is it a glass one?

I have repowered a few in earlier days, the last one I skippered had about 450hp Volvo but wasnt real fast, maybe 12-14 knots.
If you want it in survey, you will need to get approval for shaft size and material.

Sometimes changing shaft from 316 to Comsteel is enough to get the extra horses in without having to change bearings, coupling etc

 if you are looking for 25knots top, you'd need to be looking at 700Hp minimum. This of course will depend on the hull weight, fitout, etc. Glass boats tend to slow down as they soak up a bit, always quickest when they go back in after an off-season out of water, bearing in  mind you probably have a massive slab of hardwood for a keel. 

You say it only does 12-14 knots with an 8V92? What Hp rating? NA or turboed (or twin turboed?) if you have about 450Hp you should get about 14 knot cruise, as Rob says above.  What does it top out at with a clean bottom? Should be about 17 knots? What state is the prop in? My attitude to pleasure boat motors is that they never get many hours up in the course of the years, so you may as well run them hard, up to 200rpm under max, as you will never wear them out . 

As Rob also said, changing the shaft material from 316 ( if that is what is has) to a higher grade can up your rating on the same diameter, without having to go through the business of a different stern bearing, stuffer box, etc. But takng it all into account, it  won't be a cheap exercise. Even if you can source something suitable in the hgher HP range, that isn't already more than half-worn , for a reasonable price, you'd still be looking at getting new mounts, exhaust, various brackets and fittings, that can easily set you back 10 or 15 K, if it isn't a straight drop in.

I'd try running the old girl harder, if she still has life in her, and see if you can manage a 16 knot cruise..if it's spent a long time just puttering around, it will smoke and protest something shocking for a while, and then run better than ever. Jimmies don't like being nursed :-)

As a former Detroit employee what Ranmar says is correct, the two stroke will love you back if you drive it like you stole it!

Sometimes (depending on what exact engine you have, saying "8V92" is like saying it's a Ford V8, doesn't really provide much info) a horespower increase can be as simple as an injector change.

I'd suggest you talk to a knowledgeable marine engineer, someone like the Lawson boys at Pro-Marine or M&J Engineering. They will be able to guide you as to what gains can be made, and what power increase can be achieved. It might be relatively simple, or it might be cheaper to buy a new boat.

Out of interest, what maximum RPM are you getting out of her in gear? Sometimes if the prop match is way out a new prop can make a world of difference.

Good luck,

Browny.

 8/92's are a strong engine and as said you could find one for next to nothing, see MTU maybe?

But if its glass over ply, you may end up throwing money in you will never see again.
Solid glass Westcoasters are legendary, not so sure about glass/ply.
In any case Id be astonished if you were able to insure it unless the engine change is documented by a Naval Architect/engineer

 A non turbo 8v92 are only good for about 360hp, but pretty well bullet proof.

 Thanks guys. 

Reaches about 1800 rpm. 

I dont think it is glass over ply. I think it is solid glass hull with timber support frame internal which is fully glassed in.

Useful to note about the single turbo engine. 

 transfering the rpm to the shaft efficiently!

 You should be able to cruise at 1900. I'd be looking for 2200-2300 max. Prop could be the problem, or it could be a fuel system thing. I ran an 8V92 single turbo rated at 500hp in a 42 foot ali hull, quite light, and it would hit 2250 from memory. We bought the boat from the original owner, 1900 hrs on motor and hull, and it had spent it's life being run at 1600-1700, as he didn't work far away. I travelled more, and bumped it up to 1900 for cruise. It smoked and didn't seem to like it, but after just a few working days was a much happier motor. They are pretty thirsty, but also pretty reliable.

That old hull will be pretty heavy, I think you would be really asking a lot of anything to hit a 20 knot +  cruise speed with it. You'd be surprised how quick 18 knots feels after doing 12-14, if you can get that motor running and propped  right ( assuming it is not derated), you should be able to see around that. As Browndog says, it could be a case of different injectors and a fuel pump readjustment. Just be prepared for the fuel bills

 Thanks guys. 

Sadly however, the Detroit is gonna go. It has seen better days and it is time for the long budgeted new engine. So does anyone have a feel for what cruising speed I might achieve if I put a 800 hp engine in her?

thanks

Ian

You need to do your homework. You could put 2,000 hp in and still find you get the same speed. Or you could sort what you have and pick up 8 knots. Talk to some marine engineers, they will set you straight.

Cheers,

Browny.

Mate had a big Fiat V8 rated at about 800hp in a 50foot Westcoaster, cruise 17 flat out 20 knots

It all depends on hull speed, once you reach that it takes mountains of horsepower for only a small increase in speed.

Thanks everyone for your assistance. I will do as suggested and consider the old Detroit a little more closely. Also I appreciate the experience and help you all bring to this forum. 

Best regards