Monohedron vs. Warped bottom

[Paul Dean]

Monohedron vs. Warped bottom

I’m curious about the difference in efficiency and sea keeping between the traditional “warped bottom” lobster style (semi displacement?) design compared to a monohedron or variable deadrise (Ray Hunt style) deep V design at the speed range of 18 – 33 knots. What are the differences and the design features responsible? Firstly is the lobster design capable of plus 30 kt. performance with typical diesel power? If not, why?
Thanks for any input. Paul Dean

[Tom Lathrop]

Very interesting question Paul. There are some differing opinions but, from my reading, experience and model testing, here is my thought.

The warp in the aft hull bottom is intended to give the stern more lift and promote planing by presenting a flatter surface to the water. Lift per unit of surface area will be greater and total frictional drag for equal displacement will be less with the warped bottom.

The price for this is energy imparted to the water in the form of rotation caused by the twisting of the hull bottom. One prominent NA, Lindsay Lord, also said that the warp introduced suction or negative lift in the stern which caused the stern to squat. In some towing tests I did on a model in which I could vary the degree of warp in the bottom, I found that warp could cause yaw instability. That is, the boat would start to oscillate (steer itself) horizontally about a central vertical axis. At high speed, the model went so far as to turn 180 degrees and break the towing rig. I have my own theory as to the cause of this but did not study the problem enough to verify it.

If the boat is very fast, such as some racing boats like the "B Utility", the boat only rides on the very aft part of the hull which is almost flat all the way across. In this case there is no twist , and no directional stability either, so they add a metal fin in the aft part of the bottom so the boat can be steered.

For my part, I prefer the monohedron hull. It gives greater directional stability (deeper keel), handles better in all conditions and has better, and safer, manners going downwind where powerboats are at their worst.

Of course there are other factors that may lead you to turn to the warped or lobster hull. It will allow a larger and more powerful propeller on the centerline. Especially for a working boat, this may be a deciding factor.

While some of the more macho Maine lobster boat fishermen race their boats with over powered engines, they also sometimes come to grief when these hulls founder under race conditions.

In the lower end of the speed range you mentioned, it may not make a lot of difference. I don't know just where the benefits of the monohedron would take over. I use the monohedron hull and hedge with a chine flat to promote planing and spray reduction. Works for me.

I am only an amateur trying to study and understand these problems, so a more schooled fourmite may be able to enlighten the problem further.

[badges65]

Hi,
why dont you go for the best of both worlds and try an IVB!!!
it will out perform both and has greater stability and requires less HP to move it..
don

[Paul Dean]

Don
Thanks for your input. That is an interesting site with much good info about the Hickman S.sled etc. I am familaiar with this design but always dismised it as probably being a bit of a "pounder" in the rough stuff as Boston Walers tend to be. What is your experience with Walers and the sea sled design in the rough? Also I suppose this design relegates one to twin power???
Paul

[Paul Dean]

Don
Thanks for your input. That is an interesting site with much good info about the Hickman S.sled etc. I am familaiar with this design but always dismised it as probably being a bit of a "pounder" in the rough stuff as Boston Walers tend to be. What is your experience with Walers and the sea sled design in the rough? Also I suppose this design relegates one to twin power???
Paul

[badges65]

Hi Paul,
boy dont know where you got your feeling that they were a bit of a pounder!!
May be our design has rectified that as they are far smoother than any hull i have ever been in.
Go to site and look at location we regularily go out to the hen and chicks and boy have struck some real ruff stuff on the way home and have never had to go below 15 knots that area is renowned for its sea conditions we get a 2-3 metre easterly swell with a south westerly chop across the top we come home at about nor'westerly ..
didnt you notice that all boats shown except willinghoffs were running single engines they only use twins because of the excess loads they carry.
that 6.5M x 2.5M hull with the 70HP will cruise at 25kts with three up and still do in excess of 20Kts with 7 up.
don