Offshore/Outboard Bracket Design Critiques

Converting a 26' Osprey short cabin pilothouse boat from a 5.7L V8 inboard to a single 350 HP outboard. The hull is fiberglass, 22 degree deadrise with a 7" wide delta keel that runs all the way forward the bow transition. There is not hook/rocker in the last 10' of the keel, but the keel does dip slightly down in the bow. A sort of semi wave piercing/plunging keel line. Overall wet weight of the boat with the inboard was around 8000lbs. Roughly 1100 of this was the inboard and outdrive. The combined weight of the new outboard, kicker and aluminum bracket is going to be roughly the same at 1100 lbs. CoG is getting moved aft, since the bracket is hanging off the back of the transom, but the bracket will provide floatation as it will be an extension of the bottom of the hull so I don't think the CoB will move much. Here are some of the details I was hoping to get feedback on.

-Bracket will provide 750lbs of floatation. It's shorter setback and wider than most. I wanted enough floatation so that with 5 adults on the back deck and the weight of the motors, that I would still be floating somewhat level to keep the deck drain/scuppers above the waterline. I also want it narrow enough to retain my trim tabs.
-17 degree transom angle. I want the ability to stuff the bow down when traveling into rough, choppy head sea and with an outboard there is more range of trim than the old I/O.
-30" of setback. I wanted the extension as short as possible for weight balance and to make it easier to fish around. Per the outboard motor drawings, it will clear the existing transom by 1.5" at full tilt.
-The bottom will continue the existing hull, but have 1.5 degree of upwards rocker. I wanted to add rocker because when on plane, with 5-6 people in the forward pilothouse, I feel that it will be too bow heavy. My thought is that by adding rocker, it will help keep the bow up when needed. The other way to counteract the bow heavy attitude would be to reduce the floatation, but then at rest I would sit stern low. I figure I can always use trim tabs or motor trim to keep the bow down, but there's not a lot you can do to get the bow up besides move weight.

Thoughts or critiques? I've done a lot of research and am about to pull the trigger on building it. It will be aluminum and through bolts to the existing transom.
Thanks!

 

You're ready to pull the trigger and there's no sketch?
We need more info.

I've been through this experience on a commercial tour boat of similar dimensions.
But they went to twin 250HP o/b's from twin inboard/outboards (Volvo-Penta, contra-rotating props).
I can tell you the stability characteristics changed considerably.
The boat would roll normally in a seaway but then just hang in the full offset roll position like time had stopped.
Strangest, most disconcerting thing I've ever experienced.
It had all it's certification up to date.
It was better with 12 passengers on board!

 

I'm with BB here, hard to comment without drawings.

I have a few comments, but am only pseudo qualified to comment.

First of all, I am concerned you do not discuss where the keel ends aft or how, back to the drawing need.

But I thought the inboard Osprey had the keel all the way to in front of the prop, so I am wondering if that is tapered already or if you ought to. When I went and looked, they don't, so maybe I am confusing Osprey with another boat (Allmand?)with a full keel. Bottom line is you need some clean water back there.

@baeckmo is certainly going to give you a better contribution if you have even a bottom pic of existing

When you mention adding flotation; that sounds like nonsense. A bracket that is above the water is only adding mass, no flotation. If the bracket is filled with flotation foam; it offsets engine mass and bracket mass. But I am confused as to how you add 750 pounds of flotation. For example, if I add 1 cubic foot of 2# buoyancy foam to my boat, I am adding about 62 pounds of flotation at a cost of 2#. Brackets are made with high density materials. A large bracket of mass 200 pounds with 2 cubic feet of flotation foam inside is 80 pounds short on neutral buoyancy in crude terms. In order to at 750 pounds of buoyancy net of the bracket; you'd need to add like 16 cubic feet of foam. So, something is not making sense to me..sorry

You need to understand the hydrostatics better and doesn't seem like you have a good grasp.

When you remove the inboard; you are taking away a lever arm or moment. If you arbitrarily use the leading edge of the engine as the LCG; you should be able to make a chart of all the changing moments and get a better idea of the extent of the changes.

For example, the engine is a moment then of 1 foot (1ft is the midpoint) times its mass, say 800 or 800ftpds aft. Then the transmission say 300 pounds 2 feet aft is 600ftpds. Or removed 1400ftpds aft of arbitrary lcg.

Then adding a bracket say 250 pounds at 5' back from our guesstimated lcg is 1250 ftpds. And an engine of 700 at say 6' back is 4200ftpds. So you removed 1400 and added back 5450 for a net change of 4050 This means you need offsetting mass to bring the boat back to balance.

Anyhow, this is all for static equilibrium to keep the boat level at the dock. But you ought to know the numbers.

The Osprey ob conversions I saw online are really great. But do your homework.

