Jet Zip build progress

[Cribbsy]

Hey guys, thought I would post some pics of my Zip project. Happy to move to Jet ski section but just figured most of the Zip owners hang out here. I'm grafting in a Yamaha xl1200 hull section and will be a single cockpit layout. I originally built to suit a long shaft 50 merc but just wasn't happy with the look.

[Cribbsy]

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[JimmY]

Very cool, looks like your breaking new ground here.

I like the idea of stripping down the hull and building it right in to the frame. Other jet ski mods have finished the hull and then cut it up to graft in the pump.

[DSR]

Hi Greg,

Looks good sitting in the Zip hull. Did it graft into the hull as nicely as it looks like it did, or was there a lot of fussing going on?

Dave

[Cribbsy]

Hey Dave, I think I got lucky with the hull shape of the ski as it gave me a level and straight line to cut. I took to it with the grinder before I had enough time to overthink and confuse myself!! Notching/cutting the transom was bit tricky but pretty happy with how it is sitting. I am a little concerned about weight as the ski hull section is surprisingly heavy but should have plenty of power to make up for it. I'm going to epoxy in some wedges along the stringers and then run the planking over the top onto the flat area on the ski hull then glass the inside as well once it's flipped. Your right Jimmy! I'm a bit nervous chopping into the hull after so much work but I think it will turn out ok, and something a bit different

[BarnacleMike]

That looks impressive!

[JimmY]

I've heard about a couple Yamaha jet powered boats, and the early ones wander in a cross wake and have no control when coasting. Later models have a significant keel. Something you may want to look into.

[DSR]

Hi Greg,

Yeah, the weight of the ski hull was one of a few reasons that made me decide against grafting the Honda hull Into the TNT. I have a target weight of around 600 lbs complete, which is still substantial considering that it's a 12' 6" boat.
With the Zip being a larger boat, and having the powertrain weight ahead of the transom instead of hanging off the back, the Zip should balance out pretty well. From what I've gathered, the Zips have had issues with porpoising with heavier outboards, so having the weight forward of the transom should help with that also. If I remember, your Yamaha engine is a good bit lighter than my 220 lb 4-cylinder, so that's a bonus as well.

I was wondering, once you have the bottom planking on, have you figured out how much of the front edge of the ski hull will be below the Zip hull, as far as width and depth? That will have an effect on the length and shape of the keel that's added to blend the ski hull in.

Looks great Greg!
Dave

[Cribbsy]

Hey Dave, It's around 80mm(Aussie!) from keel to the bottom of the ski hull from memory. I was trying to find a balance between keeping the intake low enough but not too mutch to make it too hard to fair in. A lot of guesstimating going on so fingers crossed! I'm just trying to sort out some support between the stringers to tie in and mount the keel and lower batten at the moment

[DSR]

At 80mm, you're going to end up adding a pretty substantial keel to blend it in. Is it too late to raise the ski section up into the hull further?
It might just be the perspective of the pics, but is the ski keel parallel to the Zip keel? It just looks like it's rockered a bit.

[Cribbsy]

Hi Dave, nothing is fixed in place yet as it's just resting on those little timber tags at the moment. I've been using Shane Newcombes tiny might video as a bit of a guide I guess. I'm planing on laying the hull as per plans and then fair it in with foam. I didn't think 80mm was Much at all but can still move it no worries. I was more concerned about keeping the pump intake low enough in the water. The ski hull is parallel and tapered down slightly at front which should almost match the zip. Those stringers will be trimmed down and faired towards the bow too.

Appreciate the help and feedback, Cheers

[DSR]

Hi Greg,

Yeah, Shane's Tiny Might is a really cool little boat and he did a great job on it.
I'm definitely not going to say that he did anything wrong building his boat and the proof is in the pudding.. It obviously runs good! I'm just saying that I would do it a little differently, and the reasons behind it.

The Zip is what is called a low-deadrise warped bottom design. This is a very efficient planing hull design and the larger, and the deeper the "V", that the added keel is, will correspondingly effect that efficiency. So the smaller the added keel is, the more efficiency you retain. Also, the focus of "dropping the keel" for loading the pump, is to keep the aerated boundary layer at the surface of the water from getting into the pump. The most efficient way to do this is not to mount the intake below the bottom of the hull to try to reach solid water below the boundary layer (this works, but creates additional drag and a more pronounced "bow-down moment" that constantly tries to push the bow of the boat down when up on plane. A better way to do the same thing is to actually "steer" the boundary layer around the intake, which is exactly what the ski keel that you're adding will be doing already, and it doesn't have to be very large or deep to do it.

I would move the ski hull up far enough so that the bottom planking meets the edges of the ride plate mounting area to give you a lot less underwater appendage drag and still keep the pump happy (and less fairing to boot ). I would also actually keep the ski hull strakes and extend them forward into your fairing job a little ways (not for lift, but directional stability) keep them straight and make the edges on them fairly sharp. Don't extend them back any closer to the intake though..

The only advantage to be seen by doing a larger drop keel is better pump loading in rougher water (rougher than you would probably want to ride in a Zip in any sort of comfort..... )

Sorry for rambling, just my 2¢.......or 3.......

Thanks,
Dave

[DSR]

Hey Greg,

I just went back through the pics and I failed to notice that the mount points I was looking at were the pump mounting and the ride plate mounts are behind on the outside edges, so to raise the ski hull up any further will probably require quite a bit more work. Sorry I missed this earlier
It would still be worth it to do, but it will definitely work where you have it set now also......

Thanks,
Dave

[Cribbsy]

Hi Dave, thanks again for the info. I'm all ears and trying to soak up as much information as possible. After some more thinking I'm going to leave it close to where it's sitting now. One thing I would like to get your thoughts on is if you think I should have run the main stringers that I've added right through to the next frame. I'm a bit concerned that I've created a weak point in the keel where the ski hull meets? Or am I over engineering it? I'm going to make up a block between the keel and ski hull to bolt through and thinking of adding a ply stringer along the keel to the shape I want to fair it to and then can foam each side in after sheeting. Cheers, Greg

[DSR]

Hi Greg,

I would extend the stringers and mount them to the next frame forward to create a box. This will help stiffen the structure and spread the thrust load over a larger area and to more framing within the hull. All the thrust-reaction from the pump will be pushing against the keel and the ski hull will try to basically "twist" within the Zip hull when you turn. If I remember, I believe that your Yamaha produces close to 1000 lbs of thrust at WOT, so that gives you an idea of forces you're dealing with.

As for adding the keel, I would add a batten on either side of the keel positioned right in front of where the ski keel meets the bottom planking on the Zip and running forward from there. This will back up and reinforce the areas where the sides of the added keel will rest on the bottom so water lift at speed isn't pushing the keel up on just the Zip keel and the open spans of bottom planking between the battens already in place on the build. It wouldn't even be a bad idea to just reinforce the entire area inside the planking where the added keel attaches to the bottom (basically, filling in the spans in between the existing battens with timber)
I would also complete the bottom planking and give it a couple coats of epoxy, then build the added keel on top of the planked bottom and then sheath the whole thing with glass cloth and epoxy to finish it off. Also, make sure that you prep the ski hull properly so the epoxy can adhere to it. Aggressive sanding and washing with acetone should do the trick

I hope that helps Greg, it's looking great so far and I can't wait to see it scootin across the water when you're done!!

Thanks,
Dave