Design proposal - help needed

I'm interested in knowing why this needs to be created, and existing boats are not capable of filling the role.

 

What seaworthy low power displacement boat <15 meter cruising 15 knots exist?

This "boat proposal/question" will cross Skagerak and be exposed to the North Sea. Therefore; fuel consumption, seaworthiness and length/width is a really big issue.

 

As you're going into the ocean I would suggest some features.

First off, flat vertical glass may not be the best answer to deal with waves that you may encounter.

Glass angled forward may be good for rain but as low as your boat is steeply raked glass may be better.

You may want to consider an aircraft-like cockpit like some modern lifeboats use.

If that seems too claustrophobic, good quality glass automotive windshield glass may be an affordable alternative for the front of your accommodations. Granted such would lend itself to a wider hull but with a boat along the lines of a much smaller version of Cable & Wireless ...

http://www.boatdesign.net/gallery/showphoto.php/photo/521

... you can still have the high length to beam ratio you want.

Which brings up stability: my own preference would be a self righting boat with a ballasted bulb keel but your desire for speed may preclude that option.

Aforementioned Cable & Wireless design, a "stabilized monohull" because the amas are very small compared to the main hull, would greatly enhance your stability. With the amas located centrally rather than extreme aft (done for resistance reduction) it would be easier to design and, likewise, there are folding/swing wings to allow such a boat to be towed or docked in narrower slips.

 

As I think of it this is basicallya rowing shell hull mated to a fully enclosed cockpit to keep you dryand the boat upright. The hull design is going to be very fast for very low power, but it's also going to be incredibly uncomfortable and rolling so much so that it will likely be impossible to tolerate. It would be a perfect example of the boat can take more than the crew.

So the only way I can see to keep the desirable speed and power requirements is to move to a multihull of some sort. As weird as it seems I really do thing a power Proa maybe the best solution. It eliminates the docking issue with a stabalized trimaran, doesn't require twin everything like a cat, and isn't terribly complicated to build. My only concern is I don't know how much it will attenuate rough weather.

 

power displacement boat <15 meter cruising 15 knots exist?
This "boat proposal/question" will cross Skagerak and be exposed to the North Sea Therefore; fuel consumption, seaworthiness and length/width is a really big issue.


On a trimaran in the semi-planing speed, Fn 0,7, the bow wave forms a hollow amidships. The water surface is lower than at low speed. This makes the support hulls loses load carrying capacity and the boat becomes unstable.
We have proven experience of the phenomenon.
The support hulls must be placed where they interact positive with the wave formation at the projected speed.

js

 

Could you be more specific about the concept of "quite rough sea"? The expected wave heights, wind speeds etc...

 

I see this boat resembling a U-boat, in the "quite rough sea"

 

With the right design and weight distribution characteristics reminds of what can be expected of a seaworthy canoe for open water. With the right shape, it will go over rather than through the water. None of my boats go under the water.

js

 

Mostly deep water. Long swells. Dangerous breaking seas are relatively rare.

1,5 to 2 meter waves is normal. 3 - 4 meter also, but not that often. The boat certainly needs to survive 6 - 7 meters.

A few times a year, the waves are 12 to 15 meters, but no one are out in a small boat that days.

Normal wind speed; 5 to 10 meters pr. second. Sometimes 20 meters pr. second.

 

Not enough freeboard to stop it "submarining" at some stage. And an upswept sheerline forward would interfere with vision from the seated position.

 

Which of my boats are you referring to? What practical experience do you have of that particular boat?

js

 

Not your boat, obviously.

 

Loa 12,6 m
Boa 1,28 m
Lwl 12,0 m
Bwl 1,07 m
Displ 1,4 ton
15,0 knots 26 hp
10,0 knots 8 hp

 

With this design I really have to wonder if a wave piercing hull wouldn't work better. The ends are almost useless anyway since they would be too narrow to access, and anything to reduce pitching would be desirable.

 

This showed hull form is based on our long experience with rowboats for unprotected waters. Reducing the pitch was precisely one of the important requirements, in addition to getting the lowest possible drag at Fn 0.7 in this proposal it correspond to 15 knots.

Calculate and test, no guesswork, please.

http://sassdesign.net/Design of a fast rowboat.pdf