Would anyone like to design a 39 foot/12 metre Stabicraft-style boat?

@TANSL makes a few good points

Is this really considered a planing hull when 450hp maxes out at 20 knots?

I don't think so. This is probably an in between boat. If so, each additional 20hp maybe offers 1 knot of speed? Check with the designer, but these do not appear to be planing hulls, per se, to me and the hull designs certainly look capable of large seas. Poleson may also not like the excess speed from twin 300s either as some boats get squirrely when run too fast.

 

note the shades over the aft deck

 

A simple email to Hamilton Jet in New Zealand should be able provide some price guidelines and perhaps some advice. I do not agree with others that a jet will be "obviously" obviously more expensive than other options.

Compared to a straight shaft drive, the jet does not need a transmission, compared to a V-drive, the jet does not need a v-drive and a transmission.
Pods, like the Volvo unit are crazy expensive, certainly offer sat positioning but it all comes with a price.

So a jet drive will require an engine and pump. (certainly some mounting considerations but you will have that with any set up)

 

Agreed - its certainly not going to cost much to flick them an email and ask.

Educate me time - is the transmission just needed to give forward and reverse options - or is used to alter the rotation speed of the prop compared to the engine speed?

 

Well both.
Normally props do not run at engine speed so a transmission will have a preset gear ratio. Plus you need a neutral to start the engine and forward and reverse for direction.

With jets, the discharge has a bucket that redirects the flow forward. ( or forward and to the side) The position of the bucket then gives you forward, reverse or neutral. Ie no transmission. The jet is built to accommodate engine operating rpm so no inherent gear reduction is required

That being said, if you need a "pure" neutral, ie the boat would only move due to wind/current drift, it can be a tiny bit more difficult to adjust the bucket position to absolutely stay in the exact position as there is still water coming up the intake, and out/off the bucket deflector. For more than 99 percent of boating, this is not an issue. But if you need to be with absolute zero motor induced drift, it might be a very small consideration.

On the other side of the equation though is that if there is a wind and or current drift, you can adjust the forward/reverse thrust to slightly overcome these issues, making the jet the best choice for these set of conditions. When you throw a
transmission into gear, at idle, there will be a significant movement. Perhaps several knots

You could mention this to Hamilton and see what they say.