Alim Marine tunnel boat rebuild

Apologies Frans, but you lost me there re your proposal to glass a half-round tube along the top side of the tunnel - what is this supposed to do?
How is it going to help with the vaccuum created in the front of the tunnel?

Gonzo's suggestion above sounds good. It should be possible to do this fairly easily.

You mention a payload of 2,000 lbs - that is 10 big guys sitting on the deck. I doubt that she would want to plane in this condition, even with 130 hp on the transom (?)
I did a very rough calculation, and I would estimate that the boat will sink by approx 5 - 6" with this extra 2,000 lbs of weight - bear in mind that it will affect the stability as well.

 

Gonzo, I thought about that, and decided against that because of the trailer bunk resting on the keel area to support the loads I expect to put in this boat.

 

Bajonsailor, I don't intend on planning off with that much weight. I know what that kind of load does to a boat and the handling. I was merely trying explain my use, not expected performance in all situations. She is a work boat, not a pleasure craft.

 

With a "nozzle"(for lack of better term) to draw water or air from the transom area to the front of the tunnel, it should reduce, or break the vaccum at the front of the tunnel.

 

Looking at the pictures, and they may be misleading to the eye, the cavitation plate appears far too high to be a success, regardless of that suction effect. The length of that pocket seems not to be much, a couple of feet maybe, you can't expect water to rise up that high, in such a short distance, you might get 5" tops. Has that boat been running a 25" leg motor ? It appears to be about what would be required, if what you have fitted is a 20", but of course you could fit a pod and get it lower that way. The way I see it, when you are trying to plane the boat, the top half of the propeller disc is working in "stagnant" water initially, and if it does get to the point where the hull "breaks out", a significant part of the disc will be in the air, both modes lend themselves to cavitation/ventilation. The motor looks too high, the reports given re-inforce that idea.

 

I recently looked at a boat that had twin 20" leg motors, on a nominally 25" transom. The owner claimed it ran OK, but pulled heavily to one side, and needed hydraulic steering. I say nominally 25" because the actual height was about 22.5 inches, you could mount 25" motors OK, but the engine bracket stood high above the transom ledge. The idea was that it could be cut down for 20" legs, and still be able to be tilted up without bottoming out in the engine well. Assuming that boat actually did run OK with the cav plates 2" high, and that is possible with cupped stainless props, it shows it immediately creates other problems, in this case steering issues, and the lost thrust from not having the entire propeller disc in the water.

 

Mr. E, I measured the depth of the tunnel at 36". I looked at what you had said about the venting and believe i may have a cure, even if temporary. I'm fairly certain after some research today, I'll end up cutting 3-4" out of the transom. The stern hieght from water level to engine mount currently is 18.5". I have enough fretboard aft to do that without too much over wash in the stern. The biggest thing I don't know at the moment is where to end the pvc channel in the tunnel. Even if I have to channel it up the stern slightly, I don't know where the most effective place is to end it in the forward part of the tunnel. Or, is it a matter of trial and error?

 

The boat was originally an inboard/outdrive. The company went out of business in 1968, the year I was born. She's an old boat. Welcraft allegedly purchased thier molds. Anyway, just trying to make her functional, not perfect.

 

If you do cut say 4" out of the transom to lower the engine, it would be worthwhile to test that arrangement first before committing yourself to glassing that ventilation tube on inside the tunnel.
You initially showed a half round tube in your sketch, but in post #22 you now mention a pvc channel?

If you do lower the transom, it would probably be worthwhile building a splash well on the forward side of the transom, to contain any waves that come over the stern.

 

Ok, Mr. E. Mentioned ventilating the tunnel first. The half round piece was denoted as 3" pvc halves in my sketch. Just intending on putting a layer of glass over it to hold it. Hope I don't have to have a splash well as she is a dry boat with two 2" scuppers, but we will see. So your suggestion is to lower engine first? Six of one, half dozen of the other I guess.

 

You could run a 2" PVC pipe from the forward end of the pocket, out the back and then vertically up the transom. But whilst that may break the suction effect, if the motor is too high, it won't help you. If it is three feet long, you can probably say the cav plate needs to be around 8" higher than the continuation of the bottom apex, if that pocket was not there. It looks to be way more than that.

 

That, will be my starting point then. Yes sir Mr. E., the cav plate is currently 13" above that point. If I had a 25" outboard, that would be a simple fix. However, I don't. So, 5" out of the transom still leaves me with 13.5" of stern hieght. Not what I wanted, but it would do. Got a pretty day to fiberglass. Guess I'm cutting the transom. Gotta do something, it doesn't work the way it is. I'll let you know about the mods effects. Thanks again for the help and ideas. I wouldn't have had a clue.

 

You don't need to cut the transom, just step it (the motor) out and down with a bracket of some kind, You might pick up something simple and cheap at a boat wrecking yard. You may need to go down further than that 5 inches, chopping into your transom is not really advisable, you increase the risk of taking water over the stern in bad conditions.

 

Yes sir, you are correct. Used to run an 18' boat with this same 500 lbs pig on it. Water over the stern almost killed me last January. I don't get scared on the water much. That boat had a mere 8" of freeboard in the stern. The whole reason for this boats existence in my world. She is a dry boat. 13.5" is a lot better.

 

Well folks, thank God, motor complete and reinstalled 5" lower. Seatrial number 2 commencing at noon tomorrow. Will update performance when we are done. I will say, it does look a lot more properly placed on the boat now. And yes, will change out to the stainless prop in am before getting wet. Thanks again to all who voiced ideas and opinions. Really nice to have a place to come to for help. Hopefully she will work better.