Extension of a planing boat.

If you try to stick to the figures I mentioned abow, you will get a boat that will have extremely soft ride, good economy and works well in ALL speeds.


http://www.sassdesign.net/U-botten rev 03062010.pdf
http://www.youtube.com:80/watch?v=cXDfQqxJ_pM


js

 

Don't know what size boat your a thinking about so my experience may not apply. I made a dinghy with this design feature. I was looking for more bouyancy aft, protection of the motor at crowded dinghy docks and also to have more walk around room at the mast when the boat was stored on deck.
It certainly makes my boat just jump onto a plane. I have not noticed any adverse handling affects when making turns. The only drawback that I have noticed is at non-planning low speeds my boat kicks up a huge wake way out of proportion to the size of the boat. Attached are some photos.

 

Is it better to cut the bottom straight of at the green line?

js

 

I can't answer that question. But that portion of the hull when the motor is not attached makes a handy step to get into the boat after snorkling and diving. I can stand on the extensions and there is still lots of freeboard left. The extensions are filled with foam.

 

I like your dingy Chuck. How heavy is it? I think cutting it at the green lines would be going backwards as you loose planing area aft.

 

It is the question to balance all the centers of effort. Especially in small and light crafts can that be difficult. The driver sits aft and is steering with the tiller on the engine. By moving the driver and the engine forward can the center of gravity be optimized so that the boat has good operating characteristics over the entire speed range. Important is of course that the planing surface has the right size and form for all different loads.

Is a cut after the pink line better?

js

 

The pink line cutout won't work because you can't turn the outboard.
I think that more info about the thought behind my dinghy and it's design would beneficial. I live aboard my Gulfstar 37, Hale Moana, in the Sea of Cortez at the present time. I needed a bigger dinghy and didn't want all the problems that an inflatable has. They just don't hold up well IMHO to rigors of cruising and are extremely wet in anything other then dead flat calm conditions. Dinghy docks can get crowded so I wanted protection for the outboard and more bouyancy aft to support my 9.9 hp outboard among other requirements. So I started searching for a fiberglass hard chine dinghy. Couldn't find one. So I started looking for dinghy plans on the net. After exchanging emails with a couple of designers, I bought a set plans for Sam Devlin's guppy design. Sam was very helpful. Several of his designs already had the extensions but wouldn't fit my needs for a variety of reasons. The guppy as designed is a sailing/rowing dinghy and didn't have the extensions but it had sufficient displacement to carry the loads that I needed. With Sam's approval I modified the back half of the design by taking out all of the shape to the panels and let them just run out straight. This gave me your basic V hull with if I remember correctly about 10` of deadrise. I moved the transom forward 16 inches to create the extensions. I modified the sheer line so that the dinghy would fit on the foredeck of my sailboat and changed the interior layout to provide more support to the bottom. The finished weight is 110 lbs without outboard, fuel tank, etc.. It is 9'9" LOA with 4' beam. The 9.9 turned out to be way too much motor and I now have an 8 hp which seems about right.
No boat is perfect but this dinghy has exceeded all of my expectations. It is fast and stable and dry in all but the nastiest conditions. I can get into the dinghy from the bow, stern or side. It will comfortably carry 3 people or alot of gear, jerry jugs or what have you. It is in daily use and has been for 2 years. It gets drug up onto beaches and over rocks. It has been blown against rough pilings and barnacle encrusted seawalls with nothing more than scratches in the paint. It is one tough little boat.
As you can tell, I like my dinghy. Could go on and on.

 

Your dingy deserves its own thread Chuck.

 

Hi guys,
I would like to hear an opinion from someone who has sailed with that kind of transom in following seas.
I have like an impression that it would behave better in that condition than a straight transom, because the aft buoyancy would increase more gradually as the wave swells from abaft, thus decreasing the bow diving and the possibility of bow steering. Any thoughts or experiences about this?
Cheers

 

Then again it could funnel the breakers in causing a kick up the *** effect instead of parting the wave around the stern?

 

Daiquiri

I think you are right. It can be a similar capacity as a boat with rounded or pointed stern. It presupposes of course that the center of gravity is in the right position in relation to the center of the waterline surface and the pressure center of the planing surface. And that can be quite a delicate task to find out an optimum, as our knowledge is limited at present. That is why we are asking. But I believe the solution can provide advantages over a boat with the same displacement.

js

 

Hull extention

The one thing I noticed on a recend fishing trip to S.E. Alaska was the loading on the aft end of Boats, trolling outboards, gas tanks, fish boxs ect. One of the Boats incurred an engine flooding problem,<yes the flapper valves where in place> in part due to the transom sitting lower in the water than originally designed. This is a common problem I see all the time. So adding hull extentions- pods, would help and perhaps eliminate this problem, just one more thought on the plus side of this thread.

 

Gentlemen
I know its being a while but I must replay to the thread, I have a plate aluminum vessel with two of these pods welded onto to transom, Its 20' with a diesel I/O the two pods sit either side of the leg and extend 1.5' aft. there is a swim platform above. Your posts have answered 90% of my questions, the one that remains is, do the pods do anything for stability at rest.
My vessel is under repair, I purchased the vessel with the pods attached so its to make a comparison before and after pods. It will be while before I launch off, but I would like to know if these things actually help. The vessel has a very high floor line above water level nearly 7". D you think that having a pod either side with the middle removed would add stationary stability say like that of a twin hull boat.
Katoh

 

Not having a point of reference, about the stability of this boat previous to the pod addition, it's stability is basically what it is (and moot). The pods don't offer much in terms of hull volume, so you can't expect much improvement in static stability, though there will be a slight improvement in "stiffness", particularly in increased aft trim situations, like boardings. The same would be true of dynamic stability, though a higher percentage in stiffness could be enjoyed, but again it would be quite slight, given it's immersed volume percentage.

If you desire a self draining cockpit sole (floor line), you'll need at least 6" or 7" over the LWL to insure it'll drain. So, the 7" you see isn't that unusual. Pictures might be helpful.

 

I can't speak to the removal of the swim platform, but doubt it would have much effect on stability. The pods themselves will. Let's assume they are approximately 12 inches wide (since you didn't say), and that they are submerged 9 inches at rest. Also assuming that they are empty (or filled with air), that gives each pod a volume of roughly 1.125 cubic feet below water line. This equates to nearly 71 pounds of buoyancy in saltwater, or 142 pounds buoyancy with the two pods combined. If they weight 20 pounds each, you still have 100 pounds of extra buoyancy at the stern that you would not have without the pods. Depending on the weight distribution within the hull, this could have a significant effect on boat behavior.

Additionally, the pods will effectively make your 20-footer a 21.5-footer, increasing water line and stability.

Personally, I run a 24-footer with 1.5-foot pods and an outboard - I like the pods because it allows the boat to float shallow at rest, and allows flat planing so that the boat can "get up on plane" in shallow water, without the stern digging down and contacting substrate. That is likely not important to you since you are running a diesel I/O; however, you still benefit from extra floatation and stability in the stern.

With that being said, there is an art and a science to pods. Many folks will install them slightly above the bottom of the boat hull, or at upswept angles in an effort to maintain the ability to trim the bow higher for traveling into waves. I myself am happy with the even bottom (literal extension of the hull planing surface) since I can't trim the bow very high on a 24-foot boat regardless. You definitely down want them angled down, as that will effectively put a hook in the hull and keep the bow down permanently - not a good situation. Therefore, proper installation is critical.