LM18 Tunnel Build - NC

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Jerid Ray
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Re: LM18 Tunnel Build - NC

Post by Jerid Ray »

I also disagree with the 70hp choice as well. The awesome thing is those controls/wiring will work with a 50 or 40 hp two stoke. Ask me how I know!!! Also that 70 is a monster at 228lbs!!!!! The Tohatsu brand motors are the lightest on the market the 40/50hp weighing 209 lbs. The 30 weighing 157lbs. Having so much weight in the back will start making your skiff like a production skiff....stern fat and unbalanced. Not so awesome. These boats are meant to be light. He calls for a 40hp in the plans as the largest.

I went with a 50hp yamaha two stroke (189lbs) on my LM18 tunnel after talking to a guy with a 50hp Tohatsu on his. He is my size (6'5" 230-250lbs) and said it was a great matchup for his LM18 tunnel. But if you talk to Reid at BBC he will say that his 30hp on his LM works just fine.

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Re: LM18 Tunnel Build - NC

Post by TomW1 »

Ryan there is more than just putting that 70HP Yamaha on the transom. You will have to add extra strength to the transom, add an extra layer of fiberglass to the bottom. I ran the 70 through my calculators and your top speed will be in the high 40's, speeds the LM18 was not designed for. Jacques recommends motor HP for a reason. Ask Jeff to ask Jacques for the specs to update the transom and bottom for the 70HP. You could get by with a 50 but that is all. For example, that same Yamaha 2st 70HP pushed my bigger OD18 to the mid 30's top speed with 500lbs of passengers on her. by adding the extra weight of the transom mods and fiberglass mods plus the extra weight of the 70HP you will not be able to get as shallow as the LM18 was designed for.

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

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Re: LM18 Tunnel Build - NC

Post by jacquesmm »

70 HP is a lot. It will be OK for the transom if you keep it running low but at WOT, the boat may be difficult to control. As some have said, the extra weight will bring the transom down and increase draft.
The recommended HP is 25 to 40. With a 40, the boat will plane with 3 persons and gear.
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Re: LM18 Tunnel Build - NC

Post by fallguy1000 »

70hp will probably result in some major chine walk which will be terrifying and hopefully not toss you. Make sure you vest for sea trials. After experiencing the wot, you can always adjust the throttle down for safer operation.
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Re: LM18 Tunnel Build - NC

Post by Ryan »

Hey all! It’s been a long time, my daughters travel softball and work has kept me pretty wrapped up… knocking the dust off the boat this weekend to continue progress! My thought with the 70 and the tunnel was to go for a pretty aggressively cupped 4 blade for plane-ing out fast and running shallow long distances. If it becomes too much I’ll just sell the 70 and buy a 40 or 50.. I couldn’t pass on the deal at the time. I’m Definitely never going to run this boat wide open, as I never do my bay boat either.

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