Stern Designs for Outboard Powered Boats

[Kevin Morin]

I've posted this design review on other sites, and post it here for those who would like to have a laugh at our own expense, surely I've gone around the 'bush' more than once. Maybe you have too? Perhaps you're looking at one of the newly offered SpecMar PreCut kits in welded aluminum and you're looking at a transom and thinking about it? I'm here to help that thinking and if you'll stick around to read this thread for a while... I'll take you through some thoughts on the subject, hopefully ending with a smile?

The transom images are just the last few feet of a boat- just boxes with engine mounts- the discussion is only about that area of the boat so the images/sketches only show that much "boat".

On the metal design thread we've gotten into some discussion of the ways to remodel stock hulls. That reminded me of one of my permanent smiles in welded boat building. How to design the 'best' stern arrangement for the outboard engine?

The stern of an outboard powered welded metal boat can be arranged in different ways and since we all have different ideas about the right way things should be…. Well, we could, each of us, prefer one or another of those arrangements.

I’ve built lots of welded metal skiffs and so I’ve been involved with many people going around this decision cycle to decide which stern they like best? In fact I’ll be trying to show how we got here? (current design summary) What lead to the changes, and what thinking I’ve seen and what thoughts I’ve had on the subject.

I’ll start with a little stroll down memory lane and remind us all why things all started on the transom of the skiff? To tell the truth the real start was inside the boat, the outboard engine well, forward the transom needs a brief mention as well. In each stage, I’ll try to remind us of the thinking the justifies that stage or design arrangement and to suggest what lead on to the next design evolution.

Please feel free to explain what I miss, to remark about your location and the experiences there, and to suggest other facets that I will naturally miss.

Disclaimer: I don’t intend to find fault with your opinion, I don’t intend to suggest that as a builder I’m any way qualified to suggest my ideas are better than yours. On the other hand I will probably poke fun at the logic and reasoning I’ve learned from others’ as they’ve explored this topic. These stern arrangements could be used on any given welded aluminum planing boat so they do qualify as potential design modifications to stock plans.

Let’s begin with outboards: they started out smallish. The 100hp, or as we now know 300hp engines were lifetimes away when the initial small horse outboards first appeared. So originally outboard engines were light weight, not huge in size (compared to the original inboards) and reasonably compact. But they were also engines that needed tending. Where to put them? On the transom; (I'm not mentioning the obvious fact these initial small engines were designed to go in this location); in side a well?, in the bow inside a well? hanging off the bow?



So where did all this begin? Here, is a pretty little wooden row boat made to move with oars or maybe a sail?



Now we're getting somewhere, the boat while very nice looking is still not made of metal except for the very new (we're talking history here not the model year of the kicker) power. No longer pulled or pushed by oars, and quite a bit bigger because of the ability of the new "engine on the transom" propulsion, the skiff is deeper, wider, longer and moves incredibly faster than any human can row the previous little boat.

But the short shaft made the mount a cut down on the transom so potentially a weak link in the gunwale for shipping a follow sea? Swamping and perhaps causing the loss of the skiff if it were loaded and didn't float?

What's next? (beside bringing in metal boats) Wait; forgot to mention the reasons to stern arrangements and here is where the history gets tangled. But first a word about outboard wells.



This little boat has the engine's transom inside the actual topsides and overall aft most transom- an outboard well. There are reasons for this arrangement, slow boats that don't plane, flotation aft the engine lifting the stern in a following sea, access to the early engines, and modern ones too if the skipper (as here) is inexperienced, and if you're fishing the wheel is below the bottom inside the bottom and out of the way of gear. All these reasons lead early builders, often building for commercial fishermen, to install wells. Also the previously mentioned reliability of the early engines almost insured there was work done on the engine any give trip off the beach.

So we'll need to look back at the reasons for the first few arrangements but we'll do that next post.

Cheers,
Kevin Morin
Kenai, AK

[Kevin Morin]

If you look at the 2nd wooden skiff above; notice the outboard engine mount at the stern is cut down? The reason for this needs to be introduced as this will be a constant companion for the entire thread.

What's up? isn't that where the low point of the sides is? Yes.
Won't water come in around the engine? Yes.
So to put an outboard on the stern you lower the freeboard of an entire boat? Yes, if the outboard mount is lower than the sides and transom of the boat.