 

I would ask about the performance of the Osprey with outboard or outboards on the "Osprey boat owners forum"

A quick google search "Osprey short cabin with outboards" present many pictures and links. A call to Osprey themselves to find out if they moved the tanks etc when they built the outboard models. Many images show twins and you are only going a single?

 

Bluebell, oh I have a complete 3D model of the bracket. I measured the transom, digitized it, built the bracket, checked the floatation amount, verified everything was spot on then flat patterned the sheets. I just haven't added pictures on here so I'm not sure how. I'll figure it out how to get a picture loaded. I am a bit worried about the pitch and roll with the higher center off gravity from the outboards. I plan on relocating all my batteries as low as possible as well as adding some in floor fish boxes to hopefully keep the weight low.

Fallguy, You're off the mark a little bit, but nonetheless I appreciate your input. Up in the Northwest a lot of "brackets" are full bottoming following. Not really a "bracket" like you see in Florida boat. So you can buoyancy and floatation by basically extending the hull length. We do that for better rough water handling with various loads in our water conditions. And I have calculated the impact to the CoG. It's moves backwards roughly 9"-11". It's a heavy boat so even the weight of the engine moving a couple feet didn't move it as dramatically as I was thinking.

Barry, I'm sure you're familiar with this style of bracket. I've called Osprey. They use a non-bottom following bracket from the factory. In my opinion, simply because it's easier. They can mass produce one bracket and slap it on many different hulls. I've talked to owners who've gone the route I'm going and the handling was much better. I'm just trying to see if it can be improved on before I build mine. I would go twins, and the bracket will accommodate twins if I went that route in the future, but for now a single 350 duoprop is the plan.

 

I guess posting pictures wasn't that hard

 

So, you are referring to displacement as flotation.

Very few of the brackets I've seen are in the plane of the hull, so that combined with the verbage got me.

 

Completely understood. They are unique to our waters up here.

Internal volume of the bracket, at the estimated waterline, provides 750 lbs of buoyancy/floatation since it is sealed.

 

Not at all true, many of the most effective extension brackets all over the world closely follow the bottom configuration to extend the planing surface and extend the waterline length.
The most effective ones are extensions of the entire hull width.
A partial hull extension creates two inside corners, an awkward feature never found on a well designed vessel.

 

kapnD, I didn't realize that. Thanks for the info. What are the negative effects from having the two inside corners of a partial width extension?

 

It does not provide buoyancy or flotation, only displacement.

It reduces the buoyancy of the vessel. It changes the centers of buoyancy and flotation.

But it is important to use these terms appropriately. If you want to make the boat more buoyant for the added mass; it requires adding buoyancy foam or air.

Many vessels are not positive buoyant, so it is important for others reading this post to not become confused and believe the boat won't sink with the added section.

Calculating the changes to the boat's buoyancy has nothing to do with the displacement of the extension.

In order to provide 750 pounds of buoyancy, an air chamber inside would need to be 750#/64#/cuft or 11.7 cubic feet of air PLUS enough air to offset the mass of the bracket-highly unlikely

-edited for grammar, clarity, and an old reply

 

For a job as significant as this one, it would not be unwise to consider making sure the boat is actually pos buoyant.

The old engine bay and the stern could easily be foamed in with 2# foam and perhaps you could make the entire craft pos buoyant. There is a lot that goes into it and much depends on how far offshore you typically run. The pilothouse adds some complexity, but all another discussion....as I digress now

 

fallguy,

I believe I am mixing up displacement with floatation/buoyancy. Honestly I'm still not quite sure why they would be different in this case but I understand your explanation. The overall volume being submerged at the estimated waterline, to the outside of the plates, is 11.7 cubic ft. Multiplying this by 63.9 I get to 747 lb of DISPLACEMENT (correct term for application?). The weight of the bracket itself is 235 lbs so it yields a net buoyancy??? of 512 lbs?

Am I on the right track with terminology now?

I don't plan on adding any foam to the boat at this point.

 

Inside corners catch water, and direct it upwards!
Your outboard will be dryer on a full transom, and your boat will be quieter at rest.
The trim tabs will be more effective further aft too.

 

Well, you are getting closer, but not yet correct.

The extension adds displacement.

The extension reduces the vessels buoyancy by 235 pounds.

When talking about buoyancy; it is not the net of displacement less mass. Buoyancy can be simply defined as the ability of something to float. The bracket and the boat do not float by nature of their buoyancy! The extension offers your boat additional displacement of 512 pounds (net of its mass) in the back near where the engine will be placed.

You may feel as though I am being unneccesarily nitty at this point, but this thread may be read by others someday and words matter.

And because you are remodeling, it is important to point out things like the opportunity to make the boat pos buoyant.

Another thing for you to consider is scuppers..if they are now close to flooding; they surely will when moving lcg a foot back...etc, etc

EDIT: since the extension is sealed it actually does add buoyancy of 512...I forgot it was sealed