Building Logs

Outboard sizing

Posted by Paul

Firstly my apologies again for the state of the blog, this time its not my fault well not entirely. My service provider decided to move my site from one server to another supposedly to improve my service. I had no issue with it, nor had I requested the move. Anyway, during the move they lost most of my site, had no backup and refused for a month to even answer my complaint. Needless to say I dont recommend them nor will I be staying with them. So the site may be down again very temporarily once we figure out how to retrieve what we have lost if it is even possible and how to then move it onto another site. One of the things that amazes me is how often we hear complaints about how companies are losing their business to offshore competitors and how they cant compete, but with the levels of customer service being offered in Australia and the US (justhost is a US company) is it any wonder. We do some work with IT companies in India and they cannot do enough to ensure we are happy with their work and will re-do work as many times as we need until we are happy. The reason they do this is they are hungry for the work, and it shows. Its the kind of hunger that built the West into the power that they once were and we have grown lazy and complacent at our success and that is why the West is in decline and the East is in ascent.

OK rant over, back to the build. One of the great things about nearing the end of the build, besides being near the end, is that you get to finally do things you have played over in your mind for years and years. And one of those for me is the outboards. Most builders have their engine choice made very early in the build. If you are putting diesel inboards in then there will be many decisions that the engine choice will influence very early in the build. Outboards as an option leaves the brand decision for some time but in many ways that decision may be made for you by some of the limitations of the design.

Most designers that stipulate outboards as an option, build them into pods or nacelles that tuck up under the bridgedeck close to the hull sides. Schionning have devised a way to get the engines into the hulls. This has a couple of advantages (and a couple of disadvantages) over nacelles but overall I think the advantages outweigh the disadvantages. The disadvantages of pods is that the closer together the motors are, or more appropriately the props are the less manoeuvrable the boat becomes when just using the motors to manoeuvre. This also applies to how far aft they are, the further aft the better they steer and in the hulls they are further aft than under the cockpit bridgedeck, but this can also be a disadvantage.  A skilled multi hull skipper can turn a cat in its own length just by using the 2 motors, and that is much easier if you can get the props out as wide as possible. I on the other hand have no idea what to do yet, that is all ahead to learn but being an unskilled skipper I will want all the advantage of widely spread props I can get.

Another disadvantage of pods is that they are under the bridgedeck and therefore they lower the minimum height you have or clearance as it is commonly known among cat designers, albeit only for the width of the pods which is usually alongside the hulls and protrude out under the bridgedeck about 400mm – 500mm each, but they can still slam against waves where a cat with inboards or outboards in the hulls wont with the same bridgedeck clearance in the same sea conditions. They also sometimes (not always, some designs offer excellent access) make access to the outboards difficult for basic maintenance. With the outboards in the hulls access through the deck is fairly good, in fact I am designing my access hatches through the steps so that the motor can be raised through the hatch for removal or insitue maintenance with plenty of clear space all around for tools and workers either side of it. Another advantage of outboards inside the hulls is that they are harder to steal, not that theft is reported as a massive issue for permanently attached outboards, nevertheless outboards that are out of sight removes most temptation where opportunity arises and access to the props is easier from inside the hull rather than under the bridgedeck should the need arise to clear a net or crab pot line without having to actually enter the water to release it.

Disadvantages of in hull vs pod under bridgedeck outboard mounting is that under the bridgedeck the outboards are generally a little further forward and not as susceptible to cavitation from the hulls leaving the water at the ends over following seas. The moment the prop grabs air instead of water it momentarily revs harder and this can shorten its life, but more importantly the prop is not propelling for that moment and you dont want to lose propulsion because you also lose steerage and control, even momentarily.

