tunnel trim

[capnzee]

I have a recently purchased 20' true tunnel aluminum fishing boat currently powered with a 175 v-6 Merc. Problem is, the boat runs very "flat",(heavy in the bow), trimming the engine out does not raise the bow. The engine is set-back approximately 4 inches. The engine is mounted with the lower unit well down in the tunnel, absolutely no cavitation coming up on plane. I am planning on replacing the 175 with a 225 optimax next week, and replacing the 4" set-back with a 10". I am not a "tunnel" type guy, doing most of my boating with flat bottoms or Mod VP's. I am looking for props with bow lift characteristics (I currently have two mazco big ears, a 22" and a 24"). I'm wondering if lowering the engine would give me more bow lift--raising the engine doesn't seem to help. The boat is quite heavy, speed is satisfactory(55+), handling "sucks" as the boat wants to run like a sled. I believe moving the C/G back will help plus, the 225 is heavier--any ideas from the guys who run tunnels? Can any one agree or not with me concerning lowering the engine or moving the engine (c.g.) further aft? Any thoughts or recommendations are welcomed! Rod

[JPEROG]

I have a recently purchased 20' true tunnel aluminum fishing boat currently powered with a 175 v-6 Merc. Problem is, the boat runs very "flat",(heavy in the bow), trimming the engine out does not raise the bow. The engine is set-back approximately 4 inches. The engine is mounted with the lower unit well down in the tunnel, absolutely no cavitation coming up on plane. I am planning on replacing the 175 with a 225 optimax next week, and replacing the 4" set-back with a 10". I am not a "tunnel" type guy, doing most of my boating with flat bottoms or Mod VP's. I am looking for props with bow lift characteristics (I currently have two mazco big ears, a 22" and a 24"). I'm wondering if lowering the engine would give me more bow lift--raising the engine doesn't seem to help. The boat is quite heavy, speed is satisfactory(55+), handling "sucks" as the boat wants to run like a sled. I believe moving the C/G back will help plus, the 225 is heavier--any ideas from the guys who run tunnels? Can any one agree or not with me concerning lowering the engine or moving the engine (c.g.) further aft? Any thoughts or recommendations are welcomed! Rod

Get the new power put on there and see how it acts-might give you enough speed to create the lift you are after. If it doesn't, CG and balance is the game you will need to play.

Joe

[Mr. Demeanor]

Is this a full tunnel or a "pocket" tunnel like a lot of the flats fishing boats? What is the boat exactly?

[capnzee]

The boat is a full tunnel. The tunnel is quite wide, the boat itself is over 7 feet wide by 20' long.. I tested it before the floor and decks were put in and it ran great. It was much lighter at the time. What throws me is the boat does not react to engine trim. I am hoping that adding horse power and shifting engine weight aft by use of a 10" set-back versus the 4" will help--maybe even need more set-back. Will lowering the engine give me more "leverage" at the prop helping to hold the bow up? The boat does not have a lot of "lift" built into the tunnel. I have as much "moveable" weight as near the transom as I can get it. I think I need to get the bow higher in the air to obtain the angle of attack (lift) I need to help carry the boat. What more choices do I have: C.G., prop height, 3 blade versus 4 blade, more or less cup in the props? Thanks for the interest, I was beginning to think I wasn't going to get any "help" from the membership--again, THANKS, I have too much invested to just "give it all up!".

[Mo River Rat]

I have a tunnel v that I had motor too high, and didn't respond well to trim until I lowered motor. Bob.

[capnzee]

I have a tunnel v that I had motor too high, and didn't respond well to trim until I lowered motor. Bob. That is great supporting news--I am going to try lowering the engine and hope to gain some trim response. Thanks

[Carter Powell]

If you have a lot of added weight in the front then you need to consider a couple of things. First, you may have to add some more setback to move motor weight further reward. Secondly you need to go to a prop that has a lot of bow lift like a Mercury chopper. Remember that this type of prop will only give you that bow lift when the top blades are out of the water. If you don't have a hydraulic jackplate you should consider one.
I have seen most true tunnel boats react properly when using a chopper type prop. It may take all three things to get the front end up and flying.

Best wishes, Carter Powell

[capnzee]

I will try the new engine with a 10 in. set-back first. I have several Mazco big ear choppers with considerable cup. If this doesn't help I will add to the set-back, maybe a 15 incher. I can sure come way up with the set-back I have, but don't want to cook the Opti. I have a water pressure gage. I don't think I am getting enough lift from the runners, sponsons, canoes (what ever they are called) to lift the tunnel high enough to pack the air and "clear" the prop--trimming out will not raise the bow. Current speed is well over 50 mph but it is running like a sled. (It ran 67 with a 175 before we added the decks, consoles etc. Thanks for the input Carter.

[capnzee]

What is a good starting point for the prop height on the tunnel boat that is the subject of this thread? Tunnel is about 6 inchs deep and can plate is 1 inch below the floor of the tunnel. This puts the prop shift way down low could this be my problem with boat running flat?









I

[Mr. Demeanor]

Seems many start with the propshaft even with the bottom of the sponsons at neutral trim with a good surfacing prop. That would be with a nosecone pickup. Otherwise, height will be dependant on water pressure.

[capnzee]

replaced the 175 with a 225 opti and a 26 in. hi five prop, on a 10 in set-back. Worked great. bow lifted. Thanks for all the help! Rod