Alim Marine tunnel boat rebuild

[Frans old man]

Where to start? The beginning. First let me say to this entire site, thank you. The knowledge and expertise that I have learned and read here is fantastic and much appreciated.

I'm at a loss for what to do here. Back in January of last year, I bought a boat from an estate auction. Turns out, after some research, the hull was originally built by a company called Alim Marine from Miami, Fl. It's a 20' hull that has the most beautiful lines I've ever seen on a boat. She was upside down on the trailer with a tunnel partially completed.

Fast forward to October of last year. I started on rebuilding her to be used as my primary commercial fishing boat. Monday, I finally got everything finished and took her to the water. When I rebuilt transom and floor, i had the goal of making a dry floor boat. That came out perfect. The trouble is, she wont plain off. I set the cavitation plate flush with the top of the tunnel. When the boat tries to lift up on plain, the prop looses traction and cavitates. I know the outboard has to be lowered. What i don't know, is how much. My fix is to start cutting the transom and drilling holes till i get it right. I'm sure there is a way to get an educated guess as to how much to lower it, i just don't know how. Would appreciate if someone with any idea or knowledge could help me figure this out. I really don't want to mutilate the care and work i did to put this thing back together. Really don't have any idea what the person who started this was trying to accomplish, as they are no longer here. Really appreciate any help. Pictures attached to try and help explain the situation.

 

[bajansailor]

I see what you mean - she has one heck of a nice sheerline.

Re the 4 blade propeller shown - I am wondering if this is the correct prop for this engine? I would have thought that you would usually have a larger diameter prop than that, re the space available?

I think that your cavitation (or possibly ventilation?) problem might be due to the poor engine not getting enough water flowing into it - and if the prop is way off spec from what it should be, that won't help.
What is the vertical depth of the tunnel - I am guessing maybe 5 - 6"?
Rather than re-drilling new holes to lower the engine some more, would it be feasible to add an extension to the leg to make it longer?
I wouldn't be surprised if you have to get the cavitation plate down to an imaginary line drawn across the base of the two 'skegs' (for want of a better term) on the sides of the tunnel.
Another wild suggestion - would it be feasible to add a bracket or pod to the transom to mount the engine on, to get it further away from the transom?

Mr Efficiency should be along soon, and I am sure that he will have some good ideas to suggest.

 

[Frans old man]

No, that is not a correct prop. Was trying a couple different ones on the suggestion of a friend who thought that might cure it. But to no avail. That's when I decided to seek help here. Could put the longer shaft stuff on it, but can't source the parts. Being a brick, at 505 lbs drywieght, using a jack plate would make the bow even lighter than it already is. I will measure the hieght from what I call the stake level to the top of the tunnel tomorrow. Not just a matter of hole drilling, gotta cut the transom down too, to drop the prop down. Would a fin make any difference on th cav plate I wonder?????

 

[Mr Efficiency]

Is that pocket ventilated ? I can't see a vent. If not, that is the first job. Yea or nay, the outboard seemingly will need to be dropped lower, water won't rise to the necessary height in that short space. There are are propeller options that might assist, is that 4-blader stainless ? In compromised situations you don't want alloy props.

 

[fredrosse]

The tunnel is formed with a rather abrupt transition, and needs to be made with a better streamlined path along the centerline of the boat. That is just my opinion, but is based on more than 20 years as an engineer working in fluid flow around various obstructions. I hope the two attached sketches illustrate the problem.

Lowering the prop would improve the condition somewhat, but the existing tunnel shape will seriously impede hull planing performance.

 

[fredrosse]

Sketch of hull section, blue arrows illustrate fluid flow.

 

[Mr Efficiency]

Half the prop will be working in turbulent water being dragged along by that pocket, until the boat planes, when you are trying to plane, that is the point of greatest prop "slip", and cavitation most likely. Vent the pocket, lower the motor, and use a stainless prop, and better still one that hangs on to the water better than average.

 

[baeckmo]

That "tunnel" simply should not be there. There are two ways to attack it. The best is to rebuild the cut-away bottom panels so that they meet at the CL, as a continuation of the existing keel line. I would cut a 4 mm alu panel, bend along the keel line and fit by screws. Carefully trim down forward edges to a smooth fillet. You may just leave it open at the transom.

Next best is to fit a horizontal pad instead of the V-shaped panel, but that is sensitive to the resulting angle of attack. But if you provide the dimensions of the present cut-out, I can give more precise advice. If you feel more comfortable with glass, the flat pad probably needs a central stiffener to the tunnel roof. Alu panel will do without if given a slight V or a big radius.

This kind of tunnel/pocket has been tried in order to reduce draft, but it is very sensitive to shape and prop shaft location and ventilation at different speeds. In theory, with the pocket properly ventilated (a science on its own), the antiventilation plate would be placed at (or slightly below) about the horizontal line between the aft outer edges of the tunnel sides. It requires a propeller with a substantial trailing edge cup to work.

