Hull shape help please

Attached is a hull I stumbled across.

Why is the stern slant foreward, or does it reduce drag ? It's not very wide aft, the drag is so little you can ignore but you win stability / LWL ?

Someone pls explain.

Thanks in advance

 

Hi Fanny - - asthetics ? - rot - hull damage? - - looks like the hull was cut back - some multihulls have reverse transoms that incorporate a swim step - - we extended the main hull aft by 5 feet in our ORMA re-bulid - (The amas will receive the same treatment by 30-40") - the main hull transom angle is similar to your picture & it will incorporate a swim step feature as well as a cassette rudder system -(cassette within a cassette) - as the boat will be used for fast cruising & being able to pull the rudder blade(s) up will really make a difference in where the boat can be taken - the cassette rudder systems will allow the easy comparison of various rudder blade designs to dial-in the right blades for the boat - - I like the pictures you submitted of your project in another thread - you will be able to try various rig configurations with your boat sort of like the early trimaran experimenters did on San Francisco Bay back in the 60's - they would tear-up the boats on a weekend - rebuild during the week & go back out & tear stuff up again - thats one way to learn - - Dale S.V. VOODOO SCIENCE Miami

 

He he... are you ugly or sister or both Just pulling your leg a bit... ehem, no but seriously

There are two of those hulls, they are the same, fiberglass construction and are identical. I checked them through thoroughly, they were made like that, no mods, no extensions or rot either. The frame you see attach to another hull, so these are actually ama's for a tri.

They are unfortunately not mine - I wish they were ! They belong to someone who is restoring his tri. I just happened to like the hull and wondered why the foreward slanted stern, and if it would have any negative efects being like that.

I made a drawing of this hull. The stern ending above the waterline shorten the LWL and has to have quite a belly to carry the same weight.

The original hull (on the photo) has less of a belly and can carry a lot more weight for the same draft...

 

This is the right forum to ask this question, right ?

 

Hi Fannie - - the name S.V. Ugly Sister - thread - Trimaran Help !? - reply # 14 & # 19 ought to sort it out - - - a reverse transom can also slightly reduce the weight of the hull over the same waterline length with a plumb stern - to some it can also be a matter of asthetics - I prefer the looks of most reverse transoms - (but there are some pretty radical examples amongst the larger trimarans) - but I can't say if there would be much of a difference in overall performance between the two transom configurations - keep em' mean & keep em' lean seems to go a long ways in trimaran performance - - Dale S. V. VOODOO SCIENCE Miami

 

In the early 60's a well known designer, was beaten in a pre race heat. In an effort to save weight, the stern was cut off, in what is now known to be a reverse transom. His boat still had issues, but it was substantially lighter in the butt. The idea stuck and designers started to slope the transom inboard. It has since evolved into a fair bit more then just a weight saving gimmick, with steps, platforms, garages, cup holders, etc. installed. Mutli hulls just picked up the trend, for the same reasons, you really don't want any more weight then necessary in the ends of the boat.

 

Saving the weight aft is a good idea - it doesn't look too bad, the steps is a good idea too.

The stern being small, a relative small drag there probably won't matter too much, it should improve the hull's rocker as well as weight carrying capabilities then since the draft can be more as well.

 

A forward slanting stern helps to re-align a poor LCG problem without a major redesign...ie moves the LCG aft.

 

Ooops..sorry wrote that too quickly. Moves the midships aft, ie closer to the LCG.

 

Thanks Par, always interesting to read where something comes from.

 

Looking at the last picture I posted, how can that hull be improved / made faster / better ? assuming it's for a cat ?

 

You're limited by the hull shapes, wetted surface, L/B ratio, etc. You could attach foils to it and change the amount of wetted surface, possibly dramatically if shaped well and large enough, but this is a whole other can of worms.

In other words, some improvements could be made, but the speed gains would be very small, fractions of a knot. Wings could make huge gains, but they have their vices too.

 

Hi Fannie - 2 of those trimaran float hulls configured as a catamaran - make them go faster? - - add 6 to 8 feet to the center of each hull - gives a meaningful increase in waterline length - - but you are seriously limited by the method of construction - you said the hulls were solid glass = HEAVY - WEIGHT IS WEIGHT IS WEIGHT - - Dale S.V. VOODOO SCIENCE Miami

 

Foils is a bit like cheating eh I don't want to go that way. Ratio is about 12 : 1

Weight is a consideration, carbon fiber is out, too expensive so glass it will have to be.

Another consideration when saving weight is the bridgedeck clearance. Wind make waves and if the bridgedeck is too low it may make more drag if you splash through each wave. So at the expense of a bit more weight one could actually gain speed

To add to the length... it's 8m500 as it is. To add a few meters...larger sails and other problems. Moving towards big boats now and as you say the mechanical requirements becomes an issue, adding much to weigt too.

 

I wouldn't get too excited about making these two hulls into much more then a beach cat.

Clearly by the bottom paint they were fair well immersed. Judging by their general dimensions they don't have a lot of buoyancy to offer. Making them longer is an option, but I'll bet it'll cost nearly as much and take nearly as long as building two new, more wholesome hulls from plywood.