Tunnel for LM18

[fallguy1000]

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.

[fallguy1000]

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.

[TomW1]

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

[Ryan]

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).

[jacquesmm]

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.

[jacquesmm]

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.

[Ryan]

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.

[TomW1]

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

[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?

Thanks again!
~Ryan

[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