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Thread: Short-Shaft Outboard for a Cat-a-Raft?

  1. #1
    Member BluNosDav's Avatar
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    Question Short-Shaft Outboard for a Cat-a-Raft?

    A long-shaft (20") outboard is usually recommended for a large cat-a-raft. But, if I swapped out the straight cross-bar at the stern of my frame for a curved foot-bar, I could lower the "transom" enough that a short-shaft (15") outboard could easily reach the water.

    Has anyone else tried this approach?
    If yes, are there any problems (or advantages) that you encountered?

    I'm looking to get the lightest rope-start 20-hp outboard available, and right now, that's the Suzuki DF20 @ only 97 lbs. But, for some strange marketing reason, Suzuki only offers this in the short-shaft configuration. Their long-shaft DF20 comes with a combination electric & rope start, and the weight goes up to 108#. That's still lighter than the other brands of long-shaft rope-starts: Honda 110#, Yamaha 115#, Mercury & Tohatsu 118#. But, that 97# Suzuki sure sounds good to my aging back, and it comes with EFI, too.

    Thanx, Dave.

    PS - My other option is to over-load my Leopard as bad as I did last summer on the Forty-Mile; I could have used a 10" outboard (if they made such a thing) since my frame's floor was touching the water most of the time. But like I said, I trying to lighten things up!
    "Luckily, enforcement reads these forums, and likely will peruse this one...Especially after a link of it is forwarded to them....." - AlaskaHippie.

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    Member AK Troutbum's Avatar
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    I wonder if having the motor sitting lower like you describe will present any issues with steering since the angle of the tiller will be more upright. Might be something to consider.

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    Like AK Troutbum I have another wonder. --- My experience using a 30hp Suzuki years ago showed that when under power the space between the tubes near the back filled to overflowing with a lot of water. It was turbulent water and I still had issues with cavitating, but I wonder if that large pile-up of water between the tubes might swamp the engine? I doubt experience with a smaller motor wouldn't give the same effect. Like a lot of things, it would be interesting to try it with someone else's motor first. Any short shaft 20-40hp could be used for the test.

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    webmaster Michael Strahan's Avatar
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    Quote Originally Posted by Jim Strutz View Post
    Like AK Troutbum I have another wonder. --- My experience using a 30hp Suzuki years ago showed that when under power the space between the tubes near the back filled to overflowing with a lot of water. It was turbulent water and I still had issues with cavitating, but I wonder if that large pile-up of water between the tubes might swamp the engine? I doubt experience with a smaller motor wouldn't give the same effect. Like a lot of things, it would be interesting to try it with someone else's motor first. Any short shaft 20-40hp could be used for the test.
    For larger outboards (pretty much anything over 12 horses or so), you have to mash that water column down between the tubes. UHMW works fine for that purpose, but you need to back it with some supporting frame pieces inside the boat. You've also got to control the spray between the floor and the edge of the tubes, or you'll get twin rooster tails shooting up through there.

    -Mike
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    Member BluNosDav's Avatar
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    Quote Originally Posted by Michael Strahan View Post
    For larger outboards (pretty much anything over 12 horses or so)
    Mike,

    I remember your experiences and suggestions for controlling spray on motorized cat-a-rafts in previous threads. But, I seem to recall that was with motors much larger than 20hp? The reason that I've been looking at 20hp outboards, is because that seems to be the most popular size on normally** rigged cat-a-rafts that I've seen. (** without specialized spray-control floors). I assumed that 20hp was the best combination of (relatively) lightweight portability and adequate (but not overpowering) thrust.

    Are you saying that anything over a 9.9hp (don't know of any 12hp models) will result in excessive spray issues?

    Or were you responding to Jim's 30hp example?

    Thanx, Dave.
    "Luckily, enforcement reads these forums, and likely will peruse this one...Especially after a link of it is forwarded to them....." - AlaskaHippie.

