Gramps50 said:
Nice looking build from the video you would never guess that it was home built. From the 2 threads you and PSG-1 both have great fabrication skill when it comes to the conversions.
What's the advantage of using a jet ski motor as opposed to just using a jet outboard?
Sorry for the extremely late reply, just noticed that someone else had posted in this thread.
As for your question..... here's some advantages and disadvantages to each one:
With an OB jet, you still have the weight of an engine hanging off the transom, with an IB jet, the weight is slightly forward, making the boat less stern-heavy.
Also, provided that the engine compartment and exhaust system is properly designed, an IB jet is always going to be much quieter than an OB jet. All the OB jets I've heard were as loud, or louder than, a jet ski.
With an OB jet, the water is drawn up vertically, and then turned 90 degrees through the exit nozzle. As anyone who understands the dynamics of water flow, etc knows, anytime you make a 90 degree angle, you lose some efficiency and overall pressure.
With the IB jet, the water is drawn through a tunnel, at a very slight angle, and exits straight through, it doesn't make any turns, therefore, it is a more efficient means of transferring power.
An OB jet's intake grate still protrudes slightly below the edge of the transom, where the IB jet is flush with the hull.
On the other hand, an advantage of the OB jet, is that it can be trimmed out of the water, which is a good thing if you operate in salt, because with the IB, the pump is constantly sitting in salt water. Also, the vertical design of the OB allows all water in the cooling passages to drain out when not in use.
With an IB jet, the horizontal configuration of the engine allows water to stand in those passages, and in the water box (muffler) which means if you're running in saltwater, you either have to haul out every day, or you need a specialized drive-on dock known as a 'sport port' where you can get the pump out of the water, to flush it. Or, you can flush it while it's sitting in the water, as long as you let it idle, and don't rev it. In this manner, garden hose pressure exceeds pump pressure, so, only fresh water is flowing to the engine. But I don't like this method, as you can't "blow out" the water by revving it like you can when it's out of the water.
And finally, with the OB jet, you're still dealing with a conventional steering system, which is 3 turns lock-to-lock, unless there's some specialized system I'm not aware of.
With the IB jet, you can buy a helm that's 270 degrees lock-to-lock, which is what my boat uses, or, for even sharper response, they also make a 135 degree helm. With steering that quick, you can spin the boat 180 degrees in its own length at 30 MPH. I just don't see an OB jet with a 3 turn lock-to-lock teleflex helm having that type of steering response.
