Hello people
some thing differnt for the pros.
I have a 12ft jet boat with a A12 in it which just died so i got a A15 can you tell me what would be the best thing to do to get the most hp out of it (now for the good bit?) WITH OUT going over 5500 rpm MAX.
The jet pump cant run more than that.
thanks for the help.

I think twin carbs bigger valves and more
compression will do it for you and some
porting work done on it of course

Power with out revs......Torque!
One word dude. supercharger!
Why don't you want to exceed 5500?
If thats how hard the old a12 went a stock a15 will pull more easily. Mind you if this is for essetial marine reliability and motor longevity then thats fair enough too.
Plus with a s/c , you can make good power without using a lumpy cam with an unhappy idle. It is quite a bit of work fitting one , but it looks cool.

The jet will start to cavietate after them REVS.Years ago when i was workin for QUINTREX boats we made a jet flat bottom boat with A15 for go(stock),went very well.jet was a DOAN.Mite make one again one day about 5m long with twin jets.

You can put anything in a boat without vicroads up your clacker
So you best go with a torquey motor if possible like a z24 or anything long stroke like an L20b or even a ca20.
Youll never look back and even better if funds were reasonable you could go all alloy nissan v8 (much cheaper than toytoa v8 as no one buys them cause they fit hardly anything) especially is you get the earlier pre 90 model one a pushrod 3.9 litre capable of big torque and 196hp with micro sized single barrel carby and bad manifold and exhaust.

thanks for the info but the boat is set up for a A motor and dont have the time set up anything else.

Keep the A15 stock. It already makes max power at around 5500 rpm. Just do a re-jetting (make it richer).

The only extra thing is possibly a higher-lift cam (but same cam timing).

Nick,
A14Force is right on the money. In fact he is reinventing the wheel as supercharging both Holden and Falcon 6's back in the 60's and 70's was a common option.
The problem is as you explain cavitation and really you need to get your max. power under 5000 and run either a coarser blade or a smaller nozzel.
The supercharging option on a jet boat has always intrigued me and I firmly believe that a very good setup would be an RS Legacy Turbo engine with the Turbo biffed and the supercharger off an 2 litre Toyota Celica put on instead- aq strong factory built engine with NO lag, nice and light and able to be mounted low in the boat.....
However I digress. An MR2 suoercharger drawing through an1 3/4'' SU ( off any old BMC 1800/1750 with the needle pruned accordingly will work just fine and give you exactly what you are looking for.
Good luck.
Any questions give me a yell.
Dave

It seems to me that a couple of things come into play here.

If i assume correctly, the original A12 managed to do the deed quite nicely & you are now looking for a modest boost in power with a replacement engine. This engine simply drives a pump that pushes water, under pressure, out of a controlable jet pipe. So far so good?

If the power was increased substantially, you either run the risk of excessive cavitation, or you will need a bigger pump to handle the extra power. This is either undesirable, or costly, so i think that you are on the right track, & ddgonzal, as usual, seems to have got it right, for the right reasons.

Most A15's were from commercial models, which were better known for their torque output than their high revs, & it seems to me that this is what you want.

If a cam change is required, then a good heart to heart with your cam specialist should provide you with what you need. If supercharging [turbo] is considered, then a system that is tailored to your needs would be the best bet, but i'm inclined to think that this would overpower your existing pump.

Good luck with your a15

Previous posters have correctly pointed out that all the Datto engine is doing is running an axial flow pump that pushes water out its clacker and so, by reaction, pushes the boat along.

I reckon you should go for an A15 and then look closely at the nozzle at the outlet of the jet unit. I've never seen a Doen unit in pieces, but others have a removable nozzle with the port size tailored to suit the engine.

I believe that if you go for a more powerful engine, you may well have to increase the size of the nozzle. No big deal, put it up in a lathe and bore it until it works best. Mind you, Doen might already know the answer.

Increasing the nozzle size will allow more water thro' the jet (making use of the increased power) and will therefore provide a greater motive force, the result being a higher speed, all other things being equal.

The question that intrigues me is --How does the cylinder head like salt water?