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Hey all, I’ve been combing the forums and can’t find where anyone has built the LM18 with the tunnel? Seriously entertaining this before I get any further in my build…. Any thoughts? Probably going to be repurposing a 40hp Yamaha 2stroke(tiller) on a micro jackplate, with trim tabs. I’ve been looking at those compression plates - on the cavitation plate - that many of the tunnel hull guys run in the gulf, thinking that might be an efficient option if I decide to go this tunnel route?

~Ryan

You'll be the first if you build the tunnel version. Jacques partially designed the LM18 to be wide enough (versus the FS18) to add a tunnel, but due to its already shallow draft and the loss of performance a tunnel adds, no one has made one...yet.

The 40, 2 stroke will more than compensate for the performance drop. Go for it!

I think I’m going to! Originally I really wanted a tunnel hull (prior to buying the plans) I realize the pros and cons. A little nervous to be the Guinea pig. Lol. Never quite seen a tunnel designed like that, it does make sense though -tapers up to 2-7/8 then back down to 1-3/4 similar to an air foil, I’m assuming this is to get the water to speed back up…

What is the weight of the 40 stroke? I have a 50 on my LM18 same weight as a 40 in the 4 stroke world. It does change the attitude of the boat at rest. understanding that the tunnel will drop the tail in the water even more plan to add batteries and gas in the front to balance. For me the added weight forced me to go up 2 or 3 holes in the mounting of the motor, so having a jack plate will be key. Plus it will be good because the gear box on the 40 is much bigger than the 30's. I say go for it, I kinda wish I would have. Also, I raised my sole 1", if I were to do it again I would raise it 1.5 inches for proper drainage with more weight, as I run heavy, trolling motor+ batteries, 250 lbs 50HP, 18 gallon livewell, 9 gallon gas tank, etc.

cbuf

Good info cbuf. I’m trying to keep it light, but jackplate, trim tabs, 12v trolling motor, 10-12 fuel tank, etc. not going to get fancy with a bunch of hatches. Hopefully this will save weight! That 40 2stroke is roughly 190lbs. I’m about 98% certain I’m committing to the tunnel! I will keep in mind raising the sole slightly. I’ll keep everyone updated, some more cutting when I get off work. Hopefully this Christmas break will give me some time to get some good progress done.

ryan looking good. just follow the build and attachment instrutions for the tunnel and you will be fine. If you have a 90 2st you are saving 20 to 30lbs over a 4st based on Yamaha specs. Adding batteries, gas and trolling motor forward will offset your offset the weight added at the rear. Your build is on track

Keep the good work up. Tom

Here’s your LM18 with tunnel. Port Aransas ,Tx

Sorry the pics flipped. Jacques designed the Lm18 after taking some lines off a skiff built here at. Farley. This particular build has dropped the sheer some. The tunnel was built making parallel cuts on the hull bottom from the stern towards the bow. Notched the transom and then bent this “slot” to join at 6” depth. Builder has expert knowledge and assistance so did not post the build to this forum. I am sure the builder would entertain questions. Merry Christmas to all

Dr Pete thea is a nice looking LM18, a small console, jack plate and alll Very, very nice. Do you know how much he reduced the sheer by?

Tom

Sheer was reduced May be 2” midship. Towards the bow it was tapered up to near original line. At the stern an inch to inch and a quarter

Wow that tunnel is huge! How does it perform? Boat is beautiful by the way!

~Ryan

Not really Ryan looking at the prop, it is propably 2.5,3 x 10,12i If you click on the poc you will get an righside up photo the will let you enlarge it.

Tom

Got a question for the experts here! I am not going to put a skeg in the center of the boat per plans (since I’m doing the tunnel). I was wondering if I were to run two 1” tall strips on either side of the tunnel- if that were to negatively effect anything? Also how far from the tunnel do you recommend and and how long?

I would be a bit worried about aeration from the runners. But Jacques is the best man to answer.

The LM18 is not made for speed and does not require skegs. With the tunnel there really is not a good place to put them on a narrow boat like the LM18. Instead install reverse chines. They will do will two things reduce spray and help in turning. The negative is that the reverse chines may increase slapping a little bit but not much since they are an 1-2" below the normal chine.

Tom

Thanks Tom! I think I’ll go with your suggestion and just not put anything on it. It’s definitely going to be no speed demon with a 40!

~Ryan

I would probably add the skegs if I were you, just based on my experience with my FS14LS with no skeg(top speed around 25mph). I'll probably add a skeg or strakes at some point, other priorities at the moment.

Isn't Barraman running this boat/sim?

My lm18 runs 34mph with a 50 you will probably run around the same with a 40 because your planning lighter than I built. I'm not sure what the answers is for the skeg with a tunnel, but when you have a small boat running 30+ it is a lots faster than you think. Also, when I turn at speed, I couldn't imagine not having a skeg especially when it is bumpy because the back 6-8' of the boat is basically a flat bottom boat. Maybe two strakes, to not interfere with the tunnel. The chine might work, but it would have to be a pronounced.

Hey CBUF do you have any videos of your boat running? I’d like to see how this boat turns.