Let's recall that outboards are manufactured not built one-off by craftsmen so they have standardized parts allowing the makers to cast and machine them in thousands (100's of 1,000's?) to reduce per unit costs. That includes the aluminum castings of these engines' vertical drive shaft housing. This gets sort of confusing but; the original engines were very small horse power and therefore put on small skiffs where a 15" transom was decided to be standard which almost instantly was too low for the larger outboards of their day since they went on larger, longer, deeper boats.

The (non human) propulsion from these originally smaller engines- allowed boats to get bigger (bigger boat hauls more fish). But the manufacturers were a little behind the pace, so they waited until the demand for the next four inches of vertical drive shaft came from enough people before moving up to 19-1/2" transoms -at the time called a 'long shaft' but not to be confused with the much longer shafts in today's bigger engines. Profits were to be had with 18' and 20some foot skiffs that came about next but they had STILL little cut outs in the transoms- as the plank skiff above shows.

OK, now you're out fishing in your much-bigger-skiff-than-before, made possible by not rowing for 12 hours to go fishing- and really hit the mother lode. The bigger skiff has a small outboard BUT.... to reach the water, the stern is cut down because when the engine manufacturers first researched and sold the product they went on smaller boats with shorter transom heights. So, now, those low cut outs are really hurting the amount of fish you can haul with the bigger boats made possible by the use of these new engines and the increased amount of gear you're hauling, again, made possible by the reduction in rowing or sailing.

What to do? Well, two things; first the engine makers need to make longer shaft higher horsepower models so the transoms of the bigger skiffs can have a taller freeboard, and that happened. But not overnight. The next step while waiting for manufacturing to catch up builders began putting a 'splash well' /slop tray/ safety box around the low point of the cut down transom.



Mostly this prevented waves that come in around the engine from flooding the entire boat, they just flood for a few seconds and drain out. The box that was added to the low point cut down transom because the engines weren't made yet with taller, longer vertical shafts, was an adaptation to the constant problem of having enough boats the same to allow engine makers to build for a given vertical transom dimension. So the builders had to build to the engine makers' dimensions.

But right away you can see that tiller handles attached to the outboard engine made the boxes have to get deeper, forward into the boat AND you had to reach down to hold them, bend over to fiddle with this low cut transom mounted engine- but it still beat rowing so the splash wells were accepted.

Engines got bigger and bigger and handling them with a tiller became more of a chore, plus the larger boats being pushed made it hard to see everything squatting in the stern holding then tiller that was mounted so close to the water!

Cheers,
Kevin Morin

[Kevin Morin]

Here is the next evolution twins- how do you mount twins on the stern with the narrow splash well for one engine?



The full width splash well or slop tray as some call it allowed two engines and even a kicker to be mounted along the cut down transom, still fixed by the manufacturers of engines- not boat builders. The idea simply takes the cutdown transom's stern protection bulkhead around the engine and instead of making a three sided surround- just continues he entire stern splash tray into a bulkhead with perhaps stowage below.

The idea shown here, keeps the engine(s) mounted to the transom of the original boat hull length, it still keeps the lowered cut down safe in a following sea and as paddler notes, works for other purposes too.

But... lots of owners look at this and say "That sure does take up a lot of deck space." Which is true, and the bigger the power head the farther forward the well/tray/false transom has to be! So a 100+ hp engine starts to need 27-30" of deck space depending on the rest of the geometry of the stern that is 2 feet of deck, given up to the engine's design and mounting and the resulting hull protection of the surround.

What to do? The deck of fishing boat, commercial or recreational is some expensive real estate. The deck is where the action is, so the entire boat could be described as a means to get this deck out to the fish and back in whatever weather comes along? I know that is oversimplifying a bit but really; think for second about what the deck is? It is the reason for the boat in many cases. The deck is where the fishing happens, the deck is where the fisherman fishes so.. back to my original statement the deck is about the most premium value in the boat if she's been built to haul you off shore to fish.

Say the boat is a 24'er with about 1/2 helm/cabin and 1/2 deck. (it's usually much more like 5/8's cabin) Then she might have as much as 12 feet of deck (multiplied by her beam for area)? but the engine to push this boat HAS to take up 30" leaving 9'-6" where there was 12'- the engine takes up 1/5 the boat's deck! That's some serious area of the purpose of the boat given up to the fact outboards are manufactured with too short legs, and unchanged stern mounts for too long! For every boat that is not 50% deck space and instead has a larger ratio of cabin to LOA; the amount given up by these initial stern arrangements goes to a higher percent of the premium fishing space.