Another disadvantage, at least for my particular build and preferences is the compromise between height and depth. Outboards come in 3 sizes, short shaft (15 inch), mostly used on tinnies and speedboats, long shaft (20 inch) used on deeper V’d hulls and then extra long shaft (25 inch) used on yachts as auxiliary power. Extra long shaft is what most designers specify to get the prop as deep into the water as possible so as to avoid cavitation as waves, usually from a following sea, lift the hulls out of the water temporarily. Under the bridgedeck the motors can be tilted using the hydraulic systems that come built into the outboards so leg length has no disadvantage. With the Schionning system where the outboards are mounted in the hulls, under what will be the rear access steps means the longer the shaft, the taller the motor is inside the hull, and this has a direct impact on the height the steps can be set to in order for the motor to retract into the hull and for the prop and skeg to clear the inside of the hull so that the hole that the shaft goes through can be closed off again.


And of course there is the complexity of clearing the outboard of the water in the up position. Being fully retracted is one of the major benefits of outboards over inboards. When sailing, the prop and shaft or leg (sail drive) with an inboard is a drag on the efficiency of sailing. When fully retracted sailing is unimpeded by that drag. Other advantages are as previously mentioned, clearing snags from props with an inboard means getting wet and diving the hull, servicing the motor means a visit from a technician if you cannot fix it yourself, with an outboard you can remove it and take it to a service centre, much cheaper, and speaking of cheap, outboards are half the price of inboards or less, mine are $6000 each, even cheap inboards are about $12000 each. Some outboard brands and sizes are nearly half the cost of mine (about $4000 each for new Honda 15 long shaft 4 strokes).  So on the basis of cost, if they only last half as long as an inboard you are still better off from a purely cost point of view. Running costs of good outboards these days is comparable to diesel inboards. My outboards are said to run at low revs at 1 litre per hour and I ought to get 5 knots. I litre per 5 knots is very reasonable, put another way, assuming 200 litres aboard and used, thats about a 1000 nm range at a cost of about $250. I know a guy that spends that in a weekend on his 20ft sports fishing boat with a massive mercruiser V8! And dont even get me started on pollution, I cant stand the smell of diesel boats when new, imagine what they are like when they are blowing and belching black smoke all over the place. Modern outboards run very clean and quiet.

So to my particular dilemnas and compromises. Firstly, it would appear (although to be fair to Schionning I have not confirmed this, it is just my casual observation) that the idea of raising outboards into hulls was envisaged for 10 and 11 meter designs and as a result designed around 9.9hp motors. The larger size cats of 13 meters or more were deemed too big to benefit from outboards and were designed with inboards as the only option. Then when they designed the 1230 it is somewhat of the cross over size, still ok for outboards but probably better with inboards. But inboards never ever figured in my calculations. I cant stand the smell of diesel fuel and it would almost certainly cause me to be sea sick where I otherwise probably would not be.

I have a friend with a 40ft Crowther cat and have pictures aboard his boat sprinkled through this blog. He has Yamaha hi-thrust 9.9’s and his boat is strip planked cedar so probably just a little heavier than mine. His motors are adequate most of the time however his advise is that the 9.9’s can struggle a bit when pushing against wind and current (or tide) and that he really has to ring their necks to get enough power sometimes and he encourages me to fit bigger engines if I can.

Here is the rub though, everything on a boat is some kind of compromise. With motors its size and weight vs power. A 9.9 high thrust Yammie is about 50 kilos. The next size up in a Yamaha high thrust is the 25hp but it is a whopping 96 kilos. Way too heavy to be hung at the end of each hull. Weight hung at the extremes of the boat (forward or aft in the hulls) acts like a lever and severly exaggerates the hobby horse motion. Another minor setback for the Yamaha 9.9’s as a choice is, and I am going only on the chatter in the forums on this, but anyway, they 9.9 used to be a down tuned 15, but about 3 years ago they switched to an uptuned 8hp and by all reports it isnt a patch on the old block and has less power. Not good for me, a 9.9 was going to be underpowered to start with.

Back to the boat being designed for 9.9’s and trying to squeeze a bigger motor into the cavity is height. 4 stroke power heads are quite tall and the bigger the block (hp) the taller the cowling above the shaft which must fit in under the stair profile. I struggled with this for some time when designing the rear step profile I wanted.

