Offshore/Outboard Bracket Design Critiques

What are you talking about?? The hull extension is closed and partly submerged at zero speed. Consequently it is displacing a volume of fluid, which action produces a lift; ie a buoyant force. This buoyancy is added to the original hull. The complete shebang, hull plus extension, is in fact a "new" hull, the corresponding centers (Center of Mass, Center of Buoyancy, Center of Waterplane area, Center of Lateral area) of which can be easily calculated.

At speed, the extension will generate a dynamic lift (calculable!) which adds to the dynamic lift of the original hull. Nothing exceptional there, but for the idea of introducing a "rocker". Generally I'd say that any convexity aft in a high speed hull is a bad idea, since it may, or may not generate a downforce depending on operating conditions (it is not a dynamically stable configuration). If you think (have calculated?) that the extension will generate too much dynamic lift, then a better solution is to have the extension bottom parallell to the keel, but raised with the amount of rocker (~20 mm in this case, forming a step), and attached to the transom with a stand-off, giving access for ventilating air to reach the step down along the slot between transom and extension fwd wall.

 

He suggested a static gain of 750 pounds of buoyancy.

 

The mass of the item is 235 pounds.

The displacement is 750 pounds.

He said it is sealed, so I am wrong and it would indeed provide flotation. I missed the sealed part because originally he said it provided its displacement in buoyancy which is impossible...

Of course, making this more complex, buoyancy can be related to an air chamber above the waterline, so it is not simply displacement driven, but hull or here, extension volume driven..in which case it could be more

At no time have I been referring to dynamics.

 

kapnD, there are 18" wide trim tabs mounted to the existing transom just outboard of the extension bracket on each side. Even in the fully retracted position, I think this should knock any spray down. There is also some taper on the sides of the bracket to match the angle the tabs come off the transom at trying to minimize any gap between the bracket and the tabs.

Baeckmo,

I am thinking that the extension will provide too much dynamic lift at times on on. The balance I'm trying to resolve is when fishing, there will be 5-6 people on the back deck so I need buoyancy at rest. Both to keep deck scupper above water and to improve tracking and trolling behavior for fishing. However, in bad weather, those same 5-6 people will climb in the pilothouse that is forward located (back of pilothouse is roughly 13' in front of the transom). Now when on plane, I'm sure it's going to be a bit bow heavy. Not a big deal running into a head sea, BIG deal crossing a bar with a big following sea.

So, that is the balance I'm trying to strike. Float higher enough at rest for stern heavy, yet when on plane not be too bow heavy. Full extension brackets with no rocker fit the bill for the at rest scenario. Traditional non-floatation "bracket" fits the bill on plane. In my research and understanding, adding rocker might be an option to keep buoyancy at rest, and mitigate any bow heavy issues while on plane. Additionally, the duo prop outboard generates a lot of stern lift so it may actually exaggerate the dynamic lift issue.

That's where I'm coming from. 8000# boat. Max speed of 36-38 knots, cruise 25-30 knots. From the factory, these boats tend to ride bow high but float fairly level to slightly stern heavy.

 

If you don't like the way the boat sits at rest you could add a water ballast forward. Just a tank or even homemade box to fit the bow. I sort of hate the idea, but throwing it out there. Not like it hasn't been done before.

 

There already is a 30 gallon freshwater tank 12 gallon black tank in the bow in front if the fuel tank. Also, fuel tank is 150 gallons, starts about 5' forward of the transom.

Id like to avoid putting dead weight in the boat but if I have to I have to.

 

Based on everything you are telling us, I doubt the impacts will be significant. The center of buoyancy and the center of gravity will have both moved back after the work, so despite the lcg moving back; the impacts will be somewhat negated. I'd measure the level of the boat now and then after changes and see what the impacts are. My guess is very little...

 

Mitch,

Did you add the trim tabs or did the boat come with them?

(Sent you a Private Message as well.)

BB

 

fallguy, yeah it's a big boat. I'd say fully loaded it will be closer to 9000lbs wet. So moving that motor weight back didn't impact the CoG or CoB as much as I originally thought. I semi validated this when I moved the trailer axles 9" backwards to plan for the bracket.

Boat came with 18"Wx9"D trim tabs. Bracket is sized to still be able to fit the tabs BARELY on the boat and I will step up to 18"Wx12"D.

Thanks again all for the comments. Greatly appreciate your time and thoughts.

 

Those o/b conversions look great on the Ospreys.

