Adding wedge to fix rocker problem?

[Steve Miller, Jr]

Some of you may have seen my post concerning the launch of the skiff that I built in the designs/plans section. The boats bow rode too high in the water, and it would not plane. I made a few changes such as a different prop (now running 5500 RPM's), added a doelfin, and moved the battery all the way forward. Currently it is much better. You all made some great suggestions, However, I really don't feel comfortable cutting the bottom off. I would like to get your thoughts on adding a wedge to the bottom of the boat. I am not really looking for the boat to plane, I am rarely in a hurry, I just want it to ride at a decent speed more level than it is. How big of a wedge would level it out? Please help. Building this has been a lot of fun, but I am spent and falling in love with plastic and aluminum fast.

[Tristan]

I speak from limited experience with power boats, but I wonder if it is worth while to install a wedge(s) if you don't want the boat to plane. With a proper run your boat will either move with minimal wake slowly, plow through the water with a big wake at intermediate speed, or plane at higher speeds. It seems to me that getting her to move faster, though below planing speed, might just make her dig a bigger hole in the water. If you give her a flat run she'll plane. In otherwords, the planing hull boats I've been in either move slowly with little wake, throw a huge wake at intermediate, sub-planing speed, or plane. As for the rocker in the bow, seems to me she'd plane anyhow (in calm water) were her run flat rather than rockered. That is, it's the after part of the bottom that allows them to plane. The bow is pretty much out of the water (in calm water) when the boat planes except in a seaway. And in a seaway you'd want to throttle back anyhow I expect. Seems like two different things here. You need the after part of the bottom to be deeper (flatter run) so she'll move with the bow down. If you just put a wedge in it will serve to get her up on a plane, but might be guesswork as to how much wedge to drop the bow at sub-planing speeds (and how much wake she'd throw). Your boat might need more bouancy aft to ride level at sub planing speeds, not wedges (but wedges would help her plane). The above is from my personal experience and from memories dredged up from a boat design course taken about 30 years ago. .

[ 09-19-2005, 02:57 PM: Message edited by: Tristan ]

[Memphis Mike]

Before I installed a wedge, I think I would try some transom mounted adjustable trim tabs.

Here's one of many links.

http://www.insta-trim.com/main.html

[ 09-19-2005, 03:09 PM: Message edited by: Memphis Mike ]

[Bruce Hooke]

For those who did not follow the original thread and want some background, I believe this is the original thread (http://www.woodenboat-ubb.com/ubb/ultimatebb.php?ubb=get_topic;f=2;t=004502) .

[Bruce Hooke]

As an aside Steve, I think you may be slightly misusing the term "plane." If the boat sits ok at rest but the bow starts sticking way up in the air when you get to a certain speed then the boat is trying to plane and if you truly do not want to plane then you should simply drop down below that speed. HOWEVER, I suspect you could probably reach that speed at something like 1/4 power on the motor and your speed likely be well under 10 knots. If you want to go faster than that then you do need to get the boat up on a plane.

Have you checked the boat against the drawings to try to figure out if indeed you did make a mistake that resulted in the boat coming out with a significantly different shape than the design called for? It would be useful to know that because if the boat did come out as designed and if others have built boats to this design that do plane well then that would greatly reduce the possible issues that could be causing your problems.

[ 09-19-2005, 03:22 PM: Message edited by: Bruce Hooke ]

[RonW]

In your other post, I made the statement that it was a poor design and trim tabs, wedges, redistribution of weight is going to do nothing. The boat has too much rocker, and consequently the faster you try to go past about 6 miles a hour the more the nose will point skyward,(porposing)the more the stern will squat and it will wobble around on what is left of the stern area to support itself.

I have a 18 ft. western river dory that I built for fishing rough water, it has more rocker then your boat, it is a great rough water boat, but it will only go about 8 m.p.h. and if you get past 5 mph you have to stand up to see where you are going.

Flat bottom boats may seem easy to build, but they are very sensitive to design as to get them to do what you want and not something else. Far too many people think they can design flat bottom boats, but only end up with results like yours.
They put the rocker up front to, as they think help get it on plane, but that is just backwards.
I think they get these ideas from the pacific dory, which is flat, but only the last couple feet of the bow is highly rockered. This has nothing to do with the bottom, but the rocker is just enough to get a tilt trailer under the nose and winch it up on the trailer from the beach.
Now look at the carolina dory, it has a perfectly flat bottom from bow to stern, no rocker and it rows well as well as a planing hull.
Then look at atkin's flat bottom row and power skiffs, the bow is submerged and a small amount of rocker at the transom, just exactly opposite of what you got.
http://www.boat-links.com/Atkinco/

I will make 3 suggestions, do as Mike says, cut the bottom off and redo it, sell it and start again, or be happy plugging around at 6 m.p.h.

I looked at the website for that boat, and the picture that told me the truth, was the one where someone was standing in the middle of the boat with a exstension handle on the motor, and plugging along at about 4 or 5 m.p.h.
They where keeping the weight forward, and going slow. That is because when you sit next to the motor and give it gas, it is going to porpoise.
Due to the fact it has too much rocker up front.