 

[Mr Efficiency]

That is the only reason, reduced draft, about a foot.

 

[Mr Efficiency]

Ventilate that cubby hole, borrow a jacking plate, and fit a cupped stainless prop, that way you find out what the highest point is you can run it at. Gotta say, that boat looks like it would be quite tippy.

 

[Mr Efficiency]

Actually, the first thing I would do, is fit a decent prop, the one fitted looks like an alloy prop, that is far from ideal. It is possible the right prop will pop it up on plane, where it currently is, though to the eye it looks too high, if you were continually stopping and starting, it would be best to seek a more permanent solution.

 

[Frans old man]

WOW! Thank you all! I should have known to do a bit of research before trying to finish this thing. Ok. Yes, the reason I suspect this was originally started was for shallower draft for flounder fishing here when we used to be able to do that.
I had my suspicion that the shape wasn't correct from the beginning, but thought I might be able to work it out.
It sounds as if simply lowering the outboard will not "fix" my issue.
Holy crap! Not the first project i ever did that was junk in the end.
On the positive side, it does float, and I did learn a lot.
Gonna take a minute and digest this to see if I can understand how to be able to save this thing. Thanks again to all who replied.

 

[bajansailor]

Please don't get too despondent Frans - where there is a will, there is a way.....
You could perhaps try Mr E's suggestions above first, as they would be the easiest to do.

I have no experience of Doel Fins, but I am sure that others on here have - they are not too expensive, so it might be worthwhile trying one?
Or do those with experience say 'no way' in this situation?
Doel-Fin Stabilizer Fin, black | Davis Instruments https://www.davisinstruments.com/product/doel-fin-stabilizer-fin-black/

Failing the above, you could try Baeckmo's suggestion of 'filling in' the tunnel with a shaped aluminium plate as per his description above?

Re ventilating the tunnel, Cajunpockettunnel has posted some photos of the vents that he built on the tunnel on his skiff - would something like this work on Frans' boat?
Pocket tunnel bateau https://www.boatdesign.net/threads/pocket-tunnel-bateau.64358/page-5

 

[Frans old man]

I've been looking, reading, and thinking all day. Thanks for the encouragement sir. When I put so much of me into something, hoping and praying it works, when things don't, I get really aggrivated. I should have followed my gut in the beginning, and asked prior to starting this......but I ain't that smart.
Now that I've regained some of my senses, I'm gonna fix this somehow. Mr. Efficiency suggested venting the tunnel. I'd never heard of such a thing, but after thinking and reading today, I believe I have somewhat of an understanding of what that is.
Baeckmo suggested what I contemplated in the very beginning, put it back. Since the outboard I have is a 20" shaft, i decided against it. Trying to maintain some semblance of a budget for this, i couldn't justify that expense.
So, I'm gonna try first to lay some parameters that i think i require to get this thing functional, get a plan together, and start the changes. Any thoughts or advice would be greatly appreciated.
As i understand things I've read, I'm trying to drag an upside down bowl through the water, all the while trying to lift it up out of the water. I used to play with a cup in the tub many years ago as a child. I remember how hard it was to pick up.
My requirements for this are pretty simple, first, it must be functional, not perfect. I need the boat to plain and handle safely in the water. I don't need the absolute perfect blend of speed and efficiency. This thing is my work truck for my work on the water. I work year round, cold and hot. Swimming in January is not an option. Second, I want to do this with what I have. My meager budget has been tapped to the max doing this. I'm not complaining, nearly stating reasoning behind my choices. Third, I'm trying to keep from reworking the things I've already done as much as possible. And last, I'm going to not put any more holes in this hull than I have right now.
This boat on any given day is subject to run empty to a 2000 lbs load of gear and fish through a given day.
I don't really think there would be a practical way to accomplish the engineering/modifications for what time and budget I have. I have to get back on the water and back to work.

So, here is my idea to fix/help cure this. Any constructive help is greatly appreciated. If nothing else, it'll be something for someone else not to try.

So first, I'm going to split a length of 3" PVC and glass it to the inside top of the tunnel to end somewhere forward in the tunnel. My thinking is this should help with the vaccum created in the front of the tunnel.

Then I'm going to run the boat to see what change occurs with my stainless 3 blade 17p prop. Pull the boat out each time to change props with 4 blade props. One is 17p, theother is 19p.

If I need to, I'll cut the transom down to be able to adjust the prop further in the water. Right now, the cav plate is even with the top of th tunnel and the prop center is at the bottom side stakes.

Good plan? Bad plan? Any thoughts??? Thank you all for the help to this point.

 

[gonzo]

It shouldn't be too expensive to cut the forward end of the tunnel and make it more streamlined. I don't know how this would work, but you could try a scoop at the forward end of the tunnel. I am thinking about a flat piece of aluminum across the tunnel. It wouldn't break anything and it is really cheap to try.