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    webmaster Michael Strahan's Avatar
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    Quote Originally Posted by BluNosDav View Post
    Mike,

    I remember your experiences and suggestions for controlling spray on motorized cat-a-rafts in previous threads. But, I seem to recall that was with motors much larger than 20hp? The reason that I've been looking at 20hp outboards, is because that seems to be the most popular size on normally** rigged cat-a-rafts that I've seen. (** without specialized spray-control floors). I assumed that 20hp was the best combination of (relatively) lightweight portability and adequate (but not overpowering) thrust.

    Are you saying that anything over a 9.9hp (don't know of any 12hp models) will result in excessive spray issues?

    Or were you responding to Jim's 30hp example?

    Thanx, Dave.
    Dave,

    I'm speaking of configurations designed to get the boat up on step. For float hunting, I'm running an eight-horse. All I'm trying to do is increase my drift speed. Does that make sense?

    -Mike
    LOST CREEK COMPANY: Specializing in Alaska hunt consultation and planning for do-it-yourself hunts, fully outfitted hunts, and guided hunts.
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    Member AK Troutbum's Avatar
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    Default Short-Shaft Outboard for a Cat-a-Raft?

    I've ran everything from 20 hp down to 9.9 4 strokes on my 18 foot Leopard without too many issues with spray. I don't have any of the plastic/UHMW modifications, and for the most part when I'm running across Skilak I'm loaded with camp and people, so just pushing about 8-12 mph. This year I'll be running a five horse and we'll see how that works out, I'm just tired of packing the heavier motors. I have been able to get up on step with a 20 and even 15 hp motor, unloaded, but there's a fair amount of splashing inbetween the pontoons while doing it. That's been my experience anyway.

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    May I piggyback ride here? I will be moving to an area with rocky bottom rivers and I was checking out inflatables and Cat-A-Rafts at the outdoor shows.

    Unfortunately I have a profession that really doesn't allow me to take time off for hunting and my wallet doesn't really allow me to pay for charters. So I have a small outboard that I was wanting to maybe mount on one of these inflatables or cat-a-rafts. How important is it to get on step with one of these? My idea (until I am told it is not realistic or feasible) is to push upstream x number of miles, then spend Saturday and Sunday and possibly monday float hunting. Is that doable? Would a 7.5 be sufficient?

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    Member BluNosDav's Avatar
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    Thanx Troutbum & Mike,

    Like many cat-a-rafters, I'm looking for the best balance of power & portability. Most times my vessel is loaded for day trips with the whole family. (approximately 1000# load) We don't want to get soaking wet while motoring, so, we'll use less throttle, if spray becomes an issue. But, we also want to be able to go as fast as possible (without spray) on multi-day camping/hunting trips, when the raft is heavily loaded (1500+#) with gear and game. I also need to be able to carry the motor from the truck to the bank, and my personal limit for that is 125 pounds. In order to comply with the growing number of AK rivers that have pollution restrictions, it must also be a 4-stroke engine.

    Which size engine best meets these specifications: 9.9hp, 15hp, 20hp?

    Thanx again, Dave.
    "Luckily, enforcement reads these forums, and likely will peruse this one...Especially after a link of it is forwarded to them....." - AlaskaHippie.

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    Member AK Troutbum's Avatar
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    I wouldn't mess around with a short shaft if I were you. Long shafts work without any added mods or issues what so ever. Pretty much all the 20 hp 4 stroke long shafts (tiller steer,manual tilt, pull start) are in the neighborhood of 120#. If I was willing to lug around a 120# motor, I would opt for a 20 hp since the 15 hp is the same powerhead (same wt.) just less horses. Just my .02

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    just remember most small motors have a 5 inch piece you can buy to make it 20 inch as most people like the 20 inch a lot better
    then the 15 , they got the 15 inch because of the money, they have a 15 inch or that was what they got for the money they had to spend at the time,
    most RAFT plat forms are above the water line , so most of the motors set above the water line a distance MY 2 CTS SID

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    webmaster Michael Strahan's Avatar
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    I think I need to clarify my comments on cataraft-rigged outboards. I don't know if this will answer the OP's original question, but this thread has taken a few turns anyway, so...

