LM18 Tunnel

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PortA Brad
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LM18 Tunnel

Post by PortA Brad »

My son and I are considering building a LM18 with a tunnel. Can anyone tell me the hole shot with a tunnel and with out a tunnel? We fish the central coast of Texas and there are a lot of really shallow flats we will be fishing and are trying to figure out the best boat to build for this area.
Thanks for your help.



TomW1
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Re: LM18 Tunnel

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Hole shot depends on HP and the weight of the boat and load. But with a tunnel you will need a jack plate and start with the motor lowered at the normal position and as you get up on plane you will raise the motor up to the hump that the tunnel provides. So if you put the 30 or 40HP the max allowed for the LM18 and the right prop you will be up on plane quickly and can raise the motor almost immediately. I would learn how to do it in clear water before taking it into the flats so you are comfortable. I would be glad to work up any weight numbers for you on top speed and the prop you need also.

I think the LM18 is exactly the boat you are looking for, there in Texas. Several have been built. I don't know what motor you prefer but the Merc 30 and 3 cyl. 40 weigh less than the equivalent Yamaha's.

fishgitr in Power Boats is building a LM18 you might want to chick it out.

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

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cape man
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Re: LM18 Tunnel

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A hydraulic Jack plate works in the opposite of what you just posted. With or without a tunnel, at rest, the boat sits lower in the water so you start with the jack plate raised, and lower it as the boat comes up on plane. With a tunnel you don't need to lower it much, if at all as the water will rise up as you gain speed. Most commercial boats here in Florida (crab boats, clam farms) using tunnels don't use jack plates because of this.



A tunnel will allow you to get up and run in shallower water than no tunnel, but you will lose speed and fuel efficiency. A 4-blade prop with some decent cup will also help a lot with or without a tunnel. I don't think there will be any significant difference in the speed of the hole shot, just the depth that you pull it off in.
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swglenn
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Re: LM18 Tunnel

Post by swglenn »

My son had a commercially built fishing boat with a tunnel. As described by Cape Man he would raise the motor up into the tunnel until the foot of the motor was even with the bottom of the boat so there was nothing to drag. As the boat came out of the "hole" and the hump formed behind the boat he would gradually lower the motor. If run too long with the motor raised up into the tunnel the motor would get hot because it was getting insufficient water. He could run in 4" of water once the boat was on plane and get up in 8" of water. With a 150 HP motor he was limited to 38-40 MPH due to the tunnel hydraulic drag.

PortA Brad
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Re: LM18 Tunnel

Post by PortA Brad »

Thank you very much for the great information. I have a good understanding of how a tunnel works, as our current boat has a tunnel. We will be fishing flats that can have 10" of water then on a falling tide can go to 6" or less. Assuming a 40hp motor and full load, does anyone know the minimum depth that a LM18 with tunnel can get on plane with out scaring the bay floor? Being able to cruse the shallows will allow us to avoid the channel, which can get rough in wind and ship traffic.
Thanks,
Brad

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cape man
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Re: LM18 Tunnel

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I don't know if anyone has built the tunnel version yet.
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TomW1
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Re: LM18 Tunnel

Post by TomW1 »

cape man wrote: Wed Apr 07, 2021 6:04 am A hydraulic Jack plate works in the opposite of what you just posted. With or without a tunnel, at rest, the boat sits lower in the water so you start with the jack plate raised, and lower it as the boat comes up on plane. With a tunnel you don't need to lower it much, if at all as the water will rise up as you gain speed. Most commercial boats here in Florida (crab boats, clam farms) using tunnels don't use jack plates because of this.



A tunnel will allow you to get up and run in shallower water than no tunnel, but you will lose speed and fuel efficiency. A 4-blade prop with some decent cup will also help a lot with or without a tunnel. I don't think there will be any significant difference in the speed of the hole shot, just the depth that you pull it off in.
cape man you as usual are totally wrong as the hump builds behind the boat you raise the motor into it. When a boat is properly loaded a boat will sit on her lines and not stern down. You get your speed up from the motor being down and then raise it. I don't know why you don't understand basic hydro dynamics. If the motor is above the tunnel it has no water to push the boat with. :doh: The prop will cavitate as it will be grabbing air. Ask Richard how he works his jack plate. Also a jack plate can only lower a motor a couple of inches while it can raise it 5-6".

I agree that there will be a minor loss of speed because of the tunnel, but as a narrow boat it will be minimal.

Keep well guy.l

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

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cape man
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Re: LM18 Tunnel

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As usual I'm totally wrong? I don't understand basic hydro dynamics?

Tom, Tom, Tom. Read swglenn's post again. That's from someone using a jack plate and tunnel. I know you went after me as I was the one to correct your post, but are you also willing to sit in your chair in the mountains and tell him that he's been using it wrong?

I've been running a hydraulic jack plate for over 20 years, on three different boats. With or without a tunnel it is the same principle. I don't have to ask Richard, as I have fished in his XF20 many times. Have you ever used a jack plate? They can go up 5 to 6", but only 1 to 2" down?

If I'm wrong, it wouldn't be the first time, but I think you may be the first to say I'm usually totally wrong.

PortaBrad, sorry to muddy up your thread here. I have a LM18 hull I bought from another builder waiting for me to finish, so I am always watching here for other LM18 builds. It has some hook in the bottom and one idea I'm tossing around is to perform some surgery and add a tunnel. Like I said, I don't know if anyone has built the tunnel option yet, and when he designed this boat Jacques was shooting for a wider boat than the FS18 that would accommodate a tunnel. If you are hoping to run in 6" I think you are going to need one.
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TomW1
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Re: LM18 Tunnel

Post by TomW1 »

Yes cape man you are wrong in saying that a jack plate works in the way you say, it cannot be lowered. It is always designed to lift the motor up into the hump of the tunnel. I just talked to a Bob's Jack Plate technician at 3pm and confirmed what I knew that you cannot lower a jack plate, it will stop even at the bottom. You can raise it but not lower it. So once again you are wrong. You know nothing about propping a boat except by guess, you have just shown you know nothing about jack plates. A jack plate works by the prop being raised into the hump coming out of the tunnel as it comes up on plane, it cannot be lowered.

Quit giving people incorrect information. I have no respect for you when you do this. I would very much like to give you my respect but I can't when you pull this stuff. People have spent $1000 + using your method of finding a prop one spent $2500. And yes I have been on tunnel boats with much more HP than you have been. 10" set backs with 250 to 300HP motors 60+mph in bass boats are quite common in the Midwest where I grew up and speeds much more than your flats boats in Florida ever experience.

How about we just cut down the rhetoric as I have talked to Bob's and they confirmed my operation of a jack plate.

Good night and sleep tight.

Tom

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

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cape man
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Re: LM18 Tunnel

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Absolutely no comment. :lol:
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