Mm21 motor

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Jmk2000
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Mm21 motor

Post by Jmk2000 »

Question for the designers. So the internet info says 6-8 hp and the in the drawing I believe it says up to a 15 hp outboard. I was thinking a 9.9 would be perfect. Thoughts? My last question is shaft length. It’s says a to use a Long (20”) or extra long (25”) shaft is even better. Is there a performance difference? It seems from the drawing that the extra long shaft would be longer than the skeg. Just looking for any input thanks!

Josh

Fuzz
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Re: Mm21 motor

Post by Fuzz »

8-15 hp I do not think it will make much difference as long as it is a twin. As for how long of shaft length I would get the longest made. Longer lets you get the power head more out of the water but the big thing is getting the prop deep enough.
I have seen outboards mounted so that in any seas the prop broke out and made the motor useless. This is not a planing hull so deeper is better.

TomW1
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Re: Mm21 motor

Post by TomW1 »

Yaamaha has a 9.9HP Hi-thrust with a 25" shaft. The Hii-Thrust will give you more power than a regular 9HP motor. Also, if you have room in the motor box, I would add a 4' jack plate so you can adjust the height of the motor prop if needed.

Tom
Restored Mirror Dinghy, Bought OD18 built by CL, Westlawn School of Yacht Design courses. LT US Navy 1970-1978

rick berrey
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Re: Mm21 motor

Post by rick berrey »

It is only calling for a 6 to 8 hp , I would look at a high thrust 9.9 with a jack plate . I am not sure why a jack plate is not standard on every build .

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Re: Mm21 motor

Post by pee wee »

rick berrey wrote: Tue Feb 07, 2023 10:47 pm It is only calling for a 6 to 8 hp , I would look at a high thrust 9.9 with a jack plate . I am not sure why a jack plate is not standard on every build .
For a planing hull, jack plate and trim tabs are ways to improve the motor setup or deal with different conditions, but probably not needed for a displacement hull.
Hank

TomW1
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Re: Mm21 motor

Post by TomW1 »

peewee the reason I recommended the Jack plate was that it would allow him to adjust the prop to get the right height for max performance. I would recommend a 4-blade prop to allow better maneuvering, straight line cruising, curve holding and better holding in nasty weather. Here are 2 PT props that should work, https://www.ptprop.com/a-class-propellers1.html

Tom
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Jmk2000
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Re: Mm21 motor

Post by Jmk2000 »

Thanks for the replies fellas. I was planning on a high thrust model in a 9.9. I priced both the Suzuki and Yamaha remote controls and with trim tilt. The Yamer hammer runs about 1k more, but both were high thrust with 4 blade props 25” shaft motors.

Doug N
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Re: Mm21 motor

Post by Doug N »

Hi Josh. On my hmd19, Evan specified the 25" shaft and it put the prop below the level of the skeg. I believe that he wanted to be sure that the prop didn't come out of the water in rough weather. I think that there may be enough similarities between the designs for this to possibly apply. Here is a copy of what Evan said when I was asking about the shaft length.
'20" is o.k. for lakes, though it is not preferred for larger bodies of water where wave action makes a shallower prop more likely to ventilate.'

Doug

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Re: Mm21 motor

Post by TomW1 »

Jmk2000 wrote: Wed Feb 08, 2023 6:20 pm Thanks for the replies fellas. I was planning on a high thrust model in a 9.9. I priced both the Suzuki and Yamaha remote controls and with trim tilt. The Yamer hammer runs about 1k more, but both were high thrust with 4 blade props 25” shaft motors.
The only Hi-thrust motors available are the Yamaha (2.92gr) that you found and the Mercury 9.9 25" CT (2.42gr) The Suzuki as far as I could find has only a 2.08 gear ratio, the same as their other small motors and some of their larger motors also. A higher gear ratio allows the motor to push more weight for the HP or put a larger prop on for more speed. In this case it is the former. In any case check the gear case ratio before buying the Suzuki Plus the Suzuki weighs 12- 17 more than the other two. My advice would be the Yamaha as of right now.

Tom
Restored Mirror Dinghy, Bought OD18 built by CL, Westlawn School of Yacht Design courses. LT US Navy 1970-1978

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Re: Mm21 motor

Post by Evan_Gatehouse »

Yamaha 9.9 is fine. Longer shaft is to keep the prop immersed in rough water. The skeg is more for directional stability not to deflect debris from the outboard (hits the hull and bow first) but you can always make the skeg deeper.
designer: FB11/GV10,11,13/ HMD18/
SK17,MM21/MT24

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