What can be done?

Cheers,
Kevin Morin

[Kevin Morin]

One idea is to put the engine behind/aft/back or the transom and not cut any holes, no splash wells/slop trays/engine shrouds - just a big flat full height transom- the safety in a following is as good as the hull will ever get! NO low spots no swells overtaking and coming aboard- but how?

Outboard racers know that the farther aft of the center or gravity of the boat they can place a mass the potential faster their boat can go. So a big mass is the engine, and it could be hung off the stern of outboard racers using a metal or even cast plastic bracket.

I'm not sure who started this, but the Gill Bracket was one of the first widely recognized products to be bolted to a boats' transom ( they were a tube frame I think?) so the shape of the hull could remain full height of the topsides all around the hull, AND yet still accommodate the engine mounted to the stern. There were, and probably still are, many of these engine cantilever bracket products around as they did their job to move the engine aft, improved performance a bit and solved the loss of deck space to the previous engine mounts shown above.



Now the deck just got 2-3' LONGER, for the cost of a little metal bracket- and this could be and is built into welded boats.

Notice the bottom surface of this cantilever box engine bracket? Some of these frames were not boxed in sheet metal but were just pipe frames with a little plate to bolt the engine mount on. I show the version where the entire cantilevered bracket is of plate.

The lower surface is not at the bottom of the boat, was most often completely free of water when the boat ran on step (planing) but some of these brackets with the bottom surface as shown, DID add to the positive displacement of the stern when the boat settled off plane and sank into the design waterline at rest.

So the Gill bracket (that's probably a trade mark? so I'm using it to describe an entire series of product and my version is just for illustration) solved the deck space loss problem while improving top speed as well as improving steering by putting the wheel aft the hull a couple feet.

then what?



Some one noticed that the Gill bracket's lower surface could be used to help with planing the hull! That is- if the formerly upward slanted bottom surface of the bracket were to be fabricated to the existing plane of the bottom surfaces, shown above, the bottom of the boat would become longer and when at rest the bracket would add some more flotation to the stern by displacing more water.

The engine is still aft the hull, the transom is still full height to the topsides height, the running speed is increased by the location and there's really nothing you can do to a modern engine at sea- so why bother to have it close inboard where you can "work on it" ?

This design review has gone from open stern cut-down mounts to transom mounts with splash wells around the low point to brackets and now short narrow bottom extensions doing all the jobs of the previous arrangements as well as getting the full deck space back for use.

What happened next? Everyone knows we could "just add some swim platforms" or small side decks to allow more 'room' next to the engine(s)- right?

Cheers,
Kevin Morin

[Kevin Morin]

With a small amount of plate filling in the two open corners of the new stern extension outboard bracket we could have some MORE deck space.
This is some expensive real estate and getting a low cost addition to this expensive commodity is worth looking closely at the details.



Here are a pair of metal squares/rectangles shown outboard the central hull extension 'bracket' that holds the outboard. Since the extension is there already, it provides 50% of the support and structural stiffness required for this 'deck extension'. The transom is the other 50% support and a small amount of metal goes a long way because the two supporting structures were already afforded, by the previous decision to extend the hull for the engine.

So the next variation in a stern arrangement is to flank the engine's center hull extension with flanking plates that are used for all sorts of good and benefit the boat's overall utility. They allow boarding over and through the stern, formerly not even considered, they allow gear to be carried lashed on the stern, coolers and buoys, anchors for gear and lots of other equipment find a handy home on the steps aft.

We need to review and look at the progression in a single image, so I'll line up these various stages of arranging sterns (evolution of design) so we can make a visual comparison of the steps so far.



Here's a look, left to right, coming toward the foreground are the different basic ideas we've seen that are employed to arrange the engines on the sterns of welded boats. If this image is not clear? An important point in my sketch is that the transom plane of all these images is lined up -they're all in a common plane left to right and so is the upper row that I rotated to see them from a slightly different angle in one image.

(But... wait!) What happens when you need two engines instead of one? Well the central hull extension has to be a little wider, that may be true of adding a kicker too? Some kickers we've seen can be added to the sides of the swim platform or cantilever deck panels to the sides of the main hull extension but many times the central extension is made wider too.