    At the present time, nearly all recreational catarafts are not designed to run with an outboard motor. They are primarily intended for floating rivers or rowing on flat water. As far as I know, the power cataraft configurations first began in Alaska with the efforts of Paul Jobe, owner of Wild Alaska Rivers Company, out of Anchorage, Alaska. Paul was my hunting partner for many years, until circumstances forced us to part company. We spent many years together working on various outboard configurations, what worked and what didn't, and generally trying to get power cats on the map. Those efforts continue under Marc Cohen's leadership at Alaska Raft and Kayak, not to mention the many garage mechanics out there who are working on making this work, with varying degrees of success. The largest setup I personally worked on and actually ran, was a 22' custom-built AIRE cataraft that ran a special floor and an 88-horse jet outboard.

    It all comes down to what you're trying to do, and there are basically two things, as follows:

    GET THE BOAT ON STEP

    There are many situations where a person needs to get the boat up on step, planing on the surface of a lake, a river, or the ocean. This can be done with either a prop or a jet-equipped motor. For this application, measures need to be taken to control the interior wake. As you push the tubes across the water, each tube produces a wake which usually converges about 2/3 of the way back from the bow. This wake causes problems in two ways:

    1) Because each tube produces a wake, those wakes converge and create a standing wave between the tubes. This wave is higher than the floor of the boat, and has to be controlled. If you don't mash that wave down, you'll have a rooster-tail of water aimed right at your transom / outboard. Typically this wave is flattened with a floor system that rides under the frame (if it rides over the frame, the crossbars of the frame will restrict the flow of water).

    2) With the floor in place, water is pushed out to the sides of the floor, where it will squirt up between the edge of the floor and the tubes. Because of the pressure, you get a rooster-tail on each side of your floor, shooting into the air and soaking your outboard operator. There are various ways to control these two water columns.

    In most cases where a person is trying to get the boat up on step, an outboard of 25 horsepower or greater is required. It can be done with a 25, but your success in getting on step will be greatly influenced by the load in the boat. Keep in mind that with the move toward 4-stroke motors, you're looking at an approximate 40% weight gain of the actual motor itself, when you compare a 4-stroke to a 2-stroke. So you need to figure this additional weight into your performance calculations. You will also need a special transom. Because these transoms are custom-built, the shaft length of the outboard is irrelevant. You simply build a transom that fits what you have. For the most part, these are fixed-height transoms. They can be cut low to accommodate a short-shaft, and a transom riser can be added if you want to run a long-shaft or an extra-long shaft later. For someone building a transom for the first time, it might be wise to build it with this in mind.

    RUN THE BOAT OFF-STEP

    Some floaters put a smaller outboard on a fixed or an adjustable transom for the purpose of running across a lake, trolling, or increasing their drift speed on float trips. In this application, the outboard is not intended to get the boat up on step, therefore the motor is usually 25-horsepower or less. On rare occasions when the boat is very lightly-loaded, a smaller outboard like this will get the boat out of the hole and on step, but circumstances have to be perfect.

    In most cases where a person is trying to run upriver, they will need to get up on step, though some of the really slow, deep, Class I rivers would allow upstream runs without having to be on step. In those cases, it might be possible to run off-step. It all depends on current speed and the load you're hauling.

    Transoms for this application are often adjustable, especially for river trips where the water depth varies. The transom height can sometimes be adjusted on the fly, once the boat gets moving and the tubes start to produce a wake. This wake is higher than the normal water level, and will allow the operator to run the motor a little higher in the water, for the purpose of avoiding impact with the riverbed.

