Extension of a planing boat.

When you extend the bottom, you are effectively increasing the LWL (length at water line), which will have the effect of making the boat handle like a larger boat with little weight penalty, which should make planing easier.

These 'key slots' as they are often called, are quite popular on flats boats in both Florida and Texas. Their main advantage is to leverage the hull to reduce stern squat and allow the boats to jump on plane in much shallower water. Without the floatation pods on the stern, the back of the boat may submerge an additional half-foot or more during holeshot, whereas that is eliminated almost totally with a good pod design.

Tradeoffs? As mentioned, turning performance will be affected. The hull will turn much flatter, which can contribute to prop ventilation, much less responsive and in extreme situations where the outboard is raised high on a jackplate, contribute to the boat swapping ends during high-speed turns, a situation that can be very dangerous. The boat will also run much flatter (bow down) unless the pods are designed to be upswept going aft. Pods that are adjustable in the fashion of trim tabs may be a good solution, but AFAIK no one offers them in the aftermarket and they would likely be problematic to design from an engineering standpoint (high lateral loads during cornering would demand a very strong hinge point).

That being said, floatation pods are of great benefit in specialized situations, most notably that of taking off in shallow water. If you aren't dealing with shallow water (where your motor skeg is often bumping the bottom), then you would be better served with trim tabs.

Some folks have added the floatation pods to compensate for heavier four-stroke engines being installed on older boats that were designed for lighter two-stroke engines. However, outboard technology is reaching the point where 4-str. engines are nearly as light as 2-str. engines.

 

p.s. - I should have mentioned that these floatation pods are most popular on boats that incorporate a partial, or pocket, tunnel (which includes most 'flats' boats). Their main advantage is to add additional floatation to compensate for the buoyancy lost with the addition of the tunnel in the hull.

 

Stepped and notched

Thanks, Village Idiot

This also gives me the strength to introduce a small step, ventilated from the inside.

js

 

Hi HJS. One ot the trend is to add swimstep extensions, I have seen them out over 48" so in a situation wher you have abuild in step and add 36"to 48"that could be 72" out over the water without floatation as suport pluss a Big cooler. Question what happens to the trim and balance of the boat?

 

If one thing has become clear, it's that there is no right or wrong answer, it depends on the hull and it's use.

 

I have seen many flats skiffs here in South Florida with trim tabs installed on the back of the hull extensions. Trim tabs contribute to shortening "to plane" time and are typically considered pretty standard of flats skiffs.

Here are a couple more pictures of "extensions"

Steve

 

We have confused the issue here by including well slots, hull extensions and pods in the same discussion. Tie these to many different planing hull forms out there and it gets very difficult to gain an understanding of the various posts.

Well boats were not developed for hole shot or quick planing but were built that way as a very practical way to avoid tangling fishing gear and nets over the transom. That they serve other purposes is a plus.

A sad fact is that most planing boats available come from the builder with overly heavy hulls, motors and the necessary fuel capacity to drive the beast up onto plane. Such boats do respond much better with small additions to the aft planing surface. If they were built with a reasonable level of bottom loading in the first place, such big motors and fuel capacity would not be required and the boat would achieve planing much earlier in the speed range and be much more pleasant to use and much less expensive to run.

How often does there appear a post on this forum looking for some means to achieve planing with a Bayliner or other such boat that just won't get up and go??

End of rant.

 

Thanks Tom

Figures for a good planing boat according to my calculations:

Ap / Vol^2/3 5,5 – 7,0
Lvl / Vol^1/3 5,0 – 7,0
Lp / Bpx 4,0 – 5,5
Lcg / Apo 5% – 12% of Lp
Deadrise 10 dgr – 18 dgr
FnV 2,7 – 3,5 for economy

Not many boats out there are within these limits.


End of rant

js

 

The platform is just a platform... If it's low enough it might help at low speeds but lift out of the water at higher speeds.

The practice of putting the motor on a bracket allows you to use your power trim to adjust the attitude of the boat while on plane, and gives a little more top speed, but it doesn't do much for low speed performance or "hole shot" as the weight is so far aft. The hull extensions work the opposite way; they give you more buoyancy and lift and help at low and moderate speeds, but at high speeds they could cause the boat to run TOO flat with a subsequent loss of speed. Cavitation is a possibility in TIGHT turns at high speed but that's not a good idea anyway, I wouldn't worry about it.

A thought of mine would be to add the hull extensions as pictured but have the bottom of them an inch or so above the original bottom of the boat. They would help at low speed but would be out of the water and have no effect at high speed. However it's just an idea I have NOT tried it. I think I've seen a few productions boats with aftermarket baitwells or whatever in such a position, though.

 

That is true but there is another advantage to the longer hull with a lower trim angle. It rides better in chop with the sharp bow cutting the water rather than the chop hitting the hull bottom further back where it flattens out. It does rob a bit of top speed but some, including me, may find that a bit of top end speed loss is small price to pay for better performance in the lower speed ranges.

 

I tend to agree with you. However the stern-heavy boats can accomplish almost the same thing by trimming the engine, drive or tabs down, though maybe at the expense of extra fuel consumption.

Trim tabs are relatively cheap, and they're adjustable for varying loads and service, something the designer or builder cannot always predict. Also having the engine on a mount rather than the transom or in a well gives the buyer more room in a hull of a given length, or a shorter, probably less expensive boat for a given amount of space.

Motor trim and tabs are pretty much useless at the displacement speeds where hull extensions would shine, but I don't think low speed performance matters much to most folks; as once they're out of the no-wake zone it's hammer down...

 

If my main interests were the same as the ones you refer to, I would certainly agree with you. The hammer down folks seem to be few and far between on the longer cruising circuits. My observation is that most people get very fuel use sensitive with cruising boats. This is where the boat that can run effectively and economically at speed in the never never range of 8kts to 18kts can shine.

I do have an issue with the length of the boat idea. The boat takes up as much space as its overall length, including the motor and anything its mounted on. Also I doubt that a well designed pod can be added to a transom cheaper than just extending the hull which would offer even more room in the cockpit than a pod. The transom and trailer had better be able to support and care for that extra cantilevered load which also adds cost. I know that is true on my designs and think it would apply to many if not most others. In the end it may be just a matter of choice and is nothing to fight over. Good discussion.

 

I think the hull extensions might be preferable at the lower speeds you mention as they'll lengthen the waterline and add buoyancy aft.

 

FWIW, many mfrs. offer these pods as optional accessories on a number of their boats; however, these are aluminum jon-style boats that are generally built to be relatively robust to begin with. As an example, G3 boats offer this option on their tunnel boats, and I think more and more mfrs. are offering them as they become more popular.

You can also order the pods "pre-made" in aluminum from several mfrs. I think Beavertail sells them, as does Boyd's Welding on ebay, and Boatright Marine out of Texas. They typically run $300-400 to purchase, or you can find a good local welder to build them for you cheaper.

 

Tom you are absolutely correct, crousing speed and better fuel burn are more important than full speed. Yes folks could purchase a newer and better designed boat, but the banker may not go for it. We in the the repair and refit line of things like the challenge to improve what is on hand, extending the hull will work for some hulls better than others. A hull with I/O power adding pods is simpler, by splashing two mold section of the aft corners one has a start for a mould. So from a labor and material point less expense.