Outboard on Bracket - Advantages / Disadvantages ???

What are the advantages and disadvantages of mounting an outboard on an "Armstrong" bracket on a new boat. The alternative is to mount the outboard on the transom. "Armstrong" seems to have become a generic name for the brackets though Armstrong is one of several manufactuers. Many of the brackets have a sealed compartment which is intended to provide floatation at rest and slow speeds and offset the stern-down trim which would otherwise result from the aft movement of the CG. Some are designed so that the bottom of the sealed compartment effectively extends the bottom of the hull.

My particular interest is a 115 or 150 HP outboard on a 22 ft to 24 ft "lobster boat" semi-displacment hull such as the Crowley-Beal 23 http://bealsboatshop.com/PHDInventory/Pictures/161633/161633_25140_93415.jpg or Eastern 24 http://easternboats.com/prod-desc.php?pageid=4&boatid=7 Displacement would be around 3500 lbs to 4200 lbs, top speed in the 24 to 30 knot range, cruising speed 14 to 16 knots (Fn around 1.1) around with considerable time at 4 to 6 knots. Top speed is not a major consideration, fuel consumption is more important. Mounting the engine on a bracket on one of these boats would shift the CG around 3% aft.

My initial thoughts of the advantages are:
- More room in the cockpit.
- Transom can be full height, not cut down to mount the motor.
- "Swim" platform can be integrated with the bracket.
- Improved steering/handling at slow speeds
- Potentially slightly quieter with the motor further aft and behind the higher transom.

My initial thoughts on disadvantages are:
- Length, 24" to 30" additional. Disadvantage for trailering and docking.
- Cost.
- Engine more difficult to access when afloat.
- Weight.

Comments, particularly those based on experience, would be appreciated.

 

I had one on a 6m Stabicraft 115 hp ...18 in extension but did not go to the bottom of the hull..advantage of this is that you can lift the motor up by 2 in per 6 in of length which gives you less draught on the plane.. if its level with the hull no advantage as you cannot lift it ....really just a cheap way of making the boat longer/saving cabin space or in stabicrafts case claiming the boat is really longer than it is .....watch its welded on good as they can and did crack off where welded to transom but stabicraft wont admit it ......

 

Dont know about a work boat but i had a transom rebuild to do so chopped it off completely . Added a new transom that went aft 450mm and made a big differance . TRim works better , boat gets on the plane quicker , is real fast and the steerng is so responsive and works much better . I was able to raise my motor a good inch plus without any hassle Changed from a 20 inch to a 25 inch shaft so the transom is highter and the boat dryer .Changed the prop pitch from a 17 to 21 and goes much better
The motor well is 1/3 the size it was before so lots more room inside the cocpit . For me plus's all round I shifter the oil tank to fit up under the small aft deck and can see at a glance , also has the fuel waterseparator planted on the side ot the new fibreglass bracket i made specially ,also the battery up under the aft deck so is close to the motor and less power drop for starting etc etc . Have one on one side and one on the other side .

 

like this

 

The major drawback is than at slow speed, with waves, the propeller ventilates.

 

Thanks, that's the type of information I'm looking for.

 

well we never had any problem at low speed even in the very rough stuff off queesland but with the motor higher on the transom than a normal boat and trimmed up for best speed at WOT a sharp turn caused the prop to lift out the water so when a turn was anticpated at say 28 kts its was best to trim down a little and then up again on the straight ....thats the boat in the picture I posted.....I think it had a 20 deg V so when running on one side of the bottom you can see by simple geometry the prop goes up...if you zoom in on the photo 400% you can see its lifted to the highest hole about 3 in above the manufacturers 20 in transom ( 20 in shaft motor but I think yamaha long shaft is 22in)

 

Advantages:
CG farther aft -> can be an advantage at high planing speeds.
Prop shaft set higher -> lower drag at high speed.
Longer lever arm between prop shaft and CG -> trim tweaks can be smaller to have the same effect on the boat.

Disadvantages:
Prop can come out of the water in rough conditions (see Gonzo's post #5).
CG farther aft means more bow rise and a wider range of no-go speeds between dead slow and planing.

If I were looking at either of the boats mentioned in the original post, I don't think I'd be interested in adding an extension bracket. Neither will run fast enough for the bracket's advantages to be substantial.

 

Set up is all important when mounting any out board on any boat . Have a friend with a 150hp on a 24inch bracket and wen he go the boat it wasnt se up properly so we changed and raised the motor 30 mm and made a huge differance . Ventilating can also be ballance and how the boat is set up . 5 mm up or down can be all it takes to make the differance . feel through the wheel , listen to the motor , feel where the trim is best suited take a mental not of these things as that where your boat is at its best . Each boat is differant and no two are ever the same .

 

Gonzo got most of it. I had an I/O with a 15 Hp Johnson out on the lift bracket for trolling.
It got swamped when we were Mooching near the Active waterway and the Wakes came in. Constantly the Carb was full of water.

Finally, I took the Motor off and stored it on the transom, and put it back on when needed.

 

The only reason an outboard is a reasonable arrangement is due to the fact that they are/were light. The best place for mass in most/any vehicle is in it's center ....not at one end ...or even at both ends like a tandem canoe or push/pull airplane. If one needs to have more room in the boat buy a bigger boat or cut the transom off and extent the boat. Or if the lines allow cut it in half and add more boat. Alaska state ferries did that and many others around the world. Think of the engine and propeller separately. Where would the best place for each to be for best performance and balance of the boat? I would think 35% of the way fwd of the transom for the engine and the propeller could'nt be much better placed than it is on most inboards. I think the biggest advantage to the "Armstrong" bracket is is it's vogueness and that's very small indeed.