    MY SITUATION

    I am running a short-shaft 8-horse Yamaha two stroke on my rig (18' Super Leopard). The boat is rigged with an adjustable transom, so I can raise the motor as needed. For my purposes, a long-shaft is not needed, and could actually be a liability. While the transom would allow me to run a long-shaft without any riverbed impact issues, I would have to run the top half of the motor higher, which would position the head of the outboard higher, making it more susceptible to hitting sweepers and such. Of course if we were running woody rivers with lots of sweepers, we probably wouldn't run the outboard anyway, because we would have our hands full dealing with sweepers and strainers anyway. In that case, I would lower the transom and tilt the outboard out of the water, which would put the head low along the profile of the boat.

    I used to run this same outboard on my Leopard, and the fastest we were ever able to go with that setup was 9-10 mph running downriver (the current speed was about 3-4 mph). In order to juice the rig for every ounce of speed, I run it with an extender on the tiller handle, because sitting in the back pushes the bow up and slows me down (go ahead and say it!). I tried a commercially-produced tiller handle extension, but I didn't like it. So I usually use a thicker willow branch, duct-taped to the tiller handle instead. Works great, and even allows me to operate the throttle.

    Anyway, I saw lots of people going different directions with this, and thought I would chime in to clarify what I am saying.

    To the OP, yes, I think you could run a short-shaft.

    Hope it helps!

    -Mike
    LOST CREEK COMPANY: Specializing in Alaska hunt consultation and planning for do-it-yourself hunts, fully outfitted hunts, and guided hunts.
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    Member BluNosDav's Avatar
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    Thanx Mike, that was very helpful. I'm planning for "Off-Step" applications.

    If you have some photos, I'd like to see what your adjustable motor mount looks like. I hadn't considered changing the motor's mounting height while underway. Guess I always thought that the motor would be operating in water between the tubes, not lower than them. My Leopard usually drafts at least 1 foot when loaded. If the water ever got that shallow, I figured that the tubes would hit bottom before the prop-skeg. (Single submerged boulders that are straddled by the cat-a-raft, not withstanding.) My current transom, which is made of aluminum plate, uses the two stern frame cross-bars for support, and is about 28" above the "keels" of the tubes.

    Thanx, Dave.
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    webmaster Michael Strahan's Avatar
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    Quote Originally Posted by BluNosDav View Post
    Thanx Mike, that was very helpful. I'm planning for "Off-Step" applications.

    If you have some photos, I'd like to see what your adjustable motor mount looks like. I hadn't considered changing the motor's mounting height while underway. Guess I always thought that the motor would be operating in water between the tubes, not lower than them. My Leopard usually drafts at least 1 foot when loaded. If the water ever got that shallow, I figured that the tubes would hit bottom before the prop-skeg. (Single submerged boulders that are straddled by the cat-a-raft, not withstanding.) My current transom, which is made of aluminum plate, uses the two stern frame cross-bars for support, and is about 28" above the "keels" of the tubes.

    Thanx, Dave.
    Will this work? It doesn't show everything, but it should give you a general idea. This setup does not require a hard floor to control the water flow, as I am not using it to get on step. The wake generated by the tubes is not of a size that causes concern about splashing or anything.

    This one is on my old Leopard. The Super Leopard transom is secured to the cross-bar on the top like you see here, and to the cross-bar on the bottom, which did not exist on the Leopard frame. Does that make sense?



    -Mike
    LOST CREEK COMPANY: Specializing in Alaska hunt consultation and planning for do-it-yourself hunts, fully outfitted hunts, and guided hunts.
    CLICK HERE to send me a private message.
    Web Address: http://alaskaoutdoorssupersite.com/hunt-planner/
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    Hey AK Troutbum, I tried to reply to your PM but you inbox is full bud

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    Member BluNosDav's Avatar
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    Thanx for the photo and explanation, Mike.

    Is a tiller extension "required" to reach/operate a short-shaft motor on an adjustable mount?
    Or is it just a weight & balance aid when running with a lightly loaded raft?

    With all the discussion of converging wakes and a build up of water at the back of a moving cat-a-raft,
    I'm also starting to wonder if a short shaft outboard might be able to operate from my standard mount?
    "Luckily, enforcement reads these forums, and likely will peruse this one...Especially after a link of it is forwarded to them....." - AlaskaHippie.

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