The solution to a number of problems presented itself in the form of the Evinrude Etec 25. This is a detuned 30 so right off the bat I am ahead of the curve. Its components are built for the torque and thrust of a 30 so at 25hp tuning it is not being pushed to its limits. An Etec is a modern version of a 2 stroke motor, less moving parts means less weight for a given hp and a smaller power head means a shorter motor above the shaft length. It ticks just about every box except cost. The Etec 25 is 70 kgs.

And for years this was the plan. But you never really know until you fit one. Then it becomes practical instead of theoretical. And that is what we have finally been able to do. Years ago I raised the height of the rear bulkhead by about 200mm, I needed to fill the void in that would have been the walk through transoms, that is only applicable if you are having inboards, which I had ruled out, so not having inboards meant I was already gluing duflex back into the boat anyway so I took the opportunity to raise the height for a number of reasons, not least to get as much room in the well as possible to raise a bigger motor up into it. Another pleasant side benefit is the extra head room in the  aft cabin and laundry in each hull. But that extra height brings with it other issues, such as getting the steps to a usable height and depth, the taller they are the higher the risers need to be for a given tread depth, and of course the distance out that the steps can go is set by the hull length and unless I lengthen the boat the steps become steeper if I raise the height of the top step by raising the bulkhead. Anyway, I have had a height and depth of step worked out for some time but fitting the outboard was the final confirmation of that.


What fitting the outboard revealed, that I had not considered in all that planning was that the outboard may be too deep in the well when fully lowered. It created a doubt in my mind that the shorter shaft  of the etec was going to be viable. But herein lies the issue, Etec dont make an extra long shaft and even if they did, that extra height compromises the top step height, it simply wont fit under the height I wanted to use and raising the top step height means each step has to have a steeper rise. The 2 motors fitted side by side as it were in the pics above, show that the height for either is about the same thanks to the etec being a more compact motor despite it being double the hp and that the only difference is length of the leg as shown by the depth of the prop for a given top height.

To confirm my fears or suspicions I did 2 things. First I borrowed a 15hp Honda extra long shaft from Terry. This is a 4 stroke so it has a bigger power head and as a result it is about the same size and weight as the 25 etec I had already had fitted in the Port hull. We fitted it to the starboard hull then lowered it for a comparison. The other thing I did was had the guys from the Etec dealership come take a look. The last thing I wanted to do was swamp the motor because it was burried too deep into the water in order to get the prop through the bottom of the hull, and voided the warranty and have Evinrude wipe me because of incorrect fitting of the motors.

The Etec guys reassured me that the motors were easily high enough at the depth I wanted to lower them to get adequate prop clearance out of the hull and also that the extra long shaft when raised high enough for the prop and skeg to be clear and inside the boat would result in the top step being too high. So that confirmed the decision. Evinrude Etec 25’s will be fitted (there is no benefit in springing the extra $500 each for the 30’s because the torque at low revs is what I need not the upper rpm power of a 30hp that a planing boat might value).

Another revelation which was always a concern was the other motor dimensions, in particular the fore and aft size of them. I had made up cars to run on tracks and preditermined their size by guessing how much clearance I would need for the umbilical to clear out the back of the motor and curve around and have enough clearance in front of the bulkhead to make that curve, knowing they dont like sharp turns. The gear linkages are similar to teleflex steering in that it is a push pull cable that cannot make a sharp curve.

So with the car on the tracks and the outboard on the car, the rear edge of the motor has to clear the bulkhead behind the well. That bulkhead cannot be moved as it is the bulkhead the rudder shafts are mounted to, so moving the bulkhead if there wasnt room for the motor means moving the rudders which could and probably would effect the balance of the boat and the steering.

As it is, it fits and there is room for mini tillers to be fitted to the rudder shaft tops to fit to the hydraulic steering rams. But its very tight, and the mini tillers would be short. The shorter the tiller the harder the hydraulics have to work as they dont get as much lever action to help them. There is room in the well to mount the hydraulic rams fore and aft rather than across the boat which is the prefered option so I will explore which is the better option. In the end it may not matter as most hydraulic systems could handle that load or if mounted fore and aft the system works equally well, you just adjust the tiller angle to shaft to suit your mounting points. Across the boat keeps the hydraulic ram neatly against the rudder bulkhead but the tiller would be very short, fore and aft hangs the hydraulic ram out in the well, but I suspect a compromise of along the inside angled hull side might be the answer.


