* 500 yam GP two stroke motors, gearbox was a nightmare & would run out more expensive than buying two bran new XR2's when they first came out.
Yam put them up for the old boy to try when they raced with them.
power was great but hooking it Up was just not cost effective
But it was a Copland, Buzzi, Idea, tune the feck out of it coz it's now an inboard under the regs of the day.
Now putting the block on an outboard leg that could work if it's just for fun.

£££££££££££££££££££££££££££££

my thoughts "Spend ya time on the water, not in the garage"

 

[Ciao]

Or how about constructing an aerodynamic wing over the boat? A few race boats tried this with good results (Buzi, Cougar amongst them) until they were banned for racing. The bat boat wings clearly generate lift and add to speed.
Could be a fun and far cheaper way of going quicker, plus it will create a bit of interest on the slipway!!

 

[Ciao]

this idea..

you can make the wing trimable with hydraulic rams -

 

[Ciao]

The other thing you could try is Ocke mannerfelt Speed Rail TM

 

[darexms]

Wouldn't the speed rail cause more 'slam'?

 

[darexms]

Erm.. before I got distracted, another issue I see with 4 stroke bike engines is the entire lack of bottom end grunt. Running just a single speed gearbox will see you struggling to get through the lower rev range before any useable power comes in.

In my humble opinion of course.

 

[MrRob]

I think that problem could be over come with using a lower ratio gear box and tuning the engine. After talking to my mate who is bike nuts yesterday the said you could easily tune a honda CBR engine upto 220bhp easy and have a red line of about 15000 - 16000rpm. Put that on a 2.1 ratio back and with that rav range your talking some serious high speed!

BluFin had an interesting idea of wacking the engine onto an outboard leg. I have a friend of a friend who makes adaptor plates to fix modern gear boxes to big old V8 for the kit car industry, I'll see what he thinks!

Rob.

 

[JamesM]

I would have thought that if you tune a 1000cc bike engine upto that sort of power it would all be in a very narrow rev band right at the top of the range. That would be even worse than the standard motor. Boats need motors with lots of torque and wide power bands as they only have one gear that has to take it from a standstill to flat out.

 

[JamesM]

It might work on a very light weight hull.

 

[darexms]

I think that problem could be over come with using a lower ratio gear box and tuning the engine. After talking to my mate who is bike nuts yesterday the said you could easily tune a honda CBR engine upto 220bhp easy and have a red line of about 15000 - 16000rpm. Put that on a 2.1 ratio back and with that rav range your talking some serious high speed!

BluFin had an interesting idea of wacking the engine onto an outboard leg. I have a friend of a friend who makes adaptor plates to fix modern gear boxes to big old V8 for the kit car industry, I'll see what he thinks!

Rob.

I must agree IMO with Johnny Boat Dude. It's not the out and out power that makes a fast useable boat it's a continuous power/torque throughout the rev range. Remember most marinised car engines are de-tuned to provide a more even spread of power.

Imagine riding a bike everywhere in 4th or 5th gear.

 

[codprawn]

I must agree IMO with Johnny Boat Dude. It's not the out and out power that makes a fast useable boat it's a continuous power/torque throughout the rev range. Remember most marinised car engines are de-tuned to provide a more even spread of power.

Imagine riding a bike everywhere in 4th or 5th gear.

Couldn't agree more - a steam traction engine may only be 8 or 12hp but pulling power - will easily outdrag a modern tractor puller/racer with 1500hp!!!

 

[kiwiracer]

why dont you just go and buy a 2.5EFI Offshore, it will probably be cheaper in the long run, Mercury have already spent the dollars in R and D.