reluctant planning hull

[michael alan]

I have a new 38 ft x 10 ft beam Thai built ,old bertram design ,deep v planning hull boat with twin 300 hp outboards recently fitted .
She has a step in the last approx . 5 ft of the hull at the stern , to which the motors are both fitted .
She will attempt to plane up to 24 mph but with a lot of effort
The only way I get the boat to plane above 24 mph is by jumping a big wake of another boat and getting my stern out of the water for an instant -then she goes like a bat -suddenly accelarating up to 40 mph.!
I have tried 10 degree trim tabs on the transom to lift the stern but that does not help her to plane at high speed -she does not lift and really accelarate until I hit a very big wake -it is as if the stern is "stuck" in the water
any suggestions please ??
thanks
michael

[tom28571]

Presuming the boat is designed and built correctly, this problem almost always starts with too much weight. The massive weight relative to the bottom area in contact with the water is creating a wave that the available power cannot get the boat over without outside help, like the wakes you mention. Adjustable trim tabs is the usual fix which lift the stern and increase the boat length (hull speed) enough to get past the hump. Transferring weight forward can help also as well as adding weight forward, a hated practice since it speaks of poor design.

Different props that might add more thrust in the problem speed range could help but would probably decrease top speed some.

Its a problem common to deep V hulls which are always heavier than like sized shallow V hulls. Deep V hulls have to be heavier to keep the chines immersed and make them acceptably stable. Extra weight needs more power and fuel which makes them heavier, which needs more power and fuel, which makes them heavier and so on, and on. 600hp just may not be enough for the boat. My solution is to avoid such boats like the plague unless one is absolutely required for high speed in water conditions offshore.

The "step" you mention is not really a step in the normal sense and may actually contribute to the problem by shifting weight further aft without adding aft buoyancy and length.

Try taking all unneccessary weight off of the boat and see how it does - it just might be enuf to get that stern out of the water and you will have more information. Trim tabs are really not my cup of tea, especially at these speeds. You may try glassing trim planes right to the hull, right to the bottom paint. These should be in line with the planing surface or very small down angle at your discretion. The idea is to get much more planing area and keep that aft area out of the "hole". Again, you will gain information as to what you need to do. By chance, do you have a picture of the area?

[erik818]

Maybe you need to ventilate the step with a pipe that leads air into the step and eliminate the suction. I've heard/read about this problem but have not actually seen it myself.

Erik

[Oyvind]

I have a 32 CC with twin 225s on a Stainless Marine engine bracket and hydraulic jack plates. My hull is a 24deg offshore hull with a transom notch to improve top speed, and when trying to get on plane, the hull used to experience severe suction in the notched area, preventing the boat to plane properly. I always had to work the boat to make it "let go", and then it ran properly. Very much like your scenario.

This bothered me, so I ended up with two alternatives; 1 - ventilate the notch with pipes and valves, or 2- remove the notch. I needed the added buoyancy, so this winter I removed the notch/ filled in the gap. The boat changed completely! There is no effort in getting on plane, it takes on waves a lot better, and is just a better boat. I did, however, get tendencies to overplaning at WOT, but I have ordered a pair of bowlifting props which will sort this out.

My advice; ventilate your notch using SS through-hull fittings and pipes with shut off valves! Good luck!

[u4ea32]

Oyvind, I think the right solution is the one you did, not the one you recommend: remove the notch, make the bottom shape continue to the transom.

Tom and Mark, I agree with you: get rid of weight, and move what you've got to have further forward.

The air pipe thing is totally urban legend. To experiment, fill your sink with water. Take a small pot, submerge it in the sink so its totally full of water. Now invert the pot, so its bottom is facing up. Try and lift it. Hard to do! But now tip the pot on the side and lift it. No problem at all.

There is no suction down there. The step is like the pot on the side, not the pot inverted: all air pressure at sea level is very close to identical pressure, whether it comes through a pipe or comes from the surface of the water to the aft face of the step.

There are conflicting accounts on the effectiveness of air pipes, however the fact that people only put them in once is pretty good indication that they really don't work.

What is being observed is the very large "hump" in resistance before planing. This hump is due to wave making. Hitting a huge wave to get airborne, or at least forcing the angle of attack to change dramatically momentarily, leads to a momentary but dramatic reduction in wave drag. That allows the boat to finally break free of the hump resistance and start planing.

The heavier and fatter and deeper V the boat, the larger this hump. Make the boat lighter or longer and you'll reduce the size of the hump. Get rid of that step back there and you make the length of the bottom longer and the hump smaller.

[tunnels]

Need some pictures of what you have got !! The boat out of the water and the shape of the wake at the back of the boat when its stuck or bogged down !!
If theres a step in the back and you have 600 hp it should be flying but the design of the step that could be the big problem .Simple fix Fill it in and fair it all the way to the back .

[gonzo]

Who told you that is a Bertram design? They had moderate and not deep vee bottoms. Also, the step and outboards don't seem like anything from an old Bertram

[Alik]

Yes can try to fit trim tabs.
Check propellers condition, engines condition.
Weight - yes, likely to be an issue. In this case extension of chines could help.

[Oyvind]

"There is no suction down there. The step is like the pot on the side, not the pot inverted: all air pressure at sea level is very close to identical pressure, whether it comes through a pipe or comes from the surface of the water to the aft face of the step."

Agreed, in my case, however, there was a distinct suction phenomenon; I could see water being pulled up along the bottom surface of the engine bracket, and as soon as I could brake the hull free, one could clearly see the wake level out.

I also have worked on several stepped HiPerf boats with the same problem, where the suction occurs in both the notch and also the transversal step, if present, when the steps are not properly designed/ ventilated. Stepped boats are particularly exposed to suction in sharp turns, when coming down from plane, running in following waves or if not properly designed/ ventilated.

To overcome this problem we ventilated both the step and the notch using through-hull SS fittings, shut off valves (to avoid leakage) and high quality rubber hoses with double hose clamps. This particular boat was unable to run properly before this was done...

[tunnels]

Need photos !! theres something you are not seeing !!

[Knut Sand]

Originally Posted by u4ea32

There is no suction down there. The step is like the pot on the side, not the pot inverted: all air pressure at sea level is very close to identical pressure, whether it comes through a pipe or comes from the surface of the water to the aft face of the step.

Agree only partially to this... If the hull is still, 100% correct. If the hull is moving, you'll get a "suction" the hull will partially experience beeing pulled down into the sea, think of it as an airplanewing with the lift section downwards. The water doesnt know if its the boat or itself that moves. And being compressed under a moving hull, it got to create some increased speed to getaway, in order for energy in that area to be in balance, increased speed = lower pressure. So, for some speeds, I'd assume that the tube idea will work, close to "perfect". (A stepped hull is preeeetttyyy far of from my idea of an ideal hull shape though... ).

Come to think of it... "Proffessional boatbuilder" had an article of some of the early high speeds craft, deplacement hulls steam engines, and speed tests, there was a photo series there... you could see the boat getting lower in the water for each pass, as the speed increased... scaring stuff, steam engines and close to zero freeboard.... (a recipie for a date with St. Peter...). You could look that article up on the net. IF the pressure should remain identical, the hull would have the same freeboard for zero speed as for max speed (until "out of the water", but that's just your problem?).

I also believe Oivind's solution to be the preferred one, but to glue a (half) tube to the hull, to vent that step, is far less trouble...