Sueños Mojados

Saqa, don't mess with the canoe stern, leave it as tenant designed it. I'm surprised you havnt realised the implications of what you've drawn?

If your designed displacement doesn't immerse some of the transom, then the hull dimensions are inappropriate. So, the hull dimensions should be changed so that you get a moderate transom immersion similar to the previous schionning cats you've shown. For example, decrease the hull beam...

You really should stop looking at the resistance results given by free ship. You have no idea whether the hulls your considering are appropriate for the estimation model in the software and how close it's predictions will be. It's rather pointless. The main parameter you should consider, is the displacement to length ratio if you want efficiency, and high speeds with little horsepower. You don't need software for this, it's simply lighter and longer is faster and more efficient. No other design parameter exceeds this ratio in terms of importance if achieving efficiency your goal. I can't spell it out any clearer than that.

Second, if a smooth ride is also what your after, then you really have to decouple the hull from the sea surface. Planing hulls do the exact opposite, so don't think about it if you want to achieve your goal! Modern design utilises swath hulls to minimise the effect of waves on catamarans, then there is foil stabilisation both passive and active. This is the current business end for high speed passenger ferries, customers are always after smoother rides to keep passengers happy. Avoid any lifting features within the hull, such as hard chine edges, large horizontal surfaces etc. which will increases the harshness of the ride in choppy waters.

Did you see the latest cat designed for that famous hairdresser "Stefan"
Powered by 2x 400hp diesels and speeds up near 30kts is pretty efficient for an +80ft cat... Despite it's luxury, it's pretty light for its size...

 

I think you have do decide if the flat planing areas are in the water or not. If the flats serve a hydrodynamic purpose, the wetted surface needs to be included in the resistance. If you omit this you cant also claim hydro dynamic benefits! You cant have it both ways?

Cheers, and good luck

Edit, thats a Bob Oram design isn't it Groper? Thats the first pic I have seen of it out of the water.

 

Groper, I acknowledge what you are saying. Thing is I only know how to model with chines

What I can do though is at the time of cutting;
Cut the frames a bit wider then cut out a 'U' instead of chined 'V'
Cut the panels for the bottom plates as one piece and a bit wider then wrap it around the base of the wider frames so it looks like the shoe you made
Then I can use slits and heat to form the bow and put something like a compound curve on it

By doing that I should get reasonably close to the chined volume and from what I have learned about displacement hulls, a few inches draft either way wont hurt performance that much

I cant get michlet to run on my system. At some later stage could you please help by running a model through for me, I would really appreciate the help

I can easily narrow the beam to sink the transom a couple of inches. Is it worth incorporating the Tennant design even as a means to get the prop well wet without drowning the powerhead?

Dennis
I cant fault your logic, thanks for pulling me up on that mistake
Btw, did you see the wetted pics of my skiff? She goes alright mate

 

Here are the rest of the pics of the 28'er. I have decreased the beam to sink the transom a bit. Still at high displacement of 800kg now with 26cm draft. I have increased the thickness of the skeg trailing edge to 4" toincrease buoyancy there as I think that may help flow clearer water over the leg to the prop

I can round off all the hard edges at the time of the build. I have been thinking more on the bridgedeck, I think I can lighten it up quite a bit now and have 700mm of tunnel clearance and smooth undersides. Still have to check some figures on that so will post details on that later

LP by jonny.toobad, on Flickr
3D 001 by jonny.toobad, on Flickr
3D 002 by jonny.toobad, on Flickr
3D 003 by jonny.toobad, on Flickr
3D 004 by jonny.toobad, on Flickr
3D 006 by jonny.toobad, on Flickr
3D 007 by jonny.toobad, on Flickr
3D 008 by jonny.toobad, on Flickr
Specs by jonny.toobad, on Flickr

 

Saqa, that's no good. Trailing edges should be razor thin to mmi min mise drag!

 

Wouldnt the leg then give another face to the flow and increase the drag? I am trying to get my head around the flow and its too late in the night to have another kofiii. I can hear birds outside!

 

Doesn't that mean morning time.... Maybe you can add an onfill with internal radius to match & clear the leg... sleek!
Saqa, you love the lime green, it's good to look distinctive! Maybe you can franchice lime green sportsfishing cats around Fiji... Saqacat Charters with exclusive bookings/agents etc.
I think ... that foam sandwich would be good for your boat

Jeff.

