Bay Boat - Inboard - adding a tunnel to an inboard ski boat

[cdre]

Looking to build a gas inboard to be able to sneak up on Tarpon in rivers and coastal areas in Belize.

Thinking an old ski boat would be a great platform to start with. There's old ones around, they roughly fit the bay-boat profile, range from 18-22' and they already have gas inboard setups.




I'd like to cut out the covered bow area to add a casting platform, move what is typically a stern fuel tank to the mid-ships bilge, add a center console, and raise the prop into the hull a bit more for protection.



This "prop raising" is the element that is probably the least straightforward. My first inclination would be to cut out the center of the hull from transom forward to the shaft seal, adjust motor mounts to to achieve the level of "tuck" desired, and lay strips of wood and re-glass tunnel. I'd probably have to do some shifting around to get the rudder perpendicular again, but doing a solid cut would keep the strut and seal aligned.

How far forward should I go with the tunnel to prevent poor flow over the prop, and how high can I get away with tucking it? I was thinking something along the lines of this Shamrock boat...



I've done fiberglass work before on rotten floors, but this would be my first serious project. Looking for any feedback on my concept.

[tunnels]

The longer and more fair the forward part of the tunnel the better . I would try to get the tunnel right to the back end of the motor and gear box. and fair as possible . The pocket where the shaft exits is a place for turbulance so dont have that !!Your should be able to simply use the existng rudder and just raise it inside the tunnel . Even to the point of using the same strut prop and shaft including the flange that attachs to the back of the gear box . Would just need to realign the motor when you get it all done and do some mods to the exhaust pipes and most everything else could be very close to fitting the same as before .
The tunnel could be made before hand on a work bench over a mould and then transplanted into the boat as a fully completed item with everything in it .
First take some measurements and lenghths etc and then Draw it full size on a drawing board so you can see exactly what you have and need to do and how big it will actually be. Tunnels are inclined to make the boat sit slightly lower in the water as the buoyancy has changed and the hull may need a pair of trim tabs to get it up a little when you are on the plane . you can simply build a very small amount of turn down within the tunnel roof . but only small !!

[cdre]

Great reply! Thanks. I especially like the idea of a little turn down to help give the aft end a little up-force.

Any performance concerns I should be worried about here? Cavitation issues? Relative to the surface of the water, how high can I bring the prop before i have those kind of issues (if they exist).

Would it be unreasonable to try to bring the prop almost all the way into the hull? That is to exceed what the above example has and have over half of the prop in the tunnel?

[tunnels]

Originally Posted by cdre

Great reply! Thanks. I especially like the idea of a little turn down to help give the aft end a little up-force.

Any performance concerns I should be worried about here? Cavitation issues? Relative to the surface of the water, how high can I bring the prop before i have those kind of issues (if they exist).

Would it be unreasonable to try to bring the prop almost all the way into the hull? That is to exceed what the above example has and have over half of the prop in the tunnel?

If you aim for the centre line of the prop to be at the vee of the keel the you will end up with a good part of the prop inside the tunnel of you want to go further install a jet unit !! Draw it full size ,do the sums ,look at what you can do . The further up the steeper the tunnel entry and more chance of turbulance , There are limitations to everything !!!
What is your reasons of putting in a tunnel anyway?

[cdre]

I want to use this boat for fishing for Tarpon in Belize in the Belize river and coastal areas. The ideal boat for those areas is a flats boat with an outboard, but outboards spook the fish, and electric trolling motors just don't have the stamina to troll upriver for extended periods of time.

The idea behind the tunnel is to tuck the prop up and out of harms way a bit more than it already is. Lot's of semi-submerged debris in the river and I'd like to be able to get it in slightly skinnier water in coastal areas. I'd probably look to add some sort of "keel-guard" in front of the prop to direct any large debris down below it.

[tunnels]

Originally Posted by cdre

I want to use this boat for fishing for Tarpon in Belize in the Belize river and coastal areas. The ideal boat for those areas is a flats boat with an outboard, but outboards spook the fish, and electric trolling motors just don't have the stamina to troll upriver for extended periods of time.

The idea behind the tunnel is to tuck the prop up and out of harms way a bit more than it already is. Lot's of semi-submerged debris in the river and I'd like to be able to get it in slightly skinnier water in coastal areas. I'd probably look to add some sort of "keel-guard" in front of the prop to direct any large debris down below it.

After what you have just written i would use a V shaped strut instead of the usuall and one of two options have the prop close fitting to the hull roof with a vee Strut or give it a good amount of clearance and sacrafice a little performance so tings dont get jambed between the prop and the tunnel . Draw it out full size and look at the options

[baeckmo]

cdre, go to "Propulsion > Surface drives > DIY surface drive, post #46, attachment. There you find the critical proportions for a propeller tunnel, referred to propeller diameter, allowing you to calculate the dimensions of your own tunnel.

In particular, note the flat "roof" of the entrance; do NOT think of it as "a bent pipe", because the inflow to the propeller disc has a transverse fraction in addition to the pure longitudinal velocity. Also be careful to include the radii between tunnel and bottom, allowing crossflow in off-design operation and turns.

[tunnels]

Give me your e/mail address please i have something interesting for you to have a look at from another project i was involved with earlyer this year
stuartwrcom@gmail.com this is my address !

Originally Posted by tunnels

...... i have something interesting for you to have a look at from another project i was involved with earlyer this year

Why not sharing it with us tunnels?

[tunnels]

Originally Posted by apex1

Why not sharing it with us tunnels?

Will do as soon as i find the pictures of the full sized rough drawings i did up on the wall drawing board with the diamentions i was given .

Plus some pictures of the 147 foot boat i worked on making all the framing and internal structure etc etc a while ago that had twin semi tunnel drives with 3000hp a side

[cdre]

Thanks!