A new guy's hello and plan. Jet drive Tunnel hull.

[Coreyjon]

This may or may not be the most appropriate place to place this but its close. My names Corey, and I have a problem, I love engineering design, speed, and having fun. I'm currently a aircraft mechanic in the air force so when it comes to wrenching/building I'm good, although sometimes I think outside of my own boundaries. I've never designed a boat, and I have never built one. In fact, I never really thought about boating as a significant hobby or even thought that passionately about it before. My best friends father is into boats, even fixes up old boat motors, and we would go out on his boats every now and then, and it was always fun, and relaxing. I'm young, and while that limits my overall experience, I think I make up for it with my imagination, and drive. I didn't want to design a build my first boat, having no real experience, so I baught a set of plans for a 15 ft tunnel hull design (as I am passionate about the engineering and physics involved with flight - the tunnel hull caught my eye for several reasons) I have not received the plans yet but it does call for a 35-100 hp outboard short drive - my plan is to modify the plan somewhat to use a large jet ski engine / jet drive combo. My reasons are to reduce weight, to lower the center of gravity by being able to place the motor further down inside the hull, and to streamline the back of the boat better.

-I should state the boat weight will be in the neighborhood of around 220 lbs or so with out engine. I weight currently 213 lbs, so 440 lbs without engine, not sure how much a jet ski weights, but I think a large jet ski engine would work.

I think I found in what seams like an unlikely place (for me that is) an outlet for many of my desires, to have fun, go fast, and relax - while at the same time testing my ability to modify and redesign slightly the boat and build it. Well that's it for my first post/rambling. I'm here to learn, I'm far from a boat builder/designer, but things like theory interest me, and especially when testing those theory's can be a lot of fun.

[56caddy]

I just blew my lower unit and possible the whole 30 HP mercuray engine running in shallow water. So now I want to put a jet drive on it. It is a tunnel hull. You mentioned you are building a boat like this. Do jet drives work on tunnel hulls? Have you researched this already? Thanks.

[marshmat]

Jets can be made to work on many hulls, but not all.

One of the major factors is that the jet intake must be sucking from undisturbed, unaerated water. There can be no strakes, no fins, no irregularities in the hull for a metre or two in front of the jet. And there can be no air entrainment in the water flow to the jet.

If it doesn't look like those requirements can be met, it's probably not feasible to convert a hull to jet drive. (I should note that there are some boats, esp. whitewater boats, that can handle aerated water; this requires very specialized hull design and pumps designed for the purpose.)

If you think it's possible to get a good steady flow of water to the pump, then you might start thinking about weight distribution, trim, directional stability, etc.

Do either of you have photos of the boats in question that could be posted? It's much easier to evaluate a project with some idea of what the hull looks like.

[harlemriverman]

Fun project. good instinct on weight to thrust. And marsh, good comment on the jet intakes. Picture a trimaran, the jet intake will work fine but the weight to thrust will be a real challenge for geometry.

Do the math first. My question is whether jetski is too wimpy a platform. Are you stretching its capability so far to and beyond the limits that at the end of the day you might be better-off buying a blank hull configuration that you like, and building the internals to your own specifications? you will really heat up that engine to carry all the found drag with two tunnels at your side, which you will need for the added weight of fuel and drive train.