 

An outboard engine has a very streamlined lower unit (or foot, if you want) designed to create a smooth inflow to the prop and hence increase it's efficiency. With such a thick trailing edge of the skeg you are doing exactly the opposite - creating a lots of turbulence in front of the propeller. It will translate into a loss of thrust and increased vibrations and fuel consumption.
Do as Groper suggest you, in the limits of what is practically sound and possible. Razor-sharp is clearly not, but reasonably thinned-out is.
See, as an example, the picture in the post #61 and the first and the last pics in your post #59.
Cheeers

 

The foot is streamlined, but usually above the cav plate, it is not. You cannot have cav plates level with the static waterline, it needs immersion to allow for pitching, I would say in a boat this length around a foot at the minimum. If you have an outboard prop immediately behind a thinned-down canoe end, there will be cavitation trouble when turning. You would need to allow a couple of feet clear.

 

Above the anti-aeration (not cavitation) plate, it doesn't matters. That is the part which works behind the transom, either in a disturbed flow or in air when the transom is dry.
What matters is that the propeller receives a water which is as little turbulent as possible, in order to transmit the biggest possible useful power to the water. So if the idea is to have the lower unit of the O/B behind that skeg/keel, then the skeg should be well streamlined and not cut-off like that.
Cheers

 

Thanks for the explanation, and I largely agree, but having an outboard, which is a steering device as well, immediately behind a fixed skeg or tapered canoe stern will create cavitation problems. Also, it requires running depths that don't expose the propellor to the air with the normal gyrations of the boat. Having the skeg truncated will allow him to run the leg deep and free of interference to the prop, or drag on the upper leg. But of course, at low speeds there will be a drag penalty.

 

Guys its 10" from the waterline to the bottom of the keel and 18" from the waterline to the top of the transom. I would like to fit an aluminum bracket to the transom to mount the motor instead of mounting it on the transom itself to avoid making a splashwell. That bracket will have two vertically spaced aluminum 'T' extrusion stems that will plunge lengthwise into the hull and bolt to each bulkhead

I will be getting a pic of a bigfoot 40 and scaling it down to photoshop it on the transom in the profile pic at some stage

Jeff
Lol! I been up working on this whole night. Yeah I really like that color on the sporty vehicles. When I lived in Aus, I always had a red holden v8 but ever since they brought out that green on the VE I have had the hots for it. There is a thread about a homemade alloy sports cat in that color in the powerboats section that was built in Russia

For this build I can order HDPE sheet in that color. I would actually like to build light using polycore but will leave that for later once I have sorted the mooring area out. At the moment its going to be a very liberating boating experience being able to ground in any terain and not having to worry about fenders or scratches and most other damages!

Daiquiri, Mr E
Thats right, that part of the leg is well faired. I wish I had access to see water flow in some software! I can sharpen that skeg trailing edge and even offset it forward without making substantial changes in buoyancy but I don't have any means to see what difference it would make other then informed calls from helpful folks

 

Looks like the Bigfoots are called Command Thrust now!

This bit is worth some thought, the 40, 50 and 60 Command Thrust EFI list the same cylinder capacity of 995cc and weight of 118kg and same gear ratios and alternator specs

Is there any good reasons to consider the 40 over the 60 other then price? The 60 is also available with XL 25" shaft

 

Some measurements of the transom

3D 011 by jonny.toobad, on Flickr

Mercury scaled to place cav plate 25" below transom top edge to see how the XL shaft will sit

profileob by jonny.toobad, on Flickr

Since there is an XL shaft option, I am going to play a bit more with the simpler original shape and try to reduce draft for a drier transom and see how deep the prop sits on that too

 

My suggestion.
1. Build in plywood. Add more glass for higher wear.
2. Buy ecat 24 plans and scale them (longer as you can afford, wider to match displacement you need)
3. Listen to the negative feedback. You are too quick skip good advise and warnings. And when you stop you fail to understand what is needed. Instead of thinking of adding removable transom pods or exotic bottom shapes aim for very simple hull - but with sense. Don't use software as an excuse (my software can't do this...). You can do a lot in freeship.
4. As stated before ignore freeship's calcs.