As a fishing nut, I would never want twins, but I might want a kicker with a fat prop if I could work around it.

Sorry about my confusion over the bracket. I'm used to another forum where guys are all making their boats pos buoyant. If you remove the engine bay and transmission; there is probably an opportunity to make it the unsinkable Osprey.

 

No problem. I am bad at explaining things I've been looking at for months. This is first and foremost a fishing boat. That is the main reason why the extension is as short as possible, at 30", so it's easier to fish around. Short cabin gives me 11' of rear deck fishing space. Being only an 8'6" beam (85" bottom chine to chine), twins doesn't leave room for a kicker motor that steers or handles well. So I'm sticking with a single 30 " Suzuki 350 duo prop main and then a 30" shaft 25HP high thrust kicker mounted as close to the main as possible for fishing.

Although I do like the idea of adding foam to make the boat unsinkable, I think batteries and a removable insulated fish box are going to go where the old inboard lived.

 

About the trimming issues:

With the rough figures available, the present CoB and CoM, would be about 2.44 m forward of the original transom, with the hull in balance, free to trim. I'm assuming the engine plus i/o leg is 500 kg and its CoM lies 0.6 m fw of transom. The rest is 3500 kg with CoM at 2.7 m fw.

Now, taking away the V8 engine's trim moment 500*0.6 kpm and substituting it with something like 500*(-0.7) will result in a new total CoM at about 2.28 m fw.

Then checking for the effect of buoyancy, we start with the old value, ie 4000 kg * 2.44 m. To this you add something like 320 kg * (-0.4) m, resulting in a new CoB at 2.23 m fw. The hull will seek a new trim balance, where the CoB = CoM = 2.28 m fw; it will float slightly higher aft and with a slightly reduced trim angle. The difference corresponds to the effect of one 80 kg person moving forwards 2.5 m on deck; not much to get anyone upset. The efficiency of your present trim tabs is reduced though, since their lever arm is reduced; they should be moved aft to the new transom.

What actually is problematic in my eyes is the total weight (your'e talking about over 4 tons!) in relation to the short waterline length, resulting in a low value of ~4.4 for the slenderness ratio. That will cause a high resistance and an uncomfortably high trim angle in the hump speed region. If you are considering the effort of adding a closed "bracket", why not make it a complete extension of the hull? With some minor rearrangement of internal loads and redesign of the "tin can", you would have a much better performing 28,5 foot vessel at very little extra cost.

 

@baeckmo

Intuition suggests he is making the waterline longer with the pod. Can you explain a bit more how this is not so?

I do not wish to disagree, but just get a more detailed understanding. And this is the kind of thing that matters because the difference of adding a small pod versus an integral hull extension, in terms of work required, is major.

Most people just want to be able to run OB for tilt up for big fish, reliability, access to prop, etc

 

Of course there is an increase of effective wl length. The centers and the changes thereof are referring to the original transom position, as noted in my first sentence.

The difference between the originally suggested pod and a full beam pod is very small in terms of material and work, if done in alu. There are some shape issues to consider in this case, but not increasing the job at all.

 

Baeckmo, your estimates are very close and it's strangely odd how much I enjoy someone talking numbers to me like that . I think the CoG/CoB are actually further forward and the new outboard further aft. Not sure how much that impacts your assessment.

Overall CoG/CoB of boat with V8 was probably more like 2.8-2.9m in front of the transom. (There is a 150 gallon fuel tank that is centered roughly 3m fwd of the transom)
CoG/CoB of the V8 and the outdrive is probably accurate.
The new CoG/CoB of the Bracket and the outboard's is actually 0.91 m aft of the existing transom. So -0.91m. (I have center of mass values for the bracket/motor combo)

Vertically, I think the difference in center of gravity of the bracket/outboard will be 0.2-0.3 m higher than the V8 was. I don't really know of any way to counteract this besides moving batteries into the old V8 down low. Hoping the weight of the fuel and solid fiberglass bottom will be enough to keep it from rocking too much.

One reason for not going full width extension was to keep the trim tabs out of the way of the kicker motor and to keep them out of the way of the fishing line. I did however size and taper the bracket width so that tabs only have a 1/8" gap between the bracket sides. I also am moving the tabs outboard so they are within 3" of the outer chine and increasing the cord length from 9" to 12". Effectively, when they are slightly down and parallel with the keel, they would act as a bottom extension with a combined wetted width of 26" (along the deadrise) x 12" long.