[Stephen Hutchins]

Well, you might be able to glue some plywood to the bottom, stepped in layers fore to aft, and plane down with a power planner, using the steps as a guide. Then seal with epoxy/glass and give it a go. I can't guarentee anything, but that's what I'd try. It would add some weight back aft, which in turn might require you to move something foward.

[ 09-19-2005, 03:45 PM: Message edited by: Stephen Hutchins ]

[Bill Perkins]

Here's a drawing of the old wedges across the transom detail .The spec on the wedges reads : " 1 to 1 1/4 in. thick and from 12 to 24 in long for average power boats of 20 to 35 feet ." Why not experiment cautiously with a set of the long wedges ,scaled down from the 1 1/4 in. thickness proportionally to the length of your boat ? If the resulting handleing is freakish in any way,mabe you should change approach and go slow ,with an electric or a small gas out board .

http://www.imagestation.com/picture/sraid174/pb157edd8b4ca7556a376f704f988cb5e/f38eec1b.jpg

[ 09-19-2005, 04:08 PM: Message edited by: Bill Perkins ]

[RonW]

Rather then putting wedges on the back, you need to put about a 10inch wedge on the front.

http://www.boat-links.com/Atkinco/Oar/images/Punch-2.gif

[ 09-19-2005, 04:03 PM: Message edited by: RonW ]

[Steve Miller, Jr]

How about the automatic (non-electric) gas cylinder trim tabs? would those level out the ride, easier to try and less permanent than more epoxy and wedges. Plus, I could just take them off and return them if they didn't work. at this point I'll be happy with a slow level ride. Running out of patience and $$.

[ 09-19-2005, 04:16 PM: Message edited by: Steve Miller, Jr ]

[Bruce Hooke]

RonW,

I'm no expert in planing powerboats, but it seems to me that many of them (on the bottom) are quite "high" in the bow and that the key thing they need is a flat run aft. So, I am curious about why you are suggesting that the bow area is the problem...

- Bruce

[Bruce Hooke]

Steve,

I can't answer your trim tab question, but you could probably test the wedge idea with just some low-grade plywood and a few screws to hold it in place. Of course you would have to fill the screw holes if the idea did not work, but that would not be so hard...

- Bruce

[sdowney717]

Try something radical like this
How about a totally new idea.
starting about where your helm station is
build a new plywood bottom to the boat. When it reaches the transom it should be about 6 inches farther down. Extend this bottom about 8 inches further past the current back of the transom and then angle up a piece to meet the current bottom of the transom.
this would take out a lot of the rocker, extend the waterline lenght and give it more bouyancy in the back.
so it would create a wedge spaced chamber on the bottom.

[holzbt]

Get some of the blue foam flotation that is used for floating docks. Carve it to fit the rocker from the transom forward to about amidship. Glue it to the bottom and then shape it from amidships back in a straight or almost straight line to remove the rocker. Glass over and your boat will now plane properly. The gentle curve in the run of most of the Atkin boats is suitable for low power. If you want to use medium or higher power keep the run pretty flat and straight.

[RonW]

Bruce.
Look at the drawing I posted, it is one of Atkin's. Notice the flat bottom up front and the rocker in rear. Go to their site that I posted and look through at a lot of their flat bottom designs and read their comments. They also talk a lot about low speeds, and in their high speed flat bottoms they are only talking high teens.

For faster flat bottoms, go to glen-l and look at the pacific dory with only the nose turned up so as to get a tilt trailer under it. And their sweet carolina dory which is 100% flat from stem to transom.

I believe the boat in question here with problems, has double rocker, a little in the back and a lot up front. Double rocker will not work other then in a river dory as I exsplained above. I think the problem with this boat is all the rocker up front.

Too many people want to build flat bottom boats for easy construction, and then they are dissapointed when they try to run them at planing speeds and higher.

[Tom Lathrop]

This boat has too much rocker to plane and you will have to accept that fact. Trim tabs or wedges may push the bow down but it is never going to plane. Think about that bottom shape. All planing boats have straight or nearly straight fore and aft lines (buttocks) in the after part of the bottom. Any convex curvature (like yours) in the aft bottom develops negative lift and sucks the stern down. Yes, some boats do suck and I'm most sorry to say that if you were sold a plan for a planing boat, you have one of those. It is the aft rocker that causes most of your problem, NOT the rocker up forward.

The only way to make this thing plane is to straighten out the aft bottom, however it may be done. The other suggestions are just not going to work. Bite the bullet or buy some marshmallows. Sometimes life is hard, but think of all the experience you are gaining.

To verify this, read some from Dave Gerr, Ted Brewer or other designers on hull shape for different applications. Beware of all advice that starts out with "I don't know anything about X, but here is what I would do", or any version of that.

One way out is to build another boat for the 15hp motor and buy an old 4 or 5hp kicker for this one and be happy fishing at 6mph or sell it to someone happy at that speed..

Finally, good luck and don't let this problem stop you from going ahead.

