Transom Pods - Flotation and Extended Planing Surface

Greetings . . .

A quick update from my previous thread about a fiberglass swim platform. I sold that boat and now have a new(er) boat ( Cruisers Yacht 338 Bowrider). I have been getting things dialed-in over the past few months and I have some misgivings about the hull design of this boat.
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Most other aspects of the boat are fine. The boat is 33 feet LOA, same as my previous boat, the Beam is a bit wider (10'10" vs. 10' 2"), the hull is a bit deeper V (22˚ vs. 20˚ of the previous boat), and the new boat is 2,000 lbs heavier (built like a tank ).

One significant difference that I have noted can be seen in the hull profile (see factory drawing below).
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The transom is recessed nearly 4 feet from the back of the boat (swim deck). Normally, this would be about 2 feet, as was the case with my other boat. So, this is a 33 foot boat with the running surface of a 31 foot boat . . . essentially.

Good news: The boat really does not need an extended swim platform (although there is one available)

Bad News: The hull running surface is essentially 2 feet shorter than similar sized boats.

The shorter running surface, combined with the additional weight, and the deeper V, makes for quite a difference in both time-to-plane, and power needed vs. my previous boat of the same length. Since I plan to keep the boat, I figure that I could look into some alterations to improve the planing capabilities of the boat.

Additionally, being a large bowrider the people capacity is significant (yacht certified) and 12-14 people on-board is typical on a holiday weekend. Quite a few folks hang out in the stern area/swim deck, and I figure that some additional buoyancy would be good to add a measure of safety for the I/O engines (2 x Mercruiser 6.2 ECT 350 HP, Bravo 3 outdrives).

So, the concept of Transom Flotation Pods seems like it might address the planing surface issue as well as add some stern buoyancy at rest.

Here are a couple of examples (one from a thread here on BoatDesign.net and other one from elsewhere)


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Of course, my boat being significantly larger and of a different design, a different approach to transom pods will be needed. My plan is to work within the existing extremities of the boat and utilize the approximately 4 feet ( 44") of space under the existing swim deck.

The boat has trim tabs which can help for planing, but do nothing for buoyancy. I plan on making a pair of 'pods' to go under the existing swim deck to extend the running surface of the outer areas of the V hull. I'll re-locate the trim tabs to the ends of the Pods.

Here is my design so far . . .
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Original Hull (again)for quick comparison . . .

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I will make the Pods out of fiberglass and foam, with some internal structure for strength and mounting the trim tabs. The pods will be completely foam filled and will be somewhat detachable in case this turns out to be a 'bad idea' .

Timing-wise, I won't be fabricating anything until around October (when the boat comes out of the water for the season).

Please share any thoughts, ideas, cautions.

I'll post updates as I go . . .

 

How 'detachable' will they be - and how will you attach them? With nuts and bolts, or by fibreglass laminations, or.....?
Have you calculated how much extra buoyancy they will give you at the usual load waterline?
It looks like you will get some extra planing surface - but will it be enough to make a significant effect?
You should perhaps consider the cost and hassle factor of doing this, versus selling the boat and buying a boat that is more suited to what you have in mind.

 

The buoyancy of those boxes is marginal, compared to the total displacement of your boat with a dozen persons, fuel and equipment onboard, and that goes for the dynamic lift as well. It is not worth the hazzle. I think Bajansailor's last sentence should be considered seriously.

 

Try some Doel Fin planing assist devices.

 

Twelve to fourteen people on a 33 footer is seriously crowding it.We know that a higher deadrise hull planes less readily than a flatter hull form and the boat seems to be significantly heavier than the OP's previous boat.It may not have been the best buying choice and modifying the hull with significant additions may have repercussions for the selling price when the time comes.

 

Thanks for the replies so far . . . all good things to think about.

As far as the considerations . . .

Not selling the boat. . . Overall, the boat is good for what I need in a boat. . . Just want to improve upon the planing abilities and add some additional flotation to the stern (at rest). I bought the boat less than a year ago, (used, 2016 model, 5 years old) and it was the best option of the various styles/brands/models I considered. The boat does fine on the ocean, better than my previous boat, which was one of my goals. I could have bought the current generation Formula 330 CBR (newer cross-over version of my 'old' boat), which came in as a 'close second' in my boat search, but the few boats available were well beyond my budget range and those boats were even heavier. So, I'm not sure other choices would have been any better. (Yes, boats are compromises ) This boat 'checked more of the boxes' than the 7-8 boats I considered.

Attach/Detach: Still in the conceptual stages (thus my thread), but I am planning on a combination of adhesive and fasteners. I'm sure the forces involved here are significant, but not outrageous. I'll need to consider the routing of the hydraulic lines for the trim tabs within the design of the pods.

Added Buoyancy: I have not calculated it (obviously) since I don't have the boat out of the water to take some precise measurements. Eye-balling . . . it may add 3-4 cu. ft. of buoyancy . . . 200-ish pounds . . . at the static waterline. I'm good with that. Inboard/Outboard engines (aka I/O , sterndrive, etc.) can be susceptible to water intrusion if the stern gets too low in the water. I can also take some measurements of the 'riser height' above the static waterline to see how close they currently are to the 'spec'.

