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Thread: 20 hp jet too much??

  1. #1

    Default 20 hp jet too much??

    I got a zodiak S131 (10 foot zodiak) and would be interested in putting a 20 hp jet on it. Would that be too much motor for this boat??

  2. #2
    Moderator Paul H's Avatar
    Join Date
    Apr 2006


    It depends on what the boat is rated for. A jet looses about 30% of the engines power, so a 20 jet is equivalent to a 14 hp prop unit. That said, the 20 hp jet will be much heavier. If the boat is rated for 15 hp, you're probably ok, if it's rated for 10 hp, don't do it.

    One other point, zodiaks typically don't work well with jets.

  3. #3
    Join Date
    Sep 2006
    Douglas Island

    Default 20 hp too much?

    Always check the capacity plate to make sure you are not overloading or over-powering the vessel. A motor larger than recommended will make the stern too heavy and can cause the boat to flip. The transom will ride too low in the water and you could be swamped by your own wake or a passing boat's wake. Your boat will not sit properly in the water and will be difficult to handle.
    Too many people (and/or gear) will also cause the boat to become unstable. Always balance the load so that your vessel maintains proper trim. Too much weight to one side or the other will cause the boat to list and increase the chance of taking on water. Too much weight in the bow causes the vessel to plow through the water and too much weight in the stern will create a large wake. All of these situations make the vessel difficult to handle and susceptible to swamping.
    Remember that the capacity plate limits are suitable for normal operating conditions. In rough seas, bad weather or when operating in congested areas you will want to carry a lighter load. Never exceed the capacity plate limitations for your vessel and that includes the manufacturer's horsepower rating (regardless of whether a jet drive or prop); if you try and justify a higher horsepower based on estimated loss through a jet or even "round" up the numbers, you may be setting yourself up for serious problems including liability issues. It's just not worth it! Boat safe! Mike

  4. #4
    Member Rod in Wasilla's Avatar
    Join Date
    Apr 2006
    um... Wasilla...


    I almost don't want to admit this with the above post being what it is, but here goes.

    **DISCLAIMER: Just because I do it doesn't mean that you should do it.

    I own the same boat and it is rated for 10 hp and (I think) 700 lbs. I regularly run it with a 15 hp prop. With a light load it will scoot along at 22 mph. With a medium/heavy load it will still scoot along at 22 mph. (As long as I don't load it so I can't get it up on step... 20+ mph.) I use it mostly on a few lakes and the Little Su, but I have also taken it out of both Deep Creek and Whittier on occasion.

    For me, I'm absolutely comfortable exceeding the hp rating on this boat (please don't hunt me down and yell at me...). I understand that the ratings are for my safety. But, they only take into account the dimensions of the vessel, not the type. This is a low-center-of-gravity, high bouyancy, positive flotation, relatively wide skimming inflatable. I freely admit that I do get somewhat uncomfortable in any significant ocean chop, but I've made extremely sharp turns in calm conditions at WOT and easily maintained control. And although the vessel doesn't actually qualify due to transom height and tiller control, this is obviously the intent of Sec. 183.53(b)(4) [from the USCG Boatbuilder's Handbook]... maintaining control.

    That being said, I have also contemplated putting a 20 hp jet on this boat many times. But, even given all that I will do with it, I just can't justify doubling the weight hanging off the transom (that's about what it'd be) just for a few less inches of draft. It's already stern heavy with just me sitting in the back hanging on to the tiller of the 15. Any more stern weight and I don't know if it would ever get up on step.
    Quote Originally Posted by northwestalska
    ... you canít tell stories about the adventures you wished you had done!

  5. #5
    Member broncoformudv's Avatar
    Join Date
    Apr 2006
    Anchorage, Alaska


    I am curious why I have seen several post on here stating inflatables do not do well with jets?

    I have had friends that ran them and loved them and everyone that I have seen out and about with ones loves them.

  6. #6
    Member Rob B's Avatar
    Join Date
    Jun 2006
    Eagle River, Alaska,

    Default Zodiac Jet

    I totally agree with you Bronco. I have a 16 ft Zod and run a 40 Yamaha Jet with no problems at all. However, I rebuilt and replaced my transom last year. (Long story) When my boat was original, it still ran fine. The big problem is that the transom slopes to a V at the bottom and doesn't flow the water correctly for the jet intake. Mine now has a 10" wide flat bottom to it and I can run shallow water until the tubes on the sides hit gravel. And even then it keeps pushing now matter what is loaded inside. I say a 20HP jet on a 10 footer, go for it. You will know if it's too much or not the first time you run it.

  7. #7

    Default 20HP outboard Jet on Inflatable boats

    The smallest outboard Jet motors made are about 20HP at the pump and are rated 25HP or 30HP at the power head. If you can find one of the old 2 stroke motors they will weigh about 130 pounds. The newer 4 stroke outboard jet motors in the same HP ratings weigh about 200 pounds. I would think thatís too much weight on the back of an S131 Zodiac. The S131 is also a pretty light duty inflatable boat to use for shallow water running. Thereís not much protection under the transom or the bottom of the keel and tubes.

    There are 2 problems you have to over come when putting an outboard Jet motor on a conventional built inflatable sport boat.

    The first problem is caused by the small bend near the bottom of the transom where the inflatable keel ends and the transom starts. This bend tends to disrupt the flow of water that feeds into the jet suction foot and causes severe cavitations with the motor. There are 2 ways to over come this problem.
    1. Build trim tabs off the back of the transom that focus more water into the jet foot.
    2. Drop the jet foot down into deeper water. Below the disturbed water flow. This is a fine tuning problem between dropping the jet foot too low where it creates lots of drag and is subject to shallow water damage, and too high where you get the cavitations.

    The second problem with running an outboard jet on a conventional built inflatable sport boat is the way the bottom of the transom is built. This is the lowest spot on an inflatable sport boat and the first spot to get hit in shallow water. Most inflatable sport boats are built by just wrapping the fabric boat hull around the bottom edge of the wooden transom. This puts the boat fabric between the rock and the hard spot. Most inflatable boats use pretty light weight fabric hull fabric (840 denier to 1100 denier) these fabrics wonít hold up to this type of shallow water use.

    You might want to check out the 401AJR or 430AJR Alaskan Jet Rangers
    I designed them specifically to be used with the 2-stroke and 4-stroke 20HP Outboard Jet motors.

    Several of the members of this forum have Alaskan Jet Rangers and can give you feed back on how well they work.

    I hope this help

    Good boating

    Jim King
    Alaska Series Inflatable Boats,
    Commercial quality at Wholesale prices
    River Rafts, Catarafts, Inflatable Kayaks, Inflatable Canoes, Inflatable Sport Boats, Inflatable Jet Boats, Tenders and Dinghies.
    (907) 248-2900


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