Cheers,
Kevin Morin
Kenai, AK

[Kevin Morin]

OK, "back in the saddle again" with a few minutes to update our images so we can keep looking at the strange evolution of our welded aluminum outboard engine powered boats.

First its was.... oars or sail; then
Transom mounted engine, open at the gunwale; then
Cut down transom because the sheer went up because we could haul more with an engine in the second iteration; then
To keep the sea out a splash well/slop tray/ but not a hole in the hull 'well' which is more branch on this tree than trunk; then
A deeper fore and aft well to accommodate longer tillers, bigger block heads and more engine; then
A full width well/tray/splash bulkhead -but it began to "give up" more deck space to the engine mount; then
The first bracket to get deck space back, get more speed from existing power and give more room in the cockpit;then
Might as well get some floatation once the bracket began to be plate instead of pipe; then
Well! some decks along side the " bracket that floats" is only a few pieces of plate and the support is already there; then

(So, designs have progressed logically as each step is only a few pieces more metal, and a few hours labor. Not a new hull, not a significant adder, just incrementally increased within reason to the whole project.)

As mentioned last post the idea that's next is....
"Say! if I had two outboards instead of one.... I could put them both back here!"; then



What we have here is a just a small width change to the previous flotation and planing bottom adder bracket, that holds to engines. Maybe its wide enough for two and a trolling motor/kicker too? ( I added some green panels to show the original width bracket widened to starboard ONLY)

But then.... what's next? Well, logically a little more metal could make the side deck/swim platform more secure and we can put lockers, totes, tanks/ or coolers out there to clear out the deck!

So! what if we put wings or sloped topsides extensions on the sides of the swim platform/side decks/stern platform: then



The keen eyed AAB.com Forum reader will see that I didn't use the 'widened' float hull extension type to put the side 'wings' on but the ideas is that this is the next piece of metal that can be easily added to perhaps improve the look of the stern extension, to give some sides or coverage to the deck and to help with support is the deck will have totes/tanks/coolers for loads.

But now that there's a bracket about 2/3's the beam fit to the bottom and the boarding platform full width, with some of the sides added in: why don't we just fill in the bottom? I mean the plates are only a few square feet and the rest of the boat is all but "there"; then



Here's a full width bracket with 'wings' and a deck room for two or a big single and plenty of room for trolling motor, kicker and some coolers or tankage. Now we could reasonably put hatches in the top of the deck and make stowage, lots of room (now, under ) there, full flotation and it's completely reasonable to add watertight hatches to this serious bit of construction; right?

So where do we go from here? I mean this is a progression of thought- at least it was for me when I was building full time.

I'd say we're getting close to the 'ultimate stern arrangement'. Its helpful, I think to recall the stages we went through to get to this design evolution. Just start at the top, review the various drawings and see where it leads?

Cheers,
Kevin Morin
Kenai, AK

[Kevin Morin]

Now to get to the final ultimate design for an outboard stern arrangement!
Next is to add more to the wings so the sides are full to the sheer!; then



We only added a few pieces of metal and just filled in the topsides from being wings to being the full side ht again, I mean there's just a little metal and a few hours, nothing major here.

Where will we go next? I hope its getting clear to everyone, where we came from! But the reason is to get deck space along side the engine, and we've added a full 2' to 3' of deck by going through all the stages up to here- why not just box off the engine in a splash well and have that space as deck??



BUT WAIT, you say? that's sort of like the beginning of the transom mounted engine with a well and splash guard????? what's up...??
That's right and let's compare the two shapes.



I admit that the colors and transparent panels in the middle model are there to show the step by step additions and incremental planes of metal we added but when we're done with this stage what do we get? Well there's one to the left of the step by step model in the middle and the one on the right just shows those same panels in one color like the original.

Now what was our thought process? Let's review in an image summarizing the steps we took, all logical, all for a good reason and all incrementing the idea of the next design stage is "just a few pieces of plate and little welding" along the process of improving the stern design in ALL welded outboard skiffs.



What can I conclude from this historic design review exercise?

First there a several ways to mount the outboard engine on any given skiff, and when they are followed to the end of the logical sequence they simply extend the hull and then begin again.

Second; What can we do to improve this splash well design we ended up with? if you answered " I think we could move the engine aft on a bracket... don't you? " then I'm not doing a very good job in the writing department- as usual!

Third; Any stern arrangement you want is fine, they all have reasons, they have followings, they all work, they all fish, they all come home. Its your' money, get what you want in YOUR stern design.