The next task is to build out the buoyancy boxes that surround the outboard leg and turn the massive well into a narrow shaft. The well is about 1 meter square and the waterline about 300mm up the hull sides. That means that when flooded the cavity can carry 300 litres of water, and water is 1kg per litre. 300kgs per hull is a tad too much to be carrying around when sailing. Not to mention the effect of that 1 meter free surface of water sloshing from side to side, I imagine it would be enough to push the boat continually off course having that kind of force in each hull. The solution is to build the well out so that all that is left is a narrow column that the outboard leg slides through that could only be about 300mm x 400mm x 300mm which is about 1/10th the volume and therefore only about 30kgs. And the buoyancy that is built into the tanks offsets this weight to keep the sterns from burying that little bit as a result of the extra 30kgs load.

The buoyancy boxes will be made from ply (I would prefer duflex or even polycore as they are so much lighter but I have none left and I have ply left and rapidly running out of money) be hollow inside or maybe filled with expanding foam and be built to about 100mm above the waterline, and also form a platform for a panel that will fit to the outboard and raise an lower with it, the purpose of which is to close off the column of water from surging into the well. It wont need to be perfectly watertight as this is a wet well, but stopping surges will keep the weight down in the well as well as ensure the motors cant be swamped when the sterns are buried in certain wave actions. The plate if we can figure a way to securely fasten it, will fit into a rebate in the top of the buoyancy box to act as a tight control of the motors movement fore and aft under torque loads as well as an extra device to ensure the motors cannot move off straight ahead and “steer” off dead ahead. Steering is done by the rudders except low speed (no water flow over the rudders means no steerage) which is done by use of each throttle and forward or reverse in one or other motor.

Once all the work in the well is done, including the buoyancy tanks, the rudders hung (the other side of the aft bulkhead but the tillers would overhang into the outboard well to meet up with the steering linkages) and the hydraulic steering mechanisms mounted in the outboard well and general finishing work, the rear steps, which have been made for quite some time, can finally be fitted and glassed in. And I already have a little issue that finally having a full size motor in the well has revealed. As mentioned earlier there is the compromise of height vs depth of prop with any length outboard. The longer the leg, the higher the motor sits in the up position which in turn influence where the steps can be placed in the stairs in each hull. And whilst I have almost worked the height out, the fore and aft depth of the motor is presenting similar problems as can be seen from the various pictures below. In particular the first pic shows that the rear steps need to be moved aft about 100mm so that the motor can raise past the lower step and not hit the inside of the riser as represented by the curved ruler.


The issue of depth in the well fore and aft is not one caused by the choice of motor as the honda is as deep as the etec so this would be an issue regardless of the motors chosen. Its not a big problem to fix, but is a bit of work to extend all of the steps that abut the bulkhead by the 100mm. But it may also manifest itself in a shortened bottom step unless I extend the hull by 100mm or may more depending on how big a bottom step I want and what works from both an aesthetic and practical point of view. All extension is above waterline so it would not effect the way the boat sailed except perhaps diminish the ability for the sterns to bury which should reduce hobbyhorsing at anchor. But that decision will wait until the rudders are in, the rear steps are in and see how it all works. And if I have any money left to make the mods, although I probably have enough offcuts of duflex to do them.



After that there is a little tidy up work with the davits and wells on the duckboard and it is pretty much ready to start fairing. Its coming along. Its coming along. 

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2 thoughts on “Outboard sizing

  1. Glenn Harris

    Hi Paul,

    Did you consider the Suzuki 25HP V2 4-stroke? About the same weight and price as the E-TEC. Overall height (L shaft) about 1300mm.

    (Waterline 1320)

  2. Paul

    Hi Glenn, I did consider the Suzi and if I went with a four stroke that would have been the one I went for. In the end I went with the etec because theoretically it should need less maintenance, being a 2 stroke and it has better torque numbers so I figured it would push the cat a little easier.

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