[ 09-19-2005, 07:26 PM: Message edited by: Tom Lathrop ]

[Bruce Hooke]

OK, I see what you are getting at. I would agree that most planning craft have a pretty long straight run that starts not so far aft of the bow, and this boat certainly does not have such a run. However, if you were to put a 10" wedge under the bow to create such a run I think you would end up with a boat with an annoyingly high bow and a pretty odd looking profile. So, I wonder if it would make more sense to put in a stern wedge and then if that is not sufficient add a much smaller bow wedge than the one you are proposing.

[RonW]

Bruce : I was exaggerating a little, about adding 10 inches.
But the point I was making is when you are in the boat as the pictures shows, the bow starts pointing towards the sky automatically, so adding power from a outboard on the transom is going to do nothing but make the situation worse.
Too much rocker.
Straight runs from about the middle to the transom works in v's, but it might be interesting to note how the Atkin's dealt with flat bottoms, even in the planing flat bottoms, as to putting a very slight rocker in the rear and if anything a slight down ward on the bottom toward the bow.

Actually when you look at atkin's flat bottoms, there is a very slight rocker from the middle to the bow as well, but it is so slight that from the side view it almost looks as if it goes down.
Really a total different approach then having a flat run from the transom to the middle of the boat and then a lot of rocker from there to the bow.

[ 09-19-2005, 08:39 PM: Message edited by: RonW ]

[holzbt]

My sugestion of building the bottom down with foam aft of amidship will work. It's just a few dollars worth of foam and glass and can be removed if you don't like the results with no harm to the existing hull but a bit of glue to be sanded off.

The Atkin boats are meant for low power. Almost all of his flat bottom outboard skiff call for less than 8 HP. They sort of plane but are really a hybrid of row/low power skiff.

I don't see how straightening out the forward part of the bottom is going to make it plane. When is the last time you saw a planing boat running in smooth water with the nose in the water and the stern out. It doesn't happen. I also don't know where this no straight run in a flat bottom comes from. That would be very interesting news to the many watermen around here that used flat bottom power sharpies and garvies. If you want to plane you need a sraight run.

[Old Bingey]

It has a good shape for a displacement boat and would work well at displacement speeds with a smaller engine.

[Steve Miller, Jr]

Mlke,
Your right, my post was pretty pathetic, your post was kind of a kick in the ass, thanks. I have spent a lot of time on the project and it has been a labor of love, we fell in love with boats on our annual trip to Maine four years ago, where we rented an unpowered 17'sail boat several days in a row, and every year since then and have had a blast in Casco Bay, so no it is not a passing phase, I am in it for the long haul, and will most likely always have a boat, and it will be made of wood. After we sold our 23' Hunter sail boat in Nov. 04, I knew I needed a boat and unfortunately I picked the wrong plans for what I was looking for. I realize that I may seem to be at the end of my rope, but there is never a project that I have not completed, and although it may seem done, I realize that there is still work to be done. I will use the boat this fall and see how it goes as a slow displacement hull, and if I am still unhappy with it I may fix it this winter, or carefully pick a new boat to build. I have only been in it twice, perhaps for a total time of maybe 20 minutes since I made changes from the first launch, and most of that was at wide open throttle, so I should not have bothered you all with my question, before getting a real feel for the boat and how it performed at speeds other than full. However, I appreciate your suggestions, you certainly seem to know it all. I knew from my first boat that they are a waste of money, but this one has not been a waste of time.

[Bob Smalser]

http://static.flickr.com/29/42675819_0d22bb2e63.jpg

All I seem to do with planing boats under 30+ feet is lose fillings and rattle my kidneys, so I rarely enjoy them.

Halve the size of the motor (and weight) back there and I'd be very happy with that little launch at 6 knots, which is where I always seem to be running planing boats anyway in anything rougher than a mill pond.

Short of a total rebuild, I'd also have a hard time modifying that bottom without the end result looking like amateur patchwork.

[ 09-20-2005, 11:33 AM: Message edited by: Bob Smalser ]

[Victor]

Duh. What's rocker?

A wedge would create enormous drag, even if it worked, right? Before you do anything radical, get some manual trim tabs and affix them permanently in the full down position. Works really well for me! If it doesn't work for you, you just wasted 35 bucks.

[ 09-20-2005, 06:05 PM: Message edited by: Victor ]

[Memphis Mike]

Steve, I'd just put a smaller motor on it and enjoy it. It's a beautiful boat and your workmanship is excellent. It's just not a planing hull.

Save the 15 horses for your next project.

[pipefitter]

Repairing the bottom of that boat would be satisfying work once you got it done.Also,it would fly with that 15hp but you would still probably have to have someone in the front.It isn't as hard as it looks and the mistake looks like it is in the middle. It just sounds to me like you don't want to go 6 knots and you dont even probably have to take the bottom all the way off.Mike's picture on the previous post looked correct for that boat from where I sit.Bite the bullet and get it done.You won't ever be satisfied with it the way it is or you wouldn't have tried all the things that you have already.Would help to see a top view of the actual boat as well as to see chine area.Certainly if you are considering glass or aluminum you have nothing to lose by fixing it and I can't imagine you making it any worse.Go for it,you built the thing you should know it inside and out.