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  1. #1

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    7.5cc in a Dumas Hawk 3.5?

    I am in the process of building a Dumas Hawk 3.5 outrigger. I know that Dumas recommends a 3.5 engine for it, but I have a 7.5 K&B with a tuned pipe lying around. Has anyone put a bigger engine in this boat? And what, if any, would be the problem with puting it in there? It will fit just fine with just a little modification to the engine mount. Any input would be greatly appreciated.

  2. #2
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    RE: 7.5cc in a Dumas Hawk 3.5?

    The boat wont take it, even most $100 21's would slaughter that K&B.

    Give John Knight a shout, Hes got a few hot 21's hes looking to unload right now, you can reach him here 3hydroracer@gmail.com (if it was me, ask him for the CMB 21RS motor hes holding) Iv bought 4~6 motors from him in the past 2 or 3 weeks, and i love every one of them, Best 45 iv ever had came from him, older gold head cam H motor, I got it used from him for cheap, Hes more or less a rigger expert, and predicted my mongoose with his motor and the prop he selected to run 75mph, I came back with a solid 75.2 mph and have been buying up all his motors since then, recently got a 90evo2 motor, he tossed in a 101 P/S and some spares... Take ANY advice this man offers you...
    SNT Engines

  3. #3

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    RE: 7.5cc in a Dumas Hawk 3.5?

    Thanks for the reply, but I was trying not to spend even more money. Figured I would just use what I had. I spent a crap load on another 26cc Zen powered outrigger last week. So I was just building this one for fun. Question though, If some .21 would slaughter the K&B .45 then how would this boat hold up to those and not the K&B?

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    RE: 7.5cc in a Dumas Hawk 3.5?

    the 7.5 is larger and changes the roll point on the hull, it makes then hard to drive, plus the larger prop you would need would make the boat super squirrely, post a pic of your 7.5, depending on what you have, I might swap you for a good 21. and prop for that rigger.
    SNT Engines

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    RE: 7.5cc in a Dumas Hawk 3.5?

    PM me and we can talk more
    SNT Engines

  6. #6

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    RE: 7.5cc in a Dumas Hawk 3.5?

    How do you do that?

  7. #7
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    RE: 7.5cc in a Dumas Hawk 3.5?

    ORIGINAL: mhayes462

    Thanks for the reply, but I was trying not to spend even more money. Figured I would just use what I had. I spent a crap load on another 26cc Zen powered outrigger last week. So I was just building this one for fun. Question though, If some .21 would slaughter the K&B .45 then how would this boat hold up to those and not the K&B?
    There are several issues related to installing a larger than spec'd engine in a boat. They include extra weight, extra power, and extra vibration. The extra weight can mess up the balance of the boat, resulting in trim problems. Extra power will certainly increase the potential speed, but at a cost of handling issues. It's not uncommon to over power a boat that's setup for record trials. However, at record trials, only one boat is on the water at a time, so the water tends to be pretty smooth. The extra vibration and torque of a larger boat can result in shorter hull life.

    Your K&B .45 is an older design engine. Many of the newer .21s do have considerably more power and are significantly lighter. Getting a rigger trimmed to run right can be a challenge. If this is your first, you would be much better off sticking with an engine that's in the appropriate size range. Since money is an issue, perhaps selling the .45 and putting the money into a decent .21 would be an option?
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    RE: 7.5cc in a Dumas Hawk 3.5?

    OK thanks. This is my first rigger. I do have a full mod Zen 230 powered 43" v-hull. So Im not completely new to boats, just riggers. I do not race though, so Im not worried about all the details that go into setting up a boat for that. I just do this for fun and I also like a lot of speed. You do make a good point about the weight and balance issue, I can understand that. But is there that big of a difference between the 2? And as my engine is an older design, and the newer 3.5's have more power wouldnt the 3.5 actually cause more issues with the overpowering department of it?


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