How much improvement (planing)?: Hard to tell and no good way of knowing until it is done.

Re-sale considerations: I've actually been able to enhance the re-sale value of my previous boat (which I sold for more $$$ than I paid after owning it for 10 years). Here is my thread (and picture below) on adding a custom designed/built (by me) extended swim platform to my previous boat. Custom Extended Swim Platform https://www.boatdesign.net/threads/custom-extended-swim-platform.61646/ .

My previous boat w/ the extended swim platform that I made. . .


For this (new-2-me) boat, I would plan to 'do it right', making any modifications look 'factory'. If the modification improves the boat as I hope, and it looks good, I have no worries about re-sale value. Probably won't sell for many years anyway.

I have considered fins, tabs, etc., but the 'pods' should address both planing and aft flotation considerations. I've also thought about the balance point (which is noted in the drawings), but I'm not sure there is enough 'stuff' that can be moved forward beyond what I already have done. Once the boat is up on plane the balance point seems fine, so I wouldn't want to mess with it too much. One good thing that I noted is that the fuel tank is pretty close to the C.G., and is more wide than long, so I'm not concerned about the effects of fuel level on the balance point.

TIA, for your continued thoughts and comments.

 

FWIW . . . here is the boat without its mooring covers. It is a 'Mid-cabin Bowrider', aka 'cross-0ver' bowrider. Several of the traditional cruiser and sport boat manufacturers (USA) have introduced these style boats withing the past 5-10 years. (Formula, SeaRay, Chaparral, Cobalt, Cruisers Yacht, Regal, Monterey... to name a few).

This particular brand/model has the largest cabin by virtue of the forward walkway being all the way to the port side. The other brands have the walkway not quite as far over and the cabin space, headroom, etc. is not quite as good.

 

Here’s the answer!

 

Brilliant! That is sheer genius.
Do you know where this photo was taken?

I must admit that I had never heard of Gamefisher O/B motors, so I googled them -
https://goneoutdoors.com/information-gamefisher-outboard-motors-7719391.html

And here is one for sale -
15 hp Sears Gamefisher Outboard https://www.smalloutboards.com/g15clean.htm

 

Looks perfect . . . The guy went all out. . . Those fenders are probably $150 each .

Yup, (Sears) Gamefisher engines (made by a variety of smaller OB manufacturers, such as Eska) Those look later model year . . . 1980-ish.

 

Egads. Almost every outboard powered seine skiff added those when the 2 stroke to 4 stroke transition occurred. Floated the chunky new 4 stroked but man it made slow speed maneuver near impossible.

It's not a crazy glass project all things considered, but it's worth doing some math. Glass plus foam weight vs actual wetted volume it adds is an equation worth doing.

 

Yes, I can certainly do the water line 'net' buoyancy calculation for the pods, but I am wondering if there are any formulas about the dynamic lift that I could apply?

I am also wondering if I should make the pods 'stepped' or 'flush' with the existing hull and if they should have a slight downward 'rake' or not? (examples below)

Thoughts?
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Also, I need to re-locate the underwater lights. (see arrows in picture), since the pods will be covering them in their current location. I'm not a fan of underwater lights - for me it is just another thing to break/fix. More of a marketing gimmick IMO.

I'll probably re-locate them to an above waterline location.
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I read an article some years ago about successfully adding a stern foil under circumstances similar to the OPs.
Was unable to find it, but here’s something I found that was interesting.
Effect of Angle of Attack Variation of Stern Foil on High-Speed Craft on Various Speed with Computational Fluid Dynamics Method https://ijtech.eng.ui.ac.id/article/view/4467
US2832304A - Stern-mounted hydrofoil device - Google Patents https://patents.google.com/patent/US2832304A/en

 

Hydrofoils are probably beyond the scope of my objectives, but interesting.

I've done some sketches and a few basic calculations on the running surface impact with the pods.

Typical running weight is about 13,200 lbs.

11,500 lbs. dry weight
500 lbs. fuel
300 lbs. gear
900 lbs. 6 people (about half of max)​

Here is the standard boat . . .


Roughly 9,000 Sq. In of wetted hull supporting 13,200 lbs. or 1.46 psi (Note: I do know that the pressure on the hull (at planing speed) is not evenly distributed, but I am using the average pressure for comparison purposes)
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The modified hull . . .

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9,600 sq. in of wetted hull supporting 13,200 lbs or 1.375 psi

Additionally, the pods would provide 'approximately' 600 sq.in x 1.375 psi = 825 lbs of lift positioned at about 120" (10 ft) from the Center of Gravity (CG) which would be about 8,250 ft-lbs. of torque around the CG. I think the small amount of 'lift' being 10 feet away from the CG is probably the more significant aspect of the hull modification.

I'm not sure there are calculations for hull loading (similar to wing loading of aircraft), but maybe some of those following along may be able to provide some input.

TIA