How big of a boat are you after. The merc sj175 does offer a reasonable amount of thrust, but the downside is that in their infinate wisdom, Mercury put the exhaust from the engine in the cavity between the hull and jet unit. While this doesn't effect performance, if you have an alloy boat it can get rather loud. Put some thought into sound deadening, like hull liner throughout the inside of the boat, use urethane to glue any items that may be stitched to absorb vibration, and use a ply floor (but NOT the marine treated stuff)
A phenomenon known as porpoising can occur if you are not caqrefull of what you are doing on the bottom, particurly in the 6' infront of the transom. This area needs to be longitudinally flat. If you have a 'rocker' the boat will bounce around, or porpoise, if it has a hook it will bow steer badly, possibly causing the boat to spin in a turn. This is because there is nothing at the back of the boat causing drag.
Make the delta pad on the bottom nice and long, no-one says that the boat needs a 'keel', where the bottom sheets meet to form the vee. I tend to think of those that do as 'old school', and perhaps we should scrap the shuttle and fly the appollo missions again.
As previously stated, do not put any apendages infront of the intake. As in aerospace, the trailing edges create a vortex, much like the tips of a wing, only in water that usually means that air is bled off, if that air makes it to the intake the pump will cavitate causing loss of thrust. Look at not only straight line performance, but also where any vorticied will end up in a tight turn (if you terminate your strakes ahead of the intake), if the unit cavitates during a turn and you loose thruse, a real nice jet spin often occurs, sometimes at an inconvient moment!!!
Look at other jet boat manufacturers and what they do, we spend many hours making sure we get it right so we can sell a safe and well performing product.
Check us out at www.speedboats.co.nz
We specialise in trailerable jet boats and commercial adventure jetboats both in New Zealand and the USA.