Cbuf wrote: ↑Tue Jan 18, 2022 10:43 am My lm18 runs 34mph with a 50 you will probably run around the same with a 40 because your planning lighter than I built. I'm not sure what the answers is for the skeg with a tunnel, but when you have a small boat running 30+ it is a lots faster than you think. Also, when I turn at speed, I couldn't imagine not having a skeg especially when it is bumpy because the back 6-8' of the boat is basically a flat bottom boat. Maybe two strakes, to not interfere with the tunnel. The chine might work, but it would have to be a pronounced.

The chines are a bit of a gamble sometimes.

I would make two beaching strakes. Determine where the boat hits the beach and they will double as bottom protection and prevent side slip.

Probably in the neighborhood of 1.5" wide and 1/2-3/4" thick in the y plane.

Another thing you can do if you are worried is to make the chines non-tripping. This is easy and part of the necessary build process. You would simply avoid sharpening them back. Not sure what Jacques says about sharpening chines on these.

The best thing to do is look on the plans. Jacques always designates if the boat needs a keel or skegs added to the bottom, it should be on the Building Notes sheet or any ot the sheets before flipping. For small boats with moderate HP Jacques has said that strakes are not needed. The addition of a tunnel always is a compromise as Jacques explained before. If you do put on strakes his preferred method is to take a 2x2 and cut it on the diagonal, so you have two equal sides and a wide gluing side and an easy to shape front. He also doesn't think you need strakes until you reach speeds in the 40's.

Well unless Jacques specified them in the plans, don't put them on, they will mess with the tunnel. My two cents worth a penny tomorrow.

Tom

I appreciate it guys! I’m going to leave them off per the plans… I’m not anticipating going over 40hp unless I can score a 50 Yamaha 2 stroke for a steal! I really could care less about top speed but I’d like to be able to take 3 people and run shallow (also get on plane shallow).

fallguy1000 wrote: ↑Sun Jan 16, 2022 5:30 pm I would be a bit worried about aeration from the runners. But Jacques is the best man to answer.

Correct. It would disturb the flow of water to the tunnel.

Ryan wrote: ↑Tue Jan 18, 2022 8:29 pm I appreciate it guys! I’m going to leave them off per the plans… I’m not anticipating going over 40hp unless I can score a 50 Yamaha 2 stroke for a steal! I really could care less about top speed but I’d like to be able to take 3 people and run shallow (also get on plane shallow).

If speed is important, skip the tunnel.

Too late! The tunnel is in… How do you think this boat would perform with a 30 and a tunnel? Not concerned with top speed but I’d like to be able to run shallow and still plane out easily.

Ryan, help me with the weights and I can give you an approximate top speed with a 30HP and a 40HP by using my calculators. I will need a bunch of info so here goes:

Weight of finished boat
Which motor brand 30 40 I have all the weights by year of all motors so can look up the weight unless you buy one and know it.
How much gas
Gear, hardware, cables lighting, safety equipment, hinges, etc. I use a number for a small boat like yours of 150 unless you have a console then 200lbs.
Then people 3 at 175 525 less if smaller more if bigger
Fishing gear and a cooler weight 100-150
If you are going to have a livewell then 10 lbs per gallon.

This will give me the top speed you can obtain, and the second part of the calculation is a prop calculation, and I can give you a prop size estimate.

Well think about it. Done it for a bunch of guys here. I will recommend that you put some money aside for a powered jack plate.

Tom

Thanks Tom!

I’m going to keep this boat relatively light…
-basic hull weight plus the weight of a small center console
-still unsure of which motor I can procure at this time the only reason I’m considering a 30 is because they seem to be more readily available than 40s. So roughly 180-220lbs seem reasonable?
-10 gallon gas tank in the front
-525 would be probably pretty accurate for 3 people
-No livewell and a small yeti roadie cooler/front seat
-No fiberglass hatches
-definitely going to order a jackplate, I have an atlas jackplate on my bay boat now and love it. I’ve had a Bob’s in the past and it was good as well.

-From everything I’ve read the tunnel is going to create quite a bit more drag and be substantialy slower than the non-tunnel version.

Should I do a 6” setback jackplate or a 4” not quite sure if the tunnel will perform better at 6” or 4” back?
I’m assuming a pretty aggressively pitched 4 blade would be recommended?

Thanks again!
~Ryan

Thanks Tom!

I’m going to keep this boat relatively light…
-basic hull weight plus the weight of a small center console
-still unsure of which motor I can procure at this time the only reason I’m considering a 30 is because they seem to be more readily available than 40s. So roughly 180-220lbs seem reasonable?
-10 gallon gas tank in the front
-525 would be probably pretty accurate for 3 people
-No livewell and a small yeti roadie cooler/front seat
-No fiberglass hatches
-definitely going to order a jackplate, I have an atlas jackplate on my bay boat now and love it. I’ve had a Bob’s in the past and it was good as well.

-From everything I’ve read the tunnel is going to create quite a bit more drag and be substantialy slower than the non-tunnel version.

Should I do a 6” setback jackplate or a 4” not quite sure if the tunnel will perform better at 6” or 4” back?
I’m assuming a pretty aggressively pitched 4 blade would be recommended?

Being able to plane the boat with 3/4 of the prop in the water would be the ultimate! I’ve seen this on other tunnel boats. You think this is obtainable?

Thanks again!
~Ryan