Hull shape problems

[Les]

Hiya all, Me and my Father took on a boat building project and we have and it somewhat compleate but has a few bugs. Hoping to get some help with a few problems.

The boat is a shallow V design 25' by 10', glass hull glass over wood deck and house. Power is a TAMD41 deisel (aprox 200hp) and a Volvo 270 drive. The drive has held up for at least one season :D

When we did research on drive mounting forums said to put the vent plate level to 2" below the hull. We ended up with it level. There is a small keel 2" deep but stops 14" from the transom. When we turn the boat or in rough sea conditions it ventilates badly. We have since lowered the drive mounting 1.5" but have yet to try this out due to a short boating season here. Think this will solve this problem?

We also have trouble getting the boat on plane. The drive only has manual trim as per the movable pin and seems to be near level as crusing speed. As we reach top speed 24kts the bow does lower some but not much. At any speed over 15kts we realy take a pounding. We added some manual adustable trim tabs(24" by 10") and this helps some but at top speeds as I push the bow down the boat slows down slightly.
When checking over the hull I found that is wasnt flat. If I hold a 8' staight edge up to the bottom(aft of midships) the hull is swept up at the transom about 3/4". I could link some pics if a better explanation is needed.
Should the hull be perfectly flat from the transom forward and how far? Could this also be adding to the ventilation problem?
I may be able to glass on a 3/4' wedge to straighten this out if it would help. Any advice would be appriciated.
Thanks Les

[captword]

lowering the ventplate should help on the ventilation problem. the wedge on the stern will help adjust the running of the bow. a cheap test on the affect of wedge is to do a temp bolt on the trim tab. If your zinc on your tabs are set to the out board side of the tab you can bore another whole on the inboard side and temp bolt a wedge on the trim tab and test for size needed. you can switch wedge sizes (overbaord) untill you get the boat to run the way you want. THat way when you do add wedge to the hull, you will be closer to what you need. I have seen some older craft that the yard adds a wedge to then sands the wedge after every trial till right. having to hall out for each adjustment. the wedge will slow the boat some but you get the ride you want. when a boat slows with more wedge it may be planing at the angle that it was desinged.
as far as pounding goes, you did say that it was a shallow vee hull, and it is slightly beemy. A lot of boats will run faster in calm seas than they can run in average sea states.
howard

[tom28571]

I basically agree with the Capt. However, just adding wedges under the stern will not be the best thing since it still leaves the 3/4" rocker in the bottom. The best thing would be to do what you suggested and straighten out the hull bottom so that the run (buttock lines) is perfectly flat over the aft curved portion. Wedges will bring the bow down but not eliminate the power robbing suction caused by the rocker. If someone does not like the word "suction", call it "negative lift" or something else. I have experimented with wedges by just taping them on the bottom with a wide plastic tape made by 3M for sealing around windows on Tyvec. They always held well enough to get the angle right.

The big offshore fisherman boats built in the Capt's backyard do usually have some rocker in the aft bottom but that is another story.

[Les]

Tom your explanation of the suction was what I thought might have been wrong. As I said my trim tabs are adjustable but manualy with a tunbuckle. To me it would seem as I lower the trim tabs down I would have to fight the hull shape and create a bit of a "suction" at the stern. Also the drag from the trim tabs I would think would be more than normal as they would have to be in a lower position to counter the hull shape as well.
If I were to add a wedge my goal would be to make it fit the bottom and flatten it out as much as possible.

Pic 13 shows the shape of the stern
http://www3.ns.sympatico.ca/lesley.surette/New%20boat%20pics/

Thanks Les

[tom28571]

A couple of years ago, I corresponded with another guy who complained of the same problem. I told him essentially the same thing discussed above. His boat apparently had less aft rocker than yours although it was caused by a mistake in either the plans or cutting of the S&G panels on the aft keel.

He faired out the aft bottom of his boat and said that it planed earlier, ran faster and ran with less nose high trim, all normally good things. You can see his website and the photos of the fix at:

http://egyptian.net/~raymacke/Cbnskif30.htm

He is epoxy sensitive which caused him to use the wood fairing pieces in the photo which I think is a good idea anyway since it makes accurate fairing much easier.

[Les]

Thanks Tom for your advise and the link helped as well. I was fairly certain I had to correct this problem and this confirms it for me.
Les