Outboard brackets and displacement speed

I was pondering some design issues on an upcoming project and started to wonder. I know there is a direct link to theoretical max displacement speed and hull length, and can do the math if need be. What I don't know is if I extend the hull with a full floatation bracket does it, for the sake of this topic, extend the hull length? When I say full bracket, I mean within a few inches of the hull bottom, and within 6" of the hull sides. Obviously this adds buoyancy to the rear, but is it similar to having a longer hull and increase displacement speed?

 

Yes

 

I can't see any gain for having an outboard mounted aft of the transom on a displacement hull apart from minor interior space gain. It is putting the engine in a vulnerable position in regards to swamping, and I would expect it would be difficult to maintain efficient trim under way, but I've never done it on displacement, only ever on planing hulls. OMC produced a series of outboards with integral transom brackets called Seadrives, short lived thankfully.

 

I fail to see how this makes the outboard any more vulnerable. It's still mounted at the same relative height and position as it would be on the transom. The OMC you refer to I assume is the seadrive, I an familiar. It did not provide any additional floatation, but didn't increase vulnerability swamping that I ever noticed. Yes, it does increase interior room, and creates a nice dive platform.

 

And it is the relative mounting height that is the issue with an outboard on a displacement hull form. It is even an issue on many planing hulls comming off plane. At the point of displacement/planing transition near displacement "hull speed", many hull forms generate a significant stern wave. Improper operation or trim in this regime can lead to swamping of the engine and/or boat. There are hull forms (such as sea dorys and or sea skiffs) that specifically move the outboard forward of a full height transom to prevent this problem.

 

I guess? I'm sorry but I've been running outboard boats for 30+ years now, and I think this is not a real issue. My current boat is based on the Simmons sea skiff, and even if I slam the throttle shut at full speed it won't swamp the boat or outboard. Either way this problem seems easily solved with common sense seamanship.

Going back to my actual question, any insight?

 

Yes it will effectively extend the hull length. Not exactly the same amount as the full hull being extended that far due to it being slightly narrower and raised an inch or so.

 

It makes the outboard vastly more vulnerable to swamping in situations where waves are coming at the stern, following seas, sudden stops and heading astern all contribute, and nearly any repairer will confirm it happens regularly to motors on transom brackets whether they have flotation or not.

 

modest additional length will increase the theoretical hull speed of the boat. BUT...... not an enough increase to make much, or any, difference. Unless the extension is faired very carefully to the existing hull you may even lose some "hull speed".

I am not familiar with the Simmons Sea Skiff but if it is a planing boat then adding length is not likely to help low speed operation at all.

The familiar formula for theoretical Hull speed is equivalent to the square root of the waterline length multiplied by the constant 1.34. The result is in knots. so .......... if your boat is say...16 feet on the waterline then the so called hull speed is 4(1.34) = 5.34 knots..........now add 18 inches of carefully faired length to the waterline ...... then 4.183(1.34) = 5.60 knots. That would make a difference of 32 seconds of time per nautical mile.

Florida has lots of interesting waterways. Picturesque trip........Say you put in at Lake Monroe at Sanford, cruise up the Saint Johns river to Hontoon landing near Deland. At full on displacement speeds you would save about 8 minutes. Save more time than that if you put in at Marathon and head down the ICW to Key West.

There is an advantage for putting the outboard farther behind the boat. You gain a little bit more space in the interior of the boat and get a little bit farther away from the noise.

 

The Simmons sea skiff was specifically designed to prevent the stern swamping problem. Native to the shoal watters and bars of the North Carolina banks, it is a semi-planing boat that features a full height transom and forward motor well.

The Simmons Sea Skiff - Small Boats Magazine https://smallboatsmonthly.com/article/the-simmons-sea-skiff/

 

I'm guessing you aren't in the USA, all we pretty much use here is outboards now. They far outnumber any other means. As someone who has worked at shops in the past, and still have friends that do, I'm telling you this is not an issue. The only logged motors we had to try and save were from outboards that were fully submerged. In other words, from boats that took on water or guys who didn't have their outboards secure and dunked them. Never had to do a single depickling on an outboard because of taking one over the transom. Either way this really has little to do with my topic.

 

I said it was based on the Simmons. I built an FS17, it doesn't use a pocket transom, and is designed for full planning. No matter the sea state it still won't come close to swamping an outboard. Nor did the dozens of other boats I've had. Ok back to the subject at hand.

 

Thank you. This musing wasn't for my current boat, but one I'm looking at going forward.

I've actually been through the St John's where you are referring to. As I will be cruising slower speeds with lower power a .5knot difference over an 8hr run can be significant. That's not really the reason for the bracket though. As said, it's more for room, utility, and noise. I was just wondering the difference it could make. Many guys run brackets up to 3ft here, and nearly all will tell you they gained performance, but those guys are going fast and I'm not looking for that now.

 

Why ask questions if the answers provided by some of the most experienced guys on the forum are of no interest to you? It would be a lot more efficient if you started a second profile to answer your own questions.

 

Go back and reread the thread! They did not give an answer to the question, they gave opinions on outboards and possibly swamping, which had nothing to do with my question.
Fallguy, Ondarvr, and Messabout gave the answers to the topic, so thank you again for your insite.