Outboard or Jet Ski power?

I picked up this old three point hull and I'm trying to figure out how to power it.




Transom width is about 4', length is 160", widest point is 88" and the transom height is about 12". It used to carry a 40hp outboard in the past and apparently clocked around 40mph. Its been in that field on those oil cans for 20 years.

I'd like to hit 50mph.

Looking at most common outboards that I can afford the problem is the gear ratios are too low and prop availability won't push enough water. It looks like most older 70-75hp outboards with a high pitch prop. This would be the easiest solution but I a bit worried about weight. Something with a 2:1 ratio and a 23P prop would probably do it. Can anyone suggest a common outboard that would do that? It would have to be something from the 70s or 80s that I can afford to mess with.

The other option and one I'm actually more inclined towards is to graft in a jet ski power train. Looking at whats available I think something in the early 90s is affordable. Locally there are some 650/750 polaris skis. In a ski they seem good for high 40s mph. Is it easy to get more speed from these setups? Can I swap in an outboard powerhead that puts out more juice? Change impellers?

I don't think hole shot is going to be a huge issue I just want a fast little two seater.

Can anyone offer some insight into my quandary.

 

I would be worried about the soundness of the hull after being there for 20 years.

Definitely an outboard. You can do a bit over 40 knots (about 50 mph) with the highest pitch standard propeller of most 40 hp. I have done that with -85 Yamaha 40 and 17" standard propeller.

Then you can buy different kind of special propellers that allow even a standard 20 hp outboard to reach that speed, if the hull just has low enough drag. I had a 18" and a 22" cleaver in that Yamaha and reached 45 and 48 knots with those. The 22" was a bit too much and I only got about 5500 rpm (the limiter was at 6000 rpm, gear ratio 13:24). I sold that propeller and it was put to a 75 hp engine and it reached 55 knots.

 

That's good news. I'm looking at a few merc 50hp outboards right now that are 70s-80s vintage right now. From what I can tell they are 2.33:1 gear cases.

Using a prop slip calculator with 5% slip and 5600rpm it says I'll need a 23p prop to hit 50mph. Where can I get a 23-25p prop that will fit these motors?

 

Mercury 500 has a 2:1 gear ratio, thus 20" should be enough for 50 mph and easy to find. It is very a old technology motor and has only a bit over 40 hp at the propeller shaft and still consumes about 25 l/h. Yapanese 40 hp models from mid 80's have more power than it. I would recommend the 3 cylinder Yamaha 40/50, Tohatsu 40 or Suzuki 40 from that era.

 

What about 70hp evenrude/Johnsons from that time? Japanese outboards are pretty rare here.