The humorous thing here is that if someone took my advice, learned to build, and then designed and built their own; inevitably another (2nd)individual, sometime will approach them to build another skiff for that 2nd person. AND when the two begin to discuss a new prospective design that 2nd other person will say:

"Now: to improve your stern design....."

In my experience the first few welded aluminum boats are an extreme education in people, business and barnyard expletives so I hope you will be able to smile with me at our sometimes confusing follies as we seek the best boat or our dream boat in welded metal.

now if I just added a little plate here.....

Cheers,
Kevin Morin

[Dave Grason]

Excellent read, Kevin. I enjoyed that. This was an eye opener for me and I also found it quite humorous.

[kens]

That is a neat little read.
Those idea is already been in my mind long ago. Good to see someone explain outboards like that. I was the guy on this forum that always said outboards take up as much room as a inboard motor box. You explained the deck space issue with the splashwell (splashwell that I hate). I accepted the deck space issue with a inboard motor box. I can set a beer cooler on top of the motor box, but you cant set a beer cooler on top of the splashwell.
When you add on the outboard motor bracket, you just effectively added on more OAL to the boat. Well gosh, if you add on more OAL, then you could just as easily install a inboard v-drive within the same boat spaces. But I digress, nobody see's the outboard motor as a addition to the OAL of the boat, it doesnt exist. Outboards are only measured from bow to stern, never from bow to back side of the motor???? (tilted up no less)

[Soloboat]

Very amusing. The more things change the more they remain the same.

[Kevin Morin]

Dave, Kens, Soloboat,
thanks, glad you enjoyed the 'rant post' about stern design. Unfortunately, I've been right in the thick of those changes before...fortunately not all on one boat. Although, I do have to confess I did take off one entire stern and add another, while stretching the skiff in the process, and the stages are shown in this thread. (sort of embarrassing)

The "all it takes is one more little piece" step-by-step thinking is what sucked me in! I think it's a version of something for nothing, or something good for so little more- why not? I guess that's how a dog thinks the end of his tail will taste?

Anyway, I hope the post can have two purposes, one to smile at our mind games in boat design(?) and the second to review the many stern arrangements since they could be preferred by any given owner for their own boat?

Kens, truth be told I'm an inboard advocate myself. I live in an area where its common to put 2 X 300hp outboard on the stern of a 30-32' er and run 50mph burning 100-150.00$ an hour in fuel. I'd prefer to put in a smaller inboard burn less and go slower, having heat, hydraulics, and CG over CB for general performance.... but that's not the trend.

I'd even spend the time to build tiny inboard installations, if I were building right now, just because I'd like to power some double ended dory-like shapes that won't tolerate the engine on the stern or overhanging the water. That's not the popular design though, so this article was intended to help outboard owners/builders/designers to see the entire "bush" they're going to be "beating around" as they think about future builds.

Cheers,
Kevin Morin
Kenai, AK

[Gayle Brantuk]

Great article and explanation, Kevin. As usual!

[crowsridge]

Hi Kevin,

Is there an easy way (not taking up a bunch of your time ) for you to elaborate the construction of the offshore bracket. I took a jet hull to a boat builder and he made a bracket for me. I'm upgrading the boat and want to hang either twin 150's or a 300 and a kicker. I'm worried about the way he built the bracket and it holding a motor twice as heavy as the 2 stroke 200 that's on there now. Plus, I want a fixed kicker bracket instead of the hydraulic p.o.s. that's there now.

Thanks, Chris

[Kevin Morin]

crossbridge, I'm not sure which of the images shown (ABOVE) represent an "offshore bracket" I've heard the term used for half dozen shapes! So first you'd have to define what the added structure looks like or post picture of the hull with bracket on?

(therefore) If you have photo images that would go a long way to helping everyone reading to understand what you're trying to learn? If the bracket will hold two engines, as I get from your post (?) then it obviously has to be able to carry that wt but some models of the bigger horse engine are 20" and some 25" clamp to cav plate so that's a factor.

When the engines on are a bracket and cantilevered of the hull the at rest waterline will be one line buttttt.... when the bracket has planing surface extension that may be different... so there are plenty of "I don't knows " in your post that would have to considered to describe or elaborate on the add on.

Not sure I understand the kicker question really? sure could use a pic? If time allows?

Cheers,
Kevin Morin
Kenai, AK

[crowsridge]

Lets start with these.