Design....15'-17' alum Jet Sled w/PWC jetpump

Hello,
My name is Glassiches and I'm a water junky!

I would like to design and build an aluminum jet sled/work skiff w/center steering.
I would use it in Rivers, Lakes and on the Puget Sound (salt water)

I have a salvaged PWC Kawasaki 1071cc (120 hp) that I would like to use for power.

The PWC specs shows:
Pump ...853 lb Trust.
GVW.....rated at 1100 lbs

I was hoping to build it in the 16' range and as wide as possible.
I see they use 8 to 15 deg dead rise on a lot of the Jet Sleds and have seen the V at the Transom and also modified to Flat.

I haven't ever designed or built a boat but have done a lot of fabricating, repair and customizing.

In other words, my drawing looks like a white cloud.
I'm iching.......to turn the key!

Thanks...Glassiches

 

Hello...

http://carlsondesign.com/#Fun_Shareware

http://www.parallelgraphics.com/products/cortona/

http://carlsondesign.com/hulluser.html

SH.

 

Thanks Sean,
These look like they will be great tools to use.
Which hull design program do you use?
I looked at the Lundström Designed "Seajeep" which has some features I would like to incorporate.
It looks like TouchCad will take me a little while to understand and use but it will very powerful.
I really like the lines of your boat, very interesting design. When did you start the project, what type of power are you using?


Thanks again....Glassiches

 

jet sled design

"I would like to design and build an aluminum jet sled/work skiff w/center steering."

glassiches, a few things to keep in mind about pump propulsion- velocity, velocity and velocity. The ratings you gave are most often at speed and high rpms so you need to keep in mind that the power is strictly available at the top end -moving along at quite a clip- not standing still. So the work skiff part of your post looks like it needs definition. An axial flow pump moves water well only at higher speeds since the liquid rising into the pump volute and out the after nozzle best at 25+ knots. Work skiffs often putter around with loads in the displacement mode (under 12knts) and the two might not be easily mixed.

Skiff hulls are usually somewhat exclusive of one mode or the other since the bottom shape that lends it self well to slow speeds and work (except the work of offshore sport fishing) doesn't do too well at high speeds- unless kept in still water without much swell or chop. The higher speed sharper bottom is solely chosen to retain speed in a seaway with waves and has more roll for any given beam sea when not running.

Since the work and jet sled ideas are most often found in separate boats; I suggest you define what you're actually planning to do with the boat- before making design decisions. Sleds (boats with less than 4-6 deg deadrise AND more than 8:1 lb/hp ratio AND sides lower than 1:4 side ht./chine beam) will work great on lakes and rivers since the waves are usually smaller than Puget Snd. However, on the Sound you'll find the very flat shape will pound violently at typical jet sled speeds.

I mention these items because you're proposing to go around the "design cycle" and should keep some basics in mind as you go around. Many designers go around this 'bush'- and around and around....

Good luck with your project, Cheers,
kmorin

 

Hi kmorin,
Thanks for the input!
You are 100% right, I need to be more definitive ...I would like to modify a sled hull (slightly) to reduce the pounding affect we receive while using it on the Puget Sound.
I typically don't go faster than 40 knots in and around the sound. I would like to build the topside more like a work skiff. I believe I could look at that aspect more like outfitting it similar to a work skiff. I beach the boat daily and perhaps a little thicker bottom may help due to the rough beaches we have here.
The boat I currently use is completely flat at the transom but I have seen other sleds with the deadrise (of probably 4-6) as you pointed out.
The grapevine told me to use a "Torsion Style Frame System" in a modified V design using a slightly wider than standard reverse chine. I don't know what to make of it but the thought was the modified V (I don't know what "modified" represents) used together with a Torsion Frame would reduce the pound and the wider reverse chine would still allow it useable in the shallower waters at lower speeds.
I launch the boat and use the Puget Sound to travel to and enter the rivers very often. I would like to know what your thought is on the sizing of the boat relative to the power I have available? I understand about the HP ratings but if I used a more conservative approach.... maybe a reduced calculation of 90hp, do you think that being in the 16' x 600# range would be a rough place to start?

Thanks again.
Glassiches

 

Not my boat...

Hello...

Those pics are not of a boat that I am building - they are from Greg Carlsons website...

SH.

 

OK....I saw them on the site but thought it was yours???
Thank you again, the info is great!
...Glassiches

 

jet sled design

Glassiches, If these engines are similar to outboard engines then the last 10% of rpm delivers about 40-60% of the rated power and a jet pushing a load at displacement speeds means you'd get about 30% or even 20% of the rating to figure your design. So at "walking" speeds your 120 will deliver about 30 ponies.

If you're not hauling loads then the work part isn't clear. At mid speeds (15-25) you're most likely going to get 40-70 hp and above that things will pick right up -just like all pump powered craft.

I'm not familiar with the term "torsion style frame system"; reverse chine width will increase chine, and therefore waterplane beam, and hull will draw less at rest- you have to go around the design cycle and displacement-to-waterplane ratios to examine that. Excel is a fairly simple way to to that exercise.

Not positive about the term modified V- often V hulls are divided into two groups. The first has a deadrise (V) angle at the master section that remains constant to the stern; second the angle changes aft or flattens in body plan as the sections move aft. Both have places in planing boats, but I believe the latter is considered more difficult to optimize in a small design. So, a "modified V" may mean you've been recommended to design a hull that has more V forward for a sharper entry and then flattening aft for less draft at the pump.

Pounding is also a function of total displacement when you're on a boat of less than half a ton. Even 27deg (deep V) hull would pound, relative to your body wt, if it were driven at 40 knts and an all-up wt of 1000lb. Which rides better (?) a volkswagon bug or a '57 Cadillac (on a rough road)? Water gets hard at 27 to 30 mph and entry AND displacment effect the ride -especially below a few tons wt. The Boston Whalers (even the older flat sleds) rode nicely primarily in the smaller boats because they were/are three boats for every hull. The inside, outside and the filler- all added wt- these things were heavy and they had "ride"- the same size in welded alum will displace about 1/3 or even less than the double hull GFP. just a note about 'ride'.

round and round the design cycle
Cheers,
kmorin

 

Glassiches,
Modified-V usually means sharp deadrise near the bow, flattening as you get near the stern. In a boat as light as yours it really won't make a big difference whether you have 0 or 6 degrees deadrise. By 15 or 20 you'll notice in chop, but as kmorin says, the displacement is what will really determine the ride in a boat this size. Example: My 15-foot flatbottom weighs only 100kg dry and pounds like hell when empty; loaded, she's smoother than most 18-foot aluminum-sheet V-hulls. If you build a flattie sled it will pound like hell on plane; a sharper vee will be smoother but the weight is still the big factor.

As for the 'torsion' frame. This can mean a lot of things. Here it seems to refer to a hull designed to take high twisting loads between bow and stern, or perhaps one that uses the twisting loads on structural beams to counteract flex in the hull. Whatever it means, it is likely not going to be easy to develop without many hours on a CAD workstation and a few finite-element analysis runs. Aluminum behaves in very strange ways under torsion.

As for power. The ty[e pf jets used on big ferries, etc. are very efficient across wide loading ranges; your jet-ski engine is not. It will work, and your power sizing sounds reasonable (perhaps a bit low if you're going to be carrying a lot of stuff). But it will be hard to get it to work well going slow.

I'd sit back for a bit and take a good hard look at this project. Write it all down: what you plan to do with the boat, what kind of waters, load, all of it. Work through the whole design process step by step, not rushing anything. Once you're confident what your requirements are, and what you need, then move on to actually getting underway.... the patience will pay off with a much more capable boat at the end.